Content uploaded by Jorge Moreda-Piñeiro
Author content
All content in this area was uploaded by Jorge Moreda-Piñeiro on Aug 21, 2023
Content may be subject to copyright.
In: Cadmium in the Environment ISBN: 978-1-60741-934-1
Editor: Reini G. Parvau © 2009 Nova Science Publishers, Inc.
Table 8 Missing on Page 77.
Chapter 1
ANALYTICAL CHEMISTRY OF CADMIUM:
SAMPLE PRE-TREATMENT AND
DETERMINATION METHODS
Antonio Moreda-Piñeiro1 and Jorge Moreda-Piñeiro2
1Department of Analytical Chemistry, Nutrition and Bromatology, Faculty of Chemistry,
University of Santiago de Compostela, Avenida das Ciencias,
s/n. 15782 Santiago de Compostela, Spain.
2Department of Analytical Chemistry, Faculty of Sciences, University of A Coruña,
Campus da Zapateira, s/n. 15071 A Coruña, Spain.
ABSTRACT
A brief description of sampling strategies, preservation and handling of
environmental samples (liquids, solids and air) are commented with particular emphasis
on the subsequent cadmium determination.
The main analytical techniques for cadmium determination are briefly described, and
they are compared in base on their advantages and drawbacks. Therefore, UV-visible
spectrometry, atomic spectrometry techniques, neutron activation analysis, and some
electrochemical methods are considered. In addition, the use of automatic analyzers for
environmental monitoring are also described.
The main sample pre-treatment methods for liquid and solid environmental samples
are also discussed. Well-established methodologies as well as current trends are reviewed
for cadmium determination in environmental matrices such as water, soils, sediments,
biota, and atmospheric particulate matter.
In addition, cadmium speciation, mainly cadmium fractionation in soil, sediment,
sludge and atmospheric particulate matter (BCR method), and isolation and separation of
cadmium bound to metallothioneis in biota is also reviewed. Finally, a source of literature
on standard and official methods for cadmium determination in environmental materials
is also provided.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 2
SUMMARY
I. Cadmium. Environmental implications.
II. Sampling and sample conservation: Sampling programs. Sampling for water.
Sampling for soils, sediments and sludge. Sampling for biological matrices (biota). Sampling
for air matrices. Sampling for atmospheric precipitation.
III. Preservation and storage. Preliminary operations. Considerations for preserving water
samples. Considerations for preserving soil, sediment and sludge samples. Considerations for
preserving biota samples. Considerations for preserving atmospheric particulate matter.
IV. Analytical techniques to assess cadmium. UV-VIS absorption spectrometry. Atomic
absorption spectrometry (AAS). Flame atomic absorption spectrometry (FAAS).
Electrothermal atomic absorption spectrometry (ETAAS). Inductively coupled plasma based
techniques. Atomic fluorescence spectrometry (AFS). Chemical vapor generation– atomic
spectrometry (CVG-AS). X-ray fluorescence spectrometry. Neutron activitation analysis.
Electrochemical techniques: Anodic stripping voltammetry and potentiometric sensors.
Automatic analyzers
V. Sample pre-treatment procedures for environmental samples. General aspects. Sample
pre-treatment for liquid samples: Non-boiling evaporation; Lyophilization; Precipitation and
co-precipitation; Polymer-mediated extraction; Electrochemical deposition; Liquid-liquid
extraction; Liquid-liquid micro-extraction; Cloud point Extraction; Single-drop micro-
extraction; Dispersive liquid-liquid micro-extraction; Hollow fibre liquid phase micro-
extraction; Solid phase extraction; Electrochemical solid phase micro-extraction; Low
temperature directed crystallization. Sample pre-treatments for solid environmental samples:
Decomposition sample pre-treatments – Microwave assisted sample decomposition; Leaching
(extraction) procedures; Pressurized liquid extraction; Pressurized hot water extraction;
Enzymatic hydrolysis methods; Slurry sampling technique.
VI. Cadmium speciation. Sequential extraction schemes for metal fractionation.
Metallothioneins / metallothioneins-like proteins.
VII. Official methods for cadmium determination in environmental samples.
I. CADMIUM. ENVIRONMENTAL IMPLICATIONS
Cadmium is used in the industrial fields due to its excellent features. However, this
element is one of the most toxic metals, and such anthropogenic sources of cadmium are
negative to the environment and health. Renal toxicity, pancreatic cancer, or enhanced tumor
growth, are some of the reported illness related to environmental, occupational, or dietary
exposure to cadmium [1].
Several studies have pointed out that man's cadmium intake, as least for non-smokers,
comes principally from the ingestion of foods rather than from inhalation of cadmium in air.
However, it must be mentioned that cadmium levels of foods have substantially decreased
during the past several decades due to the progressive control of cadmium emissions to the
environment [2].
Cadmium levels in the environment vary widely. In ambient air, cadmium is mainly
associated with airbone and estimated concentrations ranging from 0.1 to 5 ng m-³ in rural
Analytical Chemistry of Cadmium: Sample Pre-treatment… 3
areas, from 2 to 15 ng m-³ in urban areas, and from 15 to 150 ng m-3 in industrialized areas,
have been established [3,4]. Differences in cadmium levels between indoor and outdoor air
are not reported. Nevertheless, this is true only in non-smoking environments because
smoking may substantially affect indoor ambient air for cadmium levels.
Cadmium air concentrations can be higher in certain occupational environments,
although occupational exposure standards in such environments have been progressive
reduced from 100 to 200 µg m-³, to 2 to 50 µg m-³, with the control requirements to maintain
cadmium-in-blood and cadmium-in-urine below certain levels to assure no adverse human
health effects from cadmium occupational exposure [5].
The average cadmium contents in the oceans can be lower than 5 ng L-1 [3], although
other reference values have also been reported; within 5-20 ng L-1 [6], around 100 ng L-1 [7],
or within 10 to 100 ng L-1 [4]. These levels can be higher in estuarine waters and around
certain coastal areas. Greater variations are quoted for the cadmium contents of rainwater,
fresh waters, and surface waters in urban and industrialized areas. Levels from 10 to 4000 ng
L-1 have been reported in the literature depending on specific location and whether or not total
cadmium or dissolved cadmium is measured [3,4].
The average natural abundance of cadmium in the earth's crust has been reported to be
from 0.1 to 0.5 mg kg-1, but much higher and much lower values have also been cited
depending on a large number of factors. For instance, cadmium levels are lower in soils
derived from igneous and metamorphic rocks (tipically from 0.02 to 0.2 mg kg-1), whereas
soils from sedimentary rocks offer much higher values (up to 25 mg kg-1).
Concerning foodstuffs, cadmium levels vary widely and cadmium contents may be
affected by the agricultural practices utilised in the particular areas such as phosphate
fertiliser, sewage sludge and manure application. Leafy vegetables (lettuce and spinach) and
certain staples (potatoes and grain) foods content relatively high cadmium values (from 30 to
150 µg kg-1). Meat and fish normally exhibit lower cadmium contents, from 5 to 40 µg kg-1.
Some animal tissues such as kidney and liver content extraordinarily high cadmium levels
(mainly associated with metallothioneins), and they can be to 1.0 mg kg-1 [3].
II. SAMPLING AND SAMPLE CONSERVATION
In addition to the developement and application of fast sample pre-treatments and
accurate analytical methods to determine trace elements, reliable environmental studies can
only be assessed after adequate sampling and sample conservation procedures.
II.1. Sampling Programs
The sampling program must ensure that a representative material is sampled, and also it
must take account of temporal and spatial variability of such environmental samples.
Representative samples are comonly obtained after random (random sampling) or systematic
(systematic sampling) selection of the environmental material from different sampling sites,
and also, after collecting different samples from each sampling point. Random sampling
consists of collecting samples randomly, ensuring that all parts of the system to be sampled
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 4
can be statistically collected. This sampling startegy allows to find “hot spots” (locations with
higher concentrations of pollutants), which can be later evaluated after a systematic sampling
strategy. In constrat, systematic sampling consists of dividing the area to be be sampled into
an imaginary square or triangular grid, and samples are collected at the nodes of the grid, at
the center of the spaces defined by a grid, or randomly within the spaces defined by a grid.
Moreover the physical state and location of the different sampling sites and the number of
field duplicate samples, the sampling plans also consider the selection and maintenance of the
sampling equipment, as well as the use of bottle blanks, field blanks, trip blanks and
equipment rinse samples (equipment or rinsate blanks). These strategies are mainly focused to
detect contamination sources, such as the effectiveness of the cleanning of sampling devices
and sample containers (bottle or material blanks), or contamination from the environment
around during the sampling and the sample transport (field blanks and trip blanks). In
addition, background samples are also useful for environmental sampling. These are samples
of the media similar to the test sample matrix and are taken near to the time and place where
the analyte may be present at background level [8]. The volume or the amount of sample to be
collected must be also especified in the sampling programme. It must be noticed that there are
several guidelines on quality control in sampling for environmental materials, such as the EN
ISO European guidelines based on the ISO international guidelines [9], or the American
ASTM guidelines.
II.2. Sampling for Water
Because surface waters include different types, such as creeks or rivers, lakes, estuaries,
seas, effluents and groundwater (well water), there is no a general procedure for sampling.
After the establishment of the location of the sampling sites (both random or systematic
sampling), either glass or plastic bottles used in sampling and conservation of samples for
trace metals determination must be previously cleanned by soaking in 10% nitric acid for at
least 48 hours, and rinsing several times with ultrapure water. Before collecting the water
sample, a general practice consists of rinsing the sample container two or three times with the
sample to be collected. However, this is not recommended for sampling waters containing
high amounts of suspended solids and/or grease because these materials can be adsorbed on
the walls of the container and the collected samples can be enriched with them.
(a) Surface waters
For creeks and rivers, samples are comonly collected from the mid-channel at the surface
(surface water) by immersing the sampling container at a fixed location and by retrieving. If it
is not possible to take the sample by submerging the container by hand, laboratory forceps or
a holder with a sliding sleeve can be used. Similarly, if the sample is to be taken from the
mainstream by reaching out from the bank then it may be best to attach the container to a
segmented rod made up to the appropriate length. In shallow streams the sample is usually
taken from approximately the top third of the water, or from a level between 20cm and 30cm
below the water surface.
Similarly, surface or depth water can be sampled in lakes at the fixed sampling locations
according with a random or systematic sampling plan.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 5
(b) Deep waters
Deep-water sampling is important because some analytes may be stratified at different
depths. Stratification is also a problem with estuary and ocean sampling and either surface or
deep-estuarine/seawater is comonly sampled.
Depth samples (sub-surface samples from rivers, lakes, estuaries and seas) are collected
by using depth samplers, such as Mayer’s submergible bottles [10]. This device consits of a
ballasted glass bottle and a short cable attached to its neck with a loop to the stopper. The
bottle is lowered to the desired depth with the stopper loosly inserted and then, the cable is
jerked to release the stopper and the bottle fills. These samplers offer as a main disadvantage
the difficulty of opening the bottle in deep water because the excess pressure of the stopper.
Flushing samplers, such as the earliest Ruttener’s or Theiler-Frieinger’s devices avoid this
problem. These samplers (cointaining around 1 to 2L) consist of a glass or polyethylene
cylinder with a pair of plate valves at each end, which are connected by a metal rod. The
sampler is lowered with the end valves open, allowing flushing. At the required depth, the
valves are closed by a mechanical messenger (a metal piece which hits the actuaring
mechanism allowing top and bottom valves to be locked). Modern Nyskin-type bottles (made
of polyethylene and 1.5L capacity) offer a similar mechanical operation.
Small pumps can be also used to collect deep-water samples, especially in large water
reservoirs such as lakes and seas when pumping does not cause disturbance.
In addition to the sampling device, water sampling requires usually auxiliary equipments
related to metereological and atmospheric conditions. Measurements of water temperature,
pH, conductivity and fluorescence (parameter related to algal bloom) are taken in the field at
the time of sampling. For rivers and creeks, additional measurements of flow volumes are
also taken at each sampling location. Different specially designed devices for flow volume
measurements are commercially available [10].
(c) Groundwater
Groundwater sampling deserves special attention. Groundwater sources include well or
piped waters and spring waters, and knownledge of the physical and chemical characteristics
of the aquifer (the rock formation containing the groundwater) would be needed before a
sampling strategy. The time of the year (sampling before or after rainy season) affects
considerably the groundwater composition, and other considerations such as sampling after
periods of high agricultural chemical usage must be taken into account. In addition, well
water sampling must not be affected by contamination from the loosened topsoil and when
constructing and monitoring wells, well materials must not influence the chemical
composition of groundwater. In this sense, cement used for pipe joints (pipes made of
polyvinyl chloride) must be avoided and threaded pipes are highly recommended. In addition,
equipment for monitoring wells must be made of stainless steel or, better, of
polytetrafluoroethylene. These devices should be designed to avoid excessive aeration so that
analyte oxidation can be minimized. As a general rule, purging wells before sampling is a
recommended procedure because stagnant water is eliminated. The method and the rate of
purging, time between purging and sampling, and sampling itself depend on several well’s
characteristics such as the diameter, depth and recharge rate of the well. To establish when
the well sample is representative, changes in the pH, temperature or conductance are
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 6
monitored in consecutive samples. Sampling devices for groundwater are electric
submergible pumps, bailers, suction-lift pumps and positive displacement bladder pumps.
Detailed description of groundwater sampling can be found in specific monographs [10].
Standardized methods for water sampling can be found in the ISO 5667 guidance,
especificaly guidelines ISO 5667 Part 4, ISO Part 5, ISO 5667 Part 6, ISO 5667 Part 9, ISO
Part 10 and ISO 5667 Part 11, related to water sampling in lakes, rivers and creeks, drinking
water from treatment works and piped distribution systems, estuaries and seas, waste waters,
and groundwater samples, respectively [11-16
II.3. Sampling for Soils, Sediments and Sludge
(a) Shallow soils
Scoops or shovels and trowels made of high density polyethylene are commonly used for
soil sampling at its surface or at shallow depths (less than 15 to 30cm). These devices are
adequate when non volatile analytes, such as metals, must be determined. The use of these
non-disposable sampling equipments creates a potential for cross-contamination between
samples, and they must be decontaminated as well as equipment blanks must be collected and
analyzed. It could be easier the use of separate sampling devices for each sampling point, and
the decontamination process can be carried out later in the laboratory. Surface and shallow
soil samples can also be taken with soil punch or thin-walled tube devices, the later ones can
also be used as shipping containers. Detailed description of these systems can be found
elsewhere [17].
(b) Deep soils
Deep soil samples are usually taken with punch and trowel digged laterally into the soil,
or using augers or soil probes and split barrel samplers. Soil probes are useful samplers when
collecting small diameter soil samples from the surface to depths of about 75cm. These
samplers are made of either chrome molybdenum type 4130 steel or of stainless steel with
stainless steel handles fitted with plastic end caps. The wide body slot allows for easy
removal of the sample as the tip cuts a core slightly smaller than the inside diameter of the
probe, to assist in easy removal of the sample. The top of the slot is closed to prevent soils
from being trapped in the upper section of the probe. The probe is pushed and/or twisted into
the soil to collect a sample by means of a detachable cross handle or a slide hammer and then
the sample is recovered by twisting and pulling up on the cross handle or reverse hammering
with the hammer attachment.
Split barrel samplers consist of a steel or stainless steel tube with the tubular section
longitudinally splitted into two equal semi-cylindrical halves. The tube is attached to a
connector head at the top and a drive tip mounted on a cutting shoe at the bottom. The system
is generally used with liners in which the sample is collected for easy removal. The liners can
be made of stainless steel or plastic [17]. At the desired depth, the drive tip is pulled and the
split barrel sampler (split spoon) is lowered. The drop hammer is then placed on the top and
the number of blows to advance the sampler is counted. Then, the drive tip is placed back in
Analytical Chemistry of Cadmium: Sample Pre-treatment… 7
the cutting shoe and the device can advance to the next testing depth. This later device avoid
cross contamination between soil layers.
Lake, river and marine sediment or sludge sampling are usually carried out with grab
samplers, which dredge bottom grab samples (single surficial sediment taken at a specific
time or over as short a period as feasible); or with corers, which drill core samples
(cylindrical section of a naturally occurring medium consistent enough to hold a layered
structure). Grab samples usually consist of the top ten centimeters of the sediment and
stratigraphic integrity of the sample is not maintained, so analysis is usually restricted to bulk
characteristics. However, the vertical integrity of the different layers underlying the sediment
(from surface to more than 50cm depth) is maintained for core samples.
(c) Surface sediments
There are several types of grab samplers, such as the Ekman grab sampler and the Van
Veen grab sampler. The Ekman type consists of a stainless steel cabinet to which two sprung
jaws are attached. A mechanism closes the jaws by remote command, which traps the
sediment within the cabinet. Lids on top of the cabinet prevent wash out on retrieval. Other
devices require that a messenger is sent down the line tripping when the sampler reaches the
bottom. In constrat, Van Veen sediment grab sampler has a clam-shell type scoop. This
design allows water to flow through the sampler to prevent premature closure of the jaws
when the opened grab descenting to the bottom. When contact is made with the sea/river/lake
floor, the stabilizing plates prevent over-penetration, so when the tow-cable is made taught
causing the jaws to close, thereby capturing a sample. This system allows that all samples are
equally taken (the same top centimeters of sediment) and minimizes disturbance.
(d) Bottom sediments
The simplest design of bottom sediment sampler is the hand corer. In shallow waters, the
sampler (cylinder) can be pushed into the sediment using the handles on a head assembly. In
deeper waters, the sampler can be dropped by attaching a line to the clevis, located on the
head assembly between the handles. A simple flap valve allows water to flow through the
sampler during descent and close tightly when removing the sampler, minimizing sample
loss. The corer can be equipped with a liner-type core tube that accepts removable plastic or
stainless steel liners, which results convenient when determining trace metals.
Other corer devices are the tube corer and gravity corer types. Tube corer types consist of
a stainless steel tube with an internal piston. The device is pushed into the sediment manually
by a pole, at the same time the internal piston is drawn up inside the tube. The vacuum
created retains the sediment within the corer tube. The device can be removed from the
sediment and the sample retrieved. This system can be also operated by attaching with a
cable.
Gravity corers are weighted tube mounted within a frame that descends by gravity from
the research vessel to the sea floor, where it penetrates the sediment to a given depth, filling
the tube with sediment in the process. The hydraulically-damped gravity corer has a slow rate
of penetration that is controlled by a water-filled piston and disturbance of the water-sediment
interface is minimal. A core-catcher on the bottom of the tube moves into place when
retrieval begins, trapping the sediment sample in the corer. Gravity corer have replaceable
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 8
tapered nosepieces on the bottom and balls or other types of check valves on the top, so water
passes through them on descent but does not wash out during recovery. They can be also
adapted to hold inert liners.
Standardized methods belonging to ISO 5667 guidance, especificaly guidelines ISO 5667
Part 12 and ISO 5667 Part 19, are related to sampling for sediments and marine sediment,
respectively [18,19]. Similarly, the ISO 10381 Parts 1 and 2 lists the guidelines for soil
sampling [20,21].
II.4. Sampling for Biological Matrices (biota)
Several aspects such as size difference between specimens, variations within a study
population, and mainly tissue differentiation, are some of the factors to be considered when
sampling biota. It must be point out that there are seasonal, feeding, spawning and other
periodic activities that may influence both the concentration and localization of the target
analytes within a specimen. In some cases, each sample must be analyzed individually, but
other times, composite samples, after homogenization, can be considered [22]. This is usual
when analyzing small animals such as mollusks and composite samples are formed from
several specimens located at the same sampling site. Similarly the whole organism can be
also homogenized in order to obtain a mean concentration of the analyte. However, tissue
differention is commonly carried out, and several samples from the target organs (kidney,
liver, flesh, etc., from animals; or leaves, roots, etc., from plants) are separately taken as
different subsamples from an organism. In order to obtain a representative sample, ramdom
sampling process is the recommended strategy. Ramdomness ensures that nonbiased samples
are obtained. In this process, every member of a population to be sampled has an equal
probability of being included as a part of the sample [23].
For collecting fish, different devices such as hoop nets, gill nets, or trot lines can be used.
However, some disadvantages associated with these samplers are related to the low number of
sample obtained and to the fact that some specimen can be killed before retrieval. Other
devices which avoids these problems are the electric shockers and slat boxes. Similarly, traps,
dart guns and nets capture animals for sampling without any drawback. For collecting
botanical specimens, different tools such as saws and pruning shears can be used [8].
Special attention must be offered for sampling plankton (phytoplankton and
zooplankton). Two main categories of samplers are considered; devices designed for
collecting the organism with the water in which they live, and devices able to sieve the
plankton in situ [10]. These latter devices provide concentrated samples. Plankton tubes and
some pumps allow collecting plankton with water. When sampling plankton in shallow
ecosystems, samples can be taken in a transparent tube closed with plugs and both ends of the
tube by hang. However, when collecting plankton from deep water, the tube must be attached
to a rope or cable. In this case, plankton tubes consist of plastic or organic glass tubes
(internal diameter from 40 to 60cm and variable lengh), with the bottom end closed with a
rubber plug but controlled by a cable passing down through the tube, and a handle at the top
end of the tube. The open device is vertically immersed and the bottom plug is closed by
jercking the cable.
Samplers which sieve the plankton are of different designs but all use a conical net for
sieving. The net is made of miller’s slik or polyamide fibre, with different mesh sizes (lower
Analytical Chemistry of Cadmium: Sample Pre-treatment… 9
than 0.039mm), and the trap at the end is fitted with an outlet valve. Some devices are the
Apstein’s sampler, the Clarke-Bumpus’s sampler, the Strom’s sampler and the Schindler’s
sampler. Detailed description of these devices can be found elsewhere [10].
Benthic organisims (microbenthos and macrobenthos) can be collected with corers driven
by hand (shallow waters) or by their own weight or hydraulically by pushing out of it with a
piston. After collecting, the sample must be washed through a sieve so that any organisims
are neither forced through the sieve nor damaged. Other devices for sampling are those called
as benthic dredges. A typical dredge consists of a heavy triangular metal frame (20cm of one
side) in which a sampling net (a bag around 0.5m long) is attached. The device is kept on the
bottom by attaching an extra weight and the system dredges the surface sediment by the
movement of the ship or boat or by thrown out from the shore. Common grabs used for
sediment sampling can be used although these devices offer several drawbacks. Other simple
devices such as scrapers for collecting growths, sieves on exterdable rods, brushes or notched
stoppers with microscope slides can also be adequate for collecting macrobenthos [10].
II.5. Sampling for Air Matrices
Concerning trace metals, negligible concentrations in the vapor phase are expected, and
these pollutants are normally associated with aerosol particles. Different aspects such as the
efficiency of the sampler and mainly the location and timing of the collection may determine
the representativeness of the collected air sample [8]. In addition, different situations have to
be taken into account when collecting atmospheric particulate matter from stationary
emission sources, such as stacks; when obtaining representative samples from dry deposition;
or when collecting indoor samples. In the latter cases, dust can be sampled using a filter bag
and a vacuum cleaner. After collection, the material is removed from the filter bag and sieved
(200-mesh sieve), then thoroughly mixed and bottled.
Two main collection techniques based on impact (impingers and impactors) and filtration
(filters) are used for sampling atmospheric particulate matter [24]. Filtration methods are
most used because impactors only allow the collection of size discrete samples. Filters are
usually made of Teflon or quartz fiber, although filters made of other materials, such as
polytetrafluoroethylene, polycarbonate, polyvinyl chloride cellulose or nylon, are
commercially available. These filters are mounted in devices (samplers) equipped with a
pump which deliveres the air sample through the filter in which the particles are retined.
When working with high sampling flows and large particle sizes, the main particle retention
mechanisim is based on the impact of the particles onto the filter. However, other processes
such as particles diffusion or sedimentation are important when sampling short particles
diameters or when working at low sampling flows.
It is important to know that the characteristics of a filter can vary during the collection
time, especially when sampling for long times. This is attributed to the decrease of pressure
moreover the filter is clogged. Special attention must be taken when using cellulose filters
because their high hygroscopic nature. In general, all filter types can vary their weigth
because the humidity. A pre-conditioning stage at a 50% relative humidity is usually
recommended [25].
Non continuous samplers based on filtration or impact mechanisims can be classified in
function of the pumped volume as high, medium and low volume samplers. High and
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 10
medium volume samplers work with flows ranging from 20 to 90 m3h–1 and they are highly
recommended for sampling non polluted air or when sampling during short times. In
constract, low volume samplers work with flows lower than 20 m3h–1 and are used for
contaminated air and for sampling during long times.
Although total suspended particles (TSP), offering different sizes, can be sampled, most
of the current analytical determinations are based on the analysis of the inhalable fraction,
which offers the most useful information when evaluating the interaction between air
pollutants and public health [26,27]. The inhalable fraction is commonly refered as PM10 and
PM2.5 which are defined as mean particulate matter which passes through a size-selective
inlet with a 50% efficiency cut-off at 10µm aerodynamic diameters for PM10 fraction or 2.5
µm aerodynamic diameters for PM2.5 fraction [28]. Therefore, most of the air samplers are
equipped with air entrance systems which allow the aerodynamical retention of PM10 or
PM2.5 fractions.
Standardized procedures for sampling PM10 fraction are englobed in the ISO 12341
guidance [29]. For PM2.5 there is a reference method by the European Community [30].
II.6. Sampling for Atmospheric Precipitation
General guidelines for sampling atmospheric precipitation (rain, snow, fog) are difficult
because several variations sources can affect the composition of the samples during the
sampling event, and also because the variable metereological conditions and an absence of an
ideal surface/place for fixing the sampling equipments. This last aspect is important because
the volume of precipitation from individual events can differ a great deal, even within relative
short distances. By this way, information on metereological conditions when sampling must
be given, as well as the sampling time in relation to the total precipitation event.8 The amount
of vertical precipitation is easily gauged by means of the metereological sations network,
which locations must be selected as representative ones of the widest possible surrounding
areas. The most suitable locations for rain gauges are open plains or slight slopes, and if
elevated features are present, the distance from them must be at least four times their vertical
height [10].
Sampling devices for precipitation must be made of inert materials for avoiding
contamination. Similarly, they must be designed in order to avoid contamination (dust, falling
leaves, insects, etc) from the surrounding environment during the sampling operation. Other
alterations can occur as consequence of evaporation when finishing the precipitation event
and the following transport of the sample. Finally, the aerodynamic properties of the sampling
equipments must ensure adequate efficiencies of sampling, which can not be higher than the
typical efficiency of the surrounding surfaces.
Simple plastic (polyethylene) buckets, equipped with a tight fitting lid for minimizing
evaporation, can be used as manually operated devices for sampling precipitation. Other
manually operated systems consist of plastic funnels (30cm diameter) with the tubular end
shortened which are inserted through a bored stopper of a sample container (typically a 2L
polyethylene bottle). This device avoids evaporation of the collected sample.
Automatic sampling equipments are preferable. These systems are equipped with a lid or
cover for avoiding dry precipitation. The lid is controlled by a sensing element wich reacts to
the first drop of rain (flake of snow), allowing the opening of the collector device. The
Analytical Chemistry of Cadmium: Sample Pre-treatment… 11
movement of the lid must be horizontally rather than vertically for avoiding interference from
wind. These samplers have a funnel or a cylinder within a hermetically sealed container,
which can be thermostatically controlled (just above zero in winter and below 4°C in
summer). There are some sampling equipments with two containers for sequentally collecting
dry deposition and precipitation. In this case, the lid is moved alternately to positions on one
or the other of two containers, depending on whether it is raining or not.10
The ISO 5667 Part 8 and ASTM D 5012-89 (1994) guidances offers the standardized
guidelines for sampling wet precipitation [31,32].
III. PRESERVATION AND STORAGE. PRELIMINARY OPERATIONS
Due to the low levels of cadmium in environmental materials, all glassware and plastic
ware used for sampling and for keeping the collected samples must be previously
decontamined (soaked in a 10% nitric acid solution for at least 48 hours and rinsed several
times with ultrapure water). Preservation conditions depend on the type of sample.
III.1. Considerations for Preserving Water Samples
When collecting waters, metals tend to be adsorbed onto the walls of the glass or plastic
containers. This fact can lead to lower cadmium concentrations than those contained in the
sample. To avoid this adsorption, nitric acid, at a concentration of 1%, is usually added to
water samples, and then, they are commonly stored at 4°C before analysis. In order to know
contamination by the addition of acids, reagent blanks and trip blanks must be prepared. It
must be taken into account that for speciation studies preservation by acidification must be
avoided because the oxidation state of the elements and/or the different metal-complexes ratio
can be changed. In these cases, freezing at -18°C is highly recommended for avoiding looses
by adsorption. A preliminary operation for waters consists of filtration by using Nucleopore
filters of 45µm to separate suspended particulate matter. This operation is highly
recommended when determining metals because they are commonly associated with
suspended particles. If possible, this operation must be carried out in the field after water
sampling and just before addition of nitric acid for preservation. However, it is not always
possible, and the filtration operation is carried out in the laboratory and after acidification. In
these cases, the acid can digest the particulate matter, especially organic particulated matter,
and changes in chemical composition can be occurred for some target analytes.
The ISO 5667 Part 3 and the APHA.1060 guidances offer the standardized guidelines for
handle and preservation of water samples [33,34].
III.2. Considerations for Preserving Soil, Sediment and Sludge Samples
In constrat to preservation techniques for water, there are not prescribed preservation
recomendatios for soils, sediments or sludge. It is highly recommended to seal the containers,
minimizing the headspace, refrigerate the samples during storage and transportation, and
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 12
analyze the samples as soon as possible. Dry soils can be kept in decontamined plastic
containers at room temperature. However, when sampling wet soils, the samples must be
dried for conservation. Water removal minimizes possible biodegrdation of the sample as
well as oxidation-reduction phenomena. Some times wet soil samples can be air-dried.
However, contamination by dust can be occurred during this operation and this drying system
is not recommended when trace elements have to be determined. Lyophilisation (freeze-
drying) procedure is a more recommended procedure. In this case, wet samples must be
previously freezed and then lyophilized at temperatures within the –80 to –40°C and at
pressures around a few millibars. After freeze-drying soil samples can be kept at room
temperature before analysis. River, lake and marine sediments, and sludge must also be air-
dried or freeze-dried to ensure integrity of the sample.
When measuring metals in soils and sediments, there are big differences in the trace
elements concentrations in function of the paricle size. Most of the soils and sediments
analysis for trace elements are carried out using the < 63µm fraction, and a sieving procedure
is the first operation before any other sample pre-treatment. Sieving is performed with Nylon
sieves (preferred to stainlees steel sieves to avoid metal contamination) manually or by mean
of automatic shakers (dried samples). Sieving can be carried out after or before drying. When
sieving after drying (dry sieving) some problems derived from compactness of the sample can
be occurred. This fact leads to the formation of large aggregates from small particles, which
are not efficiently sieved. Wet sieving is carried out by placing the sieve with the adequate
mesh in a stable position resting on a bucket. The wet sample is added to the sieve and a
minimum amount of ultrapure water should be used to wash the sediment through the bottom
sieve and all washing water should be retained in the collection bucket until the sample is
allowed to settle. Once the suspended material has settled, excess water on the surface of the
sediment should be carefully decanted. Care should be taken to remove only water and not
sediment at this stage. After this operation, the isolated fraction can be kept at -18°C or it can
be subjected to a freeze-drying process.
The ISO 5667 Part 15 gives general guidelines on the precautions to be taken to preserve
and handling sludge and sediment samples [35]. Simmilarly, ISO 23909, ISO 11464, ISO
16720 and APHA 3030 give recommendations for the preliminary treatments of soils and
sediment samples [36-39
III.3. Considerations for Preserving Biota Samples
Biota samples are much more succeptible to decomposition and even inorganic
compounds such as metals can be altered, mainly by oxidation-reduction reactions. After
sampling, biomaterials must be kept at low temperature (4°C) in inert containers and they
must be transport to the laboratory as soon as possible. If biota samples are not be inmediatly
treated, samples must be freezed at -18°C. If the analysis requires tissue differentiation, the
different parts of the organisim must be obtained and homogenized. Similarly, when dealing
with small organisims such as molusks, composite samples must be prepared by mixing and
homogenizing the soft tissues from the different specimens. Cutting blade-type homogenizers
are commonly used for blending, mixing and homogenizing soft tissues from biota samples.
These systems allow homogenize sample volumes from 0.25L to 10L by using processing
speeds from 500 to 20000rpm. Cutting blades are commonly made of stainless steel, although
some equipments work with cutting blades made of other materials to avoid metal
Analytical Chemistry of Cadmium: Sample Pre-treatment… 13
contamination. Homogenization can be also obtained by blending (Stomacher blender-
homogenizer). In these cases, the biomaterial is placed into an inert bag and the device
applies shear forces on the wet tissue without direct contact between the wet sample and the
steal paddles.
Once the sample is homogenized analysis can be directly performed with the wet sample.
The homogenized sample must be kept at -18°C to avoid decomposition and analyte changes.
Freeze-drying is also recommended for biota samples. After this process, biota samples can
stored at room temperature. In addition, because water is lost during the freeze-drying
process, an analyte preconcentration is achieved after lyiphilization. However, moisture of the
sample must be previously assessed because some analytical results are usually given in wet
mass.
Finally, a further particle size reduction of dried biological samples can be performed.
This preliminary operation can be performed by using different grinders made of different
materials (zirconia oxide is preferred when metals are determined). Vibrating ball mills or
planetary ball mills are commonly used for small amount of dried samples. New equipments
allow cryogenic grinding, which avoids analyte looses by increments of temperature during
grinding. This operation also allows a further homogenization of the sample.
III.4. Considerations for Preserving Atmospheric Particulate Matter
There are not official guidelines for preserving atmospheric particulate matter samples
adsorbed onto filters. Most of the scientists kept the quartz fiber filters (containing the
sampled particulate matter) at -18°C before analysis in accordance to recommendations by
the European Community on provisional reference method for the sampling and measurement
of PM2.5 [30].
IV. ANALYTICAL TECHNIQUES TO ASSESS CADMIUM
There are well-established analytical techniques for the determination of metals in all
kind of samples. Some of them offer an enough sensitivity to carry out directly the analysis;
however, other techniques require previous stages for analyte pre-concentration and/or matrix
concomitants removal. Because cadmium can be present at very different concentrations in
environmental materials, a careful selection of the adequate analytical technique is needed.
Several spectrometric techniques, mainly atomic spectrometric detectors and certain
electroanalytical techniques, allow sensitive and reliable determination of trace metals.
Moreover sensitivity, high background signals and non-spectral interferences can be
important troubles in some analytical techniques. This is the case of conventional X-ray based
spectrometric techniques, which offer high background and scattering radiation when directly
analyzing environmental materials. Other analytical techniques, such as neutron activation
analysis, are not widely applied because the high toxicity of wastes obtained and the special
requirements of installation. However, both X-ray based spectrometric techniques as well as
neutron activation analysis are advantageous methodologies because they allow the direct
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 14
analysis of samples without any sample pre-treatment. This is especially appealing when
coping with solid environmental specimens.
Spectrometric methods based on atomic absorption or emission phenomena, as well as
those dealing with atomic masses, are widely used because the relatively high sensitivity, low
cost, and fast analysis, which is mainly appealing for routine work.
A description of the theoretical aspects, instrumentation, experimental parameters and
application of the most advantageous techniques for trace element determination will be
developed along this section.
IV.1. UV-VIS Absorption Spectrometry
The absorption of radiation is the process consisting of the selective attenuation of certain
frequencies contained in electromagnetic radiation by chemical species in solution. Because
each species has a unique set of energy states, the absorbed frequencies are typical of each
chemical species. Therefore, after exposing a molecule in the ground state to a radiation of a
certain frequency, it is likely to be absorbed if its energy matches any of the energy jumps
quantified for this molecule. Under these conditions, the energy of the photons (radiation) is
transferred to the species, whose valence electrons are promoted to a higher energy state,
called an excited state. Molecules in the excited state can relax to the ground state by
transferring the excess of energy to adjacent molecules, by photochemical decomposition of
the excited species and the formation of new molecules, or by the re-emission of energy
(fluorescence and phosphorescence phenomena).
Because of a large number of vibrational and rotational energy associated with the
electronic states (ground and excited states), several energies, which differ little among
themselves, are absorbed by a chemical specie, and the number of transitions is very high,
resulting in a wide band. These absorption bands make use of the transitions located in the
ultraviolet (10-400 nm), visible (400-1000 nm) and infrared (0.78-1000 µm) regions of the
electromagnetic spectrum. Detailed information on UV-VIS spectrometry can be found in the
monographs by Heinz-Helmut [40] and Harris and Bashford [41].
As a consequence of the matter-radiation interaction, the beam of radiation can be
considered to be attenuated from I0 to I. Two important terms are then used in absorption
spectrometry. One is the transmittance T, defined as the ratio (%) of the intensity of the
radiation beam emerging from the solution (I) to that of the incident beam (I0)
T = I/I0.
The other term is the absorbance, A, defined as the negative logarithm of the
transmittance.
A = - logT = log(I0/I)
Unlike the transmittance, the absorbance of a solution increases as the attenuation of the
beam becomes greater. The absorbance of a sample is proportional to the total amount of
material that absorbs the incident radiation. The functional relationship between the analytical
Analytical Chemistry of Cadmium: Sample Pre-treatment… 15
signal measurement (absorbance) and the analytical parameter of interest (concentration) is
known as the Bouguer-Lambert-Beer law and is expressed as
A = log (I0/I) = abc,
where A is the absorbance of the solution; a is a proportionality constant called absorptivity,
which is a property of the material itself as well as the wavelength of the measurement; b is
the length of the path within a sample; and c is the concentration of the material that absorbs
the radiation. When the concentration is expressed in mol L-1, absorptivity is represented by e
and is called the molar absorptivity. This linear relationship between absorbance and
concentration is a generalization, and there are several deviations, mainly associated with
higher concentrations (up 0.01 M). Other deviations are consequences of the experimental
measurements, e.g., the use of polychromatic and dispersive radiation (instrumental
deviations), or they may derive from analyte association or dissociation with the solvent or
chemical changes to give products with different absorption properties (chemical deviations).
The absorption of UV-VIS radiation normally produces excitation of bonding electrons,
and a certain correlation of the absorbed wavelengths with the types of bonds in the absorbing
species can be expected. However, the most important application of UV-VIS spectrometry is
the quantitative determination of compounds containing absorbing groups. Absorbing species
(organic molecules, ions and inorganic anions) can contain π, σ and n electrons, and the
energy level of a nonbonding electron normally lies between the bonding and the antibonding
π and σ orbitals. Low energy transitions (n→σ*) are typical in saturated compounds
containing atoms with unshared electron pairs, and they lies in the region between 150 and
250nm. Transitions involving more energy exchange (n→ π* or π→ π*) occur in a more
convenient spectral region, with the 200 to 700 nm range, and they are the most useful for
organic unsaturated compounds. The unsaturated groups in absorbing species which exhibit
transitions to a π orbital (unsaturated groups) are usually called as chomophores.
IV.1.1. Instrumentation
Instruments used for transmittance or absorbance measurements consist of five basic
elements: 1) a light source of radiant energy; 2) a wavelength selector to isolate certain
wavelengths; 3) a sample cell or container; 4) a detector; and 5) a data processing unit.
The most common source used in UV-VIS spectrometry is the tungsten filament lamp,
which provides radiation within the 320 – 2500 nm range. The tungsten/halogen lamps,
containing a small amount of iodine together with the tungsten filament within the quartz
envelope, give higher intensities, and wavelengths up to 240 nm are possible. Other lamps
includes the hydrogen lamps, the mercury or deuterium lamps, and the xenon discharge
lamps, all providing intense, stable and constant radiations.
Isolation of the wavelength band is commonly carried out by means of interference and
absorption filters, or by monochromators (prisms and diffraction gratings). Filters result
sufficient to isolate a specific band of wavelengths, but to obtain absorption spectra a
monochromator is necessary.
The sample container, cell or cuvette is normally parallelepiped in shape with a standard
length of 1 cm and made of glass for the VIS region or quartz (or fused silica) for UV region.
The cell has an opening for inserting the sample and a stopper to prevent evaporation.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 16
The detectors used in UV-VIS spectrometry are photon detectors and they can be
phototubes, photomultiplier tubes, photovoltaic cells or photodiodes. All these detectors use
the photoelectric effect to convert the radiant energy into a measurable (electric) signal.
Finally, a data processing unit consists of an electronic device that amplifies, filters and
performs mathematical processes on the electric signal.
These five basic components can be assembled in different ways to produce several
instrument designs. Here we consider two general types of spectroscopic instruments: 1) the
photometer, which is a simple instrument that uses absorption or interference filters for
wavelength isolation and a photoelectric device to measure the radiant power; and 2) the
spectrophotometer, consisting of a specialized device capable of recording the various
components of complex radiation.
These instruments exist in three different configurations:
(1) Single-beam instruments; which consist of a radiation source; a monochromator; two
cells for the reference and the sample solutions (alternately inserted in the light path);
a detector; an amplifier; a reading device. These instruments require a stable voltage
source to prevent errors arising from variations in the beam intensity. Also,
differences between cells, mainly irregularities in the walls, are not easily
compensated for.
(2) Dual-beam instruments, in which the incident light beam is split by a rotating mirror-
chopper into two separate beams, one of which passes through the sample and the
other passes through the reference. The alternating beams reaching the detector thus
permit a simple mathematical treatment of signals (signal modulation). This design is
routinely used and leads to good results, since it minimizes drift of the radiation
source and the amplifier.
(3) Multi-channel instruments, which are equipped with a photodiode array detection
system. The radiation from a tungsten or deuterium lamp is focused on the sample or
solvent cell and then it passes to a diffracting grating. The scattered radiation arrives
at the diode array, which simultaneously detects and analyses various wavelengths.
IV.1.2. Cadmium determination by UV-VIS spectrometry
A typical UV-VIS spectrometric method for cadmium is based on the use of dithizone
(HDz) and chloroform or carbon tetrachloride as a chromogenic/extractive reagent. The
dithizone (diphenylthiocarbazone) offers a high capacity to form metallic complexes and for
cadmium the complex has the formula Cd(HDz)2 [42]. When using either chloroform or
carbon tetrachloride as extracting solvents, the complex exhibits an absorption maxima at
517-520nm, and the molar absortivity is around 8.6 104 [42]. Improved limits of detection of
the dithizone spectrometric method for cadmium determination in river water can be reached
by using surfactant induced sensitization with Triton X-100 [43]. The formation of a complex
between cadmium and iodide in acid medium is another recurrent approach. After cadmium
iodide formation, cadmium is converted to a strongly colored complex by subsequent
treatment of the extraction with a 2,2'-diquinolyl ketone 2-quinolylhydrazone (DQQH)
solution in benzene (maximum at 552nm, molar absorptivity of 9.15 104) [44], or with
Malachite Green (maximum at 685nm, molar absorptivity of 6.1 104) [45,46]. The method
was successfully adapted for flow-injection analysis [45]. Other reagents proposed for the
Analytical Chemistry of Cadmium: Sample Pre-treatment… 17
colorimetric determination of cadmium (Table 1) were N-phenylcinnamohydroxamic acid
[47], 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) [48, 49], triphenyl-
phosphine oxide [50],2-[2-benzothiazolylazo)-5-dimethylaminophenol (BTADAP) [48], N1-
hydroxy-N1,N2-diphenylbenzamidine (HDPBA) and 4-(2-pyridylazo)naphthol [51], 1,10-
phenanthroline and thymol blue [52], 2-[2-(4-Methylquinolyl)azo]-5-diethylaminophenol
(QADP) [53], 4-(2-pyridylazo)-resorcinol [54], phenanthraquinone monophenylthiosemi-
carbazone (PPT) [55] and p,p'-dinitro-sym-diphenylcarbazid [56]. Reactions between
cadmium and other chromogenic reagents in micellar media, such as sodium dodecyl sulfate
and cetylpyridinium chloride [57] or Triton X-100 [43, 58-60], have also proved to give
sensitive spectrometric methods to assess cadmium in environmental samples (Table 1).
IV.2. Atomic Absorption Spectrometry (AAS)
The physicochemical principles which control measurements by atomic absorption
spectrometry (AAS) are similar to those of UV-VIS for molecules; but in AAS, free atoms or
ions are involved in the absorption of radiation. Therefore, at a certain resonance line for an
element, attributed to an optical transition between atoms in the ground state and the atoms in
an excited level, the absorption of radiation can directly be related to the concentration of that
element.
An atomic absorption spectrometer consists of a source of radiation providing the
radiation to be absorbed, an atomizer or source of free atoms, an optical dispersive element,
and a detector and a data acquisition system. The atomizer is perhaps the former characteristic
of AAS piece of equipment; and the different types of atomizers (flame, graphite furnace,
heated quartz tube) lead to the various available atomic spectrometry techniques in use. The
atomization mechanisms in most of these systems involve a thermal atomization by heating at
very high temperatures, but some times, a chemical mechanisms such as atomization through
a chemical reaction which coverts the aqueous analyte into an atomic vapor, is also used.
IV.2.1. Sources of radiation
The radiation sources for AAS instrumentation are commonly line sources (LSs), which
generates a characteristic narrow-line emission of a selected element. However, new designs
of continuous sources (CSs), derived mainly from advances in solid-state array detectors and
charge-coupled devices (CCD), have led to successful results in new High Resolution Atomic
Absorption Spectrometry (HR-AAS).
(a) Line sources
There are three principal LSs for AAS: the hollow cathode lamp (HCL), the high
intensity hollow cathode lamp (HI-HCL), and the electrodeless discharge lamp (EDL)
[61,62].
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 18
Table 1. UV-VIS spectrometric determination of cadmium in environmental samples.
Sample type Chromogenic ragent / solvent Maximum spectra /
molar absortivity Reference
Aqueous solutions DQQH / benzene 552 nm / 9.1 104 [44]
Waters Malachite Green / ---- 685 nm / 6.1 104 [45]
Tap water,
vegetables Malachite Greena 690 nm / ---- [46]
Aqueous solutions N-phenylcinnamohydroxamic
acid / chloroform 380 nm / ---- [47]
Waters 5-Br-PADAP 587 nm / --- [48]
Aqueous solutions 1-(2-pyridylazo)-2-naphthol /
triphenylphosphine oxide 555 nm / ---- [50]
Water Br-PADAP / o-xylene 590 nm / 3.8 104 [48]
Water BTADAP / o-xylene 600 nm / 4.5 104 [48]
Industrial effluents,
fly ash HDPBA 400 nm / 2.0 104 [51]
Seawater 1,10-phenanthroline-thymol blue /
chloroform 410 nm / --- [52]
Aqueous mixtures QADP in aqueous ethanolb 569 nm / 1.5 105 [53]
Aqueous mixtures 4-(2-pyridylazo)-resorcinol / 510 nm / 2.5 105 [54]
Water PPT / oleic acidc 520 nm / 2.4 105 [55]
Tap water, seawater p,p'-dinitro-sym-diphenylcarbazid 630-640 nm / 2.1 104 [56]
Aqueous solutions CCE - sodium dodecyl sulfate and
cetylpyridinium chloride 414 nm / ----- [57]
Wastewater HDAA- Triton X-100 520 nm / 1.5 105 [58]
Water, waste water Cadion Py – TritonX-100 530 nm / 1.9 105 [59]
Water, plants Thiazolylazo chromogenic
reagents - Triton X-100 ----- [60]
(a) Cd(II), as a CdCl42-, is previously retained on an AG1-X8 resin
(b) Cd-QADP on line (FIA-silica gel column) separated from other ions
(c) Cd-PPT complex is floated quantitatively with oleic acid (HOL) surfactant
DQQH, 2,2'-diquinolyl ketone 2-quinolylhydrazone; 5-Br-PADAP, 2-(5-bromo-2-pyridylazo)-5-
diethylaminophenol; Br-PADAP, 2-[2-(5-Bromopyridyl)azo]-4,5-dimethylphenol; BTADAP, 2-[2-
benzothiazolylazo)-5-dimethylaminophenol; HDPBA, N1-hydroxy-N1,N2-diphenylbenzamidine;
QADP, 2-[2-(4-Methylquinolyl)azo]-5-diethylaminophenol; PPT, phenanthraquinone monophenyl-
thiosemicarbazone; CCE, N,N'-bis(2-hydroxy-5-nitrobenzyl)-4,13-diazadibenzo-18-crown-6;
HDAA, o-hydroxybenzenediazoaminoazobenzene; Cadion Py, 2-pyridinediazoaminoazobenzene
An HCL consists of a tungsten rod (anode) and a hollow cylindrical cathode made of the
target element (mono-elemental HCLs) or target elements (multi-elemental HCLs), which
characteristic radiation is to be produced. The cathode and the anode are contained within a
glass cylinder with a silica window and filled with Ar or Ne at low pressures (1 to 5 Torr).
The characteristic radiation is obtained when a potential of 300-500 V (currents about 5 to 20
mA) is applied across the electrodes, which causes the inert gas contained in the lamp to
ionize. Under this high voltage, cations acquire enough kinetic energy to impact on the inner
surface of the cathode and to cause metal atoms to sputter out of the cathode cup, producing
Analytical Chemistry of Cadmium: Sample Pre-treatment… 19
an atomic vapor. Further collisions of the sputtered metal atoms with the inert gas ions excite
these metal atoms, and they emit their intense and characteristic radiation when returning to
the ground state. The cylindrical configuration of the cathode enhances the probability of
metal redeposition at it rather than on the glass walls, and the cathode is continuously
regenerated.
The intensity of the radiation emitted by HCLs is low for cathodes made of volatile
elements, such as cadmium, and HI-HCLs or EDLs are preferred. HI-HCLs, also called
boosted discharged hollow cathode lamps, BDCHLs, are similar to HCLs, but they exhibit an
additional or secondary cathode (boosted cathode). After applying a high voltage, the
sputtered atoms from the primary cathode (atomization) are excited by applying an additional
voltage (boost discharge) across the secondary cathode (excitation). When both atomization
and excitation process occur at two different electrodes, the self-absorption phenomena is
minimized and the resulting emission is around 5 to 15 times higher than those provided by
standard HCLs.
An EDL consists of a hermetically sealed quartz envelope containing an inert gas (Ar) at
very low pressure and the element or salt of the target element (commonly iodides). The lamp
does not have electrodes and ionization of the inert gas is obtained after applying microwave
radiation (c. 100 MHz) or, as is usually the case, radio-frequency radiation (from 100 kHz to
100 MHz). Commercially available radio-frequency EDLs have a built-in starter, run at 27
MHz, which provides a high voltage spark to ionize the filler gas to initiate the discharge.
Similarly to HCLs or HI-HCLs, the inert gas ions acquire high kinetic energies and after
collisions with the metal salt, the sputtered and excited metal atoms emit the desired
characteristic spectrum.
(b) Continuous sources
A modern CS is based on a conventional xenon short-arc lamp which has been optimized
to run in the so-called `hot-spot mode´ [63]. This discharge mode consists of the appearance
of a small plasma spot close to the cathode surface. This plasma is obtained after operating
the lamp at a high xenon pressure, using short electrode distances, and new materials and
geometries for both anode and cathode rods.
IV.2.2. Background correction
When using flames and graphite furnace as atomizers it is necessary to discriminate
between the absorption by the target element and the background absorption. This latter type
of non-specific absorption is attributed to light scattering on particulates formed by
recombination of the sample matrix at cold spots, and also by broad molecular absorption
signals caused by radicals or molecules vaporized in the atomizer. For this, two main
background correction systems are used; the continuous sources based background correction
systems and the Zeeman background correction devices [61].
The first system uses a non-specific continuous radiation emitted by additional sources;
e.g., a deuterium arc lamp or a hydrogen arc lamp which provide a continuous radiation
throughout the UV region (correction below 400 nm), or a tungsten halogen lamp for the VIS
region [61]. The system works by passing alternatively the radiation provided by the
continuous sources (deuterium or tungsten arc lamps) and the radiation of the HCL or EDL
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 20
through the atomizer. When radiation from the HCL or EDL passes, the detector records the
atomic absorption and the background absorption signals, but when radiation provided by the
continuous lamp passes through the atomizer only the background absorption signal is
measured. This is because the level of atomic absorption due to the continuous lamp is
negligible since the slit width is kept sufficiently wide. Therefore, the signal recorded with the
line sources can be subtracted from that recorded with the continuous source, and the
background absorption can be removed. This method for background correction is
inexpensive, but background signals larger than 0.5 units can not be compensated for. In
addition, there are other drawbacks associated with degradation of the signal-to-noise ratio
and over-corrections in some systems, which occurs when emission lines from other elements
in the sample lie close to the characteristic emission line of the target element.
The second background correction system uses the Zeeman effect on atomic energy
levels, e.g., the splitting of atomic energy levels when exposing the atomic vapor to a
magnetic field, which lead to the formation of several absorption lines for each electronic
transition. The normal Zeeman splitting pattern leads to a central or π line, which shows the
same wavelength of the electronic transition in absence of the magnetic field; and two σ lines,
equally spaced around the π line and very close to the original electronic transition
wavelength (π line). Analyte and absorbing species (background) absorb at the π line, while
the σ lines are only absorbed by the background absorbing species. The net analyte
absorbance signal is obtained by modulating the magnetic field and by using a polarizer for
background subtraction. The advantages of this method over continuous lamps are that high
background signals (up to 2.0 units) and structured backgrounds can easily be corrected for.
However, it presents as a main drawback the slight decrease in sensitivity.
IV.3. Flame Atomic Absorption Spectrometry (FAAS)
FAAS is one of the most commonly used atomic techniques in the analytical laboratory
mainly because of its simplicity, low capital cost and inexpensive maintenance. A more
detailed description of FAAS instrumentation will be found elsewhere [61].
IV.3.1. Atomizer: flame
The typical atomizer for FAAS is a pre-mixed laminar flame, in which the fuel and the
oxidant gases are mixed in an expansion chamber prior to entering the burner. The energy
supplied by this atomizer is enough to atomize the sample and to maintain the atomic vapor
within the light path of the spectrometer. The most common flames used in AAS depend on
the nature of the oxidant gas: the air-acetylene flame and the nitrous oxide-acetylene flame.
With the air-acetylene flame (slot length of 100 mm) is possible to reach temperatures from
2000 to 2500°C, and these temperatures are high enough to atomize elements which oxides or
carbides do not offer a refractory behavior. The nitrous oxide-acetylene flame (slot length of
50 mm), which attains temperatures of c. 3000°C, is preferred when atomizing refractory
elements or those forming thermally stable oxides or carbides.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 21
IV.3.2. Nebulizer
In flame atomization the liquid sample is introduced into the flame as a fine aerosol. This
spray is generated by a nebulizer/expansion chamber arrangement, which converts the sample
solution into an aerosol showing droplets of different sizes.
Pneumatic nebulizers are the most widely used in FAAS instruments, especially the
pneumatic concentric nebulizer which consists of a concentric stainless-steel tube through
which a Pt/Ir capillary tube is located. In this nebulizer the sample solution passes through the
capillary by the action of a high-velocity oxidant gas stream parallel to the capillary axis and
at the end of the capillary (Venturi effect). When the liquid sample arise the end of the
capillary, it escapes through the exit orifice between the outside of the capillary tube and the
inside of the stainless-steal concentric tube. In this point, the high-velocity gas breaks the
liquid up into a coarse aerosol. Once the aerosol is generated, it passes through the expansion
(spray) chamber, where large droplets collect on its walls and drain away. A number of
baffles are placed in the spray chamber to ensure that only the smallest droplets reach the
flame. Although the combination of a pneumatic concentric nebulizer and the expansion
chamber is a cheap and robust system which offer good long-term stability, it offers some
drawbacks when working with liquids containing high salts content or particulate matter
mainly because the small internal diameter of the capillary. In addition, the efficiency in the
aerosol generation by the combination of a pneumatic nebulizer and the expansion chamber is
around 10-15 %, which offers poor sensitivity. In order to obtain a more concentrated aerosol,
an impact bead (made of glass or ceramics) may be placed in the path of the initial aerosol
inside the spray chamber [62]. The primary aerosol generated by the nebulizer impacts on the
bead and secondary fragmentation takes place; this increases the number of fine drops that
can reach the flame and improves the efficiency of nebulization.
Enhancement on analyte transport to the flame can be also obtained by using highly
efficient nebulizers, such as high pressure pneumatic nebulizers (HPN) and hydraulic high
pressure pneumatic nebulizers (HHPN) [64-66], ultrasonic nebulizers and thermospray
nebulizers, which lead to a more concentrated aerosol and a more sensitive FAAS
determination. A detailed description of the different nebulizers is given in the following sections
devoted to inductively coupled plasma – optical emission spectrometry / mass spectrometry.
IV.3.3. Cadmium determination by FAAS
Cadmium is a volatile and non-refractory element and therefore an oxidizing air–
acetylene flame provides enough energy for cadmium atomization without any interference
[61]. However, the main drawback when determining cadmium by FAAS is its low
characteristic concentration, around 0.02 mg L–1 at the 228.8nm resonance line for
conventional pneumatic nebulization and impact bead. The characteristic concentration at the
second main resonance line for cadmium, 326.1nm, is very high, near 6.1 mg L–1, which
results unusuable. This low sensitivity is inherent to the flame atomization process itself, and
as commented, it is associated to a poor efficiency of the conventional nebulizer systems and
to the short residence time of the analyte atoms in the optical path (typically 10–3s). Since
cadmium occurs at very low concentrations, sensitivity given by conventional FAAS is not
low enough to directly assess cadmium in most of the biological and environmental samples.
To overcome this problem, there have been developed different pre–concentration methods,
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 22
mainly based on solvent extraction and solid phase extraction. Basic principles of these
procedures, as well as current trends, are reviewed in section IV.
IV.3.4. Atom trapping systems - FAAS
Instead of pre-concentrating the analyte and/or improving sample nebulization, sensitivity
of cadmium determinations by FAAS can also be obtained after certain modifications in the
atomizer that allow analyte atoms to remain longer in the optical path. This is the case of the
use of atom trapping systems and in situ pre-concentration techniques for FAAS [67]. The
long-path absorption tube (LPAT) and the Delves’ cup microsampling techniques are the first
atom trapping systems reported. LPATs are T-shaped or open-ended tubes, firstly made of
silica, in which the free atoms from a total consumption burner, and after conventional
nebulization, are confined. The tubes or cells are placed in the optical path and just above a
flame burning at the T joint (T-shaped cells) or at a side of the open-ended tube. The Delves’
cup microsampling technique also uses the idea of collecting the free atoms in an open-ended
trapping tube above the flame but here, the sample is directly deposited in a sampling cup or
boat instead of being aspirated into the flame. The technique was simplified and improved by
incoporating a holder for the Delves’ microsampling cup [68]. This improved system uses a
nickel or stainless-steel microcrucible (cup) mounted onto a device that enables it to be
pushed close to the flame to allow the charring of the sample, and then into the flame to allow
atomization. A nickel open-ended tube is mounted in the flame and atoms enter the tube
through an opening half-way along its length. Although the Delves´microsampling cup
technique was commercially utilized, this approach, as well as LPATs, is not used nowadays
and they are not routine laboratory procedures. However, their basic principles and their first
applications, reviewed by Matusiewicz [67], can be considered the basis of the current atom
trapping systems.
More usable atom trapping systems are the so-called slotted–tube atom traps (STAT) and
water cooled atom traps (WCAT). The simplest STAT systems, firstly used by Walting
[69,70], are double–slotted tubes which are placed above the burner allowing the entrance of
flame gases and analyte atoms through the base slot [67]. Different designs showing entrance
and exit slots with 120° or 180° angle between them, as well as different length of the upper
and base slots, the internal diameter and the length of the tube, were proposed [71,72]. In
addition, modification and optimization of STATs with a row of six exit holes centrally
drilled above the entrance slot were also applied [67,72]. STATs are usually made of quartz
and can be easily manufactured in laboratory. However, the main drawback of quartz STATs
is the divitrification process, attributed to sodium migration into the silica and to metal oxides
deposits. This process continuously changes the quartz tube surface and bad repeatability of
measurements as well as analytical signal depression is occurred [67,73]. Divitrification
process can be prevented by in situ coating of the inside surface with an aluminum sulphate,
lanthanum chloride or vanadium (from ammonium vanadate) solutions [67,74]. Alternatively,
the use of tubes made of stainless-steel, silicon nitride and graphite has been reported [67].
However, most of the recent applications, including interference studies [73] and new
injection systems [75] have been carried out with conventional quartz double–slotted tubes.
Although quartz STATs can be easily manufactured in the laboratory, there are several
commercial devices based on this atom trapping technique and the atom concentrator tube
(ACT-80) by Varian is the most popular.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 23
Table 2. Application of atom trap techniques in FAAS determination of
cadmium in environmental samples.
Atom trap Sample type Sample treatment Reference
STAT Drinking water Dilution with water [76]
STAT River and drinking water Dilution with 0.1% HCl [77]
STAT Drinking, dam lake,
stream and lake water Cd-cupferron complex formation
and adsortion on activated C [78]
STAT Drinking, dam lake,
stream and lake water Solid phase extraction with
thiouresulfonamide resin [79]
STAT Drinking water None [80]
STAT Sewage sludge Hot plate acid digestion and
sequential extraction [81]
STAT Soils Acid digestion and sequential
extraction [82]
STAT Soils Hot plate acid digestion [83]
STAT Plants Dry ashing [83]
STAT Plants Dry ashing and microwave
assisted acid digestion [84]
STAT Mussels Microwave assisted acid digestion [72]
WCAT Drinking water None [80]
WCAT Water 0.01M HCl acidification [90]
WCAT River water None [91]
WCAT Potable water None [94]
WCAT Water and seawater Acidification [95]
WCAT Soils Acid digestion [95]
WCAT Soils Acid digestion and acetic acid
extraction [96]
WCAT Soils 0.05M CaCl2 extraction [97]
STAT, slotted tube atom trap; WCAT, water cooled atom trap
Table 2 summarizes the applications of STAT to assess cadmium in environmental
samples by FAAS. The developed methods were applied to water [76-80 sewage sludge [81],
soils [82,83], plants [83,84] and biota [72].
WCAT devices are U-shaped quartz tubes, positioned just below the optical path of the
spectrometer, which are normally cooled by a rapid flow of cold water through the quartz
tube [67]. This last device can be considered as an in situ pre–concentration system because
analyte atoms condense onto the outer surface of the cooled tube when cold water is flowing
through the quartz tube. On switching off the flow of coolant, the quartz tube undergoes fast
heating and the condensed atoms are released as free atoms. The tube is positioned above the
burner and the beam from the radiation source is allowed to pass just above the tube. The tube
is commonly made of quartz because it offers a high melting point moreover a low thermal
expansion coefficient. Other materials, such as copper, nickel, stainless-steel or titanium,
were investigated to trap volatile elements such as cadmium. However, problems derived
from the formation of intermetallic compounds and lack of repeatability led to consider these
cooled traps as non-usable [85,86]. Using quartz tubes, cadmium as well as other elements,
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 24
were efficiently trapped and re-vaporized. Recently Korkmaz et al. [87] have investigated the
nature of the re-volatilization process when using organic solvents aspiration. It was proved
that cadmium is trapped onto the quartz surface as free metal and as oxide, and atomization of
the analyte adsorbed onto the quartz surface occur by evaporation directly from the solid
metal or oxide or by melting [88]. In addition, the repeatability of the cadmium
determinations is improved and the lifetime of the tubes is increased when coating the quartz
tubes with vanadium oxide salts [89]. From the firstly described single-tube configurations,
double- and triple-tube WCATs were proposed, and the double-tube configuration gave the
best analytical performances [90-92. Recent studies have led to improved dual-tube WCATs
that consist of using “bent” tubes instead of earlier “flat” tubes, which enables more of the
tube to be exposed to the radiation [93]. Table 2 also lists the different applications of
WCATs for cadmium determination in environmental materials. Main applications have been
developed for assessing cadmium in water [80,90,91,94,95] and soils [95-97
The combination of STAT and WCAT systems has resulted in a new atom trap, the
slotted–tube water–cooled atom trap (STWCAT) which consists of a STAT with a single
WCAT passing through it, and mounted on a holder over the burner [98]. This system works
in a similar way than WCAT devices but with the difference of released atoms are confined
inside the STAT. Therefore, the STWCAT shows higher sensitivity and precision than
STATs or WCATs used separately. As suggested by Matusiewicz and Kopras [99], these
devices can be considered as integrated atom traps (IAT), and they can work as conventional
atom traps or using the termed “flame alteration technique”. Recent applications deal with the
determination of arsenic, selenium and antimony by IAT-FAAS after hydride generation
[100,101].
IV.3.5. Beam injection flame furnace atomic absorption spectrometry and thermospray
flame furnace atomic absorption spectrometry.
As commented, sensitivity in FAAS determinations can be improved by increasing the
efficiency in aerosol generation and the residence time of free atoms inside the atomizer.
However, sensitivity can be also enhanced if the sample is totally introduced in the atomizer,
such as for the use of the Delves’s cup system. Recent approaches, firstly proposed by Gáspár
and Berndt [75,102], use this idea of total sample consumption, and two different
methodologies, beam injection flame furnace atomic absorption spectrometry (BIFF-AAS)
and thermospray flame furnace atomic absorption spectrometry (TSFF-AAS), were
developed.
(a) Flame furnace
Both BIFF-AAS and TSFF-AAS use a cell (furnace) heated directly by the flame (flame
furnace), but discrete sample volumes are totally introduced in the flame furnace in contrast
to conventional atom trap systems (STATs or WCATs) where the sample is continuously
nubulized. In this sense, a better sensitivity when using BIFF-AAS and TSFF-AAS is
expected because the whole sample is atomized moreover the increase in the residence time
of the free atoms confined in the furnace.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 25
The furnaces or cells are commonly made of pure nickel, although other materials, such
as non-porous Al2O3 ceramic and quartz, were firstly tested [102]. However, because of
memory effects and unsatisfactory analytical results obtained for ceramic and quartz furnaces,
nickel, nickel alloys or titanium furnaces are preferred [103]. The furnace is positioned on a
standard burner head by means of a stainless steel holder to keep the tube around 5mm above
the burner head. The nickel furnaces are commonly 10cm long open-ended tubes with 10mm
i.d. and with an opening in the middle of the tube for introducing a 2mm ceramic capillary,
which is used for sample introduction. No flame gases enter the tube through this opening
because the plane of this hole is parallel of the flow of the flame gases. This fact originates a
lower temperature of the tube, around 200-300°C lower, than the flame temperature.
Therefore, a further modification of the furnace implied four or more holes (2mm diameter) at
the bottom side of the furnace and at 90° to the ceramic capillary that allow a partial entering
of the flame gases. Higher temperatures in these modified tubes than in the tubes without
additional holes were obtained, and the analytical performances for refractory elements were
improved.
(b) Sample introduction system: high speed liquid jet (beam injection)
In BIFF-AAS, the liquid sample is transported (introduced) into the flame furnace as a
high speed liquid jet. A jet impact nebulization (JIN), droplets around 50µm in size, occurs
when the liquid jet enters the furnace through the ceramic capillary and impacts at the
opposite inner wall of the flame furnace. In contract to JIN for sample introduction in ICP-
OES [104], the generated droplets in BIFF-AAS are vaporized spontaneously onto the heated
wall when impacting. This combined mechanism, jet impact nebulization and spontaneous
vaporization, is termed jet impact vaporization (JIV).
The high speed liquid jets are generated by mean of a capillary nozzle (3cm length)
which is fixed in from of the flame furnace. The solvent or the liquid sample are transported
through PEEK tubing (0.17mm i.d.) using a high performance pump, usually a pump for high
performance liquid chromatography (HPLC). Additionally, solvent/sample transport can be
also carried out at low pressure by use of a peristaltic pump combined with a micro channel
smooth jet nozzle (low sample jet generation system) [105]. The sample introduction (discrete
volumes) is achieved by using a five- or six-port sample injection valve. Further
developments have involved new designs for nozzles and also liquid sub-critical CO2 as a
carrier liquid and as gas-pressure pump for liquid jet generation [106].
(c) Sample introduction system: thermospray
TSFF-AAS uses a thermospray as a system for total sample consumption. Thermospray
nebulization occurs when passing a liquid through a heated narrow tube, which originates the
liquid stream to spontaneously boil. Typical TS required an external electrical heating system
to fix the temperature just above the boiling point of the solvent. However, in TSFF-AAS, the
TS capillary is directly heated by the flame and additionally by heat transfer based on direct
contact of the TS capillary and the furnace [102,107]. The TS capillary (around 10cm in
length) can be made of stainless steal or ceramic and it is partially introduced (ca 2 mm)
allowing the sample introduction into the nickel tube. Usually, a peristaltic pump (low flow)
[102,108] is used to transport the solvent or the liquid sample, although a high pressure pump
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 26
can be also used [109]. Similarly to BIFF-AAS, a six-port injection valve is used for sample
introduction (discrete volume).
(d) Cadmium determination by BIFF-AAS and TSFF-AAS
Compared with standard FAAS, a 7- to 17-fold improvement in the power of detection
was obtained for several elements, cadmium included. In this sense, recent studies have
shown adequate correlations between spray formation (droplets size) and cadmium analytical
sensitivity by TSFF-AAS [110].
Scarce applications of BIFF-AAS to assess cadmium in environmental samples have
found in literature, e.g. cadmium determination in mussels after ultrasound assisted acid
leaching [111]. However, TSFF-AAS has extensively been used. Most of the samples
analyzed are waters and seawater [112-116, in some cases after a pre-concentration method
[112,113,115,116]; but also the technique has been applied to assess cadmium in biological
samples [117-119 plants [120,121], sewage sludge [122], and marine sediments [114].
IV.4. Electrothermal Atomic Absorption Spectrometry (ETAAS)
Electrothermal atomization avoids most of the limitations found in flame atomization,
such as the low analyte transfer ratio to the atomizer because the low efficiency of the
nebulization process, the short residence time of the free atoms in the flame, the use of an
inner environment in the system which avoids interferences, and the possibility of control the
different stages before sample atomization.
IV.4.1. Electrothermal atomizers
The atomic absorption spectrometer for ETAAS is the same as that used for FAAS except
that an electrothermal atomizer replaces the flame/burner arrangement in the light pass of the
spectrometer. The atomizer uses a refractory material which is electrically heated by low
voltage (10V) and high current (up to 500A) [61]. Therefore, the material used for
electrothermal atomizers must exhibit high electrical conductivity, and also high thermal
resistance and high durability. Different materials such as graphite, tungsten or tantalum offer
these properties and have been used in electrothermal atomization. Because tungsten- or
tantalum- based atomizers present as a main drawback the extreme brightness at high
temperatures, graphite atomizers are nowadays the most common used. However, graphite
offers as disadvantages a high porosity and a high tendency to form stable carbides with some
elements. These problems are partially overcome by coating the graphite tubes with pyrolytic
graphite (pyrolytic graphite coated graphite tubes) [61], which minimizes analyte looses by
diffusion into the porous material, and prevents analyte carbide formation.
A conventional graphite atomizer for longitudinal heating (graphite tube or graphite
furnace) is typically a cylindrical tube (25 to 50mm long and 5 to 10mm diameter) with
graphite electrodes clamped at either end and held axially in line with the light source. The
tube has a central and small opening for sample introduction (discrete volumes between 5 and
100 µl) often with the aid of an autosampler. Graphite tubes for transverse heating (THGAs,
Analytical Chemistry of Cadmium: Sample Pre-treatment… 27
transversal heated graphite atomizers) are shorter than those used for longitudinal heating
(typically 27mm long and 5mm diameter) and the graphite electrodes are clamped
transversally, i.e., from the sides, and the electrical current is applied at the central part of the
tube. Because these tubes are shorter, sensitivity is decreased and end caps are commonly
used.
The electrodes are water-cooled and the whole atomizer is purged with two inert gas (Ar
or N2) streams. The first gas stream is external (sheath gas) and prevents the entrance of air,
while the second one flows internally into both ends of the tube and sweeps vapors generated
during the mineralization of the sample. When using longitudinal heating, the electrical
current is applied at the ends of the tube, and there is a temperature gradient along the
graphite tube: the central portion is several hundred degrees hotter than the ends. These non-
isothermal conditions can lead to condensation of the analyte or recombination with other
species at the cooler ends of the tube. These problems are minimized when using transverse
heating, which significantly reduces or eliminates condensation of the sample matrix
components and memory effects and improves the atomization efficiency for refractory
elements.
For both longitudinal and transverse heating, graphite tubes are commonly modified with
a small graphite platform, formally called L´vov platform, only loosely connected to the tube
walls and located beneath the sample entrance port [123]. The sample is deposited on this
platform and although the graphite tube is directly heated by the electric current, the platform
is heated by radiation and convection from the tube walls. There is therefore a time lag
between the heating of the tube and that of the platform, and atomization occurs only when
the surrounding gas is relatively hot and the whole operation is taking place isothermally. By
using the L´vov platform technology (platform atomization), interferences derived from
analyte condensation and/or recombination with other species are less significant than those
occurred when using wall atomization. In addition, the use of L´vov platform is highly
recommended when determining volatile elements, such as cadmium, because platform
atomization enhances of sample dissociation and more reproducible absorbance peak schemes
are obtained. Standard graphite tubes for both longitudinal and transverse heating are
commercially available as graphite tubes with integrated platform.
A typical ETAAS measurement consists of a controlled four stages program showing the
successive drying, ashing, atomization and cleaning steps. Evaporation of the solvent is
reached at the drying stage, commonly at a temperature slightly higher than 100°C. Then, the
solid residue remaining after drying is heated at temperatures between 300 to 1500°C (in
function of the analyte and the sample matrix) during the second step (ashing, pyolysis or
charring). Organic matter and also most of the inorganic concomitants of the sample are
charred and removed. The programmed increase of temperature between the drying and the
ashing steps is established with a temperature ramp of 20 to 30s to avoid sample sputtering.
After ashing, the remaining residue consists of the analyte in molecular form and a small
amount of inorganic concomitants. This residue is subjected to the third stage (atomization) at
temperatures within the 1200 – 2900°C range. The dissociation of the analyte molecular
species occurs at this step and formation of free analyte atoms is obtained. Maximum power,
i.e., rapid increase in temperature from the ashing to the atomization step is needed to avoid
looses of free atoms. In addition, purge gas flow is stopped during atomization to increase the
residence time of the free atoms into the graphite tube. Finally, at cleaning step a highest
temperature is programmed for removing residues inside the atomizer.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 28
IV.4.2. Matrix modification
As commented above, a successful ETAAS measurement implies the analyte atomization
in absence of matrix compounds. These concomitants must efficiently be removed at the
charring of the graphite furnace temperature program. However, the volatility of the target
elements and the refractory nature of the matrix concomitants can lead to an insufficient
matrix removal to avoid analyte losses, which involves the existence of chemical
interferences because of the presence of such concomitants during the atomization. Other
times, the chemical interferences arise from losses of the analyte during the charring step
because the formation of volatile compounds between the target element and some
component of the sample. Therefore, increasing the difference in volatilization between the
analyte and the matrix to obtain a less volatile analyte or a more volatile matrix is desirable to
perform a free interference ETAAS measurement. This objective is usually achieved by using
the matrix modification approach. Matrix modification, also called chemical modification,
was introduced by Ediger [124] and it can be defined as a process aiming to separate the
analyte from the matrix, therefore facilitating interference-free determinations. This process
consists of the addition of a reagent or a combination of reagents (chemical modifier), which
react with the analyte or with the matrix, thus permitting selective volatilization and,
consequently, the separation of analyte from the matrix at some point of the graphite furnace
temperature program.
A matrix or chemical modifier acts mainly in two ways. Firstly, it removes concomitants
(matrix) by reducing matrix volatility. In this case, the chemical modifier reacts with the
matrix and the product of this reaction is more volatile so it can be lost at low temperatures,
i.e. during the charring step. A classical example of this behavior is exhibited by chemical
modifiers such as ammonium nitrate, which transforms a sodium chloride matrix (boiling
point 1413°C) into volatile ammonium chloride, which sublimes at 335°C and sodium nitrate,
which decomposes at 380°C [61]. Gas matrix modifiers, such as oxygen or synthetic air
[125], or the use of aqueous oxygen peroxide [126], are other examples. These reagents
combust a sample with a high organic matrix content at low temperatures, allowing moderate
low charring temperatures and avoiding volatile analyte losses.
The second way of action of a modifier is the direct reaction with the analyte to convert it
into a phase of higher thermostability, i.e., to decrease analyte volatility. By this way, a more
efficient removal of the matrix at higher temperatures at the charring stage, but without loss
of the analyte, can be occurred. Transition metal ions, e.g. palladium and nickel, usually
added as inorganic salts (Pd(NO3)2 or Ni(NO3)2) in aqueous solution, are examples of this
type of matrix modifiers. The mechanism of stabilization of these modifiers implies the
formation of thermally stable intermetallic compounds with the analytes. Magnesium, added
as magnesium nitrate, is other example of this chemical modification. In this case, magnesium
nitrate is thermally decomposed to magnesium oxide at some point of the charring step, and
in this process traps analyte atoms within its crystalline matrix, exhibiting a thermal stability
until 1100 °C. Finally, other conventional chemical modifier is ammonium phosphate, which
is mainly used for stabilizing volatile elements such as cadmium and lead. For cadmium, the
use of ammonium phosphate leads to the formation of cadmium-oxyphosphorus compounds
that shifted the atomization of cadmium to higher temperatures [127]. However, the most
reported mixture for matrix modification is Pd(NO3)2 and Mg(NO3)2, proposed by Schlemmer
and Welz as an universal chemical modifier [128].
Analytical Chemistry of Cadmium: Sample Pre-treatment… 29
As firstly proposed by Hoenig et al. [129] some chemical modifiers behave in other way
– they increase the analyte volatility – so that concomitants (matrix) are volatilized during the
cleaning step. Organic acids, such as ascorbic acid or citric acid, are some examples. These
reagents react with volatile elements, such as cadmium or mercury [129,130], thereby
diminishing their volatilities from matrices with a high salts content as seawater. Similarly,
other way of chemical modification consists of the decrease of the concomitants volatility. An
example of the second type of behavior is the use of ammonium molybdate, which reacts with
phosphate ions to form the highly refractory ammonium molybdophosphate. This product is
volatilized at the cleaning stage and analyte atomization is free of interferences from
phosphate ions [131].
Certain organic reagents exhibiting reducing properties, such as ascorbic acid, citric acid
or hydroxylamine chloride, can be used in combination with palladium as a powerful
chemical modifier for volatile elements [132]. The formation of thermally stable intermetallic
compounds with palladium requires that palladium, usually added as aqueous palladium
nitrate (Pd2+ ions), to be reduced (Pd0). The reduction of Pd2+ ions to Pd0 is temperature-
dependent and it occurs in absence of a reducing medium at temperatures closed to 1000°C.
However, volatile elements such as cadmium can be lost before palladium ions can be
temperature-reduced, and it is results unusable as a chemical modifier for such elements.
Although some applications on palladium reduction by in situ thermal decomposition of the
palladium nitrate just before sample injection (dry mode) have been described in literature
[133], the chemical reduction of palladium nitrate is preferred. The combination of palladium
and a reducing agent guarantees the reduction of palladium at an early stage of the
temperature program, and palladium can act as a chemical modifier (formation of
intermetallic analyte-palladium species) for volatile elements.
Although chemical modifiers are commonly used as aqueous solutions, coating the
graphite tubes with the chemical modifier, mainly transition metals, is other possibility for
chemical modification [134]. Moreover the use of metal-coated graphite tubes offers the
advantage that chemical modifiers has not to be added to each sample, metal-coated graphite
tubes avoids chemical interferences arise from the reaction of some elements with the surface
of the graphite tube to form stable carbides. Such chemical modification involves coating the
graphite tube with a noble metal, such as lanthanum, zirconium and tungsten; and more
recently, iridium, platinum and thorium. These modifiers, formally coated permanent
chemical modifiers, behave in much the same way as aqueous palladium in that thermally
stable intermetallic compounds are formed on the hot inner surfaces of the coated graphite
tube.
IV.4.3. The stabilized temperature platform furnace concept
The stabilized temperature platform furnace (STPF) concept, introduced by Slavin et al.
[135], consists of a series of operating conditions that lead atomization under an isothermal
furnace. This conditions implies (1) the use of an isothermal operation mode, i.e., platform
atomization and/or transversal heating; (2) the use of the Zeeman effect background
correction system; (3) the use of a chemical modifier; (4) the use of measurements based on
integrated absorbance signal rather than peak height; (5) the use of a rapid rate heating during
atomization (maximum power heating) so that the platform effect can be enhanced; (6) the
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 30
use of gas stop during atomization; and (7) the use of fast electronics to follow the transient
signal without distortion.
Graphite furnace operation under the STPF concept leads to interference-free
determinations because an efficient volatilization of concomitants prior atomization, as well
as a better control of condensed phase and gas phase chemical interferences is reached.
Therefore, these conditions are the basis of the modern ETAAS measurements.
IV.4.4. Cadmium determination by ETAAS
Similarly to FAAS, the primary resonance line for cadmium in ETAAS is 228.8nm,
while the only alternative line (326.1nm) results unusable because the low sensitivity
[61]. At the most sensitive line, spectral interferences have been reported for the use of
ammonium phosphate as a chemical modifier, especially when determining very low
concentrations of cadmium in matrices with high iron and arsenic contents [61], and
especially with high chloride concentrations [136]. These interferences could not be
eliminated by using the Zeeman-effect background correction system, but they can be
avoided using palladium-magnesium as a chemical modifier [137]. In general, the
characteristic mass for cadmium when using longitudinally heated atomizers is 0.4pg,
while in a transversely heated atomizer it is 1.3pg [61].
The atomization mechanism for cadmium has not been well-established [61]. A
first proposed atomization mechanism implies a carbothermal reduction of the cadmium
oxide and the followed direct vaporization of the reduced metal [138]. However, other
studies lead to a thermal dissociation of gaseous cadmium oxide [139].
Because cadmium is a volatile element, chemical modification has been shown to
be important for a successful determination of cadmium in several matrices. From the
early application of ammonium phosphate as a chemical modifier [124,140], and the
use of palladium-magnesium as an universal chemical modifier [128,137], different
agents have been proposed as chemical modifiers for cadmium determination by
ETAAS, such as lithium tetraborate [141], nickel [142,143], silver [144], and even
polytetrafluoroethylene [145]. A mixture of scandium-palladium-ammonium nitrate
[146] was also used. Determination of cadmium by ETAAS in absence of chemical
modifier but using pyrolytic and non-pyrolytic boron nitride platforms and
conventional graphite tubes has been proposed [147]. However, most of the reported
ETAAS determinations of cadmium require palladium [143,148], reduced palladium
[130,133], palladium-magnesium [149-155 magnesium [156,157] or ammonium
phosphate [143,157-165 for chemical modification. Permanent chemical modifiers,
obtained after coating the graphite tubes with concentrated noble metal solutions, are
also quite popular, especially iridium [153,166-170], rhodium [153,169-173 ruthenium
[153,167-170,174-177 Tungsten [161,168-173,175,176], zirconium [176] and
molybdenum [167]. Electrodeposition was also proposed for coating graphite tubes
with palladium and rhodium [178].
Analytical Chemistry of Cadmium: Sample Pre-treatment… 31
IV.5. Inductively Coupled Plasma Based Techniques
IV.5.1. Inductively coupled plasma – optical emission spectrometry (ICP-OES)
In atomic emission spectrometry, atoms or ions from a representative amount of an
atomized sample are excited by the absorption of thermal energy, and on returning to the
ground state, these species emit radiation of the element-characteristic wavelength. The
intensity of such emitted radiation is proportional to the concentration of atoms or ions.
Atomization cells which provide very high temperatures are required for AES. This is
because in addition to the free atom production (atomization) as it does in AAS, these atoms
must be promoted to high energetic levels. The high temperatures of atomization cells for
AES can eventually produce ionization, enabling ion lines to be used. From all available
atomizers, flames (flame atomic emission spectrometry, FAES) and, mainly, plasmas
(inductively coupled plasma – optical emission spectrometry, ICP-OES), are nowadays the
most important cells for AES. The instrumentation for FAES is similar to that for FAAS,
except for the external radiation source, which is not required. The instrumentation for ICP
will be discussed in the following section.
ICP-OES is one of the most useful analytical techniques for elemental analysis and one of
the most used in routine laboratories. This technique offers as main strengths the possibility
of simultaneous or sequential determination of up to 70 elements in theory; several orders of
magnitude for the useful working range; and easy of automation, enhancing samples
throughput. In addition, the better spectral resolution achieved with the modern instruments
has increased the number of wavelengths qualify as potential analytical lines, and some
wavelengths which were considered as unusable a long time ago are now used analytically
[179]. Complete information on all aspects relating to ICP-OES can be found in some
monographs [179-181
In general, an ICP-OES determination consists of the sample (mainly in solution)
introduction into a plasma, temperature of approximately 8000°C, which thermally excited all
elements (atoms or ions), allowing the emission of radiation at their characteristic
wavelengths. The emitted radiation is passed through a diffraction grating to resolve it into a
spectrum of its constituent wavelengths. Each diffracted wavelength is then collected and
amplified to yield an intensity measurement that is proportional to the elemental
concentration.
As it shown above, the basis of ICP is plasma, sometimes referred to as the fourth state of
aggregation. Plasma is a very hot ionized gas, in which co-exist positive ions, electrons and
neutral species of an inert gas. Argon is usually chosen as the plasma gas because of its
inertness, optical transparency to the UV-VIS spectrum, moderately low thermal
conductivity, and high first ionization energy [62]. In addition, the most common plasma
sources used in atomic spectrometry are radio-frequency inductively coupled plasma (ICP).
(a) Plasma torch
The inductively coupled plasma is formed and sustained within a quartz torch placed in a
radio-frequency oscillating magnetic field. The quartz torch consists of three concentric tubes.
The outer and the intermediate tubes have tangentially arranged entry points, and the outer
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 32
channel is for the coolant gas (outer, coolant or plasma gas) while the intermediate one is the
channel for the auxiliary gas. The plasma gas flows tangentially into the torch along the outer
tube until shortly before it reaches the plasma. The gas cools the torch but also maintains the
plasma; flow rates between 10 and 20 L min–1 are typical. The intermediate tube serves to
force the coolant gas to flow tangentially along the outer tube; it also enables another gas (the
auxiliary gas) to be introduced. Typical auxiliary gas flow rates are between 0 and 2 L min–1.
Finally, the inner tube is a capillary tube through which the aerosol from the nebulization
system enters. Therefore, the inner channel is for the sample carrier gas (nebulizer gas). The
sample is introduced as an aerosol through the inner tube, also called injector. This tube is
usually made of quartz or aluminum oxide. Nebulizer flows are normally low (from 0.6 to 1.0
L min–1). Modern ICP-OES instruments allow both radial and axial viewing (the dual view
concept); in them, the plasma lies horizontally and the optics is set up for axial viewing.
(b) Radio-frequency generators
Located around the outer tube of the plasma torch is a coil of copper tubing through
which water is re-circulated. Power input to the ICP is achieved through this coil, typically
within the 0.5 – 1.5 kW range and at a frequency of 27 or 40 MHz. In order to ignite the
plasma a high frequency electrical field is applied. Two types of designs for radio-frequency
generators can be used: crystal-controlled and free-running generators. In the former, an
oscillator circuit incorporating a crystal oscillating at a fixed frequency is responsible for
inducing the electrical field. In the latter, changes in power loading are compensated by slight
shifts in the frequency of the oscillation circuit in order to bring the whole circuit back into
resonance.
(c) Nebulizers
Similarly to FAAS, liquid samples are introduced into the plasma as a fine aerosol.
Therefore, a system for converting the sample liquid into suspended fine droplets (nebulizer
system) is required. Unlike FAAS/FAES, where solution uptake is by free aspiration, the
solution to be nebulized in ICP is usually moved by a peristaltic pump.
There are different types of nebulizers: pneumatic nebulizers (PNs), glass frit (fritted
disk) nebulizers, ultrasonic nebulizers (UNs) and thermospray nebulizers (TNs)
[62,180,181,182].
(c.1.) Pneumatic nebulizers
The pneumatic concentric nebulizer (Meinhard nebulizer), used in FAAS/FAES, are the
most common pneumatic nebulizers in use today for ICP-OES. Other PNs such as the cross-
flow and Babington nebulizers are also of interest. The cross-flow nebulizer consists of two
capillary needles positioned at 90° to each other. The sample is pumped through one capillary
and the argon carrier gas is flowed through the other one. At the exit point both carrier gas
and liquid sample are in contact and the force of the escaping carrier gas is sufficient to break
the liquid up into a coarse aerosol. V-groove or Babington nebulizers are similar to the cross-
Analytical Chemistry of Cadmium: Sample Pre-treatment… 33
flow nebulizer but the modified design allows the nebulization of solutions with high solids
contents [181].
Other described PNs are the low-flow nebulizers and the micronebulizers, which deliver
a higher mass of analyte to the plasma but have low limits of detection [183,184], and
especially, the dual nebulizer sample introduction system or multimode sample introduction
system [185], which permits direct pneumatic nebulization and/or introduction of vapors
(hydrides). Recently, the direct injection nebulizers (Vulkan direct nebulizers and direct
injection high efficiency nebulizers, DIHN) are becoming popular because they transport
nearly 100% of the sample to the plasma, thereby increasing sensitivity [62]. However,
DIHNs require very low flow rates of sample uptake and the absence of suspended solids.
(c.2.) Glass frit nebulizers
Glass frit nebulizers consist of a glass frit or metal grid (grid-type nebulizers) through the
liquid sample runs over and through the nebulizer gas passes at a high velocity. This produces
that the liquid sample shears into fine droplets. It has been established that these nebulizers
have greater transport efficiency than PNs because the very fine droplets generated [62].
However, these nebulizers type suffer from salt deposition when nebulizing solutions
containing high dissolved solids.
(c.3.) Ultrasonic nebulizers
An ultrasonic nebulizer (UN) utilizes a piezoelectric ceramic membrane (oscillator)
which can vibrate at different radiofrequencies, although 1 MHz is typically the oscillation
frequency used for [62]. The piezoelectric material is connected to and AC/DC converter
which supplies the electrical energy to be converted into vibration. When the aspirated liquid
sample passes near the oscillator, the high frequency vibration makes the liquid to break into
very fine droplets. Modern ultrasonic nebulizers also comprise an electrothermal desolvation
device instead of a heater-condenser (recirculating chiller) system, which is required to
desolvate the aerosol in order to prevent large amount of solvent vapor. UNs convert more of
the liquid sample into a usable aerosol.
(c.4.) Thermospray nebulizers
Thermospray nebulizers (TNs) consist of a narrow bore stainless steel tube, which are
electrically heated to just above the boiling point of the solvent. The heating causes the liquid
stream to spontaneously boil and eject a hot and fine aerosol of fine droplets from the
capillary tube mixed with solvent vapor [62]. Instead of electrical heating, microwave
irradiation has been also proposed to heat the solvent [186]. Similarly to UNs, desolvation of
the aerosol in needed to avoid deterioration in the analytical characteristics of the
determinations. Thermostated spray chambers (TSCs) are commonly used for [187]. The hot
aerosol derived from the TN is introduced into the TSC and part of the solvent vapor is
removed from the aerosol stream. Other devices, formally called two-unit desolvation system
(TUDS) [187], use a first desolvation system to evaporate most of the solvent, and then a
second unit where the solvent vapor is removed from the aerosol.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 34
(d) Spray Chambers
The direct introduction of a fine aerosol generated by a nebulizer into a plasma could
extinguish or induce cooling of the plasma. Therefore, the aerosol is directed through a spray
chamber (nebulizer chamber) within the large droplets are discharged to waste. In addition to
the reduction of the amount of aerosol reaching the plasma, the spray chamber reduces the
aerosol particle size and decrease the turbulence associated with the nebulization process
[181]. Spray chambers are usually made of glass, quartz or inert polymers (Ryton or several
fluorine-based polymers). There are different available designs such as the single-pass type,
the double-pass or Scott-type and the cyclonic spray chambers.
The single-pass, also referred as direct or cylindrical spray chamber, consists of a
cylindrical tube with an impact bead just in front the nebulizer. The fine aerosol from the
nebulizer impact onto the bead and a further fragmentation of the droplets is obtained. The
fine aerosol generated after the impact arise the plasma torch and the remaining liquid sample
is delivered to waste. This type of spray chamber is usually connected to a desolvation system
to reduce sample load into the plasma. The simplest system consists of a thermostated water
jacket where the spray chamber is fitted.
The Scott chamber is the most common type. It is formed by two concentric tubes. The
nebulizer is fitted in the inner tube and the interaction of the coarse aerosol from the nebulizer
with the internal surfaces of the inner and the outer tubes (double-pass) leads to a production
of a finer aerosol. This fine aerosol reaches the plasma, while the larger droplets emerge from
the tube and, by gravity, exit the spray chamber via a drain tube. The liquid in the drain tube
is kept at positive pressure, which forces the small droplets back between the outer wall and
the central tube, where they emerge from the spray chamber into the sample injector of the
plasma torch. A drawback of this design is the presence of dead volumes which implies large
rinsing times between samples to avoid cross contamination.[181] Both single-pass and
double-pass types have been superseded by the cyclonic spray chamber mainly because the
sensitivity reached (around 50% better sensitivity). The cyclonic spray chamber operates by
centrifugal force. Droplets are discriminated according to their size by means of a vortex
produced by the tangential flow of the sample aerosol and argon gas inside the chamber.
Smaller droplets are carried with the gas stream into the plasma torch, while the larger
droplets interact with the walls and fall out through the drain. A more concentrated aerosol is
obtained with this spray chamber, which implies a higher sampling efficiency and sensitivity.
However, the use of the cyclonic chamber has been reported to give slightly inferior precision
for certain types of samples [188].
(e) Optics and Detectors
Optical components can be arranged in two principal types of instruments: those that
measure all wavelengths at the same time (simultaneous spectrometers), and those in which
one wavelength is measured after another (sequential spectrometers). The classical optical
mounts based on the Paschen-Runge or Czerny-Turner mounts, which used to be the basis of
many simultaneous and some sequential spectrometers, have now been superseded by the
echelle mount. In such a mount very good resolution is obtained with a mechanically ruled
grating, which typically has only 50-100 grooves per mm. Both a prism and a grating are used
as cross-dispersing media, the former to sort the orders of the VIS range, and the latter to
Analytical Chemistry of Cadmium: Sample Pre-treatment… 35
perform this task for the UV range. In such configurations, modern ICP-OES instruments use
solid-state detectors, mainly charge injection devices (CIDs) and charge-coupled devices
(CCDs). Complete information of different configurations as well as detectors can be found
elsewhere [62,179,181].
IV.5.2. Inductively coupled plasma – mass spectrometry (ICP-MS)
ICP-MS uses the same sample introduction systems (peristaltic pump, nebulizer and
spray chamber) as ICP-OES, as well as a horizontally configured (axial configuration) argon
plasma torch. The specific parts of an ICP-MS are the detector (mass analyzer) and an
interface between the plasma torch and the mass analyzer (ion detector).
(a) Interface
Because ICP source works at atmospheric pressure and the mass spectrometer is a high-
vacuum (c.a. 10–9 atm), a suitable interface allowing the coupling of the plasma torch with the
ion detector is needed. This interface (ion sampling interface) consists of a water-cooled outer
sampling cone which is axially aligned with the plasma torch. The sampling cone is made of
nickel (high thermal conductivity and resistance to corrosion) and shows a narrow orifice (1
mm) which reduces the pressure to approximately 10–3 atm. The region behind the sampling
cone is maintained at a moderate pressure by a vacuum pump. The pressure differential
created by the sampling cone allows that ions from the plasma and the plasma gas itself pass
through the narrow orifice. Because large flows of plasma gases pass through the sampling
cone, a second cone also made of nickel and with a 0.75 mm orifice (skimmer cone) is placed
behind the sampling cone. The pressure in the intermediate vacuum chamber after this second
cone is maintained at around 10–7 atm. At this instant, the ions form a beam that can be
electrically focused on to the entrance of the mass analyzer by means of a series of ion lenses.
(b) Collision/reaction cells
If the ICP-MS system contains a collision/reaction cell, then this is located after the
interface and before the mass analyzer. This system is used in modern instruments to reduce
or avoid molecular interferences [189,190]. A reagent (gas) is introduced at this point, which
react with the potentially interferences and minimize or avoid their interfering effect on a
certain mass-to-charge ratio (m/z ratio). Based on the different ion-molecule reactions (charge
exchange, proton transfer, hydrogen-atom transfer, hydride-atom transfer, adduct formation
or condensation reactions) [181,191], different gases can be used: He (collision gas), H2 or
NH3 (charge-exchange gas), O2 or N2O (oxidizing gas), H2 (reducing gas) or CH4 (other
reaction gas).
(c) Mass analyzers
Mass analyzers act as a filter, transmitting ions with a pre-selected m/z ratio, and
allowing ions separation. There are different types of mass analyzers which can offer different
resolution and sensitivity. The most common analyzers are the quadrupole mass spectrometer,
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 36
the sector-field or double-focusing magnetic sector – mass spectrometer, and the time of
flight mass spectrometer.
Quadrupole mass analyzers consist of four cylindrical metal rods arranged in parallel and
equidistant from a central axis, and operating as electrodes. By applying a combination of
radio-frequency (RF) (the RF voltage of each pair are 180° out of phase) and direct current
(DC) voltages (the DC voltage can be positive for one pair and negative for the other), only
ions in a certain m/z range are allowed to pass. The system can be operated allowing the
transmission of only one m/z ratio (single-ion monitoring) or by continuous varying the
RF/DC ratio allowing ions of consecutively higher m/z ratio to pass to the detector (multi-
element analysis). Mass resolution of quadrupole mass spectrometers is unit-mass resolution.
The sector-field mass spectrometer (double focusing mass spectrometer or high
resolution mass spectrometer) consists of an electrostatic analyzer combined with a magnetic
analyzer to focus the ion beam. The system also uses narrow entry and exit slits which allow
the number of ions to pass through the detector. Ions from the source are accelerated through
the entrance slit of the electric sector, which acts as an energy filter. After a narrow band of
ions with certain kinetic energies has been selected, the ion beam is focused on to the
magnetic sector, where the ions are deflected in accordance with the m/z ratio (a high degree
of deflection for ions with high m/z ratios). In the same way as for quadrupoles, a mass
spectrum is obtained by scanning the magnetic field and allowing ions of consecutively
higher m/z ratio to pass the exit slit of the magnetic sector in the direction of the detector. A
better spectral resolution (lower than unit-mass) is obtained with this system, removing most
of the interferences from polyatomic species.
Time of flight (TOF) mass analyzers use an electric voltage (voltages from 103 to 104 V)
to accelerate the ions trough a 1 m long analyzer rod, and the velocities of the different ions,
which are dependent of the (m/z) ratio, is the basis of its separation: all the ions have the same
kinetic energy, their velocities along the analyzer rod must be inversely proportional to the
m/z ratio. In this way, those ions with lower m/z ratios reach the detector first. The times to
reach the detector (the time of flight) are commonly within 1 to 30 µs. To obtain pulse of
positive ions, a device to generate pulse of electrons or photons is needed. The frequencies
commonly used are between 10 and 50 kHz.
(d) Ion detectors
The channel electron multiplier is the usual ion detector for mass spectrometry. The
operating principles of this system are similar to those of the photomultiplier tube but in this
case, it must be operated at a very low pressure. This device consists of a curved glass tube (1
mm i.d.), with the inner surface of coated with a resistive material (lead oxide); and which
flared end of the tube. When the instrument is operating in the pulse counting mode (the most
sensitive mode), ions are attracted into the funnel opening by a high applied voltage (up to -
3500 V). When these ions collide with the inner coating, a significant number of secondary
electrons are ejected from the resistive surface and accelerated down the tube. They then
collide with the inner walls of the tube and cause further electrons to be ejected from that
surface. As a result, an exponential cascade of electrons is produced, which eventually
reaches saturation point and results in a large electron pulse (a gain of 107 to 108 over the
original collision). The second operation mode is the analog mode; this works in a similar
way to the pulse counting mode, but lower voltages (between -500 and -1500 V) are applied
Analytical Chemistry of Cadmium: Sample Pre-treatment… 37
and the multiplier does not become saturated. The pulses therefore vary in size, and there is a
gain of only 103 – 104.
IV.5.3. Isotope dilution analysis
As any mass spectrometer can measure isotope ratios [62,181], besides to external
calibration and the standard addition technique, ICP-MS can perform isotope dilution analysis
(IDA) as unique method of calibration [181,192]. IDA is based on the fact that most of the
elements have more than one stable isotope and the normal isotopic distribution of those
elements is fixed. The analysis consists of measuring the sample (normal isotopic distribution
of an element) and the same sample after spiking with a certain isotope (artificially isotopic
distribution). After applying a mathematical equation, the concentration of the element in the
sample can be obtained. IDA can be applied to those elements with more than one isotope,
and it can be used in multi-element determinations. However, this approach offers as main
disadvantage the high cost of the enriched stable isotopes and the large time of analysis.
IV.5.4. Vapor introduction in ICP techniques
As commented, sensitivity in ICP based techniques, mainly for ICP-OES, is limited by
the low efficiency in conventional sample introduction devices (nebulization for liquids).
However, the efficiency when introducing vapors is close to 100%, with the high increase on
sensitivity. Chemical vapor generation techniques, mainly cold vapor and covalent hydrides
generation, will be discussed in section IV.7; however, once the vapor is generated, it can
efficiently be introduced in a plasma torch. This section is devoted to vapor introduction in
ICP based techniques, and the different modified nebulizer-spray chamber arrangements will
be briefly described.
Besides the direct connection of the gas-liquid separator with the plasma torch via a
PTFE tubing [193,194], one of the simplest systems, proposed by Feng et al. [195,196], uses
a T piece to mix the sample and the reducing agent and a Teflon tubing to transport the vapor
to a modified Burgener-type nebulizer (the nebulizer has two tubes; one for a conventional
generated aerosol; and other for gaseous species introduction) which is connected to a
modified Scott spray chamber (three channels, one additional channel for the vapor
introduction). Gas-liquid separation takes place in the spray chamber and the system can be
used simultaneously for conventional liquid nebulization and vapor introduction.
Another simple variation consists of using part of the spray chamber as a reduction
(vapor generation) cell. Based on this idea, a concentric nebulizer coupled to a commercial
Jobin Yvon cyclonic chamber has been used for vapor introduction in ICP techniques. The
cyclonic chamber exhibits three entries for sample, hydrochloric acid and reducing solutions,
and an exit for liquid waste, all located in the bottom of the chamber. The vapor generation
occurs inside the cyclonic chamber and he vapor is transported by the Ar nebulizer to the
plasma torch [197-200 A further device is the called multimode sample introduction system
(MSIS) patented by McLaughlin et al. [185], and which consists of a cyclonic spray chamber
with two integrally affixed cylindrical ports (one for introducing a nebulizer into the chamber,
and the other for facilitating volatilization of analytes within the chamber. Gas-liquid
separation also occurs inside the spray chamber and the system allows a conventional use for
liquid and sample introduction for vapor [201,202].
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 38
IV.5.5. Discrete sample introduction in ICP techniques
(a) Electrothermal vaporization
Similar to the Delves’ cup microsampling approach in FAAS, discrete sample
introduction in ICP techniques can be advantageous when analyzing samples that are limited
in volume, that contain high dissolved solids or large amounts of organic solvents, or even
high solids contents. The whole sample (a discrete sample volume) is introduced into the
plasma and all of the analyte is available for analysis, leading to improved sensitivity.
The current technique for discrete sample introduction into plasmas is the electrothermal
vaporization (ETV) which is based on pipetting the aqueous (slurried) sample or placing the
solid sample onto a graphite furnace. The graphite furnace is subjected to a controlled
temperature program (similar to ETAAS) to remove the sample matrix (ashing) and finally to
vaporize the analyte. By this way, interferences arising from concomitants of the sample
matrix are minimized. From the first use of conventional heated graphite atomizer [203],
important improvements on designs and optimization of ETV devices have been carried out
[204]. In addition, for analytes which form very refractory oxides the use of matrix modifiers
usually assist the vaporization. Fluoride-based modifiers [205], such as PTFE as a suspension
or Freon gases, are widely used because they vaporize these analytes as their fluorides (lower
vaporization temperatures than those required to volatilize the analyte oxides) [62].
As has been pointed out by Resano et al. [206] ETV results highly convenient for solid
sample introduction in ICP-OES/MS (solid and slurry sampling) [207,208]. In addition, ETV
also results adequate for liquid microsample introduction in plasma-based techniques [209]
and even for speciation studies [210]. Detailed information on the vaporization mechanisims
of analytes in ETV as well as chemical modification can be found elsewhere [211].
(b) Laser ablation
Another method of solids introduction in ICP-based techniques is laser ablation. The
approach uses a laser, commonly an Nd:YAG laser operated in the near infrared at 1064nm,
which is focused on to the surface of the sample, allowing a part of the sample to be
mobilized (ablated) and transported direct to the plasma torch via the argon carrier.
Occasionally, the laser ablation can be occurred either just above or below the surface of the
sample. Although Nd:YAG laser operates at 1064nm, it can work at different wavelengths
such as 532, 355, 266 or 213nm with optical frequency doubling, tripling, quadrupling and
quintupling, respectively [181]. Laser ablation has several advantages over other introduction
techniques: (1) very limited amount of sample are usually enough to perform the analysis; (2)
the technique is applicable to any solid sample; (3) sample pre-treatment is omitted; (4) there
are not reagents consumption and any waste is generated; (5) spatial characterization
information can be obtained; and (6) by repetitive ablation of the same spot, depth profiling
information of a sample can be assessed. However, laser ablation has some limitations,
mainly associated to an accurate calibration (it is difficult to find matrix-matched standards
with known composition which are very closely matched with the samples). Heterogeneous
samples can lead to inaccurate results because the small sampling area (around 10 - 100 µm
diameter spot) can not be representative. This fact is also an advantage, as shown above,
Analytical Chemistry of Cadmium: Sample Pre-treatment… 39
because a spatial composition can be reached after several ablation spots along the whole
sample. Other drawbacks arises from the fact that the amount of sample ablated is dependent
upon laser and sample properties and certain fractionation for samples containing low-melting
point elements can be occurred [181].
A laser ablation device coupled to a plasma source consists of an ablation chamber,
where the sample is placed and fitted with a fused silica window; a lens to allow focusing; an
adjustable platform for positioning in the three x-, y- and z- directions; a camera for remote
viewing of the sample surface; and a Nd:YAG laser. The detection is usually carried out with
a CCD. Once the sample has been ablated, an argon carrier gas transports the ablated
materials to the torch.
There are three different calibration strategies when using laser ablation coupled to ICP:
matrix-matched direct solid ablation, dual introduction (sample-standard) and direct liquid
ablation [181]. Matrix-matched direct solid ablation is the most common approach and
consists of using matrix-matched standards (certified reference materials with close
composition to the samples) for calibration. Sometimes, it is possible to produce matrix-
matched standards, such as after addition of standard solution to a powdered material or by
co-precipitation of the analyte over a powdered matrix. Calibration by the dual introduction
(sample-standard) system is based on the sequential introduction into the plasma of a
conventionally nebulized standard and the laser-ablated material from the sample. A
comparison between the analytical response from the ablated sample and the standard is then
made. This approach has as a disadvantage the different response when the analyte is from the
aerosol (wet) or from the ablated material (dry). Finally, the direct liquid ablation uses
aqueous solutions as standards. A reagent, called chromophore, is usually added to the
standards to allow the ablation process to occur within the surface layers of the liquid and to
avoid aerosol generation.
IV.5.6. Continuous sample introduction in ICP techniques
Flow injection (FI), chromatography and capillary electrophoresis are methods of
continuous introduction in ICP-based techniques. FI leads to an automation of the sample
introduction and allow stages such as matrix removal or pre-concentration before sample
nebulization. FI techniques as well as continuous flow (CF) will be discussed in following
section devoted automated sample pre-treatments.
Because the importance of the different toxicological effects of an element based on its
chemical form, speciation studies are an active area in modern analytical chemistry. These
studies imply a chromatographic / electrophoretic separation, and thus, liquid chromatography
(LC), gas chromatography (GC) and capillary electrophoresis (CE) are nowadays common
continuous sample introduction systems for ICP-based techniques. High performance liquid
chromatography (HPLC), mainly using anion or cation exchange columns, offers few
problems with respect to physical coupling with ICP techniques. This is because the HPLC
flow rates are compatible with the sample uptake rate of an ICP instrument. Therefore, the
coupling consists of a simple connection of the output from the HPLC to the input of the
nebulizer through PTFE or PEEK tubing. The only problem that can arise is the mobile phase
composition, mainly if high proportion of organic solvent is present, which have the effect of
quenching plasma or even plasma extinction. In such cases, a desolvation device can be
useful.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 40
Similarly, the interfacing of GC with ICP is not also difficult, although is less
straightforward than for LC. This is because the volatile analytes must be transferred from the
GC to the plasma at a high temperature to avoid analyte condensation. This is commonly
achieved via a heated transfer line at temperatures ranging from 200 to 250°C which allows
the volatile analytes and the carries gas to be directly transported to the plasma torch. In this
case, nebulization is not necessary and high analyte transport efficiency is obtained (higher
sensitivity than that obtained when coupling HPLC).
IV.5.7. Cadmium determination by ICP-OES/MS
Cadmium can be sensitively determined by ICP-OES (estimated limit of detection of 3
µg L–1) for the emission wavelength 228.802nm. Other alternative lines are 214.438 and
226.502nm. All these lines are below 300nm and spectral interferences from argon emission
lines (300 to 600nm) are not important. In addition, useful emission lines for cadmium are not
within the molecular band spectra originate from OH radicals (280 – 330nm), oscillation of
NO (230nm), N2 molecular bands (370nm) or even oscillation of carbon compounds (450 to
650nm) when high organic matter or organic solvents are introduced in plasmas. Therefore,
cadmium determination by ICP-OES can be expected to be free of molecular spectral
interferences. In addition, spectral interferences derived from other atomic species with a
direct wavelength coincidence or a partial overlapping of the cadmium emission line are also
not important because the improved spectral resolution in modern ICP-OES instruments. The
classical example of As (at 228.812nm) partial overlapping of Cd (at 228.802nm) is
minimized with the improved resolution of modern instrumentation. Nevertheless, alternative
emission lines (214.438 and 226.502nm) can be used in ICP-OES with bad resolution to
avoid atomic spectral interference by arsenic.
Similarly, cadmium determination by ICP-MS is very sensitive, with a limit of detection of 0.06
µg L–1 when using the most abundant isotope for cadmium, 114Cd (28.73%). Other naturally
occurring stable isotopes are 112Cd (24.13%), 111Cd (12.80%), 110Cd (12.49%), 116Cd (7.49%), 106Cd
(1.25%) and 108Cd (0.89%). There are potential interferences on cadmium determination by ICP-MS
when using the isotope 114Cd. Interferences include an isobaric overlap from 114Sn and polyatomic
overlaps from molybdenum oxides (98Mo16O+, 96Mo18O+), ruthenium oxides (98Ru16O+) and
molybdenum hydroxide (97Mo16OH+) [212]. These last interferences are successfully compensated
by the reaction cell technology (use of O2 as a reagent gas) [213], while isobaric interference of 114Sn
is usually minimized by adequate correction equations provided by the software. Potential isobaric
interferences for 112Cd are also 112Sn, while polyatomic interferences can be observed from
zirconium, molybdenum, ruthenium, and calcium and argon (96Mo16O+, 96Zr18O+, 96Ru18O+,
95Mo16OH+, 40Ca40Ca and 41K40Ca) [212].
Because of the multi-element capacity of ICP-OES instrumentation, there is huge
literature on the determination of cadmium in different environmental samples. However
since cadmium concentrations are very low in many sample types, specially water and
seawater, separation/pre-concentration methods are required. Special attention must be given
to liquid sample with high dissolved solid contents, such as seawater, because the high salts
content can extinguish the plasma when a direct introduction is performed. Most of the
methods for determining cadmium in both liquid and solid environmental samples require a
certain sample pre-treatment. These procedures will be discussed in the sections devoted to
sample pre-treatments. Cadmium determination by methods involving chemical vapor
Analytical Chemistry of Cadmium: Sample Pre-treatment… 41
generation will be discussed in section III.2.7. However, ICP-OES methods based on special
sample introduction approach, such as ETV (for liquid and solid samples) or laser ablation
(mainly solid samples without sample pre-treatment), are going to be reviewed as follows.
(a) Cadmium determination by ETV-ICP-OES/MS
Because of the volatility of cadmium there is risk of analyte losses during conventional
sample pre-treatments for solid samples, mainly acid digestion procedures. As it will be
commented in the section devoted to sample pre-treatments for solid environmental samples, this
has led to the development of methods in AS based on the direct sample introduction into the
atomizer. Both approaches, slurry sampling or solid sampling, have been hugely developed, and
most of the applications of ETV are related to direct solid sample introduction techniques.
Table 3 lists several applications of ETV for environmental samples. Seawater [214-216
and water [217,218] have been analyzed for cadmium by direct introduction [214,216] or
after certain sample pre-concentration treatments [215,217-219 Similarly, solid environmental
samples have been also analyzed after wet decomposition methods [219] or mainly by using
the slurry sampling [220-228 or the solid sampling approaches [224,229-233
(b) Cadmium determination by LA-ICP-OES/MS
Laser ablation (LA) has widely been used as sample introduction technique mainly for
ICP-MS because avoids sample pre-treatment as well as it allows to know the spatial
distribution of a target element in a solid sample. Inorganic materials which are difficult to
dissolve using conventional pre-treatment methods have been successfully analyzed by LA-
ICP-OES/MS. Cadmium, considered as a chalcophile element has been determined in
atmospheric particulate matter, after retention/adsorption onto quartz filters or PTFE-
membrane filters, together with other chalcophile elements, and also simultaneously with
other siderophile or lithophile metals [234-238 Other environmental materials such as
steelmaking flue dust [239], soils [240,241], sediments [240], mollusk shells [242-244
sewage sludge [245], ice cores [246] or compost [247], have been also analyzed for cadmium
by LA-ICP-MS. An interesting environmental application consists of the
determination/spatial distribution of trace elements, such as cadmium, in tree rings in order to
find correlation with the pollution history of a certain area [248-250 LA-ICP-MS has been
also used as a powerful detection system to determine the levels of cadmium-binding proteins
in biota after electrophoretic separation methods [251,252]. Finally, LA-ID-ICP-MS has been
also applied to liquid samples (drinking water) to assess cadmium and other trace
elements[253]. In this case, the discrete sample introduction LA approach consisted of
quantitatively ablated the liquid sample previously mixed with sodium acetate from a
polystyrene substrate.
IV.6. Atomic Fluorescence Spectrometry (AFS)
AFS makes use of the same basic instrumental components as atomic absorption
spectrometry; however, it measures the intensity of the light emitted by atoms that have been
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 42
excited to higher energy levels by absorption of radiation. In contract to molecular
fluorescence spectrometry, the most useful type of fluorescence for atoms is the resonance
fluorescence, which involves a fluorescence emission radiation of the same wavelength as
that used for excitation. This emitted radiation can be measured with or without being
spectrally resolved (dispersive atomic fluorescence spectrometry, DAFS; or non-dispersive
atomic fluorescence spectrometry, NDAFS). AFS is very sensitive for many elements,
especially for covalent-hydride-forming elements by combining this technique with the vapor
generation methods.
Table 3. Cadmium determination in environmental samples by ETV-ICP-OES/MS.
Sample type Technique Sample treatment Reference
Seawater ETV-ICP-MS None [214]
Seawater ETV-ICP-MS on-line separaion [215]
Seawater ETV-IDA-ICP-MS None [216]
Water ETV-ICP-MS Hollow-fiber liquid phase
microextraction [217]
Water ETV-ICP-MS Single drop microextraction [218]
Tap water/ lake water ETV-ICP-MS Zr-coated graphite adsorption
bar microextraction [219]
Plants ETV-ICP-MS Acid digestion / Zr-coated
graphite adsorption bar
microextraction
[219]
Fish ETV-ICP-MS Slurry sampling [220]
Fish ETV-IDA-ICP-MS Slurry sampling [222]
Coal fly ash ETV-ICP-MS Slurry sampling [221]
Soil ETV-ICP-MS Slurry sampling [223]
Soil / sediment ETV-ICP-OES Slurry sampling / solid
sampling [224]
Coal fly ash ETV-IDA-ICP-MS Slurry sampling [225]
Sediment ETV-IDA-ICP-MS Slurry sampling [226]
Sediment ETV-IDA-ICP-MS Slurry sampling (chemical
vapor generation and trapping
onto Ir tubes)
[227]
Soil ETV-ICP-MS Slurry sampling [228]
Soil / sediment / plants ETV-ICP-MS Solid sampling [229]
Airbone particles ETV-ICP-MS Solid sampling [230]
Particulate matter ETV-ICP-MS Solid sampling [231]
Plants ETV-ICP-MS Solid sampling [232]
Plants ETV-ICP-MS Solid sampling [233]
ETV-ICP-MS, electrothermal vaporization – inductively couple plasma – mass spectrometry; ETV-
IDA-ICP-MS, electrothermal vaporization – isotope dilution analysis – inductively couple plasma
– mass spectrometry
Analytical Chemistry of Cadmium: Sample Pre-treatment… 43
IV.6.1. Source of radiation
Continuous sources, such as tungsten or deuterium lamps, can be used with the
advantage`of multi-element determinations. However, these sources offer low radiant
densities and as the sensitivity in AFS is directly proportional to the source intensity, the
limits of detections reached by using these lamps are not low enough for most of the current
analytical determinations. Similarly, line sources such as HCLs are also insufficient to
guarantee a high intensity of excitation radiation. Previously described EDLs and HI-HCL
(BDCHLs) however, can do so; and BDCHls are commonly used in modern atomic
fluorescence spectrometers.
Since atomic fluorescence is mainly resonance fluorescence, ICPs into which a
concentrated solution is introduced can provide an alternative quasi-monochromatic radiation
source for multi-element determinations.
Finally, lasers in the continuous and in the pulse mode provide high intensity
monochromatic radiation to saturate atomic transitions. These lasers have several dyes as the
active medium to cover the whole wavelength range. For laser-excited AFS (LEAFS) or
laser-induced atomic fluorescence spectrometry (LIF) the laser must be capable of generating
wavelengths throughout the VIS and UV regions in order to excite as many elements as
possible.
IV.6.2. Atomizers
Conventional air-acetylene or nitrous oxide-acetylene flames have usually been used as
atomizers for AFS. However, some problems have been found when determining elements
with thermally stable oxides; and also, high background signals from the flame are commonly
observed. Attempted solutions of this problem have included the separation of the flame with
a quartz tube (torch), or with a shear inert gas (argon), which prevents the entrance of air.
Despite the use of these approaches, however, all such flames have a high background
emission. In contract, flames in which an inert gas (argon or nitrogen) is burned in hydrogen
have low background emissions, although the temperature is also low. These flames, called as
diffusion flames, are unsuitable for work involving refractory elements, but the temperature in
these flames is quite sufficient to atomize hydrides; and as it will be commented in next
section, they are normally used in AFS instruments with hydride generation systems for
sample introduction.
ICP can be also used as an excellent atomizer which avoids problems for elements with
thermally stable oxides. Similarly, graphite furnaces offer high atomization efficiencies and
extremely low detection limits [191].
IV.6.3. Cadmium determination by AFS
Because the high excitation energy for cadmium and the absence of stable oxides for this
element, AFS offers very low limits of detection for this element, and the technique results
appealing for analyzing environmental and biological samples. Most of the current
applications of AFS to assess cadmium use non dispersive instruments (NDAFS) and the
chemical vapor generation approach (covalent hydrides) as sample introduction technique. A
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 44
complete review of these methods is discussed in the section devoted to chemical vapor
generation, while in this section is summarized the scarce literature related to the remaining
methods.
LEAFS has been used for the direct determination of cadmium in Antarctic and
Greenland ancient ice and recent snow at ultratrace levels. The atomization unit is an
electrothermal atomizer allowing cadmium determinations down to the 0.1 pg g-1 levels [254-
257 The technique has been also applied to assess cadmium in snow [258] and river
sediments [259]. A microwave plasma torch (MPT) has been also applied as an atomization
cell for AFS.260 A HCL working in the pulsed mode was used as an excitation source, while
aqueous samples were introduced by a pneumatic nebulizer. The detection limits reported for
cadmium in waters was 0.25 µg L-1. More recently, a single ring electrode radiofrequency
capacitively coupled plasma torch (SRTr.f.CCP) operated at 275 W and 27.12 MHz, has been
also proposed as atomization cell in AFS [261]. In this case, an EDL was used as primary
radiation source, while detection was carried out by a charged coupled device. The limit of
detection for cadmium determination in waters and soils by the proposed EDL-SRTr.f.CCP-
AFS method was 4.3 µg L-1, which leads to promising cadmium determinations in
environmental samples.
IV.7. Chemical vapor generation– atomic spectrometry (CVG-AS)
Moreover the use of thermal energy, mainly derived from flames, plasmas or
electrothermal heating, to produce atoms there are chemical methods capable to give atomic
vapor or molecular vapor. The first case is usually termed cold vapor generation, and mercury
cold vapor, after a chemical reduction, is the most typical example. Similarly, most of the
molecular vapors are also obtained after a chemical reduction, and the reduced products are
commonly volatile hydrides. The technique is then referred as hydride generation, and
covalent hydride forming elements, such as arsenic, are commonly known. However, the
generation of volatile species from elements can be reached by using other chemical reactions
instead of a chemical reduction. Literature shows several examples, mainly alkylation, halide
generation, metal carbonyls generation and chelates or oxide generation [262]. In addition,
new developments involving UV assisted CVG approaches or trapping strategies such as
solid phase microextraction (SPME) or single droplet microextraction (SDME) have updated
the classical cold vapor / hydride generation methodologies. This fact has led to a resurgence
of interest in CVG because the number of elements that can be determined after chemical
vapor generation has increased in the last years [263,264,265]. Therefore, moreover the well-
known chemical vapor generation for mercury, arsenic, selenium, antimony, lead, bismuth,
tin, germanium and tellurium, new elements have been added to the chemical vapor forming
elements group, such as cadmium, chromium, manganese, nickel, copper, silver or gold
[266,267].
After chemical vapor generation, atomic absorption, atomic or fluorescence emission or
mass counting can be monitored, leading the different analytical techniques based on CVG
(chemical vapor generation – atomic absorption spectrometry, CVG-AAS; chemical vapor
generation – inductively coupled plasma optical emission spectrometry or mass spectrometry,
CVG-ICP-OES/MS; and chemical vapor generation – atomic fluorescence spectrometry, CVG-
Analytical Chemistry of Cadmium: Sample Pre-treatment… 45
AFS). A monograph by Dědina and Tsalev devoted to hydride generation and atomic absorption
spectrometry can be used for understanding principles of CVG methodologies [262].
IV.7.1. Generation and transport of vapor
First chemical vapor generation developments were based on the use of tin(II) chloride as
a reducing agent for mercury and the metal/acid reducing system (most often zinc /
hydrochloric acid), known as Marsch reaction, for covalent hydrides [268]. However, the low
reducing power of these reagents and the slow kinetics of the Marsch reaction, have been
superseded by using sodium tetrahydroborate as a reducing agent [269]. The decomposition
of this reagent in acidic solutions is complete within a few microseconds and the resulting
nascent hydrogen is capable to reduce analytes fast and quantitatively. The high reactivity of
sodium tetrahydroborate is even important at the pH of ultrapure water, and to avoid sodium
tetrahydroborate decomposition, this reagent is usually prepared in alkaline solution, mainly
in sodium or potassium hydroxide (0.05 to 2.0%(m/v)).
The design of the chemical vapor generation and also the influence of the oxidation state
of the analyte can also have influence on the vapor releasing from the acidified solutions. In
batch systems the pH of the resulting solution varies continuously during the reaction because
the reducing agent is added for a time period over the acidified sample. This can lead to a
hydride re-absortion in the solution and a decrease on the hydride releasing efficiency. In
addition, variations in the amount of nascent hydrogen lead to tailing of the atomization
signals. In flow systems (continuous flow, CF, and flow injection, FI) the vapor releasing is
influenced by the flow rate and the lengths of the reaction coils [61]. However, one of the
most critical parameters affecting the chemical reduction is the oxidation state of the analyte.
Although chemical reduction of mercury, tin or germanium are not influenced by the
oxidation state when using sodium tetrahydroborate as reducing agent, the remaining covalent
hydride forming elements are markedly affected. Thus, hydrides from arsenic and antimony
give poorer responses when these elements are at highest oxidation state (+5), while the
generation efficiency is close to 100 % for the +3 state. Similarly, selenium and tellurium
hydride generation is more efficient when these elements are as +4 state (no measurable
signals from analytes in the hexavalent oxidation state). In such cases, these elements must be
pre-reduced, commonly using potassium iodide, potassium bromide or L-cysteine, or
increasing the hydrochloric acid concentration or heating. In contrast, the hydride generation
efficiency for lead is poor when this element is as +2 state (the most common oxidation state
for lead). In this case, lead is oxidized to a +4 state by using mainly hydrogen peroxide prior
reduction [61,62]. This drawback can some times be advantageous for speciation studies of
inorganic species of arsenic, antimony or selenium based on the direct determination of the
inorganic specie at the most favorable oxidation stage, and the total inorganic species after a
pre-reduction step. These speciation studies, also referred as first order speciation [270], can
be also achieved by combining two different reducing agents, such as tin(II) chloride and
sodium tetrahydroborate for inorganic mercury (Hg(II)) after tin(II) chloride reduction and
inorganic mercury and methyl-mercury after reduction with sodium tetrahydroborate. In
addition, the reaction medium (pH and/or type of acid) has also been found to affect the
reduction of inorganic arsenic and even certain organic arsenicals [262].
Reduction can also be carried out electrochemically (electrochemical hydride generation,
EcHG). In such cases, an electric current instead of a reducing agent (sodium
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 46
tetrahydroborate) is used for reduction. Both batch and CF or FI approaches have been used
and reduction is occurred in special electrolytic cells. Lower blanks, lower limit of detection
and less extension of interferences caused by concomitants (especially transition metals) are
the main advantages derived from EcHG methods [262,271].
After an efficient vapor generation (releasing), the vapor must efficiently be transported
to the atomizer. In addition to the hydrogen released during the reaction, which can be used as
a transport gas, a further inert gas (argon or nitrogen) is used. Moreover the vapor transport,
the inert gas has a second function which consists of enhancing the vapor releasing from the
acidic solutions (stripping). Therefore, the purge gas is usually mixed with the reaction
solution (acidified sample and sodium tetrahydroborate) in both CF and FI systems. Although
first applications used a vapor collection stage (absorption in solutions, adsorption onto
solids, collection under pressure or in cold traps), most of the current developments uses a
direct transfer to the atomizer or a collection in the atomizer (in situ pre-concentration). The
direct transfer of the vapor avoids problems derived from unstable hydrides, which can be
decomposed during transport, or problems derived from re-solubilization or adsorption onto
tubing surfaces. In general, a moderate gas flow rate must be used to reduce interactions of
the vapor with the surfaces and avoid analyte losses. In addition, efficient drying systems for
removing moisture (GoreTex membranes or hygroscopic ion exchange membranes) avoid
analyte losses by re-solubilization.
Most of the current methods for CVG (CV/HG) are based on continuous operation mode
(CF or FI) [262]. In addition, auxiliary lines can be used in flow modes to automatically
perform pre-reduction or oxidation stages before chemical reduction. CF modes employ a
separate delivery of sample and acid to be mixed in a mixing coil, while FI systems use an
injection valve to inject a discrete volume of sample to the flow of the carrier (acid). The
acidified sample (CF) or the carrier with the injected sample (FI) merge the reducing solution
flow upstream a reaction coil where the hydride formation takes place. In the batch mode, the
acidified sample is transported to a stirred glass cell containing the reducing solution or to a
plastic vessel without stirring but with an entrance line through the reducing solution is pumped.
After the chemical reduction, the liberated vapor is flushed with an inert gas into the atomizer.
For CF and IF modes a gas-liquid separator is used to separate the chemical vapor from
the liquid reagents before introducing the vapor into the atomizer. Gas-liquid separators can
be classified into three basic types: hydrostatic separators, separators with forced outlet and
membrane separators [262]. Hydrostatic separators are U-shaped siphons with the liquid
column to balance small pressure variations. These separators offer a relatively high dead
volume and work satisfactorily at low pressure. Gas-liquid separators with forced outlet
consist of small cells (total inner volume around 2.7mL) with the purge gas entrance upstream
the separator vessel and with an outlet connected to a peristaltic pump to draw the reacted
liquid to waste. These separators can handle overpressure better than hydrostatic types.
Finally, membrane gas-liquid separators use a micro-porous membrane made of
polytetrafluoroethylene (PTFE) or silicon rubber, which are permeable only for gases. The
separation is then based on the diffusion of the vapor through the membrane while the liquid
flow through the separator to waste. The main advantage offered by these separators is the
removal of moisture formed in other types of gas-liquid separators [262].
Analytical Chemistry of Cadmium: Sample Pre-treatment… 47
IV.7.2. Atomization of vapor. Atomizers
Except for cold vapor (monoatomic vapor), once the vapor has been generated it must be
atomized before monitoring the atomic absorption or emission signals, or atomic mass
counting. Although conventional flames and non heated flame-in-tube (FIT) atomizers were
firstly used for atomization [61,262], current atomizers consist of inert hydrogen diffusion
flames, narrow-bore quartz tubes (mainly electrically heated), graphite furnaces and plasmas.
As commented, the atomizer can act as a vapor pre-concentration medium just before
atomizing. This is the case of the graphite furnace atomizers (in situ trapping) or the silver or
gold wires for mercury direct amalgamation.
Diffusion flames are often hydrogen flames, which produce low temperatures (within the
350 – 100°C range) and which are the preferable atomizers when using atomic fluorescence
detectors. Modern diffusion flames have been designed specifically for gaseous sampling and
they are quite different from conventional air/nitrous oxide-acetylene flames used for liquid
sampling. The miniaturized design consists of a small argon-hydrogen flame burning on a
borosilicate glass tube (burner section around 5mm). This design improves the signal-to-noise
ratio as well as minimizes vapor dilution into the flame gases and the background absorption
of the flame.
Heated quartz tube atomizers (QTAs) are normally T-tubes with the bar-tube aligned in
the optical path of the spectrometer. The central arm of the T-tube serves for delivery of
vapor from the gas-liquid separator. These tubes (bar-tube typically about 15cm long and
diameter around 10mm) are heated by an electrical resistance device (controlled temperature
up 1100°C) or by a conventional air-acetylene flame. When using electrical heating, the
atomizer is closed at the ends by two removable quartz windows. However, the bar-tube ends
are opened when the quartz tube is heated by a flame. In such cases, quartz windows are
replaced by attached graphite rings at the ends of the bar-tube to prevent ignition of hydrogen.
Plasmas have also been used as excellent atomizers for vapors. The sensitivity of plasma-
based instrumentation for the determination of vapor forming elements increases markedly
when compared with conventional nebulization because the improved transport efficiency
(close to 100 %). The vapor carried by a flow of gas from the gas-liquid separator can be
directly introduced into the plasma, although some approaches use combined vapor generator-
gas-liquid separator devices for vapor introduction into the plasma. These developments were
briefly commented in the section devoted to plasma-based methods section IV.5.4).
Graphite furnaces have been used as efficient atomizers for vapors, especially hydrides,
because their inherent high atomization temperatures. First attempts used a direct transfer of
hydride from the generator to the furnace, which was previously heated at the optimum
atomization temperature (on-line atomization). However, hydride losses by adsorption on the
cooler metal or graphite parts of the atomizer occurred, which implied a poor sensitivity. In
addition, the life time of the hydride transfer tube was short because it was subjected at a high
temperature (atomization temperature). Therefore, an alternative approach based on an in situ
trapping of the hydrides in the graphite tube at low temperatures (or even at room
temperature) was developed. In this case, the graphite furnace (graphite tubes) is used as a
trapping medium and also as an atomizer. This approach offers several advantages over on-
line atomization, such as a low temperature for trapping (from room temperature to 600°C),
which avoids vapor transfer tube damage and minimizes vapor losses by capturing on cooler
parts of the atomizer. In addition, the graphite tube acts as a pre-concentration medium and a
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 48
high amount of vapor from large volumes of samples can be efficiently trapped before
atomization. Therefore, graphite furnace has offered appealing perspectives for chemical
vapor atomization [264].
Current vapor transfer tubes in modern CVG-ETAAS are fine capillaries made of quartz
which are coupled to the arm of conventional autosampler devices. After a fixed temperature
of the graphite tube is reached (trapping temperature), the transfer tube is automatically
inserted through the sampling port of the furnace until finishing vapor evolution from the
generator. Then, the transfer tube is automatically removed and the following stages of the
graphite furnace temperature program (atomization) are run [272].
Although pyrolytic coated graphite tubes with L’vov platforms were firstly used, better
trapping efficiencies were then obtained when using coated graphite tubes, mainly zirconium
coated graphite tubes (permanent coating) or graphite surfaces treated with palladium (non
permanent coating). Other noble metals such as iridium, platinum, ruthenium or rhodium
were also investigated as efficient trapping medium for hydrides [61,262]. The mechanism of
hydride trapping in untreated graphite tubes differs when the trapping occur in heated and
nonheated graphite furnaces. Sturgeon et al.[273] proposed an analyte deposition in the
preheated graphite tube via thermal decomposition. The deposited metal is then reoxidize
during the trapping step, and then it is atomized identically to that occurring for conventional
ETAAS. The mechanism for unheated graphite tubes (or trapping at low temperatures)
implies chemisorption of hydride to the surface rather than a thermal decomposition [274].
Trapping mechanisms when using palladium coated graphite tubes are explained taking into
account the ability of palladium to participate in hydrogen abstraction reactions [262].
The atomization mechanisms are strongly dependent on the atomizer used, but for most
of available atomizers (diffusion flames and heated quartz tubes) atomization occur via free
hydrogen radicals generation. These hydrogen radicals are produced by the reaction of
hydrogen (excess of nascent hydrogen) with oxygen (from the air dissolved in the working
solutions). This mechanism explains the decrease on the atomization efficiency in heated
quartz tubes, even at high temperatures, in absence of hydrogen. In addition, hydrogen
radicals formation also explain the efficient atomization in quartz tubes at low atomization
temperatures when using oxygen mixed with the purge gas [262].
As previously commented, atomization mechanism in graphite furnaces occurs
identically to that occurring for conventional ETAAS. This fact is well accepted when high
atomization temperatures (above 1600°C) are used. However, a mechanism based on
hydrogen radicals formation has been also proposed when atomizing a low temperatures (e.g.
about 1200°C for selenium) [61,262].
IV.7.3. Cadmium determination by CVG-AS
Although the first approaches for cadmium vapor generation were reported in 1989
[275,276], chemical vapor generation has not extensively been applied for the determination
of this element [277]. The reasons are attributed to the inherent instability of cadmium
hydride (some authors state that volatile cadmium species are cadmium hydride, and other
ones that it is a cold vapor); and difficulties with generation, which implies very narrow
conditions, mainly hydrochloric acid concentration, for an efficient generation. Cadmium
vapor generation has been carried out by hydride generation (presumable CdH2) using sodium
tetrahydroborate [194,202,276,278-307 although some reports use an alkylation reaction
Analytical Chemistry of Cadmium: Sample Pre-treatment… 49
[275,308-310 mainly ethylation with sodium tetraethylborate (Cd(C2H5)2) [275,308,310]. In
order to increase the efficiency of cadmium vapor generation, the use of different reagents
such as thiourea [281,282,287,288,292,293], L-cysteine [301], 8-hydroxiquinoleine
[194,296], phenanthroline [296], sodium iodate [297], as well as organized media (mainly
vesicles of didodecyldimethylammonium-bromide, DDAB) [278-280,283,284,289,290], and
certain metallic catalysts (cobalt [194,281,282,285-287,292,301], gallium [285,286], or nickel
[288,292,296]) has been proposed. In addition, EcHG for cadmium has been also tested [311].
Table 4 lists the different methods for cadmium determination based on vapor generation. It
can be seen that cadmium is sensitively assessed in environmental samples (water, sediments,
soils and biota) by coupling the vapor generation technique with all the available atomic
detectors (AAS, AFS, ICP-OES and ICP-MS).
Table 4. Cadmium chemical vapor generation – atomic spectrometric
methods for environmental samples.
AS technique CVG conditions Sample type Reference
Batch-AFS (Ar-H2
diffusion flame) Ethylation (NaBEt4), pH
2.15 Tap water, seawater [275]
Batch-AAS (flame
heated QTA) NaBH4 in DMFa and HCl Aqueous solutions,
DDCb extracts [276]
Batch-ICP-OES/
FAAS NaBH4 in HCl Seawater [278]
Batch/CF-AAS (QTA) NaBH4 in HCl and
vesicles (DDAB)c Aqueous solutions [279]
Batch/CF-AAS (QTA),
CF-ICP-OES NaBH4 in HCl and
vesicles (DDAB) Aqueous solutions [280]
CF-AFS (FIT) KBH4 in HCl, thiourea,
Co(II) Tap water, sediments [281]
CF-AAS (QTA) KBH4 in HCl, thiourea,
Co(II) Tap water, sediments [282]
CF-ETAAS (palladium
coated graphite tubes) NaBH4 in HCl and
vesicles (DDAB) Aqueous solutions [283]
CF-ETAAS (palladium
coated graphite tubes) NaBH4 in HCl and
vesicles (DDAB) River water [284]
FI-ETAAS (iridium,
tungsten and zirconium
coated graphite tubes)
NaBH4 in HCl, Co(II) or
Ga(III) Seawater [285]
FI-ETAAS (iridium
coated graphite tubes) NaBH4 in HCl, Co(II) or
Ga(III) Seawater [286]
FI-IDA-ICP-MS NaBH4 in HCl, thiourea,
Co(II) River water, seawater,
sediments [287]
FI-AAS (QTA) NaBH4 in HCl, Ni(II) Water, soils, geological
materials [288]
Batch/CF-ETAAS
(conventional pyrolytic
coated graphite tubes –
the pyrocoating was
mechanically removed)
KBH4 in HCl, vesicles
(DDAB), thiourea, Co(II) Seawater, soils,
sediments, plants [289]
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 50
Table 4. (Continued)
AS technique CVG conditions Sample type Reference
FI/CF-AAS (QTA) NaBH4 in HCl, vesicles
(DDAB) Tap water, seawater [290]
FI-AAS (QTA) NaBH4 in HCl Sewage sludge,
waste water [291]
FI-AAS (Pyrex glass
cell) NaBH4 in HCl, thiourea,
Ni(II) Plants, soils [292]
FI-AAS (Pyrex glass
cell) NaBH4 in HCl, thiourea,
Ni(II) Plants, soils [293]
FI-AAS (QTA) NaBH4 in HCl, KCN Sewage sludge,
Antarctic krill [294]
FI-NDAFS (Ar-H2
diffusion flame) KBH4 in HCl, thiourea,
ascorbic acid Seawater, soils, total
airborne particulate
matter
[295]
CF-NDAFS (Ar-H2
diffusion flame) NaBH4 in HCl, 8-
hydroxiquinoline/
phenanthroline, Ni(II)
Soils, plants [296]
FI-NDAFS (FIT) KBH4 in HCl, NaIO4 Aqueous solutions [297]
FI-AAS (trapping into
heated QTA) NaBH4 in HNO3,
Trapping at 350°C (1.4V)
and revolatilization at
1030°C (4.1V)
Seawater, oyster [298]
FI-AAS (QTA,
Multimode Sample
Introduction System
(MSIS))
NaBH4 in HCl, thiourea,
Co(II), Ni(II) Water, mussels [202]
FI-AAS (QTA) NaBH4 in HNO3,
thiourea/L-cysteine,
Co(II)
Waters, plants, soils [299]
FI-IDA-ICP-MS, CF-
ETV-IDA-ICP-MS
(iridium coated
graphite tubes)
NaBH4 in HCl River sediments, marine
sediments [300]
FI-ETAAS
(iridium/zirconium
coated graphite tubes)
NaBH4 in HNO3 Plants, whale liver [301]
CF-AFS (Ar-H2
diffusion flame) NaBH4 in HCl Plants, soils [302]
FI-AAS (QTA) NaBH4 in HCl Water [303]
CF-ICP-OES NaBH4 in HNO3, 8-
hydroxiquinoline, Co(II) Aqueous solutions [194]
CF-AFS (Ar-H2
diffusion flame) NaBH4 in HCl Seawater [304]
CF-AAS (QTA)
(trapping into a heated
tungsten coil)
NaBH4 in HCl Seawater, oyster [305]
Analytical Chemistry of Cadmium: Sample Pre-treatment… 51
Table 4. (Continued)
AS technique CVG conditions Sample type Reference
AFS (QTA) KBH4 in HCl, thiourea Plants [306]
FI-AFS (QTA) NaBH4 immobilized onto
cellulose, HCl Waters, seawater [307]
CF-AFS (Ar-H2
diffusion flame) Ethylation (NaBEt4), pH
8.0 – 8.5 Tap water, sludge [308]
CF-ETV-ICP-OES In situ alkylation
(C2H5)MgBr Mussel, plant,
rice, vehicle exhaust
particulates
[309]
FI-IDA-ICP-MS Ethylation (NaBEt4) in
HNO3 Water, sediments,
lichens [310]
DMF, dimethylformamide
DDC, diethyldithiocarbamate
DDAB, didodecyldimethylammonium-bromide
AAS, atomic absorption spectrometry; AFS, atomic fluorescence spectrometry; CF-AAS, continuous
flow – atomic absorption spectrometry; CF-AFS, continuous flow – atomic fluorescence
spectrometry; CF-ETAAS, continuous flow – electrothermal atomic absorption spectrometry; CF-
ETV-IDA-ICP-MS, continuous flow – electrothermal vaporization – isotope dilution analysis –
inductively couple plasma – mass spectrometry; CF-ETV-ICP-OES, continuous flow –
electrothermal vaporization - inductively couple plasma – optical emission spectrometry; CF-ICP-
OES, continuous flow – inductively coupled plasma optical emission spectrometry; CF-NDAFS,
continuous flow – non-dispersive atomic fluorescence spectrometry; FAAS, flame atomic
absorption spectrometry; FI-AAS, flow injection – atomic absorption spectrometry; FI-AFS, flow
injection – atomic fluorescence spectrometry; FI-ETAAS, flow injection – electrothermal atomic
absorption spectrometry; FI-NDAFS, flow injection – non-dispersive atomic fluorescence
spectrometry; FI-IDA-ICP-MS, flow injection –isotopic dilution analysis - inductively couple
plasma – mass spectrometry; ICP-OES, inductively couple plasma – optical emission
spectrometry; FIT, flame in-tube atomizer; IDA-ICP-MS, isotopic dilution analysis - inductively
couple plasma – mass spectrometry; QTA, quartz tube atomizer
IV.8. X-ray Fluorescence Spectrometry
X-ray fluorescence spectrometry (XRF) is a non-destructive analytical technique used to
identify and determine the concentrations of major and trace (at the µg g–1 level) elements
present in solid, powdered and liquid samples. One of its main advantages is that minimal
sample-pretreatment is required, being appealing methodologies for routine analsyis.
IV.8.1. General aspects
XRF is based on the emission of a characteristic radiation in form of photons
(fluorescence) of an atom. However, the physics of XRF differs from AFS in the fact that the
emission of such radiation is attributed to transitions of electrons from higher energy levels to
lower energy orbitals after the removal of tightly-held electrons from the inner orbitals of the
atom (ionization). Short-wavelength X-rays or gamma rays are energic enough (energy
greater than the ionization potential of most of the elements) to expel electrons from the inner
orbitals of an atom. The removal of such electrons renders the electronic structure of the atom
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 52
unstable, and electrons in higher orbitals fall into the lower orbitals. Therefore, the released
energy (photons) is equal to the energy difference of the two orbitals involved and it is
characteristic of the atom. However, when removing an inner electron from an atom, there are
only a limited number of ways in which electrons from outer or higher energy orbitals can
drop into its place. Taking in mind the nomenclature of N, M, L and K energy levels (from
higher to lower energy), the main electronic transitions are referred as Kα, which implies a
L→K transition (a electron from the outer L energy level to the inner K level); Kβ, which is
related to an M→K transition; Lα, which implies a hole in the L energy level and a M→L
transition; and so on. As consequence, different fluorescent photons, with a characteristic
energy equal to the difference in energy of the initial and final orbital, will be obtained,
leading to a characteristic XRF spectrum for each element.
IV.8.2. Instrumentation
XRF instruments comprise an X-ray source for ionization, a wavelength selector for
restricting the wavelength range of incident radiation, a sample holder, a detector and a signal
processor. Wavelength selectors (filters or monochromators) lead to wavelength-dispersive
instruments (WDXRF or WDS); X-ray photometers or X-ray spectrophotometers,
respectively. However, instead of separating the wavelengths of the radiation, the fluorescent
radiation can be analyzed by sorting the energies of the emitted photons. In this case, the
instruments are commonly referred as energy-dispersive systems (EDXRF or EDS). A third
type spectrometer, which is considered as a special variant of EDXRF [312], is the total
reflection X-ray fluorescence spectrometry (TXRF). TXRF occurs when X-ray beam exhibits
total reflection for a particular angle of incidence. In this case, the sample is excited by the
primary X-ray beam at a glancing angle less than the critical angle at which total external
reflection occurs, and the primary exciting radiation is incident on a plane polished surface,
which serves either as a sample support or is itself the object to be examined. A detailed
description of XRF and other X-ray based techniques can be found elsewhere [313,314].
(a) X-ray sources
Conventional X-ray tubes, also called Coolidge tubes, consist of a cathode (tungsten
filament) and an anode made of pure metals (silver, chromium, molybdenum, rhrodium or
tungsten) mounted in a vacuum glass tube, although homogeneous alloy of molybdenum and
tungsten has also used [315]. When the cathode is heated by establishing an electrical current
from a high voltage power source (typically from 30 to 150kV), a thermoionic emission of
electrons from the cathode is obtained, electrons which are accelerated to the anode. X-ray
emission from the anode is a consequence of high energy electrons passing through the tube
which are progressively decelerated by the material of the anode, and the continuous X-ray
radiation obtained is referred as a "bremsstrahlung" radiation. Anyway, the X-ray spectrum
obtained depends on the anode material and the accelerating voltage. Galodinium anodes
have been demonstrated to generate high energy polarizing X-ray beam [316,317], and the
technique is commonly referred as high-energy polarized X-ray fluorescence spectrometry
(HE-P-XRF). Modern X-ray generators use pulsed electrical currents, typically between about
1ms to 1s, rather than high voltage power sources, which allow a more accurated control of
the X-ray unit and reduced X-ray exposures.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 53
Gamma ray sources can also be used without the need for an elaborated power supply. In
these sources, different radioactive substances, which give continuous or lines spectrum, can
be used. The most common radioisotopes are tritium adsorbed onto non-radioactived titanium
(31H-Ti) and 14761Pm-Al, which give continuous X-ray spectrum. Other isotopes, such as
5626Fe, 5727Co, 10948Cd, 12553I or 21082Pb, give lines X-ray spectrum. The X-ray intensities
provided by radioisotopes are several orders of magnitude weaker than conventional X-ray
tubes. Although their limited sensitivity, these sources are mainly used in portable
instrumentation.
There is a new X-ray source type consisting of a synchrotron which offers an intensive,
polarized and collimated X-ray radiation (synchrotron radiation, SR) and has led to
synchrotron radiation induced X-ray fluorescence spectrometry (SR-XRF) [318]. Although
some application can be found in literature, the major drawback of SR is the limited access to
the facility.
(b) Wavelength selectors
Filters and monochromators can be used in WDXRF, although the use of filters is limited
by the few number of target-filter combinations. Most of the WDXRF use a monochromator
consisting of a single crystal mounted on a goniometer which separates the photons by
diffraction. Crystals with simple structure tend to give the best diffraction performance.
Different materials are used, e.g. germanium (Ge), graphite, lithium floride (LiF), sodium
chloride (NaCl), indium antimonide (InSb), ammonium dihydrogen phosphate (ADP),
tetrakis-(hydroxymethyl)-methane: penta-erythritol (PE), potassium hydrogen phthalate
(KAP), rubidium hydrogen phthalate (RbAP) or thallium(I) hydrogen phthalate (TlAP). By
varying the angle of incidence and take-off on the crystal, a single X-ray wavelength can be
selected according with the Bragg equation. It must be noticed that there are not a dispersing
crystal useful for the whole X-ray spectrum, and commercially monochromators are
commonly available with at least two interchangeable crystals. Instead of crystals, some
modern instruments use layered synthetic microstructures as dispersing systems. These
sandwich structured materials comprise successive thick layers of low atomic number matrix
and monoatomic layers of a heavy element, and they diffract any desired long wavelength.
Two different configurations (flat crystal with Soller collimators or curve crystal with
slits) can be used to precisely define the incident and exit angles. In the first configuration, a
Soller collimator is a stack of parallel metal plates, spaced a few tenths of a millimeter apart.
The use of beam collimators offers some drawbacks such as intensity reduction and
scattering, however, the simplicity of the geometry and low cost is especially useful for
variable-geometry monochromators. In the second configuration, the Rowland circle
geometry ensures that the slits are both in focus, but in order for the Bragg condition to be
met at all points, the crystal must first be bent to a radius of 2R (where R is the radius of the
Rowland circle), then ground to a radius of R. This arrangement allows higher intensities with
higher resolution and lower background. But the mechanics of keeping Rowland circle
geometry in a variable-angle monochromator is extremely difficult.
There are not dispersing elements for EDXRF. As it will be commented in following
sections the fluorescent X-rays emitted by the sample are directed into a solid-state detector
which produces a continuous distribution of pulses, the voltages of which are proportional to
the incoming photon energies. This signal is processed by a multichannel analyzer (MCA)
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 54
which produces an accumulating digital spectrum that can be processed to obtain analytical
data.
(c) Detectors
(c.1.) Detectors for WDXRF
There are different types of detectors for WDXRF which offer high pulse processing
speeds in order to cope with the very high photon count rates.
Gas-filled transducers
These devices consist of a chamber containing a gas (ionization chamber), which it can
be sealed (sealed gas detectors) or it can be open, allowing the gas to flow through it
continuosly (gas flow proportional counters). For sealed gas detectors, an inert gas (argon,
krypton or xenon) at a few atmospheres pressure is used. However, a mixture 90% argon
(helium or neon) and 10% methane (P10) is used for gas flow detectors. Methane is
commonly used to suppress the formation of fluorescent photons caused by recombination of
the argon ions with stray electrons. A wire anode made of tungsten or nichrome (20-60 µm
diameter), a cathode and a X-ray transparent window are the other components of these
devices. The window needs to be conductive, thin enough to transmit the X-rays effectively,
but thick and strong enough to minimize diffusion of the detector gas into the high vacuum of
the monochromator chamber. Materials often used are beryllium metal, aluminized PET film
and aluminized polypropylene.
For both systems, radiation (X-ray photons) enters a chamber through the transparent
window and the gas is ionized (loss of outer electrons) as consequence of X-ray photon-gas
atom interactions. This photoelectron has a large kinetic energy and loses this excess of
energy by ionizing several hundred additional atoms of the gas. Photoelectrons move toward
the central wire anode while cations are attracted toward the cylindrical metal cathode under a
fixed applied potential. Depending of the applied potential, the number of photoelectrons that
reach the anode is different. The number of electrons attacting the anode can be constant and
they represent the total number formed by a single photon. This is the case of the called
ionization chamber counters. However, the number of electrons can rapidily be increased with
the applied potential due to a secondary ion-pair production caused by collisions between the
accelerated electrons and gas molecules. In this case, the system works as a proportional
counter. Finally, a third type of system (Geiger detector) can be obtained if the electrical
signal is amplified.
Gas flow proportional counters are used mainly for detection of longer wavelengths while
sealed gas detectors are used for detecting wavelengths in the 0.15-0.6 nm range.
Scintillation counters
These detectors consist of a scintillating crystal, typically made of sodium iodide doped
with a little amount of thallium iodide (around 0.2%) wihich is attached to a photomultiplier.
Organic scintillators such as stilbene, anthracene and terphenyl can also be used. These
crystals must be protected with a relatively thick aluminium/beryllium foil window, which
Analytical Chemistry of Cadmium: Sample Pre-treatment… 55
limits the use of the detector to wavelengths below 0.25 nm. When the incoming radition
traverses the crystal, the X-ray photons are absorbed and after a certain time (about 0.25µs),
this excess of energy is released as photons of fluorescence radiation. Thousands of such
fluorescent photons are produced by each primary X-ray photon absorbed. The flashes of
light produced in the crystal are transmitted to the photocathode of the photomultiplier tube,
which convert the the radiation energy in an electrical pulse which is amplified and counted.
Semiconductor detectors
Semiconductor detectors (solid-state detectors) can be used for both WDXRF and
EDXRF, although historically their use for WDXRF had been restricted by their slow
response. The lithium-drifted silicon detector, Si(Li), is one of the most semiconductor
detectors used. Si(Li) consists of a 3-5 mm thick three layers silicon junction (p-i-n diode): a
p-type semiconducting layer that faces the X-ray source, a central intrinsic zone (the lithium-
drifted non-conducting i-layer), and an n-type layer. Under a negative applied voltage, when
an X-ray photon enters the detector, its energy is aborbed and it causes a swarm of electron-
hole pairs to form (electrons are promoted from the valence to the conduction band, leaving
positive hole in the valence band). To obtain sufficiently low conductivity, the detector must
be maintained at low temperature, and liquid-nitrogen must be used for the best resolution.
With some loss of resolution, the much more convenient Peltier cooling can be employed.
More recently, high-purity wafer silicon with low conductivity have become routinely
available. Cooled by the Peltier effect, this provides a cheap and convenient detector,
although the liquid nitrogen cooled Si(Li) detector still has the best resolution.
(c.2.) Detectors for EDXRF
In energy dispersive analysis, dispersion and detection are a single operation.
Proportional counters or various types of p-i-n diodes (Si(Li), Ge(Li) or silicon drift detector,
SDD) can be used. They all work as previously commented and its critical factor is the
detection speed because the spectrum must be built up by dividing the energy spectrum into
discreet bins and counting the number of pulses registered within each energy bin.
(d) Signal processors
Signal processing is achieved by pulse-heigth selectors which reject pulses of about 0.5V
or less, avoiding the transduction and amplification of noise. There are also electronic circuits
which reject not only low-heigth pulse but also those above a maximum level. The pulses
generated by the detector are processed by pulse-shaping amplifiers. It takes time for the
amplifier to shape the pulse for optimum resolution.
IV.8.3. X-ray fluorescence spectrometers
As commented, X-ray fluorescence spectrometers can be of two types: wavelength
dipersive and energy dispersive instruments.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 56
(a) Wavelength dispersive spectrometers
WDXRF uses a Coolidge tube as a source of radiation due to losses suffered in the
monochromator. There are single channel (sequential) and multichannel (simultaneous)
configurations.
Sequential spectrometers have a single variable-geometry monochromator (with
interchangeable dispersing crystals) and a single detector assembly (but usually with more than
one detector arranged in tandem). The instrument is programmed to move through a sequence
of wavelengths, in each case selecting the appropriate X-ray tube power, the appropriate crystal,
and the appropriate detector arrangement. The main disadvantage of these systems is the
relatively long analysis time, particularly when many elements are being determined, not only
because the elements are measured in sequence, but also because a certain amount of time is
taken in readjusting the monochromator geometry between measurements.
Simultaneous spectrometers have a number of channels dedicated to analysis of a single
element. Each channel consists of a fixed-geometry crystal monochromator, a detector, and
processing electronics. This allows a number of elements to be measured simultaneously, and
in the case of high-powered instruments, complete high-precision analyses can be obtained in
short time (less than 30s). Another advantage of this arrangement is that the fixed-geometry
monochromators have no continuously-moving parts, and so are very reliable. Moreover the
relatively high cost of these instruments, simultaneous detection offers other disadvantages
such as the number of elements that can be measured is limited to 15-20, because of space
limitations on the number of monochromators that can be arranged.
(b) Energy dispersive spectrometers
An EDXRF consists of a Coolidge tube or a radioactive material as a source of radiation,
a semiconductor detector and different electronic componens for energy discrimination.
These systems are very simple because monochrmoators are not included in the design and
avoid large distances between the sample and the detector, which results in a 100-fold
increase sensitivity. These instruments are commonly multichannel spectrometers, allowing
the simultaneous measurement of all the emitted X-ray lines.
IV.8.4. Limitations of XRF
Although XRF offers several advantages such a minimum sample pre-treatment, high
speed of analysis, relative simple spectrum with few lines and absence of spectral line
interferences, XRF method have some limitations. Sensitivity is not high enough for the
determination of concentrations at trace levels and detection of lighter elements (atomic
number below 23) is difficult because the simultaneous competitive process (Auger emission)
which reduce the fluorescence intensity.
In addition, matrix effects can be also found as consequence of the number of photons
leaving he sample can be affected by the sample itself (physical properties of the sample).
The most important matrix effect are derived from the fact that the emitted X-ray radiation
from a target element can partially be absorbed by other elements in the sample, and this can
lead to the same element concentration gives different analytical signals in function of the
Analytical Chemistry of Cadmium: Sample Pre-treatment… 57
sample components. These types of “mass absorption” are well known for many matrices and
corrective calculation can be made when knowing the composition of the sample.
Sometimes matrix effects can arise from X-ray enhancement that occurs when the
secondary X-rays emitted by a heavier element are sufficiently energetic to stimulate
additional secondary emission from a lighter element. This phenomenon can also be modeled,
and corrections can be made.
The third type of matrix effects is attributed to sample macroscopic effects, which consist
of effects of inhomogeneities of the sample, and unrepresentative conditions at its surface.
These artifacts are inherent to the method of sample preparation and they can not be
compensated by theoretical corrections, and must be "calibrated in". This means that the
calibration materials and the samples must be compositionally and mechanically similar, and
a given calibration is applicable only to a limited range of materials.
IV.8.5. Cadmium determination in environmental samples by XRF
Cadmium can be determined by XFS at the spectral Cd K line of 0.05357nm (line Kα1).
This line is the most intense and free of lines overlaps for cadmium determination. Solid
environmental materials can be directly analyzed by XRF techniques but sometimes a sample
preparation involving grinding the sample into a powder that is either analyzed as a loose
powder or pressed into a pellet is required. This procedure increases uniformity and
homogeneity of the sample. In some cases, fusion can be needed to pre-treat the solid sample
in order to minimize or avoid matrix effects. Liquid samples can be directly analyzed when
the liquid is compatible with the sample cup and sealing film, however, a sample pre-
concentration stage can be needed for some applications.
Detailed data on XRF methods for different material types can be found in a review by
Potts et al.[318]. In this section, brief information of recent applications of XRF for assessing
cadmium in environmental materials is summarized.
WDXRF has been used for the analysis of aerosol loaded onto filters [319,320],
atmospheric dust and vehicles exhaust particles [321], marine sediments [322], soils [323],
plants [324], and water [325,326,327] and waste waters [328]. EDXRF was also applied for
cadmium determination in soils [329,330], rocks [329], marine sediments [331,332], biota
[333], liquid wastes [334], and waters [335] and seawater [316,317] after certain pre-
concentration stages. TXRF was also used for domestic sludge [336], marine sediment [337],
atmospheric particulate matter [338,339], biota [340,341] and waters (river water [342,343]
and rainwater [338]).
IV.9. Neutron Activitation Analysis
Similar to XRF techniques, neutron activation analysis (NAA) is a non-destructive
analytical methodology which gives quantitative and qualitative information of a number of
major components and trace elements (at the µg g–1, or even ng g–1 levels) in different samples.
Moreover the non-destructive character as well as its inherent high sensitivity, NAA enables the
simultaneous determination of many elements. However, NAA is not a common methodology
in many laboratories because the high cost of the equipment, and mainly because the special
conditions required for laboratories and highly qualified staff to cope with radioactivity.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 58
IV.9.1. General aspects
NAA consists of irradiating the sample (which contains the naturally occurring stable
isotopes of the target elements) with neutrons. After the element-neutron interaction (neutron
capture), the target elements are transformed into radioactive isotopes. The compound
nucleus of the activated elements can almost instantaneously be de-excited into a more stable
configuration through emission of one or more characteristic prompt gamma rays. However,
in many cases, this new configuration can yield a radioactive nucleus which also decays by
emission of one or more characteristic delayed gamma rays, but at a much slower rate
according to the unique half-life of the radioactive nucleus. Depending upon the particular
radioactive species, half-lives can range from fractions of a second to several years.
Therefore, with respect to the time of measurement, NAA falls into two categories; prompt
gamma-ray neutron activation analysis (PGNAA), where measurements take place during
irradiation,; or delayed gamma-ray neutron activation analysis (DGNAA), where the
measurements follow radioactive decay. This latter operational mode leads to the most
common NAA technique.
Instead of neutron capture, other nuclear processes such as those based on (n,gamma)
reactions can be used for activating nuclides.
Qualitative information from NAA is based on the energy of the emitted gamma quanta
(E) and the half life of the nuclide (T½). Simmilarly, the quantitative data is given by the
intensity (I), which is the number of gamma quanta of energy E measured per unit time.
IV.9.2. Instrumentation
NAA instrumentation consists mainly of a source of neutrons and a gamma-ray detector.
(a) Irradiation (neutron) sources
Kinetics of neutron activation process is out of the scope of this chapter and a brief
description of the different neutron sources is only given. Detailed description of such neutron
activation mechanisins can be found elsewhere [344,345].
(a.1.) Isotopic neutron sources
An isotopic neutron source uses an alpha emitting radioactive material which is mixed
with beryllium, allowing an (,n) reaction for generating neutrons as follows:
9
4Be + 42He → 126C + 10n + 5.7 MeV
Different radioisotopes such as 227Ac (T½ of 22 years), 226Ra (T½ of 1620 years), 239Pu
(T½ of 2.4 104 years) or 210Po (T½ of 138 days) can be used as emitters, giving neutron
energies around 4 MeV.
Other isotopic neutron sources are based on certain artificially produced radioisotopes,
such as 252Cf (T½ of 2.6 years) which undergo spontaneously fission, producing 3.76
neutrons of 1.5 MeV per event (one milligram of 252Cf emits 2.28 109 neutrons per second).
Analytical Chemistry of Cadmium: Sample Pre-treatment… 59
These neutron sources generate a stable neutron flux but its intensity is low when
comparing with other neutron sources.
(a.2.) Neutron generators
The neutron sources consist of accelerators where a convenient target material is
bombarded by accelerated charged particles, commonly deutrons (deuterium ions), and the
neutrons are produced in a nuclear reaction. Tritium, an isotope of hydrogen, previously
adsorbed onto titanium or zirconium, is the most frequently target material used in such
devices and the nuclear reaction carried out is denoed as 3H(d,n)4He:
2
1H + 31H → 42He + 10n + 14.0 MeV
The monoenergetic neutrons flux produced by this accelerator is of approximately 100
neutrons cm-2 s-1.
(a.3.) Nuclear reactors
Nuclear reactors for uranium fission offer the highest available neutron fluxes, and hence
the highest sensitivities for most elements. Different types of reactors and different positions
within a reactor can vary considerably with regard to their neutron energy distributions and
fluxes due to the materials used to moderate or reduce the energies of the primary fission
neutrons. Among of the three principal components (thermal, epithermal, and fast) of neutron
energy distributions, the thermal neutron component is the most useful as a source for NAA.
This component consists of low-energy neutrons (energies below 0.5 eV) in thermal
equilibrium with atoms in the reactor's moderator. In most reactor irradiation positions, 90-
95% of the neutrons that bombard a sample are thermal neutrons. In general, a one-megawatt
reactor has a peak thermal neutron flux of approximately 1 1013 neutrons cm-2 s-1.
(b) Gamma ray detectors
(b.1.) Scintillation counters
Scintillating crystals based on sodium iodide doped with a little amount of thallium
iodide, described in section III.3.2.c.1 as X-ray detectors, can also be used to detect gamma
ray. Similarly to X-ray detection, the incoming gamma ray is absorbed by the crystal, and
after a certain time the absorbed energy is released as photons of fluorescence radiation. After
attaching the scintillating crystal to a photomultiplier tube, the released fluorescent photons
are converted in an electrical pulse which is amplified and counted. The efficiency of these
detectors is quite high, but their energy resolution is poor comparing to other gamma ray
detectors.
(b.2.) Semiconductor detectors
Most of the detectors for gamma ray are semiconductor detectors, especially made of
germanium, HP-Ge. These detectors consit of large volume cylinder of coaxial geometry in
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 60
which the outer surface serves as one electrode and a central conducting cylindrical core acts
as the other electrode after applying a high voltage between them. When gamma rays interact
with semiconductor, free electrons and holes are produced (electrons are promoted from the
valence to the conduction band, leaving positive hole in the valence band). Under the applied
voltage, the electrons and holes behave as negative and positive charges and are collected at
the electrodes and the amount of charge collected is correlated with intensity of the gamma
ray. One practical drawback of HP-Ge detectors is that it must be operated at liquid nitrogen
temperatures to keep a low thermal noise.
Semiconductor detectors based on silicon can be also used for detecting low-energy
gamma-ray. Other semiconductor materials, such as cadmium telluride (CdTe) or mercuric
iodide (HgI2) can be used in some applications. However, these materials offer poorer energy
resolution than germanium and silicon, although they can be operated at room temperature.
The energy resolution of HP-Ge detectors is around 1.9-2.2 keV, which is quite high than
that obtained for scintillating counters (approximately 90 keV).
(c) Signal processors
As commented for XRF, pulses derived from both scintillation counters and
semiconductor detectors are selected by means of electronic discriminators which reject those
electronic pulses that fall below or above a specific voltage level. After amplification, these
devices are connected to single-channel analyzers (SCAs) or multichannel analyzers (MCAs),
which are computer based systems for an automatic spectrum evaluation.
A SCA uses two electronic discriminators, called upper and lower level discriminators.
Pulses from the amplifier are fed to the analyzer, and if the pulse height falls between the
lower and upper discriminators the usual logic is to allow such a pulse to be counted. The
voltage levels of the two discriminators are adjustable so that the gap between them
corresponds to a group of pulse heights within a fixed energy interval. These groups of pulses
are related to a particular radionuclide and the voltage gap between both discriminators is
frequently referred to as an energy window. By varying the position of the window (i.e.,
changing the voltage levels of each of the discriminators) it is possible to measure gamma
rays of different energies (from different activated elements). This process offers as a
disadvantage the large time when measuring many different energy gamma rays from several
elements. For such cases, a MCA is a much more efficient system to use.
MCA consists of an analog-to-digital converter (ADC) which takes the analog voltage
pulse from the detector and converts it to a digital signal proportional in magnitude to the
height of the analog pulse. The MCA contains a number of equal width storage bins (called
channels, up to a maximum of 8192). The width of the storage bins corresponds to a specific
energy interval and can be set by the user, depending on the application.
IV.9.3. Analysis by NAA
(a) Absolute analysis
Since the theory of NAA is well founded, an “absolute” standardisation procedure can be
applied. In this case, the quantitative measurement can be obtained by determining the
Analytical Chemistry of Cadmium: Sample Pre-treatment… 61
neutron flux and counting the absolute gamma rays, according to equations related to the
measured intensities of gamma ray of certain energy. However, these absolute methods can
offer systematic errors due to uncertainties of nuclear data taken from literature, especially
those on decay schemes and activation cross-section.
When using nuclear reactors as neutron sources, there is another approach (k0-
standardization technique of NAA) originally developed by Simonits et al.[346] for
multielement analysis, which also avoids the use of standards for multi-element
determinations. K0-standardization involves the simultaneous irradiation of a sample and a
neutron flux monitor, such as gold, and the use of a composite nuclear constant called k0-
factor. The k0-factor is independent of irradiation and measuring conditions [347]. For the
calculation of elemental concentrations, the k0-NAA uses input parameters such as
subcadmium to epithermal neutron flux ratio, the epithermal neutron flux shape factor, and
the absolute efficiency of the detector. These parameters are dependent on each irradiation
site in a reactor and counting facility, and they must be determined for standardization.
(b) Classic relative methods
Classic relative methods, based on the simultaneous irradiation of samples and standards
under identical positions, and followed by measuring the induced intensities of both the
standard and the sample in a well known geometrical position, are also convenient
methodologies. Based on this method, there are two types of NNA determinations according
to a destructive or non-destructive process of the sample. As commented, for both analysis
types, several standards, and also the sample, contained in sealed quartz or polyethylene vials,
are simultaneously irradiated for a fixed time (from several minutes to hours). After finishing
the irradiation stage, both standards and sample are let for a certain time (cold-down time) in
order to allow desactivation of those radionuclides with short T½ which can be a source of
interferences. This time can be within few minutes to several hours.
(b.1.) Non-destructive methods
In non-destructive methods, pulses (counts) from sample and standards are directly
counted after the cold-down stage. In this case, the detector must guarantee a good selectivity
for distinguishing gamma rays of different energy. The amount of the analyte (analyte weigth)
in the sample is obtained by applying the following equation:
w
x = ws (Rx/Rs)
where wx and ws are the analyte weight in the sample and the standard weight, respectively;
and Rx and Rs are the counts for the analyte in the sample and for the standard, respectively.
Non-destructive methods are very simple and avoid any sample manipulation. However,
the success of the analysis are related to the high energy resultion of detectors and also to the
complexity of the sample and absence of certain elements which can lead gamma rays of
similar energy than those generated from the target analyte.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 62
(b.2.) Destructive methods
In destructive methods, the anlyte must be isolated from the sample just before gamma
ray counting. Analyte isolation commonly occurs after dissolving the irradiated sample and
extracting the analyte by precipitation, liquid-liquid extraction, ionic exchange or
chromatography. Once the analyte and the standard are isolated, the gamma ray counts are
recorded and the amount of target element is calculated by the equation shown above.
IV.9.4. Cadmium determination in environmental samples by NAA
There are reported several applications of NAA to assess cadmium and other trace
elements in environmental materials, especially solid environmental samples. Moreover NAA
based on thermal neutron activation, different NAA methodologies based on epithermal
activation (ENAA), cyclic activation (CNAA), Prompt gamma activation (PGNAA), with or
without Compton suppression, have been implemented in several laboratories and they have
been used in environmental studies. Some recent NAA determinations include the analysis of
soils [348-350 and sediments [348,351-356 atmospheric particulate matter [357-366 fly ashes
[367,368], and biota (mollusks [369], seaweeds [370], linchens [371-374 moss [372,375-380
and plants [354]).
IV.10. Electrochemical Techniques: Anodic Stripping Voltammetry and
Potentiometric Sensors
Voltammetry comprises a large group of instrumental techniques in which the potential
of an electrode (indicator or working electrode) immersed in solution is controlled so as to
force a redox reaction to occur at the electrode surface. The rate of this reaction is
proportional to the current flowing through the electrode, and as conditions are chosen so that
the rate of the redox reaction is controlled by the rate of transport of the reacting species to
the indicator electrode surface, the current flowing through the cell is proportional to the
concentration of the reacting species. For these techniques, the plot of the measured electric
current versus the potential of the electrode is commonly named as voltammogram.
Usually, a set of two or three electrodes is dipped into the analyte solution, and a
regularly varying potential is applied to the working electrode relative to the second electrode
(the other half of the cell). The working electrode is an ideally polarizable electrode (i.e., the
electrode shows a large change in potential when an infinitesimally small current is passing
through), such as the dropping mercury electrode (DME), with surfaces as small as possible
so that there is a high concentration of active species in the surrounding area, and therefore a
strong polarization. The second electrode has two functions; firstly it must have a known
potential with which to gauge the potential of the working electrode; and secondly, it must
balance the electrons added or removed by the working electrode. However, it can be difficult
for this second electrode to maintain a constant potential while passing current to counter
redox events at the working electrode. To solve this problem, two electrodes are used, the
reference electrode which is a half cell with a known reduction potential and which acts as
reference in measuring and controlling the working electrodes potential and at no point does it
Analytical Chemistry of Cadmium: Sample Pre-treatment… 63
pass any current; and the auxiliary electrode, which passes all the current needed to balance
the current observed at the working electrode. To achieve this current, the auxiliary electrode
will often swing to extreme potentials at the edges of the solvent window, where it oxidizes or
reduces the solvent or supporting electrolyte. A three electrodes set (working, reference, and
auxiliary) is the basis of the modern voltammetry instruments.
Potential modulation is usually carried out by a simple staircase ramp, but modulation
based on pulses offers advantages in sensitivity and speed. There are three pulse techniques
for modulation, which leads to three voltammetry methods, the normal pulse voltammetry
(NPV), the differential pulse voltammetry (DPV) and the square-wave voltammetry (SWV).
NPV uses a series of potential pulses of increasing amplitude and the current measurement is
made near the end of each pulse. DPV also works with potential pulses but these pulses are of
fixed amplitude and are superimposed on a slowly changing base potential. In this case,
current is measured just before the application of the pulse and at the end of the pulse (two
points for each pulse). Finally, SWV uses a symmetrical square-wave pulse of amplitude ESW
superimposed on a stairscase waveform of step height ∆E, where the forward pulse of the
square wave coincides with the stairscase setp.
There is a wide variety of voltammetry techniques such as linear sweep voltammetry
(LSV), anodic stripping voltammetry (ASV), cathodic stripping voltammentry (CSV), or
adsorptive stripping voltammetry (AdSV). Polarography is also a voltammetric technique for
which the working electrode is a DME. This electrode type is useful for its wide cathodic
range and renewable surface. Detailed information for voltammetry methods is given in
monographs by Bard and Faulkner [381], Wang [382] and Brainina and Neyman [383].
IV.10.1. Anodic stripping voltammetry
Anodic stripping voltammetry (ASV) is the electroanalytical technique that has the
lowest limits (ppb or lower) of detection for metal determination. The technique is based the
electrochemical deposition or accumulation (reduction) of the analyte at the working
electrode (anode or cathode) of constant surface and controlled potential electrolysis; and the
subsequent stripping (oxidation) from the electrode by a voltammetric technique or chemical
reaction. A certain waiting time between deposition and stripping (typically 30s) is needed.
The accumulation of the analyte onto the surface of the working electrode implies a pre-
concentration of the analyte (pre-concentration factor within 100 to 1000). This is the main
advantage of ASV over other voltammetric techniques and it is directly related with its
inherent high sensitivity. If the experimental conditions during the pre-concentration stage are
constant, the measured voltammetric answer (the peak area or peak height of the
voltammogram) is related with the concentration of the active species and it can be used for
quantification. Moreover, the potential for re-dissolving the deposited metal can be used for
cualitative purposes. With more than one metal ion in the sample, the ASV signal may
sometimes be complicated by formation of intermetallic species. This may distort the
stripping signal for the analyte of interest. These problems are minimized or even avoided by
adjusting parameters such as the deposition (oxidation) time and the deposition (reduction)
potencial.
ASV is used for the determination of amalgaman-forming metals (metallic ions). Other
similar techniques are CSV, which is employed for determining species that form insolubles
salts with mercury on the electrode surface. These species are typically inorganic anions, such
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 64
as halides, or oxyanions or certain organic compounds. In addition, AdSV is quite similar to
ASV, but the pre-concentration step of the analyte is accomplished by adsorption on the
electrode surface or by specific reactions at chemically modified working electrodes rather
than accumulation by reduction. This stripping voltammetric type is commonly used for
organics.
(a) Instrumentation
Conventional instruments (cells) consist of three electrodes: a working electrode that
should be easily polarizable (usually a microelectrode based on mercury MDE); an auxiliary
electrode, which usually is a platinum wire or a small mercury deposition; and a reference
electrode, which usually is calomel electrode (Hg/Hg2Cl2, KCl). The MDE is a micro working
electrode, which the mercury drop is formed at the end of a glass capillary (length 10 – 20cm,
inside diameter 0.05mm). The mercury droplets are highly reproducible diameter and of a life
time ranging from 2 to 6s. MDEs offer several advantages over other working electrodes (also
referred as solid electrodes), such as the constant renewal of the electrode surface, the fact
that the charge transfer overvoltage of hydrogen ions present in the aqueous solvent is high on
Hg, and that many metals are amalgamed by Hg. There are two types of MDEs; the hanging
mercury drop electrode (HMDE), most often used; and the static mercury drop electrode
(SMDE). The HMDE consists of a very fine capillary tube connected to a mercury-containing
reservoir. Mercury is forced out of the capillary by an arrangement controlled by a
micrometer screw, which allows drops of very reproducible surface areas. SMDE uses a
method of drop formation in which the mercury drop is dispensed rapidily and then allowed
to hang stationary at the capillary tip. This second electrode allows more reproducible and
stable drops.
Because the high toxicity of mercury, different working electrodes based on more
environmentally friendly materials have been developed and successfully applied for the
voltammetric determination of several metals. The most popular electrodes are those based on
bismuth films (BiFEs, bismuth film electrodes) [384] or glassy carbon electrode (GCE)
electrodes. Advances in nanotechnology have led to the development of different electrodes
based on nanoparticles, mainly bismuth nanoparticles and carbon nanotubes. Similarly, the
screen-printing technology has found an appealing development area in producing disposable
or single-use electrodes, commonly refrerred as screen-printed electrodes (S-PEs). These
electrodes avoid the memory effect between samples as well as the electrode fouling
phenomenon.
To carry out the measurements, an excess of a strong electrolyte (KCl) is added to the
sample. Thus, the ionic strength and the conductivity remain constant. The dissolved oxygen
must be removed because oxygen is easily reduced and it can be a significant interference.
Oxygen removal is performed by bubbling an inert gas (N2) for a few minutes and then
keeping a stream of nitrogen on the surface to prevent oxygen re-dissolution.
(b) Cadmium determination by ASV
Table 5 lists some recent applications of ASV methods to assess cadmium in
environmental samples. It can be seen that the conventional HMDE [385-392 is used for
several determinations, although coated mercury film electrodes [393-395are also used.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 65
BiFEs using different supports [391,396-407 and modified BiFEs with fibrinogen [408],
poly(sodium 4-styrenesulfonate) [409], or carbon nanotubes [410,411] have also been used
for cadmium determination. A bismuth/glassy carbon composite material by incorporating
bismuth powder in the composite electrode has been also described [412]. GCEs [413-415
modified carbon paste electrodes [416,417], graphite-epoxy composite electrodes (GECEs)
[418], bare gold-disk micro-electrodes [419], tin film electrodes [420], bismuth nanoparticles
modified boron doped diamond (Bi-BDD) electrode [421], and S-Pes [422-425 are also
available (Table 5).
Table 5. Selected application of ASV determination of cadmium
in environmental materials.
Sample type Working electrode Technique Ref.
Seawater HMDE DPASV [385]
Soil extracts HMDE DPASV [386]
Plants HMDE DPASV [387]
Water HMDE DPASV [388]
Atmospheric particulate
matter (PM10) HMDE DPASV [389]
Waste water HMDE SI-DPASV [390]
Seawater, hydrothermal
fluids HMDE /BiFE Single-run ASV [391]
Plants HMDE LSASV [392]
Estuarine water Poly-L-lysine-poly(sodium 4-
styrenesulfonate)-coated Hg
film electrode
SWASV [393]
Water Hg-coated glassy carbon DPASV [394]
Spicules of marine sponges Hg-film electrode plated on an
epoxyimpregnated
graphite rotating-disk support
SWASV [395]
Soil extracts BiFE LSASV / SWASV [396]
Tap water BiFE SWASV [397]
Tap water / soil BiFE SWASV [398]
Tap water / plant extracts BiFE DPASV [399]
Tap water / seawater BiFE ASV [400]
Water / plant extracts BiFE DPASV [401]
Rocks BiFE SI-SWASV [402]
Tap water BiFE LSASV [403]
Tap water BiFE SWASV [404]
Iron ores BiFE FI/SI-SWASV [405]
Water BiFE SWASV [406]
Tap water BiFE SWASV [407]
Environmental materials Fibrinogen-BiFE SWASV [408]
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 66
Table 5. (Continued)
Sample type Working electrode Technique Ref.
Tap water / snow Poly(sodium 4-
styrenesulfonate)-BiFE SWASV [409]
River water Carbon nanotubes-BiFE SWASV [410]
Water Carbon nanotubes-BiFE SWASV [411]
Tap water Bismuth/glassy carbon
composite SWASV [412]
Waste water GCE DPASV [413]
Soil / indoor atmospheric
particulate matter GCE SWASV [414]
Estuarine water GCE SWASV [415]
Tap water Modified carbon paste
electrodes SWASV [416]
Syntehic samples Modified carbon paste
electrodes DPASV [417]
Water GECE DPASV [418]
Seawater Bare gold-disk micro-
electrodes SWASV [419]
Tap water Tin film electrodes DPASV [420]
Standards Bi-BDD electrode SWASV [421]
Soil extracts S-PE LSASV [422]
Water / soil S-PE DPASV [423]
Seawater S-PE SWASV [424]
Fish S-PE SWASV [425]
BiBDD, bismuth boron doped diamond; BiFE, bismuth film electrode; DPASV, differential pulse
anodic striping voltammentry; FI-SWASV, flow injection - squared wavelength anodic striping
voltammentry; GCE, glassy carbon electrode; GECE, graphite-epoxy composite electrode; HMDE,
hanging mercury drop electrode; LSASV, linear sweep anodic stripping voltammetry; SI-SWASV,
sequential injection - squared wavelength anodic striping voltammentry; S-PE, screen-printed
electrode; SWASV, squared wavelength anodic striping voltammentry
IV.10.2. Pontentiometric sensors: Ion selective electrodes (ISEs)
Pontentiometry consists of the measurement of a thermodynamic equilibrium potential in
an electrochemical cell, without net current flows. The direct determination of the activity
results from the thermodynamic equilibrium relationship between the activity of an ion and the
potential of a cell. These techniques provide a method for distinguishing between free or ionized
and bound or complexed ions in a sample, an also between the activities of different oxidation
states of an ion. Therefore, several dissolved ions or certain dissolved gases can be measured
with high selectivity and sensitivity and these electrodes are also called sensors. When ions in
solution are monitored, the sensors are known as ionic selective electrodes (ISEs). The use of
ISEs in environmental analysis offer several advantages over other methods of analysis, mainly
the cost of initial setup to make analysis is relatively low. Detailed information on ISEs can be
found in the monographs by Bard and Faulkner [381] and Evans [426].
Analytical Chemistry of Cadmium: Sample Pre-treatment… 67
(a) Instrumentation
The basic potentiometric sensor setup includes a meter (capable of reading millivolts) and
two electrodes, the first one is the indicator or membrane electrode (ion specific electrode
surrounded by a thin film of an intermediate electrolyte solution and enclosed by an ion
permeable membrane) and other one is the reference electrode. Both electrodes are immersed
in the sample solution. The indicator electrode consists of an internal reference electrode with
an internal filling solution and a membrane located at one end. The membrane separates two
liquids, the internal filling solution and the sample solution. The liquid that fills the
membrane electrode is a solution containing the ion which will be analysed in the sample, and
which concentration remains constant. The sample solution, outside, also contains those ions,
but the concentration is unknown. The membrane allows that these ions (and only these ions)
migrate through the membrane from the two sides. The speed of migration is dependent on
the existing concentrations on such ions at each side of the membrane, which creates an
imbalance of charges on both sides of it. This fact is reflected in the emergence of a potential.
Since potentials due to the reference electrodes are constant, signal variations are only
attributed to the potential dependent on the ion concentration in the sample solution. All these
considerations and the implementation of the relevant equations lead to the conclusion that
the potential measured by a potentiometer connected to the two electrodes is a constant value
EK plus a value depending on the concentration of ion in the sample solution, according to the
following Nernstian expression
E
measured = EK + (RT / nF) ln C,
where C is the analyte concentration, the value EK, encompasses all potential constants and it
can be determined after calibration; R and F are constants (8.314 JK-1 mol-1 and 96485 °C
mol-1, respectively) and T is he temperature expressed as K.
Among the different working electrodes, membrane electrodes are quite popular for ISEs,
and several types of membranes (glass membranes, solid-state membranes, liquid membranes,
gas-permeable membranes or biocatalytic probes) can be used. Glass membranes-based
electrodes, including chalcogenide glasses, are one of the most used. Moreover glass
membrane electrode for pH measurements, there are several glass membrane based electrodes
commercially available for different metallic ions such as cadmium (Cd2+). In this case, the
cadmium ISE consists of a membrane glass of AgS2(CdS) with an internal filling solution of
potassium nitrate. Solid-stated membranes consist of crystal or pressed pellets containing the
salts of the target ion (e.g., silver halides or metal sulfides) which has a low solubility and is
electrically conductive. Solid-stated ISEs are typically used to measure ions like halides and
certain cations such as cadmium (Cd2+). This sensor is equipped with an Ag/AgCl reference
electrode and an electrolyte filling solution of potassium nitrate. As reviewed by Bakker and
Pretsch [427], hydrophobic polymeric membranes doped with active sensing ingredients,
typically a lipophilic ion exchanger and a highly selective ionophore are appealing solid-
stated membranes for ISEs because the high sensitivity achieved (ppb or lower). In these
membranes, an ion-exchanger is responsible for attracting a fixed concentration of the analyte
into the membrane phase, while an ionophore selectively binds this analyte ion.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 68
(b) Cadmium determination by ISEs
There are described several Cd2+-ISEs in function of the membrane type. Moreover
chalcogenide glass (CdSAgIAs2S3) membranes [428], CdSe based- solid-state membranes are
also used [429]. Poly(vinyl chloride) (PVC) or polyestirene containing different membrane
carriers (Table 6) have been also synthesized and applied for selective cadmium
measurements [430-432 Similarly, coated surface graphite rod electrodes containing
cetylpyridinium-tetraiodocadmate or cetylpyridinium-tetrabromocadmate have been also
proposed [433]. Other group of ISEs are those based on composite materials formed by the
combination of an inorganic ion exchanger of multivalent metal acid salts and an organic
conducting polymer (polyaniline, polypyrrole, polythiophene, etc.), which provides a new
class of organic-inorganic hybrid ion exchangers. An example of such systems is the ion
exchanger polyaniline Sn(IV) tungstoarsenate [434] or the cation-exchanger polyaniline
Ce(IV) molybdate [435]. Other polymeric membranes (PVC) containing certain neutral
ionophores such, as Schiff bases, have also proposed [436-439As commented, ISEs based on
polymeric membranes containing an ionophore and an ion exchanger offer the highest
sensitivities and can be applied for the determination of traces.
For cadmium, polymeric membranes based on the ionophore, N,N,N’,N’-tetradodecyl-
3,6-dioxaoctanedithioamide (ETH 5435) have been described, showing LOD of 11 µgL–1
[440]. Other polymeric membranes containing different ionophores (Table 6) have been also
described [441-446
IV.11. Automatic Analyzers
The emergence on the market of automated systems for analysis (automatic analyzers)
has been one of the major advances in analytical chemistry in recent decades. Automatic
analyzers provide analytical data with minimal operator intervention. Automation implies the
partial or complete replacement of human involvement in an operation or sequence of
operations. Initially, these systems were designed to address the needs of clinical laboratories,
but at present they are used in such diverse areas as industrial process control (process
analyzers) or routine determinations of various substances in the air, water, and soil
(environmental monitoring). Monograph by Valcárcel and Luque de Castro [447] offers
detailed descriptions of the different automatic analyzers.
Advantages of automation can be summarized as follows:
(1) Rapidity of the process, which allows the treatment and analysis of large number of
samples. This is useful in environmental monitoring and process analysis.
(2) Reduction of human participation, which avoids errors and increases analytical
quality (high repeatability and accuracy for long time periods is obtained).
(3) Increase on human and environmental security because automation avoids dangerous
handled manually operations, and decreases human accidents and environmental
risks.
(4) Lower consumption of sample and/or reagents, which is very important for scare of
precious samples or expensive reagents (reduction of costs).
Analytical Chemistry of Cadmium: Sample Pre-treatment… 69
Table 6. Cadmium – ionic selective electrodes.
Membrane type Some remarks Ref.
Glass membrane Membranes based on a chalcogenide glass material
(CdSAgIAs2S3) deposited as thin film onto different
transducer structures
[428]
Glass membrane CdSe instead of CdS. The reference electrode is an
Ag/AgCl [429]
Solid-stated membrane PVC containing [1,1'-bicyclohexyl]-1,1', 2,2'-tetrol as a
membrane carrier [430]
Solid-stated membrane PVC containing tert-Bu thiacalix[4]arene or
thiacalix[4]arene as membrane carriers [431]
Solid-stated membrane Polyestyrene containing 3,4:12,13-dibenzo-2,5,11,14-
tetraoxo-1,6,10,15-tetraazacyclooctadecane as a
membrana carrier
[432]
Solid-stated membrane Graphite rod electrode coated with cetylpyridinium-
tetraiodocadmate or cetylpyridinium-tetrabromocadmate [433]
Solid-stated membrane Polyaniline / Sn(IV) tungstoarsenate as organic / inorganic
composite material as a cation exchanger [434]
Solid-stated membrane Polyaniline / Ce(IV) molybdate as organic / inorganic
composite material as a cation exchanger [435]
Solid-stated membrane PVC containing 5-[((4-Me phenyl) azo)-N-(6-amino-2-
pyridin) salicyladimine] [436]
Solid-stated membrane PVC containing 5-[((4-Me phenyl) azo)-N-(2-diamino-2-
cyano-1-Et cyanide) salicylaldehyde] [436]
Solid-stated membrane PVC containing N,N'-[bis(pyridin-2-yl)
formylidene]butane-1,4-diamine [437]
Solid-stated membrane PVC containing N-(2-yridinylmethylene)-1,2-
benzenediamine [437]
Solid-stated membrane PVC containing N'-[1-(2-furyl)methylidene]-2-
furohydrazide [438]
Solid-stated membrane PVC containing N,N'-(4-methyl-1,2-
phenylene)diquinoline-2-carboxamide [439]
Solid-stated membrane PVC containing N,N,N’,N’-tetradodecyl-3,6-
dioxaoctanedithioamide (ionophore), sodium tetrakis[3,5-
bis(trifluoromethyl)] (ion-exhanger)
[440]
Solid-stated membrane 8-hydroxyquinoleine (ionophore), sodium tetra-Ph borate
(ion-exchanger) [441]
Solid-stated membrane 8-hydroxyquinoleine (ionophore), sodium tetra-Ph borate
(ion-exchanger) [442]
Solid-stated membrane tetrathia-12-crown-4 (ionophore) phenylborate (ion-
exchanger) [443]
Solid-stated membrane dicyclohexano-18-crown-6 (ionophore), sodium tetra-Ph
borate (ion-exchanger) [444]
Solid-stated membrane dicyclohexano-24-crown-8 (ionophore), sodium tetra-Ph
borate (ion-exchanger) [445]
Solid-stated membrane K hydrotris[N-(2,6-xylyl)thioimidazolyl] borate
(ionophore), sodium tetra-Ph borate (ion-exchanger) [446]
Solid-stated membrane K hydrotris(3-phenyl-5-methyl-1h-pyrazol-1-yl)borate
(ionophore), sodium tetra-Ph borate (ion-exchanger) [446]
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 70
Automatic analysis can be classified according to the way in which samples are
transported and manipulated. Therefore, there are two modes, batch and continuous modes.
1. Discrete or batch analysis. In this operation mode, each sample preserves its integrity
in a vessel transported mechanically to various parts of the analyzer, where the different
analytical stages are carried out in a sequential manner. Each sample finally arrives at the
detector where the relevant signals are recorded.
Batch analysis can be classified according to the manner in which the sample is
transferred to the final operation, that is, with or without final transfer of the sample. In
analyzers with final transfer, the reaction mixture is transported to the detection system,
where the analytical measurement is carried out in a fixed cuvette. In analyzers without final
transfer of the reaction mixture, all the stages of the process take place in the same vessel.
2. Continuous analysis. These systems are characterized by the use of a continuous
stream of liquid. The samples are sequentially introduced at regular intervals into a channel
carrying a liquid that does or does not merge with other channels carrying reagents, buffers,
masking agents, and so on, upon reaching the detector. The final reaction mixture yields an
analytical signal that is duly recorded. Continuous analysis can be adapted for analysis of one
or several analytes by using a single-channel (for the determination of one analyte) or multi-
channel configuration (for several analytes).
There are two types of continuous systems: segmented flow analysis (SFA) and un-
segmented-flow analysis. SFA are characterized by a segmented flow of air bubbles, the aim
of which is to preserve the integrity of samples. Un-segmented-flow analysis, mainly flow
injection (FI) and continuous flow (CF), provide a flow that is not segmented by air bubbles.
IV.11.1. Un-segmented-flow analysis: Flow injection and continuous flow
FI is a simple, versatile and low cost methodology, and it allows fast analysis (lower than
1.0 minute) which improves the sampling frequency (300 - 400 h-1). The rapidity of FI is due
to neither physical equilibrium (homogenization of a portion of the flow) nor chemical
equilibrium (reaction completeness) is attained by the time the signal is detected. Hence, FI
can be considered a fixed-time analytical methodology. As consequence of these
characteristics, batch analysis and SFA has been replaced by FI. As it will be shown in
section V, automatic sample pre-treatment for liquid and solid samples have been extensively
adapted to FI methodologies.
In FI, the sample and reagents are incompletely mixed and there is a concentration
gradient that varies along the system as a function of time. The continuous detectors provide a
transient signal, which is recorded. The physical foundation of FI is related to the behaviour
of the sample plug inserted in the flow, which is characterized mathematically by means of
the so-called dispersion (the shape of the profile yielded by the injected sample portion along
the system).
The main elements of these analyzers are:
1. A propelling system, which is a peristaltic pump to propel fluids to the different
elementary units along the system. It must provide a pulse-free and perfectly
reproducible flow of constant rate. Flexible plastic tubing is squeezed by a series of
Analytical Chemistry of Cadmium: Sample Pre-treatment… 71
rollers (8 or 10 rollers in circular configuration), which starts the flow of the enclosed
liquids as a result. These systems can work with several streams (between 1 and 16,
but 4, in most cases), and the flow rate is determined by the internal diameter of the
tube (i.d. 0.25 – 4.0 mm, which allow flows of 0.0005 – 40 ml min-1) and the pump
rotation speed (> 30 rpm).
2. An injection system, which is a six-way rotary valve (three inlets and three outlets)
inject the sample (sample volume of 5.0 to 200 µL) into the carrier stream in a highly
accurate and reproducible manner without disturbances. This operation is fast to
achieve a high sampling rate. The rotator valve, controllable by an electric motor, can
adopt two positions. In the filling position, the sample fills the loop; in the injecting
position, the carrier sweeps the sample toward the reactor).
3. A separation/pre-concentration system, which permits continuous separation
techniques, for example, filters or KR (on-line co-precipitation), membrane or
gravitational liquid-liquid separators (on-line liquid-liquid extraction) and adsorbent
columns or exchange resins (on line solid phase extraction), as it described in section
V. Simmilarly, microwave or ultrasounds devices can be also coupled for assisting
solid-liquid extraction (section V). As it will be commented, in these last cases flows
of extractants are passed several times through a column containing the solid sample.
4. A transport and reaction systems, which are helically coiled PTFE tubes of a given
diameter (i.d. 0.1 and 2mm) that are connected to the other elements of the system.
This system can be thermostated if necessary (reaction development must be carried
out at a fixed temperature). The length of the tubes and the flow rate fix the reaction
time.
5. A detector system, which can be an optical or an electroanalytical instrument.
CF is similar to FI. The main difference between CF and FI is the long analysis time
required to stabilize the hydrodynamic conditions. In addition, other differences are the use of
large i.d. tubes, faster flow rates, and special requirements for the rinsing stages.
IV.11.2. Segmented-flow analysis
In SF, the bubbles prevent contamination between successive samples, reduce sample
dispersion, and facilitate mixing of sample and reagents in a way that enables physical and
chemical equilibrium to be attained before the sample reaches the detector. The main
elements of this analyzer are as follows:
1. A sampling system, sampling is carried out with the aid of a moving aspirating tip,
air bubbles are also aspirated between samples
2. A propelling system, a peristaltic pump
3. A separation/pre-concentration unit
4. A reaction system
5. A debubbler, which is a high-precision device that removes the bubbles. This device
is not essential if certain software treatments that discriminate signal parasites caused
by bubbles are used
6. A detection system similar than FI or CF.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 72
Many of the automatic analyzers described have been adapted to automatic water and air
analysis (off-line and on-line water analyzers) to facilitate the analysis and monitoring of
constituents and contaminants of water and air.
V. SAMPLE PRE-TREATMENT PROCEDURES
FOR ENVIRONMENTAL SAMPLES
V.1. General Aspects
Sample pre-treatment procedures are necessary to isolate the desired components from
complex liquid and solid matrices, and also because most of the analytical instruments can
not directly handle the liquid or the solid matrix. The basic concept of sample pre-treatment
procedure is to convert a real matrix sample into a sample suitable for analysis. This process
almost inevitably changes the interactions of analytes with their matrices. These interactions
are determined by the physical and chemical properties of both analytes and matrices, and
they affect the applicability of different sample pre-treatment techniques and analytical
methods as well as their efficiency and repeatability.
Sample pre-treatments can be achieved by using a wide range of techniques; from the
most simple and inexpensive, which do not require special piece of equipment, such as
distillation and liquid-liquid extraction (LLE) procedures; to the most sophisticated pre-
treatments such as pressurized liquid extraction (PLE), which piece of equipment, and also
mantainance, is of high cost. Independenly of the degree of sophistication, sample pre-
treatment methods have the same goals:
(1) The increase on the selectivity of certain analytical techniques by removing potential
interferences, e.g., high saline matrices (Cl- and Na+ and, to a lesser extent, SO42- and
Mg2+) form sea water sample; high concentrations of ions such as Ca2+ and HCO3-
from river waters and Ca2+, HCO3-, Na+, Mg2+, SO42- and Cl- from groundwaters;
organic matter form biological samples or mineral matrices (silicates and aluminum
and iron oxides) from sediments, soils or atmospheric particulate matter. In this
sense, the matrix components removal can also avoid the potential deterioration of
the analytical instruments.
(2) The improvement of the sensitivity of the analytical techniques by increasing the
analyte concentration (pre-concentration methods), e.g., trace metals concentration in
open sea waters and atmospheric particulate matter are in the range of ng L-1 and ng
m-3, respectively.
(3) If necessary, the analyte convertion into a more suitable form, e.g. metals
derivatization to organic chelate by using adequate ligands in liquid-liquid extraction,
liquid-liquid micro-extraction or solid phase extraction (SPE) methodologies;
organometallic compounds derivatization into apolar volatile species by Gridnard
reaction (ethylation, propylation or pentylation) in LLE or solid phase micro-
extraction (SPME) methodologies; etc.
(4) The transfer of the analyte to a compatible solvent for a certain analytical technique
to allow the analytical determination, e.g., back-extraction LLE procedures into
Analytical Chemistry of Cadmium: Sample Pre-treatment… 73
aqueous / acid solution for further metal quantification by atomic or emission
spectrometric methods, or a change from a polar to a non polar solvent or viceversa
for organic compounds determination by chromatographic techniques.
(5) If possible, the isolation of certain species (speciation studies), e.g. chromium can be
determined by the reaction between Cr(VI) and 1,5-diphenylcarbazide, which forms
a complex that could be extracted into an adequate organic solvent or adsorbed into
adequate solid support, allowing Cr(VI) to be determined in the presence of Cr(III).
(6) The establishment of a robust and reliable method that is not affected by variations in
the sample matrix.
Although, many traditional sample pre-treatment methods are still in use (e.g.
precipitation, co-precipitation, liquid-liquid extraction, solid phase extraction, acid extraction
or acid digestion), there have been trends in recent year towards:
(1) The use of small initial sample sizes, e.g. sample volumes around few milliliters are
usual for single-drop micro-extraction (SDME) or dispersive liquid-liquid micro-
extraction (DLLME) procedures.
(2) The ease of automation, which minimizes the sample and extract handling and avoids
analyte losses and sample contamination; e.g. microwave or ultrasound assisted
extraction techniques and pressurized liquid extraction.
(3) The miniaturization, e.g., miniaturized devices available for operations such as
liquid-liquid extraction and pre-concentration into solid supports by flow injection
analysis (FIA) and new miniatured LLE procedures such as single-drop micro-
extraction (SDME), dispersive liquid-liquid micro-extraction (DLLME) or hollow
fibre liquid phase micro-extraction (HFLPME).
(4) The use of non exhaustive extraction procedures in which only a small fraction of the
analyte is extracted / pre-concentrated for further analysis, e.g., in SDME or DLLME
pre-treatments the analyte extraction is incomplete; the high repeatablity of
measurement of time in the sequence of operations (as a result of automation)
ensures that the sample and the patterns are exactly the same process and exactly in
the same period of time, achieving a high reproducibility.
(5) The high selectivity in the extraction, e.g. the development of new solid phases for
SPE such as molecular / ionic imprinted polymers for organic compounds and metal
pre-concentration from liquid samples, respectively; carbon nanotubes, nanometer-
sized materials, ordered mesoporus materials, etc.
(6) The reduction of the pre-treatment time; e.g. microwave assisted extraction (MAE)
and pressurized liquid extraction (PLE) dramatically shorten the extraction times,
(completed extractions after 1 to 5 minutes).
(7) The increase on the extraction efficiency without degradation or losses of the target
analytes, and decrease on the number of extraction cycles, e.g. 2 - 3 extraction steps
are typical for LLE methodologies, while one extraction step is enough for SDME or
DLLME; the extraction cycles for organic compound isolation by soxhlet (typically
around 24 cycles) are dramatically reduced by using PLE (typically 1-3).
(8) The high throughput performance, which implies that the number of samples that
could be pre-treated per unit of time should be similar to the number of samples per
unit of time that modern analytical detectors can analyze, MAE and PLE procedures
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 74
allow the pre-treatment of high number of sample per unit of time; thus, these
procedures could be on-line coupled with analytical instruments.
(9) The on-line coupling with analytical instruments, transferring the treated sample to
the detection systems; e.g. metals pre-concentration from liquid matrices by using
ion-exchange columns in a flow injection analyzers coupled to atomic absorption or
emission spectrometry; or metals determination from solid samples by using
continuous digestion / extraction devices coupled to spectrometric instruments.
(10) The increase on the operator safety and the decrease on the environmental impact of
the analytical methodologies. Four top priorities concerning the elimination or
reduction of organic solvents and/or highly toxic or ecotoxic reagents, the prevention
of waste generation and the reduction of energy consumption, were identified in the
analytical laboratory [448], which connects with the principles established by the
Analytical Green Chemistry (AGC): the use of chemistry techniques and
methodologies that reduce or eliminate the use or generation of feedstock, products,
by-products, solvents, reagents, etc. that are hazardous to human health or the
environment [449].
V.2. Sample Pre-treatment for Liquid Samples
The extraction of trace cadmium and other metals at low concentrations in complex
environmental aqueous matrices, such as saline waters, has often been difficult for analytical
chemists. There are two important problems:
(1) The saline composition of water matrix (mainly for sea water sample), which
produces a high background signal and non-spectral interferences when
spectrometric techniques (most usual) are used for quantification. In addition, the
occurrence of a high background and scattering radiation is a limiting factor for
direct analysis in most natural waters by XRF.
(2) The low trace metal concentration in environmental waters, e.g., trace metal
concentrations around few ng L-1 to 10 µg L-1 are typical for waters. Cadmium
concentration in surface sea water is between less than 5 ng L-1 (for nutrient-depleted
surface water) and 15 ng L-1. This value could be increased up to 100 ng L-1 in deep
seawaters and to 50 ng L-1 in seawater from up-walling areas [3,4,6,7]. High average
concentrations for cadmium, between 1.0 ng L-1 and 10 µg L-1, are found in river
waters, while values less than 100 ng L-1 are reported in lake waters.
Most of the chemical elements are present in seawater, but most of them are at trace
level. Six components (chloride, sodium, sulphate, magnesium, calcium and potassium)
account for over 99% of the composition of solutes. Table 7 lists the most abundant
components of seawater of 35‰ average salinity [450]. The composition of fresh water
(ponds, lakes, rivers and streams, underground water, rain and snow) contains less than 0.5
parts per thousand of dissolved salts, being bicarbonate (48%) the major component of river
waters. Ions such as Ca2+, Na+, Mg2+, SO42- and Cl- are present in fresh waters as minor
components. Table 7 also shows the average composition in river waters [450].
Analytical Chemistry of Cadmium: Sample Pre-treatment… 75
Table 7. Water composition according to reference [450].
Concentration (mM)
Ion Seawater (salinity of 35 ‰) River water
Chloride (Cl-) 545 0.16
Sodium (Na+) 468 0.23
Magnesium (Mg2+) 53.2 0.15
Sulphate (SO42-) 28.2 0.069
Calcium (Ca2+) 10.2 0.33
Potassium (K+) 10.2 0.03
Bicarbonate (HCO3-)
2.38 0.86
A comparison of the low concentrations of metals in natural waters (cadmium in the
range of less than 0.045 to 90 nM) to the high concentrations of the saline matrix (Table 7),
shows very high matrix / analyte ratios.
To overcome these problems, early emerged extraction / separation techniques offer not
only the ability to isolate the metals from the matrix solution, thus they reduce, control and
even eliminate the interferences, but also the opportunity for metals to be pre-concentrated
and determined at very low levels.
An ideal method for pre-concentration of trace metals from natural waters should have
the following characteristics:
(1) It should simultaneously allow the isolation of the analyte from the matrix and yield
an appropriate enrichment factor.
(2) It should be a simple process, and the use of few reagents must be minimize to avoid
contamination, hence low sample blank and low detection limit can be obtained.
(3) It should produce a final solution that is readily matrix matched with solutions of the
analytical calibration method.
Sample pre-treatments, such as evaporation or lyophilization, precipitation or co-
precipitation of cadmium from waters and cadmium electrodeposition on wires, were the first
applied procedures to achieve this task. Liquid-liquid extraction (LLE) and solid phase
extraction (SPE) have been extensively used for metals pre-concentration from natural water,
and most of the standardized and official methods for metals determination in waters enclosed
these techniques.
Although the high applicability of LLE and SPE, new extraction techniques are
nowadays available, which allow a huge versatility for selecting the most suitable approach
for an analyte. Single-drop micro-extraction (SDME), dispersive liquid-liquid micro-
extraction (DLLME) and hollow fibre liquid phase micro-extraction are new miniaturized and
not exhaustive sample LLE procedures which main advantages are their speed, negligible
volume of solvents used and their ability to detect analytes at very low concentrations.
A description of the theoretical aspects, experimental parameters and advantages and
disadvantages of these sample pre-treatment methods will be developed in the following
sections. In addition, the application of those techniques to the isolation and pre-concentration
of cadmium from environmental liquids samples will be also commented.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 76
V.2.1. Non-boiling evaporation
(a) General aspects
Evaporation procedure is the easiest and simplest pre-treatment procedure for liquid
samples. In non-boiling evaporation procedures, the sample (in the range of 50 to 1000 mL) is
transferred into a Erlenmeyer flask, vessel or bulb, and then placed into a thermostat
containing several fluids (water, water-ethylene glycol (1:1), triaryldimethane, etc) or placed
directly on a heating plate and then, evaporated to a drop (c.a. 50 – 100 µl). Temperature
setting is adjusted to avoid any boiling, and the process occurs at temperatures within the 80 –
90 °C range. In some cases, the water vapor evolved can be transferred, using an inert gas
flow rate, to an ice bath where water vapor is condensed (distillation). The evaporation is
continued to near dryness, and for a typical 70 mL sample volume, this procedure takes about
2 – 15 hours. After evaporation, few milliliters of ultra-pure water or diluted mineral acids are
added to residue for dissolution. Metals in the final solution are then analyzed.
Evaporation is a slow process and it is generally carried out in open systems, and
therefore it is subjected to possible contamination from the laboratory environment. In
addition, there are risks of contamination from container walls (mainly from glass vessel) and
possible loss of volatile metals. Various improvements have been made to allow a more
efficient control of contamination problems when coping with low metal concentrations. They
include:
(1) The use of clean room conditions, such as work stations fitted with special high
efficiency particulate (HEPA) filters or carrying out the whole evaporation procedure
inside an all-plastic vertical laminar-flow clean bench fitted with HEPA filters.
(2) The choice of Teflon sample vessels for the evaporation containers, which avoids the
possible dissolution of several elements from glass vessels. In addition, sample
droplets adhesion on the Teflon’s walls is negligibly small [451-453].
(3) The use of cleaning procedures, the Teflon beaker is first taken out of three heated
acid cleaning bath and rinsed with ultra-pure water.
(4) The use of ageing and pre-conditioning procedures, by filling the cleaned Teflon
beaker with ultra-pure water and heating it for several minutes. Thus, heavy metals
contamination released from walls of beaker is discarded.
(b) Non-boiling evaporation methods for cadmium pre-concentration
Water removal by non-boiling evaporation techniques has not been extensively applied
for the pre-concentration of cadmium and other trace metals from environmental samples due
to the saline matrix is also pre-concentrated. This approach is more useful for metals pre-
concentration in high purified water. Thus, there are few references on non-boiling
evaporation for cadmium pre-concentration from environmental liquid samples: snow
samples [451- 453], river water [454], and rain water [452]. Table 8 summarizes the main
analytical characteristics of these procedures.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 77
V.2.2. Lyophilization
Lyophilization, also called freeze-drying, is a method commonly used for drying
biological samples to stabilize and preserve them for storage and transport. Also it has been
used for drying natural waters as pre-concentration technique.
(a) General aspects
The freeze-drying process is essentially a low temperature, low pressure distillation of
water which sublimes from the surface of the ice and condenses in a cold trap. Ice is directly
converted to water vapor, without passing through the intermediary stage of a liquid. Thus, no
heating is required for water removal from sample. In the case of solid samples, the result is a
sample whose structure is largely preserved, which can be stored at room temperatures and
pressures. For natural waters, the result is a residue which is then irradiated with thermal
neutrons flux for neutral activation analysis (NAA) or dissolved in few milliliters of water or
mineral acid for further analysis by atomic spectrometric techniques. The heat of sublimation
removed from the sample, together with the low pressure conditions and the insulation
provided by vacuum, is sufficient to keep water with salinities below 9 mg g-1 frozen during
the period even when the apparatus is at room temperature.
In lyophilization procedures, the temperature of water sample is first lowered to the
freezing point. Then, the sample is inserted into a vacuum chamber of the lyophilizer (freeze
dryer) and the system is subjected to vacuum (10 Pa of residual pressure, approximately). The
water vapor obtained by sublimation is automatically removed from the vacuum chamber and
collected into a condenser attached to the vacuum chamber. The extremely low pressure
causes water molecules to be drawn out of the sample.
(b) Lyiophilization methods for cadmium pre-concentration
Simmilarly to evaporation techniques, there are few references about cadmium (and other
metals) pre-concentration form natural waters by lyophilization [455-459 This is because the
matrix sample is also pre-concentrated after this procedure, and lyophilization is completed
after a time (about several days). However, possible contamination from laboratory
environment and the risk of volatile metals losses are avoided, except for Hg [456].
V.2.3. Precipitation and co-precipitation
(a) General aspects
Precipitation and co-precipitation procedures are based on a selective or specific
precipitation of the analyte by the addition, under certain conditions (pH, temperature,
stirring, etc.), of different precipitating and/or co-precipitating agents. The precipitate
obtained is then separated from aqueous medium by filtration, centrifugation or flotation, and
it is re-dissolved in a small volume of mineral acids (nitric acid, hydrochloric acid,
hydrobromic acid), hydrogen peroxide or organic solvents (acetone, ethanol, 4-methyl-2-
pentanone, IBMK) for further analysis by atomic spectrometric techniques. Solid particles
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 78
could also be introduced as solid or slurry into atomic spectrometric atomizers or irradiated
with thermal neutrons flux for further analysis by NAA. Great care must be taken to ensure
that no precipitate is lost during either filtration or centrifugation steps.
Precipitaion procedures are mainly used for isolating elements which are present at high
concentrations in the sample. Because of the relative high concentration of the target metals,
precipitation is achieved by directly adding an organic or inorganic precipitant. However, co-
precipitation approaches are preferred when the target metals are at low concentrations. In
this case, carrier element and an organic co-precipitant are added to sample. Carrier elements
are metals which easily form insoluble hydroxides, oxides or which can be easily reduced.
Some examples of carrier elements are copper, zinc, iron, lanthanum, indium and magnesium.
As organic co-precipitants, reagents such as dithiocarbamate, diethyldithiophosphate,
heterocyclic azo-dyes, quinoline, etc., are used. In this case, co-precipitation involves two
steps: (1) first, the trace elements react with an organic / inorganic compound and form a
solid phase; and (2) the major precipitate reacts with other metals to form chelates.
A quantitative precipitation of the analyte from solution must be reached. Colloidal
suspensions (particle diameters from 10-7 to 10-4 cm) due to poorly soluble substances must
be avoided. The relative super-saturation affects the particle size and it is expressed as Q – S /
S, where Q is the instantaneous concentration of the added species and S is the equilibrium
solubility of the compound that precipitates. Particle size seems to be inversely proportional
to relative super-saturation. The electric double layer formed during precipitation keeps the
colloidal precipitate particles from coming into contact with each other, thus preventing
further coagulation. The formation of a colloidal suspension can be minimized or prevented
by several methods, for instance, under constant stirring; or by carrying out the precipitation
at a temperature close to the boiling point of water. This is because heating increases overall
thermal motion, affecting the mobility of adsorbed ions and colloidal precipitate particles.
The final effect of temperature is that there are collisions among particles that increase the
particle size as a result of coagulation.
When the flotation approach is used for isolating the precipitate, the formation of a
colloidal suspension does not give any problem. In this technique, precipitate isolation
occurrs in a flotation cell in which a bubbler produces numerous tiny N2 bubbles (diameter
below 0.5mm). Several surfactants (such as sodium oleate and sodium dodecylsulfate) are
added to the sample while it is gently stirred to obtain an efficient mixing and the flotation of
the precipitate with the aid of numerous tiny bubbles. This operation usually takes several
minutes. The foam containing the precipitate from the flotation cell is sucked off into a
sampling tube, and ethanol is usually added to dissolve the foam.
As commented, temperature is a critical parameter. However, precipitation and co-
precipitation procedures are affected by other experimental parameters such as the stirring
time and the standing time, the pH, the amount of precipitant and/or co-precipitant and the
sample volume. In addition, centrifugation rate and time (for those procedures that use a
centrifugation step for isolating the precipitate from aqueous sample) and the amount of
surfactants, stirring time and bubbles flow (for flotation procedures) are also important
factors.
As disadvantages, these procedures can involve sample contamination because they
require the addition of large amounts of reagents, which result in high concentrations when
comparing to trace elements concentrations (generally at µg L-1 and ng L-1 levels). In
addition, these procedures require a high volume of sample and they are tedious and time
Analytical Chemistry of Cadmium: Sample Pre-treatment… 79
consuming. The use of thermoresponsive polymers can reduce or remove some of these
drawbacks (inherent to off-line coprecipitation procedures). In addition, it is desirable the
implementation of rapid, convenient and sensitive on-line procedures by flow injection
systems, thereby reducing sample and reagents consumption while enhancing throughput.
(b) Precipitation and co-precipitation methods for cadmium pre-concentration
Cadmium has been pre-concentrated mainly by co-precipitation (Table 9) using both
inorganic and organic precipitates. As inorganic precipitants, In(OH)3 [460-464 Hf(OH)2
[465], Fe(OH)3 [466], Sn(OH)4 [467], La(OH)3 [468, 469], Zr(OH)4 [470], Mg(OH)2 [471],
Dy(OH)3 [472], Y(OH)3 [473], Cu(OH)2 [474] and MnO2 [475] have been fully applied. In
addition, cadmium retention onto the surface of reduced iron and /or palladium (reductive
precipitation by sodium tetrahydroborate) [476, 477], or reduced palladium by using H2 gas
flow [478] has been also addressed.
Simmilarly, diferents organic compounds precipitates from chelating agents such as
phenanthraquinone monophenythiosemicarbazone (PPT) [55], ammonium pyrrolidin
dithiocarbamate (APDC) [479- 481], 1-(2-pyridylazo)-2-naphthol (PAN) [482]
hexacyanoferrate [483], dibenzylammonium dibenzyldithiocarbamate (DDDE) [484],
thionalide [485], 8-hydroxiquinoline (8-HQ) [486, 487], ditizone (H2DZ) [488], 1-(2-
thiazolylazo)-2-naphthol (TAN) [481], N-nitroso-phenylhydroxylamine [481], dibenzyl-
dithiocarbamate (DBDTC) [335] and diethyldithiocarbamate (DDC) [489] have been used. In
addition, the combination of a chelating agent and an inorganic metal as a carrier element has
been extensively used. Therefore, APDC after Co(III) [490,491], Zn(II) [492] or Bi(III) [493]
addition as carriers; hexamethylene-ammonium hexamethylene dithiocarbamate (HMA-
HMDTC) after Fe(II) [494, 495] or Co(II) [496] addition as carriers; oxine after Mg(II) [497]
addition as a carrier, DDC after Cu(II) [498], Co(II) [499, 500], or Ni(II) [501, 502] addition
as carriers; 8-HQ after La(III) [503] addition as a carrier; heptyldithiocarbamate (HpDTC)
after Pb(II) or Co(III) [504] addition as carriers; and rubeanic acid after Cu(II) [505] addition
as a carrier, have been used for cadmium pre-concentration from natural waters.
V.2.3.1. Flow injection on-line pre-concentration systems by co-precipitation
AS commented, one of the major advances in analytical chemistry in recent decades has
been the development of automated systems for analysis, i.e. batch analyzer and continuous
analysers (mainly FI and CF), which provide analytical data with minimal operator
intervention. Thus, several separation methods based on ion exchange, liquid-liquid
extraction, chemical vapour generation and precipitation and co-precipitation have been
successfully adapted to FI and CF on line pre-concentration thereby resulting a variety of
rapid, convenient, sensitive and reliable spectroscopic methods for trace metal analysis.
(a) Flow injection methods with filtration stage
FI co-precipitation systems with filtration (in-line filters) often use a stainless steel HPLC
screen or a PTFE membrane on a polypropylene support as filtering devices. After complete
co-precipitation and retention of the precipitate, a small volume of an acid is used to dissolve
and flush the target element to the detector. For cadmium (Table 10), FI systems based on the
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 80
use of filters have been found in literature. In these cases, indium hydroxide [461,464] and
pyrrolidinedithiocarbamate (PDC) chelate of cobalt (carrier element) [491], were used as
reagents.
The use of filters in these on-line procedures can be a problem if large amounts of
precipitate are formed and clogg the filter. In these cases, large volume filters can be used,
although large volume filters impair the attainable enrichment factor through dispersion and
retarde the dissolution of the precipitate.
(b) Flow injection methods without filtration stage
There are two different approaches of FI methods for on-line co-precipitation which
avoid the use of in-line filters: the use of knotted reactors (KR) and the filter-less
magnetically assisted collection.
KRs are generally made from PTFE tubing, although other materials can be used for on-
line precipitation or co-precipitation. They present a 100-150 cm length and 0.5-1.5 mm inner
diameter and are made from tubing by tying interlaced knots. The KR produced from PTFE
tubes was able to retain metal complexes under appropriate experimental conditions. Two
factors are responsible for the retention of the complex molecules on the wall of the KR. The
first factor is related to the fact that the molecular species are launched at the inner wall of the
KR´s by centrifugal forces generated by secondary flows in three dimensionally disordered
systems. The second factor is attributed to the nature of the material that form the KR and the
nature of the complex.
Table 9. Non-boiling evaporation procedures for cadmium pre-concentration
from natural waters.
Brief description Sample Pre-
concentration
time / h
LOD Concentration
factor Analytical
technique Ref.
Use of Teflon
bulbs in presence
of HF and HNO3
(for solubilization
of alumino-silicate
particles) and clean
room conditions.
Snow 9 – 12 - 60 - 100 ETAAS [451]
Use Teflon sample
vessels and
conventional rotary
evaporator
Rain
water 7 - 100 ICP-OES [452]
Use of Teflon
vessels,
sophisticated
cleaning, ageing
and pre-conditi-
oning process
Snow and
ice 7 – 8 0.05 pg g-1 - FAAS [453]
Analytical Chemistry of Cadmium: Sample Pre-treatment… 81
Table 10. Precipitation and co-precipitation procedures for cadmium
pre-concentration from natural waters.
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Flotation by addition of 2.0mL of
PPT (5 10–4 M), 2.0mL of acetate
buffer (pH 6.5) and 3.0mL of oleic
acid (4 10–5 M) to the sample. After
heating (40°C for 5min) the floated
layer is measured
spectrophoomterically (520nm)
Tap
water,
river
water,
seawater
-- -- UV-visible
spectro-
metry
[55]
Co-precipitation by addition of 8-
HQ (100mg as 8-HQ) and
magnesium ions (20mg) to 100-400
mL of sample at pH 9.0 (ammonia
solution). After ageing (1.0h at
70°C on a hot plate) and filtration
on a glass filter, the filter was dried
at 110°C for 1h. A portion of the
precipitate was used for direct solid
sampling technique.
Drinkin
g water -- - EDXRF [335]
Co-precipitation by addition of 1.0
ml of In (10 mg mL-1) to 1200 mL
of sample at pH 9.5 (sodium
hydroxide, 0.3M). Precipitate was
floated with the aid of tiny N2
bubbles and supported by the stable
foam layer of sodium oleate (2 mL,
1.0 mg mL-1) and NaDDS (1.0 mL,
4.0 mg mL-1). After floats collected
in a small sampling tube, the
precipitate was dissolved with 2.0
ml ethanol (99.5% (v/v)) and 2.0 ml
of HNO3 (14.0M) and dilute to 5
mL.
Seawate
r - 240 ICP-OES [460]
Co-precipitation by addition of 1.0
mL of In (120 mg) to 20 L of
sample at pH 9.5 (30 mL of
hydrogenocarbonate / carbonate
buffer). Precipitate was floated by
the addition of ethanol (10 mL)
with the aid N2 bubbles (at 1.0 mL
min-1). After foam-precipitate
collected in a 300 mL beaker, the
precipitate was dissolved with 60 -
70 mL of HCl (8.5 M) and dilute to
10 mL.
Fresh
water - 2000 FAAS [461]
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 82
Table 10. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Co-precipitation by addition of 1.0
mL of In solution (10 mg mL-1) in
0.1M HNO3 to 100 mL of sample at
pH 9.5 (sodium hydroxide solution
3.0M). After centrifugation
(3500rpm for 10min), the
precipitate is dissolved in 5.0 mL of
HBr (0.5M).
Tap and
river
water
- 20 ETAAS [462]
Co-precipitation by addition of 1.0
mL of In solution (0.1 mg mL-1) to
100 mL of sample at pH 9.5
(sodium hydroxide solution 0.1M).
After filtration on 1.0µm Nuclepore
membrane filter, precipitate was
ultrasonically dissolved (5.9min) in
0.5 mL of HNO3 (1.0M). The
solution was diluted to 1.0 mL.
Fresh
and
river
water
2.0 ng L-1 100 ETAAS [463]
Continuous flow on line co-
precipitation with In and filtered.
The filtered precipitate is
continuously dissolved in HNO3
(3.0M) and transported directly into
the nebulizer. The sample
throughput is 20 per hour.
Water 1.43 µg L-1
10 ICP-OES [464]
Co-precipitation by addition of 10.0
mg of Hf (as HfOCl2 solution) to
50-400 mL of sample at pH 9.5
(ammonia solution 50%). After
filtration through a 0.45µm
cellulose membrane by vacuum
filtration, the precipitate is
dissolved in 1.0mL of conc. HNO3
(for ETAAS) or 2.0mL of conc.
HCl (for DPP) and dilute to 25 mL.
River
water - 2-16 ETAAS /
DPP [465]
Co-precipitation by addition of 5.0
mL of Fe(III) (2000 mg L-1) to
1000 mL of sample at pH 9.0
(ammonia solution). After filtration
through a 0.45µm cellulose
membrane by vacuum filtration, the
precipitate is dissolved in 2.0mL of
HNO3 conc. and dilute to 50 mL.
Seawater
- 20 ETAAS [466]
Analytical Chemistry of Cadmium: Sample Pre-treatment… 83
Table 10. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Co-precipitation by addition of
1.0mL of Sn (20 mg mL-1) to 200
mL of sample at pH 5.0-6.0
(sodium hydroxide solution 5.0 and
0.3M). After centrifugation
(1000rpm for 10min), the
precipitate is dissolved in 0.5mL of
HNO3 (14.0M) with the aid of
ultrasonic irradiation. The solution
was diluted to 10 mL.
River
water 0.5 nga 20 ETAAS [467]
Co-precipitation by addition of
3.0mL of La (0.1M) to 1000 mL of
sample at pH 9.5 (sodium
hydroxide solution). Precipitate was
floated with the aid of tiny N2
bubbles and supported by the stable
foam layer of a 1:8 mixed of
sodium oleate and NaDDS (each
0.1 % (w/v)). After floats collected
in a small sampling bottle, the
precipitate was dissolved with
HNO3 and dilute to 25 mL.
Waste
water - 40 FAAS [468]
Co-precipitation by addition of
1.0mL of La (10 mg mL-1) to
sample at pH pH 9.5 (ammonia
solution). After ageing for 30min
and filtration, the precipitate was
dissolved with 1.8mL of HNO3
(2.0M).
Lake
water 0.2 pg mL-
1 25 ICP-MS [469]
Co-precipitation by addition of
5.0mL of Zr (10 g L-1) to 100-1000
mL of sample at pH 9.0 (ammonia
solution 7.0M). The precipitate was
aged (1h at 90°C) and then
collected on a membrane filter
(8.0µm). The precipitate is direct
analysed by AAS.
Natural
water 0.38 ng L-1
- ETAAS [470 ]
Co-precipitation by addition of 20-
40 µL of NH3 into 1.3mL of
sample. Under these conditions Mg
present in the sample was
precipitated as Mg(OH)2. After
centrifugation, the precipitate is
dissolved in 100 µl ml of HNO3
(5.0 %).
Seawater
5.0 pM 13 ICP-MS [471]
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 84
Table 10. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Co-precipitation by addition of
1.0mg of Dy(III) to 100 mL of
sample at pH 11 (ammonia
solution). After centrifugation
(3500rpm for 10min), the
precipitate was dissolved with
1.0mL of conc. HNO3 and diluted
to 2 mL.
Natural
water 14.1 µg L-1
50 FAAS [472]
Co-precipitation by addition of
1.0mL of Y (1.0 g L-1) after
addition of 3.7mL of sodium
hydroxide solution (3.0M) to 100
mL of sample. After ageing
(10min) and centrifugation
(3500rpm for 5.0min), the
precipitate was dissolved with
5.0mL of HNO3 (1.0M).
Seawater
0.06 µg L-1
20 ICP-OES [473]
Co-precipitation by addition of
1.0mL of copper(II) solution (0.1%
(w/v)) to 200 mL of sample at pH
9.0 (sodium hydroxide). After
centrifugation (3500rpm for
10min), the precipitate was
dissolved with 0.5mL of conc.
HNO3 and diluted to 2mL.
Tap
water 3.0 µg L-1 100 FAAS [474]
Co-precipitation by addition of
10.0mL of KMnO4 (1 % (w/v)) and
20mL of 0.1% D-glucose to
2000mL of sample at pH 3.5-4.0.
After filtration on filter paper, the
precipitate (MnO2) is dissolved in
2.0mL of HCl (50 % (v/v)) and
20mL of H2O2 (30 % (v/v)). The
solution was diluted to 25mL.
Ground
water 1.0 µg L-1 80 ICP-OES /
FAAS [475]
Reductive precipitation by addition
of 5.0mL of sodium
tetrahydroborate (6.0%), and Fe3+
and Pd2+ (each a 2.0 mg L-1) to
900mL of sample at pH 8.0-9.0
(ammonium solution). After ageing
(15h) and filtering through a
0.45µm cellulose nitrate membrane,
the precipitate is dissolved with
HNO3/HCl conc. (3.0mL of each)
and dilute to 25 mL.
Seawater
1.0 ng L-1 36 ETAAS [476]
Analytical Chemistry of Cadmium: Sample Pre-treatment… 85
Table 10. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Flow injection on-line co-
precipitation with sodium
tetrahydroborate (2% at 0.5 ml min-
1) and Fe3+ and Pd2+ (30 µg mL-1) in
ammonium hydroxide (0.09M at
1.7 ml min-1) medium. After
magnetic collection, the precipitate
was dissolved in HCl (2.0M)/HNO3
(2.0M) at 0.25 mL min-1 and
transported direct to the graphite
furnace.
Seawater
2.8 pga 400 ETAAS [477]
Co-precipitation by addition of
1.0mL of Pd (10 µg mL-1), which
are subsequently reduced by
introduction of H2 gas (for 5min),
to 100 mL of sample at pH 2.0
(nitric acid solution). After
filtration, the precipitate is
dissolved with HNO3/HCl (1:1).
The solution was diluted to 2.0mL.
Seawater
- 50 - 500 ICP-OES [478]
Co-precipitation by addition of
2.0mL of APDC (2.0 % (w/v)) to
150mL of sample. The precipitate
is filtered off, and dissolved in
HNO3, and the solution heated to
near dryness and the residue is
dissolved to 2mL in 1% HNO3 - 1%
HClO4
Seawater
- 75 ICP-OES [479]
Co-precipitation by addition of
2.0mL of TAN, APDC and
ammonium salt of N-nitroso-
phenylhydroxylamine (8.0 % (w/v))
to 500-1000 mL of sample at pH
6.0-7.2 (ammonium solution). After
filtration the precipitate was
irradiated.
Tap and
seawater
- ~104 NAA [481]
Flow injection on-line co-
precipitation with PAN. After
precipitate collection in a knotted
reactor, the precipitate was
dissolved/eluted in hydrochloric
acid and transported directly into
the nebulizer. Sampling frequency
of 48 h-1.
Tap
water 0.10 µg L-1
18 FAAS [482]
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 86
Table 10. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Co-precipitation by addition of
10mL of DDDE (1.0 % (w/v)) to
sample at pH 5.0 (ammonium
hydroxide solution), after reduction
step by heating (to boiling) with
25mL of conc. HCl, 5.0mL of KI (4
% (w/v)), 5.0mL of ascorbic acid (5
% (w/v)) and 2.0mL of sodium
thiosulfate (0.7 % (w/v)). Direct
analysis, after filtration.
Tap
water - - EDXRF [484]
Co-precipitation by addition of
5.0mL of 8-HQ (2.0 %) to 100-500
mL of sample at pH 7.0-8.5
(ammonia solution). After heating
for 3h at 70°C and several hours of
ageing, the precipitate was
collected on a glass filter. A portion
(0.1 to 1 mg) is used for direct solid
sampling technique.
Seawater
1.4 ng L-1 - ETAAS [486]
Precipitation by addition of 2.0mL
of DBDTC (1.0 %(m/v) in
methanolic solution) to 200mL of
sample at pH 4.0 (0.1M potassium
hydrogen phthalate buffer). After
ageing (15min at room
temperature) and vacuum filtration
through a 25mm diameter 0.45µm
pore size cellulose nitrate filter
paper, the filter was air dried and
mouned for XRF analysis.
Fresh
water 0.6 ng L-1 - ETAAS [487]
Flow injection on-line co-
precipitation with DDC (0.025%
(m/v) at 0.9 mL min-1). After
precipitate collection in a KR, the
precipitate was eluted in ethanol
(6.0 mL min-1) and transported
directly into the nebulizer.
Sampling frequency of 30 h-1.
Rain
and
seawater
3.44 µg L-1
33 FAAS [489]
Co-precipitation by addition of 2.5mL
of Co(III) (0.02 % (w/v) and 5mL
of APDC (2 % (w/v)) to 1000mL of
sample at pH 4.0 (ammonium
acetate 34 % (w/v) solution). After
filtration through a fine-porosity
funnel under vacuum, the
precipitate is dissolved in 5.0mL of
HNO3 and dilute to 25mL.
Tap
water 1.0 µg L-1 40 FAAS [490]
Analytical Chemistry of Cadmium: Sample Pre-treatment… 87
Table 10. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Flow injection on-line co-
precipitation with Co(III) (20µg
mL-1 at 3.3 mL min-1) and APDC
(2.0 % (m/v) at 0.4 mL min-1).
After precipitate collection in a
PTFE membrane on a
polypropylene support filtering
device combined with a 1.5m
reaction coil, the precipitate was
dissolved in (1:1) conc. HNO3 and
conc. H2O2 and transported directly
into the nebulizer. The sample
throughput is 20 h-1.
Rain
water 0.7 µg L-1 28 ICP-OES [491]
Co-precipitation by addition of
APDC and Zn to sample. After
filtration, the precipitate is
dissolved in HCl (for ASV) or
direct placed into the atomizer (for
ETAAS).
Seawater
- - ETAAS /
ASV [492]
Co-precipitation by addition of
APDC and Bi(III) to the sample at
pH 4.0. After filtration, the
precipitate was irradiated.
Tap,
river
and lake
water
- - NAA [493]
Co-precipitation by addition of
1.5mg of Co(II) and 2.0mL of
HMA-HMDTC (0.01M) to
1000mL of sample at pH 6.0
(potassium hydroxide solution).
Precipitate was floated with the aid
of tiny air bubbles and supported by
the stable foam layer of NaDDS
(1.0mL, 0.5 %). After floats
collected, the precipitate was
dissolved with 5.0mL HNO3 (65%)
and diluted to 25mL with HNO3
(4.0M)
Natural
water 3.0 ng L-1 40 ETAAS [496]
Co-precipitation by addition of
2.5mL of oxine solution (20 mg
mL-1) and 0.5mL of magnesium
solution (10 mg mL-1) to 200mL of
sample at pH 9.0-9.5 (ammonia
solution, 1.0 and 5.0M). After
filtration on a 1.0µm Nuclepore
membrane filter, the precipitate is
dissolved in 5.0mL of HNO3 (1.0M).
The solution was diluted to 10mL.
River
and
seawater
1.0 ng L-1 20 ETAAS [497]
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 88
Table 10. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Flow injection on-line co-
precipitation with Cu(II) (120mg)
and DDC (2.1 % (w/v)) in HCl
(0.5M) medium. After precipitate
collection in a KR, the precipitate
was dissolved in IBMK and
transported directly into the
nebulizer.
Tap,
river
and
waste
water
0.23 µg L-1
26 FAAS [498]
Co-precipitation by addition of
0.6mL of cobalt solution (1.0 mg
mL-1) and 4.0mL of Na-DDC (5.0%
(m/v)) to 200mL of sample at pH
5.7 (2.0mL of ammonium acetate
buffer). After filtration (0.45µm
cellulose nitrate membrane and
vacuum filtration), the membrane
with the precipitate was digested
with 0.5mL of conc. HNO3 and the
residue diluted with 2mL.
Seawater
BCR-
403
- 100 ETAAS [499]
Co-precipitation by addition of
600µg of Co(II) and 200mg of Na-
DDC to 400mL of sample at pH 6.0
(acetate/acetic acid buffer). After
filtration (cellulose nitrate
membrane filter and vacuum), the
precipitate and the membrane are
dissolved in 2.0mL of conc. HNO3,
the solution was evaporated near to
dryness and the residue was dilute
to 2mL with HNO3 (1.0M).
Seawater
4.0 µg L-1 200 FAAS [500]
Flow injection on-line co-
precipitation with DDC-Ni(II) in
HNO3 medium. After precipitate
collection in a KR, the precipitate
was dissolved in IBMK and
transported directly into the
nebulizer. Sampling frequency of
60 h-1.
River
water 0.2 µg L-1 65 FAAS [501]
Co-precipitation by addition of Ni
(3.0mg) and 2.0mL of DDC (2.0%
(m/v)) to 500mL of sample at pH
9.0 (ammonia solution). After
filtration on a sintered-glass filter,
the precipitate was dissolved with
HNO3/acetone (1:1) and dilute to
10mL.
River
and
seawater
1.2 pg mL-1 50 ETAAS [502]
Analytical Chemistry of Cadmium: Sample Pre-treatment… 89
Table 10. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Precipitation by addition of 8-HQ
and La3+ to 1000mL of sample at
pH 9.0 (ammonia solution, 2.0M).
Precipitate was floated with the aid
of tiny N2 bubbles and supported by
the stable foam layer of sodium
lauryl sulfate. After floats collected
on a fritted glass filter with a
suction, the precipitate was
dissolved with 5.0mL ethanol and
1.5mL of conc. HNO3 and dilute to
25mL.
Water - 40 FAAS [503]
Co-precipitation by addition of
1.0mg of copper (II) (1ml of 1000
mg L-1) and 2.5mg of rubeanic acid
(0.5ml 0.1 % (w/v)) to 50mL of
sample at pH 7.0 (phosphate
buffer). After centrifugation
(2500rpm for 20 min), the
precipitate was dissolved with
0.5mL of HNO3 conc. and diluted
to 2mL of Milli-Q water.
Tap,
bottle
mineral
and lake
water
0.58 µg L-1
25 FAAS [505]
a Absolute detection limit
APDC, ammonium pyrrolidin dithiocarbamate; ASV, anodic stripping voltammetry; DBDTC;
dibenzyldithiocarbamate; DDC, diethyldithiocarbamate; DDDE, dibenzylammonium dibenzyldithiocarbamate;
DPP, differential pulse polarography; EDXRF, energy dispersive x-ray fluorescence; ETAAS, electrothermal
atomic absorption spectrometry; FAAS, flame atomic absorption spectrometry; HMA-HMDTC,
hexamethylene-ammonium hexamethylene dithiocarbamate; IBMK, 4-methyl-2-pentanone; ICP-OES,
inductively coupled plasma-optical emission spectrometry; KR, knotted reactor; NAA, neutron activation
analysis; NaDDS, sodium dodecylsulfate; PAN, 1-(2-pyridylazo)-2-naphthol; PQPD, 5,8-polyquinolyl
polydisulfide; PPT, phenanthraquinone monophenythiosemicarbazone; PTFE, poly tetrafluoroethylene; TAN,
1-(2-thiazolylazo)-2-naphthol; UV, ultraviolet; 8-HQ, 8-hydroxiquinoline.
After adsorption of the precipitates from hydrophobic complexes of metal ions on the
walls of the KR, an elution/dissolution step using an acid solution is mainly used to sweep the
target metal to the detector.
KRs offer several advantages for collecting the precipitate: (a) relatively high capacity as
consequence of the large internal surface area; (b) low back pressure (even for high flow
rates) due to the removal of any filter; (c) contamination are avoided due to the inert tube
material; (d) the reactor is easy and cheap to make; and, (e) the lifetime is unlimited and the
reactor requires no maintenance [61].
Since the first application of an on-line co-precipitation system with KR by Fang et al.
[506] for lead determination, several methods have been developed for the pre-concentration
of trace elements from waters. For cadmium (Table 10) FI co-precipitation systems using KR
were based on the use of HMA-HMDTC and Fe (carrier element) [494], DDC and Cu (carrier
element) [498], (DDC)-Ni [501], DDC [489], and 1-(2-pyridylazo)-2-naphthol (PAN) [482].
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 90
The filter-less magnetically assisted collection system consists of a 2.0m length of
Teflon, microbore tygon or silicone tubing (1.27 mm i.d.), where the coprecipitate analytes
(by tetrahydroborate reductive precipitation of added iron and palladium in alkaline medium)
are collected. This tube is wrapped around a (5.7 x 1.4 x 1.5 cm) rare earth cobalt ceramic
magnet of 8.58 KG field strength block. After collection, the precipitate is dissolved in a 20
ml volume of mixed acid and directly transported to measure instrument.This approach has
been applied for the determination of several trace metals, such as cadmium, in seawater by
FI on line ICP-OES and ETAAS [477].
As commented above, the heart of the manifold used for on-line co-precipitation is the
KR or the filtering device in which the precipitate is collected. Other components of the
manifold are the peristaltic pumps and the injection valve (these components have been
described in section IV.11).
V.2.3.2. Polymer-mediated extraction
Recently, an appealing alternative to classical co-precipitation methods consists of using
polymer-mediated extractions of hydrophobic metal chelates for pre-concentration. This
approach is based on the thermoresponsive precipitation phenomenon of several water soluble
polymers.
(a) General aspects
Thermoresponsive polymers are water-soluble at room temperature, but become
sparingly soluble above their critical solution temperatures (ca. 32°C) to form gum-like
precipitates (polymer phase) which can be separated from the bulk aqueous solution. During
the precipitation, any hydrophobic substances (derivatized phosphate as molybdophosphate-
melachite green, fatty acids, phospholipids, polycyclic aromatic hydrocarbons, alkylphenols,
chlorobenzenes, chlorophenols, pesticides and polyethoxylated derivatives) in the aqueous
solution can be solubilized and incorporated into the polymer assembly [507-511 However,
hydrophilic components remained in the bulk aqueous solution. In these methods, sample and
polymer solutions were mixed and the analyte-bound polymer was precipitated in a solvent, e.
g. acetone. Since the analyte and the polymer are thoroughly mixed and interacted in the
solution phase, the analyte is completely bound to the polymer. Because the polymer phase
was highly condensed, the concentrations of analytes were easily increased up to 100-fold.
This phenomenon can be applied for the pre-concentration of heavy metals only after
converting metal ions into hydrophobic metal chelates or ion-pairs of charge chelates, by
adding appropriate chelating agents.
Thermoresponsive polymers such as poly (N-isopropylacrylamide) polymer [PNIPAAm]
and poly(vinyl methyl ether) polymer [PVME] have been extensively used for metals pre-
concentration. On the other hand, efforts on the introduction of chelating functionalities into
the thermoresponsive polymers have been performed for achieving a more efficient collection
of heavy metals ions from water matrices. Poly(N-isopropylacrylamide-co-4-vinylpyridine)
polymer, [PNIPAAm-co-methacrylhydroxamic acid] copolymer and [PNIPAAm-Im],
[PNIPAAm-COOH] and [PNIPAAm-IDA] (a [PNIPAAm] polymer with imidazole,
carboxylic acid and iminodiacetic acid groups, respectively, have been also applied.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 91
An important advantage of the the polymer mediated extraction is the ability to
concentrate a variety of hydrophobic analytes (with high pre-concentration factors) by very
simple and fast procedures and without interferences because the analytes are determined in a
simple matrix (dissolved polymer). The polymer precipitates could be condensed into a small
pastelike solid, which can be easily removed from the solution by just heating and shaking the
solution for a few seconds. Thus, instruments, such as centrifuges, are not necessary for the
phase separation. The polymer phase was re-dissolved at room temperature with a small
amount of water or organic solvent and all the analyte bound to polymer is completely
transfered into the solution phase. Because of the ease of pyrolysis of the polymer, this phase
can be sampled in graphite furnaces for ETAAS determinations.
Other advantages of this methodology are related to safety because it requires only very
small amount of relatively non-flammable, non-volatile, and non-toxic reagents, and it avoids
the use large volumes of organic solvents. The non-toxic properties of PNIPAAm have been
clearly observed and have a negligible effect on the user´s health and the environment.
(b) Polymer-mediated extraction for cadmium preconcentration
As shown in Table 11, cadmium, together with copper and nickel, as a
pyrrolidindithiocarbamate chelate, was co-precipitated with [PVME] [512], while using
decyltrimethylammonium chloride (DTMAC), cadmium was pre-concentrated
with[PNIPAAm-Im], [PNIPAAm –COOH] and [PNIPAAm –IDA] [513]. Other water soluble
polymers, but without thermoresponsive properties, have been also proposed for cadmium pre-
concentration. This is the case of polyacrylic acid polymer [514] which has been applied for the
simultaneous pre-concentration of cadmium, copper and zinc from waste water.
V.2.4. Electrochemical deposition
As commented, electrochemical pre-concentration is an essential part of various
electrochemical methods for trace element determination such as ASV. This modality utilizes
the electrolysis laws in which cationic species are deposited on the electrodes surface into an
electrolytic cell. The application of a controlled potential allows the selective separation and
pre-concentration of trace metals.
(a) General aspects
Electrolytic cells are constructed in stationary [515-524 or flow-through [525,526]
arrangements. The electrolysis can be accelerated by rigorous mixing of the solution and by
maximizing the ratio of electrode surface to sample volume. This can be achieved in a flow
system by passing the sample solution through a porous working electrode made from
crushed reticulated vitreous carbon or glassy carbon [525] However, large volumes of the
eluent are required for washing out metals in these cells. Therefore, the cells with porous
electrodes are often used for on-line sampling of solutions in atomic absorption spectrometry.
In this case, a flow-through cell with a thin space between the electrodes is advantageous
when the distance of electrodes determines the volume of the cell. Thus, electrochemical
micro-cell [525,526] (3 µL volume), with a thin stream of sample solution flowing between
two electrodes, is used.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 92
Table 11. Co-precipitation procedures by polymer-mediated extraction for
cadmium pre-concentration from natural waters.
Brief description Sample LOD
Concentration
factor Analytical
technique Ref.
Water sample (50-100 mL) was treated
with NH3 (4.0M) to pH 2.0-3.0. The
sample was treated with 1.0mL of
APDC (1.0 mg mL-1) and 5.0mL of a
PVME (6.0 % (v/v)) with stirring at
50°C for 10min. Aggregates were
beaker with 10mL H2O and dissolved in
1.0mL acetone followed by 3.0mL of
HNO3 (0.1M). After combining the
washings, the solution was diluted to
5.0mL with HNO3 (0.1M).
River
and
seawater
- 10-20 ETAAS [512]
Water sample (20mL) was treated with
0.1g of PNIPAAm-COOH or
PNIPAAm-IDA, 0.017g of sodium
nitrate and 5.0mL of DTMAC (0.2M)
with stirring at 50°C. Aggregates were
beaker with N,N-dimethylformamide
and the solution was diluted to 1.0mL.
River
and
seawater
- 20 ETAAS [513]
Co-precipitation by addition of 1.0mL
of polyacrylic acid (0.56g) y 50mL of
acetone. The pH of the solution (50mL)
was adjusted at pH 3.0 by using
hydrochloric acid or sodium hydroxide
(0.1M) solution. After decantation, the
precipitate was dissolved with 5.0mL of
water.
Waste
water - 10 FAAS [514]
APDC, Ammonium pyrrolidin dithiocarbamate; DTMAC, Decyltrimethylammonium chloride;
[PNIPAAm-COOH], poly (N-isopropylacrylamide) polymer with carboxylic acid groups;
[PNIPAAm-IDA], poly (N-isopropylacrylamide) polymer with iminodiacetic acid groups; PVME,
Poly(vinyl methyl ether) polymer.
In general, electrolytic cells are composed of three electrodes. The reference electode,
which is usually a saturated calomel electrode (SCE) and saturated Ag/AgCl, the working
electrode, and the auxiliary electrode. Analyte deposition after reduction ocuurrs at the
working electrode (cathode), which can be made of different materials, such as high melting
metals wires (platinum, iridium or tungsten) [515,516,518,522,524], graphite rods, probes, or
disk electrodes [520,521,523,526] or glassy carbon [519,525]. Conventional HMDE can be
also used as a working electrode [517]. Finally, the auxiliary electrode is commonly the
platinum electrode.
The analytical procedure consists of three steps:
(1) Electrochemical deposition of the analyte at the working electrode by keeping it at a
suitable potential (~ -1.0 V) for a fixed time. During this step, the metal ions are
separated from the sample matrix and are deposited on the electrode.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 93
(2) Dissolution of the analyte from the electrode by setting the potential of the cathode at
0 V. When atomic spectroscopy techniques are used, this second step (dissolution of
the analyte from the electrode) can be avoided due to the fact the working electrode
could be inserted into the flame (FAAS determination) or into the electrothermal
furnace (ETAAS determination).
(3) Analyte measurement by electroanalytical techniques such as ASV or by atomic
spectrometry techniques. When using spectrometric techniques, atomization is
carried out by an electrical heating of a W-wire in a quartz tube or after inserting of
the electrode with deposited analyte into the graphite atomizer with or without
touching the wall of the atomizer. There are commercially available or specially
constructed graphite electrodes, e.g graphite disk electrodes, graphite ridge probes,
graphite rods or tungsten wires for sampling introduction in ETAAS. Other times, the
electrode can also form a part of the atomizer as a graphite platform (platform which
is inserted inside the graphite tube) or it can be the graphite tube itself. In this case,
the electrode with the deposite (graphite tube) is fixed between the contacts of the
graphite furnace. It is also possible the dissolution of metal deposits and the
subsequent injection into the atomizer as a liquid solution. When an electroanalytical
technique is used for final determination, the deposition potential of the metal has a
significant influence on the signal to background ratio. In addition, the presence of
oxygen, surface active compounds and inert salts in the sample can give several
interferences.
Effective electrochemical pre-concentration has some important requirements:
Electrodeposition of most of heavy metals competes with the reduction of the hydrogen
ions. To avoid extensive hydrogen ion reduction an electrode with high hydrogen overvoltage
is required. Mercury is the best material for this purpose. Other materials with significant
high hydrogen overvoltage are glassy-carbon and glassy-carbon covered by a mercury film.
Both are useful, but in the later case the mercury film, deposited on the solid electrode,
provides a larger hydrogen overvoltage than that on uncovered glassy-carbon. Once the metal
is deposited, great care must be taken of the mercury film on the glassy carbon surface and it
must be kept all the time at a secure negative potential, with no guarantee that it will keep
constant activity.
Reduction of the hydrogen ions, together with metal ions, during the pre-concentration
process means that the current efficiency of the pre-concentration is small, but analyte
deposition directly on the graphite electrode is possible. This is all that is needed for the
subsequent determination by atomic spectroscopy techniques. However the hydrogen
overvoltage on pyrolytic graphite or on electrographite is smaller than on glassy-carbon and
the performance of such electrodes may significantly change due to surface corrosion during
atomization cycles. As the roughness of the graphite surface increases, the electrochemical
deposition of more electronegative ions such as Cd2+ becomes more difficult.
As commented above, an important factor in electrochemical pre-concentration is a ratio
of the working electrode surface area to the volume of the electrolysed sample. To achieve
high electrochemical recoveries while depositing from flowing solution, the mass transfer of
the analyte species to the electrode surface should be as great as possible. To obtain this
condition, an electrode with the maximal area must interact with the minimal volume of
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 94
sample solution. Bulk pre-concentration usually requires few hours to transfer most of the
metals from the solution to the electrodes. Much more effective is a flow through cell with a
thin channel between the electrodes or a large surface area working electrode, where the
solution circulates over the electrode.
Several variables such as pH of sample (the optimum pH conditions depended on the
particular metal and the optimal deposition potential for a particular metal; at more negative
potentials increasing amount of hydrogen were released on the cathode, so that the optimal
pH was shifted to higher pH values); the deposition potential (at more negative potentials
hydrogen evolution on the cathode deteriorated precision); electrolyte concentration (the
electrodeposition of metals was also effected by the concentration of the supporting
electrolyte; the amount of deposited metals increased up to with concentration of electrolyte,
for higher concentration the signal decreased slightly); deposition time (the dependence of the
signal on the deposition time is lineal for deposition times lower than 15 min.) and optimum
distance between electrodes, electrodes surface area, etc. must be optimized.
(b) Electrochemical deposition for cadmium preconcentration
Electrodeposition is a very advantageous technique for trace metal determination in sea
water, where alkali metal halides form an ideal medium for the electrodeposition. Table 12
summarizes methods based on electrodeposition as pre-concentration methods for cadmium.
V.2.5. Liquid-liquid extraction
Liquid-liquid extraction (LLE) is the oldest and the most widely used sample pre-
treatment method for liquid samples. Extraction of low concentrations of inorganic and
organic analytes from complex aqueous samples into organic solvents is the aim of this
approach.
(a) General aspects
LLE procedures are based on the relative solubility of the target compound in two
immiscible phases, which leads to an improvement on the selectivity by isolating the analyte
and an increase on sensitivity by transferring the analyte to a small solvent volume. The
isolation and pre-concentration is attained at the same time using the solvent more suitable for
analytical signal acquisition. An ideal LLE procedure must be guarante a quantitative
isolation of the analyte fom the aqueous matrix sample moreover interference species remain
in the aqueous phase.
The pre-concentration of trace metals by LLE procedures is commonly achieved by the
addition of a chelating agent to a buffered aqueous sample, and by extracting the formed
metal complexes into an organic phase. Chelating agents are organic compounds with
functional groups containing nitrogen (as amines, amides or nitriles), oxygen (carboxylic,
hydroxyl or ether functional groups) or sulphur (thiols, thiocarbamides) atoms, which are
capable of chelating trace metals. Therefore, the most suitable compounds with the highest
affinity towards selected metals can be chosen according to the functional groups present.
Chelating agents used for LLE procedures should offer two characteristics. Firstly, these
Analytical Chemistry of Cadmium: Sample Pre-treatment… 95
substances must gurantee the complexition/extraction of most of the target metals; and
secondly, the extraction of these metal-complexes must be equally well over the some fairly
wide range of pH so that there would be some allowable error in adjusting the pH of the
solution. This last task is not always possible because most of the chelating agents show
strong pH dependence. Thus, a pH adjustment to with in one pH unit or less is required which
can result in serious errors in routine applications.
In order to fix the pH, the use of buffer solutions is mandatory in routine extraction work.
The buffer solution must offer the following characteristics: (1) buffer solution must be
stable, (2) buffer solution must have a high buffering capacity, and (3) buffer solution must
not participate in any reaction. Citrate- and acetate- based buffer solutions are the most useful
for fixing the pH before LLE, although acetate buffer is unfavourable for lead and silver
extraction due to acetate could combine with these metals to form stable Pb- and Ag-acetates
which were not readily extracted.
In addition, the solvent used to extract the metal complex must also show several
desirable characteristics:
(1) Solvents must extract the desired metal chelate by using one or few extraction steps,
i.e. two extraction steps are generally required for quantitative recovery of most
common trace metal when Freon-TF is used.
(2) Solvents must be immiscible with the aqueous solution, i.e. the water solubility of 4-
methyl-2-pentanone (IBMK) is relatively high, which originates salt carry-over and
matrix interferences when this organic phase is used.
(3) Solvents must not form emulsions because emulsion formation hinders phases
separation.
(4) Solvents must have good burning characteristics (some common analytical
techniques for determination are FAAS, ETAAS and ICP-OES).
(5) Solvents must enhance rather than suppress the atomic signal sensitivity as compared
to metal sensitivity in water.
(6) Finaly, the extracted metal complexes must be stable in the organic solvent.
Table 13 summarizes the physical properties of organic solvents which are commonly
used in LLE of trace elements [527].
Most of these solvents offer several drawbacks, e.g. the direct analysis of IBMK extracts
is limited by the poor stability of dithiocarbamate complexes [528] (dithiocarbamate complex
stability is greatest in chloroform, follows IBMK and Freon-TF). In addition, IBMK
solubility in water is higher than those offered by cyclohexane or tetrachloromethane and dual
extractions were required for the quantitative recovery of most common trace metals when
using Freon TF.
As it was commented, the efficiency of LLE process depends on the affinity of analytes
with the extracting solvent, the pH of the aqueous phase and the stability of the chelate
formed. Thus, several variables such as the chelating agent type, the concentration of
chelating agent, the organic phase nature, the organic phase / aqueous phase ratio, the pH of
aqueous phase (selection of the buffer solution), the acid and/or metal concentration in the
back-extraction stage, the number of sucesive extractions, and the shaking and standing times,
must be carefully selected.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 96
Although LLE procedures have proven to be reliable and efficient techniques, these
methodologies are time, reagent and labor-intensive consuming, which has the tendency for
emulsion formation and poor potential for automation. As LLE is a multi-step proces, it often
results in analyte losses. In addition, LLE requires large amount of high purity solvents, which are
expensive and toxic, and the procedure generates hazardous laboratory wastes, which disposal or
treatment can be harmful to the environment and the public health. Moreover, a derivatization step
previous to extraction into the organic phase is mandatory for metals preconcentration; and a back-
extraction of the pre-concentrate chelate into the aqueous phase is generally required for detection
and quantification by several spectrometric techniques.
(b) Liquid-liquid extraction for cadmium preconcentration
Analytical procedures for cadmium separation and pre-concentration by LLE are listed and
commented in Table 14. Sodium diethyldithiocarbamate (NaDDC) [529-532 ammonium pyrrolidin
dithiocarbamate (APDC) [455,533,534 APDC and NaDDC mixtures [499, 535, 536, 537, 538,
539, 540, 541, 542, 543] are commonly used as chelating agents for cadmium (and for other trace
metals) extraction because the low pH dependence. 8-hydroxiquinoline (8-HQ) [106] and H2DZ
[455, 544] can be also used. However, APDC is the most popular because its solutions are stable in
acidic conditions and it operates in a broad pH range without any decomposition. Other reported
chelating agents for Cd isolation by LLE are 1-(2'-pyridylazo) naphthol (PAN) [545], capriquat
[546], thiothenoyltrifluoroacetone (1.0 nM) – TOPO [547], 1,1,1-trifluoro-4-mercapto-4-(2-
thienyl)but-3-en-2-one – TOPO [548] and pelargonic acid [549].
The buffer solutions used in these procedures are mainly based on citrate [532,535] and
acetate [499,531,533,541,542,544,548]. Solvents such as IBMK [532,533,535,538,539], 2,6-
dimethyl-heptan-4-one (DIBK) [537,541,542], trichlorofluoroethane (Freon TF)
[499,529,530,534,536,539], chloroform [455, 539 540,543,545], cyclohexene [547,548] or
tetrachlomethane [531,544] are commonly used. All these solvents offer a relatively low
solubility in water, high complex extraction efficiencies and poor emulsion formation.
After LLE, the organic phases can be introduced into conventional atomizers [532,534,535,544-
548]. However, the direct analysis of organic phases causes several solvent effects due to changes in
the density, surface tension and viscosity which has a decisive effect on the droplet sizes of the
aerosol (FAAS), or which affect the drop deposition into the graphite furnace by the autosampler
(ETAAS). These solvent effects may cause a poor accuracy and repeatability when FAAS and
ETAAS are used as quantification techniques. Finally, other solvents such xylene commonly
originates turbulent and unstable flames, while chloroform tends to evaporate quickly leaving the
solid complex behind, which clogs the spray chamber.
Thus, direct analysis of organic extracts by atomic spectrometry instruments is not
advisable due to the low repeatability and sensitivity, and an organic solvent evaporation step
followed to aqueous dissolution [531] or a back extraction step into aqueous medium
[499,529,530,533,536-540,541,542] are usually performed. Several acid back-extraction
procedures by using nitric acid [499,529,530,533,536-539] have been reported for cadmium
determination. However, acid back-extractions are slow processes and times close to 20 min
are required to break the complex. Thus, metal exchange back-extraction by using metals
such as Hg [540,541,542] or Pd, which offer greater chelate stability constants (in the range
of 1040) than the analyte complexes overcome the slow kinetic of acid back-extraction. In
these cases, quantitative back extraction can be completed after tmes within 1 to 3 minutes.
Table 12. Electrochemical deposition procedures for cadmium pre-concentration from natural waters.
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Electrochemical pre-concentration was performed at a controlled potential
(-1000mV vs Ag/AgCl for 2.0min and 0.1KCl as supporting electrolyte) on to a
W wire filament from 20mL of sample. After deposition, the whole system was
rinsed and the potential of the cathode was set at 4000mV for 5s to analyte
dissolution.
Seawater 0.1 µg L-1 - ETAAS [515]
[516]
Electrochemical pre-concentration was performed by electrolysis on a hanging
mercury drop electrode (-1000mV for 11min) from 50mL of sample. After the
electrolysis, the drop mercury (1.8mm2 of surface) is transferred to a graphite
boat and removed by evaporation, and cadmium is determined by atomization.
Seawater - - ETAAS [517]
Electrochemical pre-concentration was performed at a controlled potential
(-1000mV vs Ag/AgCl for 2.0-5.0 min) on to a Pt wire filament. After the
electrolysis (25mL of sample at pH = 5.0 by using a acetate buffer), the Pt
spiral filament was directly heated in a flame or placed into a quartz tube to
increase the sensitivity of the method.
Seawater 0.1 µg L-1 - FAAS [518]
Electrochemical pre-concen-tration was performed at a controlled potential
(-800mV SCE for 20min) on to a graphite rod. After the electro-lysis (pH = 7.0,
potassium chloride (0.1M) as supporting electrolyte), the graphite rod was rinsed
and inserted in the graphite furnace.
Seawater,
mineral water,
spring water
and river water
4.0 ng L-1 - ETAAS [520]
Electrochemical pre-concentration was performed at a controlled potential
(-1000mV vs Ag/AgCl for 2.0-6.0 min) on a graphite disk electrode. After the
electrolysis (70mL of sample at pH = 4.5-5.0 with NaNO3 (0.01M) as
supporting electrolyte), the electrode was placed by automatic sampler into a
graphite tube atomizer for analysis by ETAAS.
Seawater 6.4 ng L-1 - ETAAS [521]
Electrochemical pre-conce-ntration was performed at a controlled potential
(-1000mV (vs SCE for 2.0min) on a three coil tungsten wire (cathode). After the
electro-lysis (20mL of sample and sulphuric acid (0.1M) as supporting
electrolyte), the wire was rinsed and placed in a graphite furnace.
River water 0.01 ng
mL-1 - ETAAS [522]
Table 12. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Electrochemical pre-concentration was performed at a controlled potential (-
1000mV (vs Ag/AgCl)) on the graphite ridge probe from 50mL of sample
deposited in a 100mL polyethylene cell magnetically stirred. After the
electrolysis (pH = 4.75 by using an acetate buffer and sodium nitrate (0.01M) as
supporting electrolyte), the electrode-probe was rinsed and inserted in the
graphite furnace.
Seawater ng L-1
- ETAAS [523]
Electrochemical pre-concentration was performed Pt (plate of helix), Ti, Ta, W,
Mo, Ni, glassy-carbon and spectrographic graphite electrodes. After electrolysis
(performed at room temperature for 15min. in 20mL of a supporting solution on
vigorous stirring at constant potential of 1000mV), electrodes were manually
inserted into the burner flame.
River and
seawater 4.0 ng L-1 - FAAS [524]
Flow-injection on line electrochemical deposition on the glassy-carbon
electrode in a flow-through micro-cell of volume 3 µl under galvanostatic
control (-1.0 V vs sat. Ag/AgCl). After deposition, the whole system was rinsed
and the potential of the cathode was set at 0 V to analyte dissolution, which is
transported to the flame by HNO3 (0.1 M) flow.
Fresh water 80 ng L−1 26 FAAS [525]
Flow injection on line elcectrochemical deposition on the graphite electrode
under conditions of controlled current (1.0mA during 10min) in a flow-through
mode (0.35 mL min-1) in a microcell of 2.6µL volume. After electrolysis (pH =
4.5 by using a sodium acetate buffer and ammonium chloride (0.1M) as
supporting electrolyte), deposited metal was dissolved in 40µL of HNO3 (0.1M)
and the whole volume was direct injected into the atomizer.
Tap water,
surface water
and seawater
25 ng L-1
- ETAAS [526]
SCE, saturated calomel electrode.
Table 13. Physical properties of the organic solvents [527] used for cadmium extraction by LLE and LLME.
Solvent Boiling point / °C
Density /
g cm-3 Water solubility /
mg L-1 Surface tension /
dyn cm−1 Viscosity / cP
Vapour pressure /
Torr
Benzene 80.1 0.87 1791 28.2 0.60 95.2
Chloroform 61.2 1.48 8500 26.53 0.54 194.8
Cyclohexane 80.7 0.78 55 24.65 0.90 97.8
Nitrobenzene 210.8 1.20 1900 (20°C) 42.17 1.62 (30°C) 0.28
Tetrachlorometane 76.6 1.58 770 26.13 0.90 115.2
Toluene 110.6 0.86 515 27.92 0.55 28.5
Xylene 138.4 0.86 156 28.48 0.64 8.7
1-octanol 195.2 0.82 538 26.92 7.30 0.08
1,1,2 trichloro-2,2,1-trifluoroethane
(Freon-TF) 47.6 1.57 200 23.9 0.68 270
2,6-dimethyl-heptan-4-one (DIBK) 166.0 0.81 500 20.7 0.89 1.7
4-Methyl-2-pentanone (IBMK) 117.4 0.80 17000 23.64 (20°C) 0.55 18.8
Table 14. Liquid-liquid procedures for cadmium pre-concentration from natural waters.
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Complexation by addition of 2.5mL of APDC (1.1% (m/v)) / DDC (1.1% (m/v))
to 200g of sample at pH 4.5 (1.5mL of ammonium acetate buffer). After shaking
with 20mL of Freon-TF (2.0min) and phase separation (5.0min), the extraction
was repeated and the organic extracts were combined. Addition of 400µL of
HNO3 for back-extraction (shaking time of 1.0min and 15min of reaction) and
6.0mL of Milli-Q water (shaking time of 1.0min and 5min phase separation).
The final volume is 14mL.
Seawater - 40 ETAAS [499]
Table 14. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Complexation by addition of 1.0mL of APDC/DDC (0.5% (m/v) each one) to
100mL of sample at pH 4.5 (ammonium acetate buffer 2.0M). After shaking with
20mL of Freon-TF (2.0min) and phase separation, the extraction was repeated
with 10mL of Freon-TF and the organic extracts were combined. Addition of
100µL of conc. HNO3 for back-extraction (5min of reaction) and 4.9mL of Milli-
Q water (>30 min phase separation).
Estuarine and
river water - 20 ETAAS [529]
Complexation by addition of 20mL of APDC/DDC (2.0% (m/v) each one) to 350mL
of sample at pH 8.0 (ammonia solution). After shaking with 35mL of Freon-TF
(6.0min) and phase separation, the extraction was repeated and the organic
extracts were combined. Addition of 100µL of HNO3 for back-extraction (shake
time of 30min and 5.0min of reaction) and 2.5mL of Milli-Q water (shaking time of
30min). The back-extraction was repeated and the acid extracts were combined.
Seawater 0.04 nM 70 ETAAS [530]
Complexation by addition of 4.0mL of DDC (1.1% (m/v)) to 500mL of sample
at pH 4.5 (10mL of ammonium acetate buffer 0.2M). After shaking twice with
5.0mL of CCl4 (2.0min) and phase separation, organic extracts were evaporated
to dryness (30°C) and the residue was mineralised with 0.1mL of conc. HNO3.
Dilution up 2.0-5.0 mL with H2O.
Seawater and
river water 10 pg 100-250 ETAAS [531]
16mL of conc. HNO3 was added to sample (400mL) and the mixture was heated
on a hot plate (<500°C) until the sample was reduced to 100mL. Complexation
by addition of 5.0mL of DDC (10 % (m/v)) and 10mL of (NH4)2SO4 (40%) to
treated sample at pH 6.5 (10mL of ammonium citrate solution (25%)). After
shaking with 20mL of IBMK and phase separation (1.0min), the organic phase
was aspirated and the cadmium determined.
River water - 20 FAAS [532]
Complexation by addition of 2.0mL of APDC (1.0% (m/v)) to 200mL of sample
at pH 4.0 (ammonium acetate buffer). After mechanical shaking with 7.0mL of
IBMK (25min) and phase separation (20min), 5.0mL of HNO3 (4.0N) was added
for back-extraction (shaking time of 20min and 20min of standing).
Seawater 3.0 ng L-1 40 ETAAS [533]
Table 14. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Complexation by addition of 25µL of chelating agent / pH adjustment (APDC
2.0% (m/v) / NaHCO3 5.0% (m/v) to 1250mL of sample (pH 4.5). After
mechanical shaking with 250µL of Freon-TF (10min) and phase separation
(5.0min), organic extracts were direct analysed
Seawater 18 ng L-1 - ETAAS [534]
Complexation by addition of 5.0mL of APDC / DDC (1.0% (m/v) each one) to
200mL of sample at pH 4.0 (4.0mL of citrate buffer). After shaking with less
than 35mL of IBMK (0.5-1.0 min) and phase separation (5.0-10 min), organic
phase is direct nebulised.
Natural water - - FAAS [535]
Complexation by addition of 3.0mL of APDC / DDC (1.0% (m/v)) to 500mL of
sample at pH 5.0 (3.0mL diammonium hydrogenocitrato 0.5M). After shaking
with 20mL of Freon TF (150s) and phase separation, the extraction was repeat
using 10mL of Freon TF. 0.2mL of conc. HNO3 (shaking time of 20min and
5.0min of standing) and 10m of water (shaking time of 20min) were added for
back-extraction.
Seawater 1.0 ng
Ll-1 50 ETAAS [536]
Complexation by addition of 20mL of APDC or DDC (5.0% (m/v)) to sample at
pH 3.0-6.0 (ammonium acetate buffer). After shaking with 8.0mL of DIBK
(2.0min for APDC and 1.5min for DDC) and phase separation, organic phases
are directly nebulised.
River water 0.2 µg L-1 - FAAS [537]
Complexation by addition of 4.0mL of APDC / DDC (1.0% (m/v) each) to 400mL
of at pH 4.0-4.5 (nitric acid). After mechanical shaking with 20mL of CHCl3 (20
min.) and phase separation (5.0min), 4.0mL of Hg(II) (1000 µg L-1) was added
to 16mL aliquot of organic phase for back-extraction (shaking time of 3.0min).
Seawater 30 ng L-1 - ETAAS [540]
Complexation by addition of 100 µl of APDC - DDC (0.5% each) to 80 ml of
sample at pH 4.5 (0.73 ml of ammonium acetate buffer). After shaking at 1000
rpm with 5.0 ml of DIBK (10 min) and phase separation (5.0 min.), 4.5 ml
portion of the organic phase was shaken for 2.0 min with 1.0 ml Hg solution
(100 mg l-1) for back-extraction.
Seawater - 72 ETAAS [541]
Table 14. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Complexation by addition of 100µL of APDC - DDC (0.5% each) to 80mL of
sample at pH 4.5 (ammonium acetate buffer). After shaking with 5.0mL of
DIBK (10min with a mechanical shaker) and phase separation (5.0min), organic
extracts (4.5mL) were centrifuged. Addition of 1.0mL of Hg back-extraction
solution (100 mg L-1) for 2.0min.
Seawater 0.2 ng L-1 16 ICP-MS [542]
Complexation by addition of 1.0mL of APDC / DDC (1.0% (m/v)) to 250mL of
sample at pH 4.0 (2.0mL of ammonium acetate buffer). After shaking with
8.0mL of chloroforme (2.0min) and phase separation (5.0min), the extraction
was repeat using 6.0mL of chloroforme. 4.0mL of HNO3 7.5M was added for
back-extraction. Combined extracts were evaporated to dryness and the residue
oxide with 250-500 µL of conc. HNO3.
Seawater 0.2 ng L-1 200 FAAS [543]
Complexation by addition of 3.0mL of H2DZ (0.78mM) - tributylphosphine
oxide (0.1M) mixture in CCl4 to sample at pH 4.0 (10mL of ammonium acetate
buffer). After shaking for 20min and phase separation, the organic phase is
analysed.
River and
seawater - 100 ETAAS [544]
Complexation by addition of thiothenoyltrifluoroacetone (1.0nM) - TOPO
(1.0mM) to 100mL sample. After shaking with cyclohexane and phase
separation, the organic phase was analysed.
River water <0.5 ng
mL-1 - ETAAS [547]
Complexation by addition of 5.0mL of 1,1,1-trifluoro-4-mercapto-4-(2-
thienyl)but-3-en-2-one - TOPO in cyclohexane to 30mL of sample at pH 5.0
(5.0mL of ammonium acetate (0.1M) or NaB4O7 (0.05M) buffer). After shaking
for 20min and phase separation by centrifugation, the organic phase was
analysed.
Seawater - 6 ETAAS [548]
Flow injection on-line liquid-liquid extraction preconcentration/separation
system associated with a newly designed gravitational phase separator, by using
APDC (0.5 % (m/v) at 1.0 mL min-1) in IBMK (at 0.45 mL min-1) medium at pH
2.0 by using nitric acid. Sampling frequency of 33 h-1.
Tap water, river
water and
seawater
20 ng L-1 155 FAAS [554]
Table 14. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Complexation by addition of 100µL of APDC - DDC (0.5% each) to 100mL of
sample at pH 4.0 (1.0mL of citrate buffer). After shaking with 1.0mL of DIBK
(1.0min) and phase separation (10min), 750µL portion of the organic phase was
transferred to a polyethylene cup for back-extraction for 1.0min with 500µL of
PdCl2 (100mg) and 10mL of HNO3 (65%).
Seawater and
marine
interstitial water
- 150 ETAAS [564]
Complexation by addition of 3.5ml of H2DZ (0.01 % (m/v)) in xylene to 500mL
of sample at pH 5.0 (boric acid and sodium hydroxide). After shaking and phase
separation (4.0min), 600µL of HNO3 was added for back-extraction (1.0min of
shaking time and phase separation).
Tap water 8.2 – 11.1
ng L-1 250 - 331 FAAS [565]
[566]
APDC, ammonium pyrrolidin dithiocarbamate; DDC, diethyldithiocarbamate; DIBK, 2,6-dimethyl-heptan-4-one; ETAAS, electrothermal atomic absorption
spectrometry; FAAS, flame atomic absorption spectrometry; Freon-TF, 1,1,2 trichloro-2,2,1-trifluoroethane; H2DZ, ditizone; IBMK, isobuthyl methyl
ketone; ICP-MS, inductively coupled plasma-mass spectrometry; PAN, 1-(2-pyridylazo)-2-naphthol; TOPO, 1,1,1-trifluoro-4-mercapto-4-(2-thienyl)but-3-
en-2-one.
*
Table 15. Cloud point extraction procedures for cadmium pre-concentration from natural waters.
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
10mL of sample (at pH 3.2) containing APDC (0.04 % w/v) and triton X-
114 (0.2 % v/v was heated (40°C for 10min) and then centrifuged
(3200rpm for 5.0min). After cooling (5.0min), the surfactant-rich phase
was separated off and 600µL of ethanol (20 % v/v) and HNO3 (10 % v/v)
was added.
River water 40 ng L-1 13 TS-FF-AAS [115]
CPE using PAN and Triton X-114 . Mineral water 25 ng L-1 59 TS-FF-AAS [116]
15mL of sample containing PAN (0.25 % w/v), triton X-114 (0.05 % w/v)
and borax buffer solution (0.05M, pH 8.0) was heated (40°C for 5.0min)
and then, centrifuged (3500rpm for 5.0min). After cooling, the surfactant-
rich phase was separated off and 100µl of a methanol containing HNO3
(0.1M) was added.
Seawater < 0.4 µg L-1 120 FAAS [581]
To a 10mL sample, 0.1mL of PAN (1.0 x 10-2 M), 0.2mL of triton X-100
(1.0 % w/v), 0.25mL of NaCL (30 % w/v) and 1.5mL of phosphate buffer
(pH 8.5) were added and then the mixture was heated (70°C for 15min) and
centrifuged (3500rpm for 15min). After cooling, the surfactant-rich phase
was separated off and 200µL of HCl (2.0M) was added.
Tap water 5.9 ng L-1 50 ETAAS [582]
CPE using PAN (5:1 M excess) and triton X-114 (0.25 % w/v). Tap water and
river water - - ICP-OES [583]
40mL of refrigerated sample containing DDTP (1.0 % w/v), triton X-114
(0.05 % w/v) and diluted nitric acid solution (pH 0.5) was heated (40°C for
15min) and then centrifuged (3500rpm and 30°C for 25min). After cooling,
the surfactant-rich phase was separated off and 1.0mL of a mixture of 60
%(v/v) methanol and 40 %(v/v) of water, also containing 1.0 %(v/v) nitric
acid was added.
Seawater and
river water 6.0 ng L-1 29 ICP-MS [584]
Table 15. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
8.0mL of sample containing DDTP (0.05M), triton X-114 (0.043 % w/v),
NaCl (1.0 % w/v) and dilute HCl solution (0.32M) was hand shaken and
then, centrifuged (2500rpm for 10min). After cooling, the surfactant-rich
phase was separated off and acidified ethanol (0.1M HNO3) was added.
Mineral water
and lake water 0.9 µg L-1 - FAAS [585]
50mL of refrigerated sample containing TAN (1.0 x 10-3 M), triton X-114
(0.05 % w/v) and 1.0mL of borax (0.1M) buffer solution (pH 8.6) was
heated (40°C for 15min) and then centrifuged (5000rpm for 15min). After
cooling, the surfactant-rich phase was separated off and 200µL of a
acidified methanol (HNO3 0.1M) was added.
Tap water, river
water and
seawater
99 ng L-1 58 FAAS [586]
To 40mL sample, 1.0mL of H2DZ (0.975 M), 200µL of triton X-114 (10 %
w/v) and 1.0mL of borate (0.2 M) buffer (pH 9.0) were added and then the
mixture was heated and centrifuged (3000rpm for 30min). After cooling
(for 15min), the surfactant-rich phase was separated off and 500µL of
HNO3 (1:1 v/v) was added.
Industrial
produced water 0.1 µg L-1a
21 ICP-OES [587]
20mL of sample at pH 9.0 containing DDC (0.01 g L-1), triton X-114 (1.5 g
L-1) was heated (40°C for 20min) and then centrifuged (3000rpm for
6.0min). After cooling (5.0min), the surfactant-rich phase was separated off
and 100µL of acidified methanol (HNO31.0 % v/v) was added.
Seawater 2.0 ng L-1 - ETAAS [588]
10mL of sample containing DDC (3.0 x 10-4 M), triton X-114 (0.05 % v/v)
and phosphate (0.005M) buffer solution (pH 7.0) was heated (40°C for
10min) and then centrifuged (3500rpm for 10min). After cooling (5.0min),
the surfactant-rich phase was separated off and 200µL of a THF was added.
Tap water, river
water, seawater
and waste water
0.31 µg L-1 52 FAAS [589]
500 or 1000mL of sample containing APDC (0.2 % w/v), triton X-114
(0.05 % w/v) and dilute HCl or NaOH solution (pH 4.0) was heated (90°C
for 15min) and then centrifuged (3500rpm for 15min). After cooling, the
surfactant-rich phase was separated off and acidified methanol
(HNO31.0M) was added to the surfactant-rich phase and sonicated for
5.0min to complete dissolution.
Lake water - - FAAS [590]
Table 15. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
To a 10mL sample, 1.0mL of solution containing 1.0 x 10-2 M of PMBP, 36
g L-1 of triton X-100 and borax buffer (pH 9.0) were added and then the
mixture was heated (80°C for 25min) and then centrifuged (3000rpm for
5.0min). After cooling, the surfactant-rich phase was separated off and
300µL of acidified methanol (HNO3 0.1M) was added.
Tap water and
lake water 0.64 ng L-1 23 FAAS [591]
Sample containing 8-HQ and triton X-114 (0.06 % v/v) was heated and
then centrifuged. After cooling the surfactant-rich phase was separated off
and acidified methanolic solution was added.
Tap water and
river water 0.15 ng L-1 - FAAS [592]
100mL of sample containing 8-HQ (1.0mM) at pH 7.0 and triton X-114
(1.0 % v/v) was shaken and heated (65°C for 15min) and then centrifuged
(3000rpm for 10min). After cooling, the surfactant-rich phase was
separated off and 3.0ml of acidified methanol (HNO3 1.0M) was added.
Lake water 16 ng L-1 - FAAS [593]
To 50mL sample, 0.5mL of Br-PADAP (0.25 % w/v) and triton X-114 (3.2
% w/v), 0.55mL of TRIS / HCl (0.4M) buffer (pH 9.0) and 1.0mL of NaCl
(5.0 % v/v) were added and then, the mixture was heated in a microwave
oven (10 % maximum power) and centrifuged (2500rpm for 15min). After
cooling (for 15min), the surfactant-rich phase was separated off and 400µL
of HNO3 (1:1 v/v) was added.
Waste water 81 ng L-1 22 ICP-OES [594]
To 25mL sample (pH 6 by using ammonium acetate 2.5 x 10-3 M), 50µL of
TTA (0.5M) and 1.0mL of triton X-114 (2.5 % w/v) was added and then
the mixture was heated (65°C). When solution is clouded, it was on.line
loaded into a cotton column (at 7.5 mL min-1), and then eluted at 3.0 mL
min-1 using propanol (0.5M) acidified with HNO3 (75:25 v/v) and directly
introduced into the nebulizer of the ICP-OES.
Tap water, well
water, seawater
and mineral
water
0.1 µg L-1 71 ICP-OES [595]
40mL sample containing DAD1 (2.75mM) or DAD2 (1.75mM) and triton
X-114 (0.25 % w/v) was heated (45°C for 15min) and centrifuged
(4000rpm for 10min). After cooling, the surfactant-rich phase was
separated off and 2.0mL of acidified methanol (HNO3 1.0M) was added.
Tap water 0.45 or 0.44
ng mL-1 18 or 20 FAAS [596]
Table 15. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
10mL of sample containing GBHA, triton X-114, SDS and sodium
hydroxide-sodium tetraborate buffer (pH 12.4) was centrifuged at room
temperature (incubation step is not necessary).
Tap water, lake
water and
seawater
7.0 ng L-1 22 ETAAS [597]
Sample containing triton X-114 (0.2 % v/v), iodide (30mM) and dilute
H2SO4 solution (1.0M) was shaken, heated (40°C for 5.0min) and then
centrifuged (3800rpm for 5.0min). After cooling, the surfactant-rich phase
was separated off and 0.4mL of ethanol was added.
River water,
seawater and
waste water
1.0 ng L-1 11 FAAS [598]
To 46mL of sample, 1.0mL of PONPE 7.5 (0.06 % v/v) and 3.0mL of boric
acid buffer (1.2 x 10-2 M) pH 8.0 were added. The resultant solution was
immediately turbid at room temperature. After centrifugation (4000rpm for
10min), liquid phase was separated completely by a 25mL syringe without
cooling in an ice bath.
Snow, rain,
subterranean,
canal water and
waste water
0.56 ng L-1 62 CV-AAS [599]
To 5.0-10 mL of sample, 3.0mL of Tween 80 (4.0 % w/v) and 5.0mL of
phosphate buffer (pH 8.5) were added and then the mixture heated (60°C
for 60min), cooled in a refrigerator (+4°C for 20min) and then centrifuged
(3500rpm for 20min). The surfactant-rich phase was separated off and
1.0mL of HNO3 (1.0M) in methanol was added.
Tap water and
mineral water 1.7 µg L-1 10 FAAS [600]
a LOQ (limit of quantification)
APDC, ammonium pyrrolidin dithiocarbamate; Br-PADAP, 2-(5-bromo-2-pyridylazo)-5-(diethylamino)-phenol; CV-AAS, cold vapour atomic absorption
spectrometry; DAD1, N,N’-bis((1R)-1-ethyl-2-hydroxyethyl)ethanediamide; DAD2, N,N’-bis((1S)-1-benzyl-2-hydroxyethyl)-ethanediamide; DDC,
diethylditiocarbamate; DDTP, ammonium O,O-diethyl-dithiophosphate; ETAAS, electrothermal atomic absorption spectrometry; FAAS, flame atomic
absorption spectrometry; GBHA, glyoxal-bis (2-hydroxyanil); H2DZ, ditizone; ICP-OES, inductively coupled plasma-optical emission spectrometry; ICP-
MS, inductively coupled plasma-mass spectrometry; PAN, 1-(2-pyridylazo)-2-naphthol; PMBP, 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone; PONPE 7.5,
polyethyleneglycolmonop-nonylphenylether; SDS, sodium dodecyl sulfate; TAN, 1-(2-thiazolylazo)-2-naphthol; THF, tetrahydroforan; TRIS,
tris(hydroxymethylaminomethane); Triton X-100, 4-(1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol; Triton X-114, octylphenoxypolyethoxyethanol;
TS-FF-AAS, thermospray flame furnace atomic absorption spectrometry; TTA, 1-(2-thenoyl)-3,3,3-trifluoraceton; Tween 80, polyethylene glycol sorbitan
monooleate; 8-HQ, 8-hydroxiquinoline.
Antonio Moreda-Piñeiro and Jorge oreda-Piñeiro 108
The implementation of LLE techniques in on-line mode coupled to atomic spectrometry
instruments has been developed to minimize many of these drawbacks moreover offer the
advantages of the automatic methods of analysis. Most of the drawbacks of conventional LLE
procedures are also overcome by miniaturizing the whole procedure, mianly by drastically
reducing the extractant phase volume. This has lead to new extractive techniques based on
liquid-liquid microextractions. Some examples of these methodologies are the liquid-liquid
micro-extraction, the single-drop micro-extraction, the dispersive liquid-liquid micro-
extraction or the hollow fibre liquid-phase micro-extraction. These new methodologies will
be described in next sections.
V.2.5.1. Flow injection on-line pre-concentration systems by LLE
Since the introduction of flow injection liquid–liquid extraction (FI-LLE) as a mean for
metal pre-concentration in flame atomic absorption spectrometry (FAAS) by Nord and
Karlberg [550, 551], the number of contributions in this field was rather few compared to
other on-line pre-concentration systems [552]. This is due to the limited range of flow rate
ratios of aqueous to organic phase of the segmented stream that on-line phase separators can
handle and, in addition, the insufficient stability, versatility and robustness of them.
(a) General aspects
The extraction process in these FI modes can be divided into three steps: (1) solvent
segmentation, (2) coil extraction, and (3) phase separation.
In the first step, the two initial streams of the two phases converging on it. Thus, regular
segments of both immiscible liquids entering into the reaction coil. The commonest type of
segmentor consists of a tube with three openings. The aqueous phase enters through a glass
capillary and the organic phase through a platinum capillary perpendicular to the former. Two
adjustable concentric PTFE tubes shut off the tube at the other opening in the same direction
as the aqueous phase. The length of the inner tube is adjustable allowing the checking of the
length segments. In the second step, the segmented phases are carried to an extraction coil
(PTFE) in which the analytes are transferred from the sample solution into the organic
solvent. In the last step, the solvent extract is separated from the waste sample solution and
finally carried into the detector. The separation is rarely quantitative and usually analytical
recoveries between 80-95 % are achieved. However, the phase in which the determination is
to be carried out should be kept completely free from the other.
Moreover the solvent segmenter and the reaction coil, an essential component of the
manifold used for on-line LLE is the phase separator in which phases are isolated. Various
types of phase separators have been reported. One of them is a micro-porous PTFE membrane
(0.7-0.9 µm pore diameter) which offers a selective permeability towards the phase which
wets the membrane material, and allows the organic phase crossing it to be completely free
from aqueous phase [553]. The main drawback offered by membrane type phase separators is
the short lifetime of the membrane and the permeation of small amount of aqueous phase
especially when high flow rate ratios of aqueous to organic phase and high segmented flow
rate is required. Thus, it is difficult to maintain the hydrophobicity of membrane for long time
and to prevent leakage of aqueous phase.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 109
Gravitational separators are a second type [554]. These devices are T-shaped separators
which use the gravity with or without a sort of phase guide made of a material wetted by one
phase but not by the other. These phase separators are promising because large range of flow
rate ratios of aqueous to organic phase can be used.
The state-of-the-art of automated solvent extraction procedures exploiting flowing stream
techniques is thoroughly presented and discussed in the paper of Miro et al.[555] and
Anthemidis et al.[556]. While FI-LLE was originally associated with the use of a solvent
extractor involving a segmentor and a phase separator, current analytical trends are focused
on the design and characterization of novel strategies for the removal of the classical
instrumentation aimed at improving the efficiency of phase separation as well as the
repeatability, sensitivity and accuracy of the analytical chemical assays.
(b) Flow injection on-line LLE for cadmium pre-concentration
Few FI-LLE procedures for metals and inorganic compounds have been reported.
Developments for cadmium pre-concentration are listed in Table 14. These applications
include cadmium extraction from tap water, river water and seawater [554], Cd,
simulatenously extracted with copper and zinc, or with cobalt, copper, iron nickel and lead,
from aqueous standard solutions [545,553]; and cadmium from sediments [557].
V.2.5.2. Liquid-liquid micro-extraction
Liquid-liquid micro-extraction (LLME) is a solvent micro-extraction technique in which
the phase ratio values are higher than 100 [558]. The technique is faster and simpler than
conventional LLE methods. It is also inexpensive, sensitive and effective for removing
interferences. Compared with conventional LLE, LLME may provide poorer analyte
recoveries, but the concentration in the organic phase is greatly enhanced. In addition, the
amount of organic solvent used is reduced and only one step of manipulation is necessary,
which reduces problems of contamination and loss of analytes.
The Murray flask and its following adaptations were firstly described for the pre-
concentration of organic pollutants (organochloride pesticides [559,560] and
organophosphorus pesticides [561]) using LLME in aqueous matrices. The Murray flask
consists of a 1000mL flask with a capillary tube at the top and a lateral arm near the base. The
capillary tube has the function of collecting the small volume of organic solvent after
agitation of the sample. The lateral arm is used to move the organic extraction solution to the
capillary tube using either water or air. The use of air allows multiple extractions of the
sample since the aqueous volume is maintained.
The first application of modified Murray flask for LLME of metals is attributed to
Carasek [562] who developed a LLME for the determination of gold in seawater by FAAS.
The method uses 800µL of xylene after gold chelation with ammonium O,O-diethyl-
dithiophosphate (DDTP). The extraction is then carried out until the aqueous to organic phase
ratio achieved a 1000-fold preconcentration of metal
Flow injection systems have been also proposed for LLME [563]. In this case, the metal
(lead) is complexed by dithizone at pH 9.3 (borate/NaOH buffer) in carbon tetrachloride. The
extraction process is performed in a miniaturized way by placing a small plug of the organic
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 110
solvent containing dithizone at the detection point. Then, a large aqueous sample volume was
passed through the organic plug, which was gradually enriched with the analyte.
Despite the advantages of this technique, few works have been reported using LLME
and/or micro volume back-extraction for the pre-concentration of metals. For cadmium pre-
concentration from environmental samples (Table 14), only three developments have been
addressed [564-566].
V.2.6. Cloud point Extraction
Cloud point extraction (CPE) was initially introduced for the pre-concentration of metals,
in the form of their hydrophobic complexes, by Miura et al. in 1976 [567], as an alternative
method for avoiding the use of organic solvents. Then, it was extensively exploited for
inorganics [568-574 and organics isolation [568-570, 575, 576]. Current trends and new
developments of CPE are mainly refereed to automation and to the use of ionic liquids and
micellar formations [572, 577, 578].
(a) General aspects
Aqueous solutions of certain surface-active agents (such as non-ionic or amphoteric
surfactants) display the so-called cloud point phenomenon in which the aqueous surfactant
solution (surfactant above the critical micellar concentration, CMC) suddenly becomes turbid
because of a decrease in the solubility of the surfactant in water [570]. Surfactant agents are
amphiphilic molecules with distinct hydrophobic and hydrophilic moieties; a polar or ionic
group connected to a long hydrocarbon tail (linear, branched or containing aromatic rings). At
low concentrations, surfactant molecules are mainly as monomers. When their concentration
increases above a certain threshold (CMC) surfactant monomers spontaneously accumulate to
form colloidal-sized clusters, known as micelles. Depending on the specific surfactant and
solution conditions, micelles can adopt a variety of shapes, ranging from roughly spherical to
ellipsoidal.
CPE is based on certain properties of non-ionic or amphoteric surfactants at levels upper
to their CMC. If some condition, such as temperature or pressure, is appropriately altered or,
if an appropriate substance (electrolyte) is added to the solution when non-ionic surfactant are
above CMC, the system composed by a unique phase is separated into two isotropic phases.
One of these phasea contains a surfactant at a concentration below, or equal to CMC, and the
other phase is a surfactant rich phase [573]. Above “cloud point”, hydrophobic species
(hydrophobic organic compounds or metal ions after reaction with a suitable hydrophobic
ligand) present in sample can be entrapped in the micelles of the surfactant, thus can be
separated from the bulk sample matrix and can be concentrated in the small volume of the
surfactant-rich phase [579].
In the micellar structure, surfactant aggregates orientate their hydrocarbon tail towards
the center of the formation, creating a non-polar core. Hydrophobic and covalent compounds
initially present in the aqueous solution are favorably partitioned in the non-polar
microenvironment. The whole process resembles traditional LLE, the only difference is that
the ‘‘organic’’ phase is generated within the aqueous phase, converting a previously
homogeneous solution to heterogeneous one by simply gathering its previously scattered
hydrophobic suspensions [577]. Although the exact mechanism via which this phenomenon
Analytical Chemistry of Cadmium: Sample Pre-treatment… 111
occurs is yet to be defined, several studies have shown that such phase separations result from
the competition between entropy (which favors miscibility of micelles in water) and enthalpy
(which favors separation), so the clouding and phase-separation procedure is reversible [577].
This behavior, showed by numerous hydrophilic groupings, is especially observed with
polyoxyethylene surfactants, and it can be attributed to the two ethylene oxide segments in
the micelle that repel each other at low temperature when they are hydrated, and that attract
each other as the temperature increases due to dehydration. This effect causes a decrease in
the effective area occupied by the polar group on the micelle surface, increasing the size of
the micelle, which can be considered to become infinite at the cloud point, leading to the
phase separation. Any hydrophobic species, such as metallic chelates, remain preferentially in
the surfactant-rich phase, and they can be extracted or pre-concentrated. More detailed
discussion on CPE phenomenon can be consulted in specialized literature [570,577, 580].
When applying CPE techniques for isolating trace metals, a previous complexation stage
is required. This fact usually leads to the optimization of several parameters such as pH and
ionic strength. In general, the following factors must be optimized to achieve a successful
CPE procedure [577]:
(a) Selection of the appropriate chelating agent. The ligand is selected with the
requirement that the derived complex is sufficiently hydrophobic, possesses a high
partition coefficient and is formed quickly and quantitatively with the least possible
excess. Based on their reactivity and formation constants with the target metal
species, some of the most widely applicable reagents are carbamates, pyridylazo,
quinoline and naphthol derivatives. These molecules are universal chelators that form
hydrophobic compounds with most of metals and they can be applied when an
element-specific detector is available. Other reagents, such as O,O-
diethyldithiophosphate, have been used for more specific applications.
(b) Concentration of chelating agent. The concentration of the chelating agent has to
compensate sufficiently for any consumption of the reagent by other metals.
Additionally, chelating agents with lower partition coefficients than others have to be
in sufficiently large excess to extract efficiently.
(c) pH. Although the chelate formation is strongly dependent on the pH, this variable
offers little influence on the extraction efficiency of the complexes formed, since
these complexes are bulky, uncharged and covalent.
(d) Ionic strength. Although ionic strength has proved to have a negligible effect on the
performance of CPE, increasing ionic strength enhances phase separation. Moreover,
the addition of a salt can markedly facilitate the phase-separation process. This has
been demonstrated when working with some non-ionic surfactant systems, since the
density of the bulk aqueous phase is changed.
(e) Surfactant selection. The non-ionic surfactant must offer high extraction efficiency
for the analyte. In addition, the two phases must be easy to separate after centrifugal
settling and cooling stages should be omitted. Moreover surfactant must be not
expensive, it must be present a low cloud point temperature, which simplified the
procedure due to incubation step is avoided. Triton X-114, Triton X-100, PONPE 7.7
and Tween 80 are the most used surfactants for metal extraction by CPE. The cloud
point achieved with these reagents is low, 23, 65, 25 and 65°C, for Triton X-114,
Triton X-100, PONPE 7.7 and Tween 80, respectively.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 112
(f) Surfactant concentration. There is a narrow range within which easy phase
separation, maximum extraction efficiency and analytical signal are accomplished.
Increasingly, outside this optimal range, the pre-concentration factor decreases.
However, if the surfactant concentration is lower than therecommended value, poor
repeatability is achieved.
(g) Incubation temperature. Temperature seems to play an additional role in enhancing
pre-concentration efficiencies and enhancement factors, and it is reported that the
application of high temperatures leads to dehydration of the micelle, and the increase
of the phase-volume ratio, and thus the signal enhancement by a factor higher than 3.
(h) Incubation time. Metal reaction with chelating agents and their transportation inside
the micelle are kinetically controlled (although thermodynamically favored,
simulating the shift of equilibrium towards precipitation). It is therefore essential to
maintain the reaction time above a minimum threshold for quantitative extraction. In
most studies, a reaction time of up to 10 min is reckoned to be optimum.
(i) Centrifugation time. Centrifugation time hardly ever affects micelle formation but it
can accelerate phase separation similar to conventional solid phase (precipitate) –
liquid separations. Centrifugation times around 5–10 min have been found to be
adequate in most of the CPE procedures.
The use of CPE procedures offers an interesting green alternative to the conventional
extraction systems because CPE avoids the use of large amounts of expensive, toxic and
flammable organic solvents. Commercial surfactants are environmentally friendly and cost
effective, and the amounts used for effective extraction schemes are minimal compared to the
amounts of organic solvents used in conventional extractions.
Other advantages of CPE procedures are related to the quantitative recoveries and large
pre-concentration factors (between 10 to 100) [571] because the analyte is transferred from a
great volume of original solution to a reduced surfactant-rich phase volume, which rarely
exceeds 100µL per 10mL of sample. Pre-concentration factors can also be modified on
demand by varying the amount of surfactant. In addition, the presence of surfactant can
minimize losses of analytes due to their adsorption onto the container.
CPE can be applied for extracting thermally sensitive analytes, such as molecules of
biological and environmental interest because the mild conditions inherent to CPE techniques.
The surfactant-rich phase is compatible with most mobile phases used in hydrodynamic
analytical systems, while it increases atomic signal in FAAS and wettability of the graphite
surface in ETAAS techniques.
There are, of course, several limitations of CPE procedures, mainly attributed to the
manipulation of the surfactant-rich phase obtained. As this phase is viscous, it can not be
injected directly to conventional analytical instruments, so it must to be diluted with an
aqueous or organic solvent to reduce its viscosity. This fact decreases the theoretical pre-
concentration factors. Moreover, surfactant-bearing chromophores interfere with UV
detection by overlapping with the analyte signal. This problem is not important when atomic
detectors are used.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 113
(b) Cloud point extraction for cadmium pre-concetration
Two different approaches have been published on the pre-concentration of cadmium and
other metals from natural waters by CPE (Table 15).
In the first approach, cadmium is pre-concentrated by CPE after the formation of
sparingly water-soluble complexes with a suitable chelating agent such as 1-(2-pyridylazo)-2-
naphthol (PAN) [112,581-, 583], O,O-diethyldithiophosphate (DDTP) [584, 585], 1-(2-
thiazolylazo)-2-naphthol (TAN) [586], H2DZ [587],diethyldithiocarbamate (DDC) [588,
589], ammonium pyrrolidin dithiocarbamate (APDC) [115,590], 1-phenyl-3-methyl-4-
benzoyl-5-pyrazolone (PMBP) [591], 8-hydroxyquinoline (8-HQ) [592, 593], 2-(5-bromo-2-
pyridylazo)-5-(diethylamino)-phenol (Br-PADAP) [594], 1-(2-thenoyl)-3,3,3-trifluoraceton
(TTA) [595], N,N’-bis((1R)-1-ethyl-2-hydroxyethyl)ethanediamide (DAD1) and N,N’-
bis((1S)-1-benzyl-2-hydroxyethyl)-ethanediamide (DAD2) [596], and glyoxal-bis (2-
hydroxyanil) (GBHA) [597].
The second approach (simplified procedure) is based on direct application of CPE
procedure without any ligand. By this method, cadmium has been pre-concentrated without
adding chelating agents (i.e. by using iodide) [598] or by without any ligand (ligandless cloud
point extraction) [599, 600].
The surfactants which are used in cloud point extraction are mostly nonionic surfactants,
such as (1,1,3,3-Tetramethylbutyl)phenyl-polyethylene glycol (Triton X-114) [115,112 ,
581,583-,592-598], 4-(1,1,3,3-Tetramethyl-butyl)phenyl-polyethylene glycol (Triton X-100)
[582,591], 7.5, polyethyleneglycolmonop-nonylphenylether (PONPE 7.5) [599] and
polyethylene glycol sorbitan monooleate (Tween 80) [600].
Usually, after phases are separated by centrifugation, the test tubes are cooled in an ice
bath, thus increasing the viscosity of the micelles, which adhere to the bottom of the vial, so
removal of water is achieved by simple decantation. Complete removal of water traces can be
attained with a Pasteur pipette or by evaporation under a nitrogen, argon or helium stream.
The inherent high viscosity of the surfactant-rich phase must be decreased before performing
measurements. Most commonly, a methanolic solution of nitric acid, as a diluting agent, is
included in analytical schemes to accommodate this need [581,584,586,588
,590,591,592,600]. There is an optimum volume for methanol with respect to the analytical
signals. This should be a compromise between the capability of methanolic solution to lower
the viscosity, thus improving the homogeneity of the solution, and the amount of organic
solvent that can, without problems, be tolerated by some detectors (for example, flames and
plasmas). Ethanolic [115,585,598] and propanolic [595] solutions of nitric acid and diluted
nitric acid [594] or hydrochloric acid [582] solutions, and tetrahydroforan (THF) [589] are
also used for the same purpose.
As commented above, although higher temperatures are commonly applied for attaining
the cloud point, other possibilities, such as pressure or the use of additives (including
electrolytes), have been proposed. These strategies allow that the cloud point can be attained
at room temperature with inherent advantages to the process. The addition of electrolytes can
increase or decrease the surfactant cloud point, being these effects respectively known as
“salting-in” and “salting-out”. Studies on the effects of some electrolytes, as NaCl, KNO3 and
MgCl2, on the cloud point behavior of non-ionic surfactants have been reported [601, 602]. It
was observed that the presence of electrolytes decreases the cloud point (salting-out effect),
whih results in a more efficient extraction. The lower cloud point is attributed to electrolytes
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 114
promoting dehydration of the polyoxyethylene chains [603]. The salting-out phenomenon is
directly related to desorption of ions to the hydrophilic parts of the micelles, which increase
inter-attraction between micelles and consequently leading to the precipitation of surfactant
molecules [602]. Therefore, Coelho et al. [585] and Filik et al. [597] have applied a mixed
micelle mediated system for Cd pre-concentration where the phase separation (at room
temperature) was induced by hydrochloric acidl or NaCl for lowering the cloud point.
V.2.6.1. Flow injection on-line pre-concentration systems by CPE
For on-line coupling of CPE to FI, the flow rates must ensure the complete adsorption of
the surfactant-rich phase, preventing any back pressure effects and blocking. Low flow rates
are not expected to impair or to enhance the extraction efficiency but increase the time of
analysis. Higher velocities cause flow disturbances and insufficient retention of the micellar
phase on the reactor. The length of the column of the reactor is other important parameter.
Short columns are inadequate for retaining the total amount of the surfactant phase, while, for
longer columns, the signal decreases due to insufficient washing and blockage of the flow
tubes. Other parameters, such as the amount of cotton additive and the elution flow rate, can
affect the extraction efficiency. If large amount of the absorptive material is produced,
blockage of the tubes and insufficient wash-out of the micellar phase can occured. However,
if elution of the surfactant phase is too fast, incomplete desorption can be attained; while, for
lower elution rates, the time of analysis can be significantly increased [577].
The combination of the FI technique with CPE has been reported in several publications
for off-line or on-line pre-concentration procedures for cadmium and other trace metals.
(A) Off-line coupling of CPE to FI
The first paper on the off-line coupling of FI with CPE was reported for the
determination of uranium (IV) by using PAN/Triton X-114 (complexing/surfactant) in tap
and river water [604]. Since this pioneering work, several metals were conveniently
determined by off-line coupling of FI with CPE [577]. In these papers, FI was used to
introduce the surfactant rich phase into various analytical devices. In that way, the dissolution
of the surfactant-rich phase can be performed in small volumes (higher pre-concentration
factors) and it minimize repeatability problems that manual applications may cause.
(B) On-line coupling of CPE to FI
Off-line CPE involves a series of fussy procedures including incubation, centrifugation,
surfactant-rich phase separation and dilution of surfactant-rich phase. All these procedures
may result in poor precision, low pre-concentration factors and time consuming procedures.
To avoid these problems, Fang et al. [605] proposed the on-line incorporation of CPE to flow
injection analysis. Since then, few studies have exploited the analytical advantage offered by
the on-line application of this technique for metal pre-concentration [577]. This method is
based on the on-line mixing of the sample with a surfactant rich solution, which allows phase
separation in the flow on a high surface material, generally a glass tube packed with a suitable
filtering material (cotton, nylon or glass-wool) that can quantitatively intercept the surfactant
Analytical Chemistry of Cadmium: Sample Pre-treatment… 115
aggregates. Subsequently, the surfactant-rich phase containing the analytes is desorbed by a
proper elution agent and is transported on-line towards the measuring device.
Yamini et al.[595] (Table 15) have applied this last approach in the determination of
cadmium and other trace elements in water by using TTA/Triton X-114. When solution is
clouded, it was on line loaded into a cotton column. Under these conditions, the surfactant
rich phase was trapped inside the column and the retained chelates were eluted using acidified
propanol and directly introduced into the nebulizer of an ICP-OES.
V.2.7. Single-drop micro-extraction
(a) General aspects
Single-drop micro-extraction (SDME) is a miniaturization of the traditional LLE.
Classically, this pre-concentration technique is based on the use of a micro-drop water
immiscible of solvent exposed to the aqueous sample donor solution (direct-SDME or
continuous flow micro-extraction (CFME)). However, other variation of the technique
consists of thin water immiscible organic extraction phase (of lower density than water)
layered over the aqueous sample (liquid–liquid–liquid micro-extraction, LLLME). In
addition, when analytes are volatile or semi-volatile, pre-concentration can be achieved by
exposing the drop to the headspace above the sample, which leads to the headspace single-
drop micro-extraction (HS-SDME) [606]. SDME is not exhaustive, and only a small fraction
of analyte is extracted/pre-concentrated for analysis.
The first SDME application was reported by Liu and Dasgupta [607] for the
determination of NH3 and SO2 in air samples by using an acidic aqueous micro-drop for
extracting the analytes. In 1996 the same authors develop another extraction system (drop-in-
drop system) [ 608], where a micro-drop of chloroform (13µL), suspended in a larger aqueous
drop, extracts sodium dodecylsulphate as ion pair with methylene blue. Once the analytical
signal due to the organic drop was measured, the organic phase was pumped away, and the
analytical cycle could be repeated. Jeannot and Cantwell [609] introduced other micro-drop
extraction technique for extracting 4-methylacetophenone, which consists of a micro-drop
(8µL) of n-octane suspended at the end of a Teflon rod immersed in a stirred aqueous
solution. After sampling the rod was removed from the sample solution and an aliquot of the
organic drop was analysed. The same authors suggested an alternative drop-based extraction
technique, which involved an improvement over the previous attempts, using an organic drop
(1.0µL) directly suspended from the tip of a microsyringe needle immersed in a stirred
aqueous solution [610]. After extracting, the micro-drop was retracted back into the
microsyringe needle and analysed.
From these first publications, SDME has been used an extraction technique for several
organic analytes [611]. The first publication of this methodology for inorganic species date
from 2003, and it is attributed to Chamsaz et al.[612] who used the headspace mode to
determine As(III) and total As by ETAAS. Since this pioneering application, the number of
applications for metal pre-concentration has significantly grown, and the potential of SDME
for pre-concentrating metals well-established [606, 613, 614].
For extracting trace elements by SDME variables affecting the formation of metal
complexes (chelating agent selection/concentration and pH of the complex formation and
extraction) must be taken into account. In addition, other variables such as solvent selection,
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 116
solvent drop size, extraction time and magnetic stirring must be optimized in SDME
procedures. In addition, the sample flow rate must be also considered when CFME is used.
(A) Solvent selection. Several solvents such as carbon tetrachloride, dichloromethane, 1,2
dichloroethane, chloroform, nitrobenzene, toluene, benzene, IBMK and DIBK can be
selected according to the polarity of metal complex formed.
(B) Drop size. Drop size shows an important influence on the extraction efficiency. It has
been found that the use of a large organic drop can result in an increase on the
analytical response of the instrument. However, larger drops are difficult to
manipulate and it is easy to dissolve or displace into aqueous solutions especially
suffering comparatively fast stirring and long extraction time. It was also found that
when the drop volume is larger than 4.0µL, the micro-drop may drop down from the
needle of the microsyringe.
(C) Extraction time. Extraction time is also an important factor because the mass transfer
is a time dependent process. Therefore, the reaction time for the transfer of metal
complex into the drop solvent increases the extraction efficiency up to reach the
equilibrium (around 15min) and leveling off at higher extraction time. Thus, SDME
will attain the maximum sensitivity after the equilibrium between aqueous and
organic phase has been achieved. However, large extraction times may result in drop
dissolution and can produce drop losses.
(D) Magnetic stirring. Magnetic stirring was used to allow the mass transfer process,
enhancing extraction and reducing the time required to reach thermodynamic
equilibrium. Magnetic stirring gives a dramatic increase on the analytical response of
the instrument, but high stirring rates (>400-500 rpm) usually lead to drop instability,
drop displacement or drop dissolution. These problems are attributed to the relatively
large vortex formed in the lower region of the drop, and the main evidence is a bad
repeatability for measurements.
(E) Sample flow rate. The flow rate of sample solution affect extraction dynamics
remarkably since the thickness of the interfacial layer surrounding the microdroplet
will vary with the change of flow rate. However, for high flow rate, although
extraction efficiency was better, air bubble formation frequently occurred, which lead
to quantification problems.
SDME is a simple, low-cost, fast and virtually solvent-free sample preparation technique
based on a high reduction of the extractant phase-to-sample volume ratio. However, SDME is
not very robust, and the droplets can be lost from the needle tip of the microsyringe during
extraction. More reliable methods are obtained when working for short times and at low
stirring rates. This increases precision of the SDME procedures, although decreases
sensitivity.
(b) Single-drop microextraction for cadmium pre-concentration
Although there are four modes of SDME for extracting/pre-concentrating inorganic
analytes (direct-SDME, CFME, HS-SDME and LLLME), only direct-SDME and CFME
modes have been applied to cadmium extraction from waters (see Table 16).
Analytical Chemistry of Cadmium: Sample Pre-treatment… 117
(A) Direct single-drop micro-extraction (Direct-SDME)
Direct-SDME is based on the exposure of a micro-drop of water-immiscible extractant
phase suspended from the tip of a microsyringe needle to a stirred aqueous sample. After
extracting for fixed time, the drop is retracted back into the microsyringe needle and finally
injected into the detector to obtain the corresponding analytical signal.
Because of two liquid phases are in direct contact in absence of any porous solid support
at the interface, when one of the phases is mechanically stirred (i.e. directly convected), the
other phase is also subjected to a convective mixing. This occurs because the first phase
transfers momentum to the second phase as a result of frictional drag at the liquid-liquid
interface. Mass transfer of the analytes from the aqueous sample to the extraction micro-drop
(by convective-diffusive mass transfer process) continues until thermodynamic equilibrium is
attained or extraction is stopped [610]. Direct-SDME requires the use of a water-immiscible
extractant phase and analytes more soluble in the extractant phase than in the sample solution.
The main drawback of Direct-SDME is the instability of the drop at high stirring rates or
temperatures, especially when samples are not perfectly clean. Moreover, solvents with
relatively high water solubility and low boiling points are not suitable for Direct- SDME
because of their high rate of dissolution or evaporation. Acidic samples, such as digests
obtained after acid digestion procedures or the presence of large non-polar species that can
saturate the organic phase could be troublesome when applying this micro-extraction mode
[606].
Four different Direct-SDME-ETAAS methods were proposed for the determination of Cd
in natural waters (Table 16). Dithizone was used in two of them as a complexing agent by
addition to the sample and subsequent extraction of the resultant cadmium dithizonate by a
micro-drop of chloroform [615] or toluene [616]. A method based on the formation of a
cadmium ion pair in a drop of nitrobenzene and ammonium tetraphenylborate after formation
of a cationic complex in the sample solution by addition of 2-(5-bromo-2- pyridylazo)-5-
diethylaminophenol (5-Br-PADAP) [617] was also developed. This last method involves an
automatic SI system coupled to ETAAS, in which Cd is on line chelated with ammonium
diethyldithiophosphate (DDPA) and it is on line extracted into a 60µL micro-drop of DIBK
[618]. The extraction procedure was performed into a newly designed flow-through extraction
cell coupled on a SI manifold. As the complex Cd(II)-DDPA flowed continuously around the
microdroplet, the analyte was extracting into the solvent micro-drop.
(B) Continuous-flow micro-extraction (CFME)
Continuous-flow micro-extraction is another mode of SDME, where the sample, instead
of being stirred, is pumped continuously at a constant flow rate and, when the extraction
chamber is full of sample, a drop is formed at the tip of a microsyringe needle. From this
time, sample is flowing through the chamber in contact with the drop. In contrast to the other
SDME micro-extraction modes, a solvent drop makes always contact with a fresh and flowing
sample solution.
In CFME the flow induces mass transfer in the drop via momentum transfer (the
extraction phase indirectly experiences convection as a consequence of convection of the
aqueous sample). The rate of extraction increases with increasing flow rate of the aqueous
solution, consistent with a decrease in thickness of the Nernst diffusion films [619]. This
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 118
mode facilitates mass transfer and high pre-concentration factors can be achieved. Sample
flow rate should ensure an effective micro-extraction of analytes without drop dislodgement
or bubble formation. Xia et al. [620] have reported a modification of this micro-extraction
mode, called cycle-flow micro-extraction, which the waste outlet of tubing was put into the
sample reservoir, and the sample volume was set as 1 mL. The extraction was performed in
the same way as continuous-flow microextraction, and after the extraction was finished, the
droplet was injected to ETV- ICPMS for further analysis.
Two different CFME-ETV-ICP-MS methods were proposed for the determination of Cd
in natural waters (Table 16). In the first, Cd is simultaneously chelated and extracted by
exposing an 8-HQ-chloroform drop to the aqueous sample [218]. In the second one, Cd is
chelated and extracted using a micro-drop of benzoylacetone (BZA) in benzene [620].
V.2.8. Dispersive liquid-liquid micro-extraction
(a) General aspects
Dispersive liquid–liquid micro-extraction (DLLME) is a micro-extraction technique
introduced by Rezaee et al. [621] in 2006. DLLME is based on the cloudy solution formed
when an appropriate mixture of an extraction solvent (i.e., a few microlitres of an organic
solvent with high density such as tetrachlorometane, chloroform, carbon disulphide,
nitrobenzene, bromobenzene, chlorobenzene or 1,2-dichlorobenzene) and a disperser solvent
(i.e., methanol, ethanol, acetonitrile or acetone) is quickly injected into the aqueous sample.
The water-immiscible extractant solvent should have higher density than water, while the
disperser solvent should be miscible in the extractant solvent and the aqueous sample.
When the mixture of extractant phase and disperser is rapidly injected into the sample, a
high turbulence is produced. This turbulent regimen gives rise to the formation of small
droplets, which are dispersed throughout the aqueous sample. Emulsified droplets have large
interfacial area. The nature of the emulsifier (disperser solvent) can also have an influence on
droplet size distribution, the mean droplet size, and also on emulsion viscosity [622]. Liquid–
liquid dispersions play an essential role in separation processes and reaction systems. This is
because the large interfacial area due to dispersion facilitates mass transfer and reaction rate
[623]. Turbidity is formed in the aqueous phase because the fine dispersion of the extractant
distributed throughout the aqueous sample, which is facilitated by the disperser solvent. After
formation of the cloudy solution, the surface area between extraction solvent and aqueous
sample is very large, so the equilibrium state is achieved quickly and the extraction time is
very short. After centrifuging the cloudy solution, the sedimented phase at the bottom of a
conical tube is recovered and analyzed by the most appropriate analytical technique.
As it has been commented for SDME, parameters, such as the nature of the chelating
agent, the concentration of chelating agent and the pH, play a unique role on metal–chelate
formation and subsequent extraction. On the other hand, several variables, such as the
extraction solvent and disperser types, the volumes for both extraction solvent and disperser,
the extraction time and the ionic strength, must be carefully optimized.
(A) Solvent extraction selection. Special attention deserves the selection of the extraction
solvent. This solvent must higher density than water, high extraction capabilities for
the target analytes, and low solubility in water. In addition, a stable cloudy solution
Analytical Chemistry of Cadmium: Sample Pre-treatment… 119
must be formed and it must be easily removed by the auto-sampler for injection into
the atomizer (ETAAS).
(B) Extraction solvent volume. Extraction solvent volume is an important factor becuase
the use of low volumes decreases the volume of the sedimented phase. This can offer
problems when removing the sedimented phase for injection into the atomizer
ETAAS. On the other hand, the enrichment factor decreases for high volumes of
solvent because the volume of the sedimented phase increases. Subsequently, high
enrichment factors can be obtained when using low volumes of the extraction
solvent.
(C) Disperser selection. The main criterion for selecting the disperser solvent (acetone,
acetonitrile, methanol or ethanol) is the miscibility of the disperser in the extraction
solvent and in the aqueous sample.
(D) Disperser volume. It is clear that by increasing the volume of methanol, the solubility
of complexes in water increases. Therefore, the extraction recovery decreases.
(E) Extraction time. Extraction time is defined as the time between the injection mixture
of disperser and extraction solvent and the starting to centrifuge. This variable is not
significant on the extraction efficiency. An explanation can be the infinitely large
surface area between extraction solvent and aqueous phase after formation of cloudy
solution. Thereby, complex formation/transfer from aqueous phase to extraction
solvent is fast. Subsequently, equilibrium state is achieved quickly therefore, the
extraction time is very short.
F) Ionic strength. Ionic strength has no significant effect on the enrichment factor. It is
maybe because of two opposite effects of salt addition; one of them is the increase on
the volume of sedimented phase that decreases the enrichment factor, and another is
the salting-out effect that increases the enrichment factor. Therefore, the enrichment
factor is nearly constant by changing this parameter.
As advantage, DLLME does not require a syringe as a drop holder during the extraction
process, and problems such as drop dislodgment are avoided. Nevertheless, this technique is
limited to a small number of extractants, which should efficiently extract the analytes and
which must have higher density than water, must form a stable cloudy solution and must be
easily removed. In addition, this micro-extraction technique appears to be difficult to
automate [606].
(b) Dispersive liquid-liquid micro-extraction for cadmium pre-concentration
Due to DLLME has been recently described, few methods for the determination of metals
have been developed [606]. In all cases, a complexing agent is added to the sample before the
mixture of the extractant and the disperser solvent. DLLME has been mainly used in
combination with ETAAS for quantitation. For instance, Au, Cd, Co, Ni and Pb have been
determined in different water samples by ETAAS after extraction of the corresponding
complex into a chlorinated organic solvent, generally tetrachlorometane. DLLME procedures
applied to Cd extraction [624,625] are summarized in Table 17.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 120
V.2.9. Hollow fibre liquid phase micro-extraction
(a) General aspects
In order to overcome drawbacks associated to SDME, Bjergaard and Rasmussen [626],
introduced an alternative concept for liquid phase micro-extraction (LPME) based on the use
of single, low-cost, disposable, porous, hollow fibers made of polypropylene. In this concept,
the analytes of interest are extracted from aqueous samples through a thin layer of organic
solvent immobilized within the pores of a porous hollow fiber, and into an acceptor solution
inside the lumen of the hollow fiber. Hollow fibre liquid-phase micro-extraction (HF-LPME)
is a technique which allows extraction and pre-concentration of analytes from complex
samples in both a simple and inexpensive way. Two reviews describe this appealing
technique in detail [627, 628].
In HF-LPME the organic phase is protected by the fiber, and it appears that the hollow
fiber decelerates the process of organic solvent dissolution into the bulk solution. Another
factor which contributes the improvement on sensitivity is that the surface area for the rod-
like configuration of the two-phase HF-LPME system is larger than the spherical surface
adopted by the drop-based SDME methods. The disposable nature of the hollow fiber totally
eliminates the possibility of sample cross-contamination and ensures precision. In addition,
the small pore size prevents large molecules and particles present in the donor solution from
entering the accepting phase and at the same time, most of macromolecules do not enter the
hollow fiber because they are not soluble in the organic phase present in the pores, thus
yielding very clean extracts.
Solvent impregnation of the fibre is essential since extraction occurs on the surface of the
immobilized solvent. The pores of a porous hydrophobic polymer membrane are filled with
an organic liquid, which is held by capillary forces. The extractant must have a polarity
matching that of the hollow fibre to be easily immobilized within the pores.
Two sampling modes are used in LPME: two phases and three phases.
(A) In the two phases LPME sampling mode (HF-LPME), the analyte is extracted from
an aqueous sample to a water-immiscible extractant immobilized in the pores of a
hydrophobic hollow fibre, typically made of polypropylene and supported by a
microsyringe. In this sampling mode, the acceptor phase is organic and the extraction
process occurs in the pores of the hollow fibre, where the solvent is immobilized.
(B) In the three-phases sampling mode, hollow fibre liquid– liquid–liquid micro-
extraction (HF-LLLME), the analyte is extracted from an aqueous sample through
the water-immiscible extractant immobilized in the pores of the hollow fibre and then
back-extracted into an aqueous phase inside the lumen of the hollow fibre. The
acceptor phase is aqueous in this micro-extraction mode. This operation mode is only
applied to analytes with ionisable functionalities.
Moreover the nature and concentration of the chelating agent and the pH, several factors,
such as the nature of the extraction solvent, the sample stirring speed and the extraction time,
must be optimized in LPME procedures. In addition, the stripping solution concentration
must be also considered in the three-phase sampling mode.
Table 16. Single-drop micro-extraction procedures for cadmium pre-concentration from natural waters.
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
After the extraction system has been filled with the sample solution
(at pH 6.5 by using potassium acid phthalate / potassium hydroxide), a
10µL micro-syringe was used to push out smoothly 4.0µL of 8-HQ
(0.1M)-chloroform to form drop above the PFA tubing outlet in the
extraction chamber and sample flows around the organic drop
continuously (at 0.05 mL min-1). After extracting for 15min, the
microdrop was retracted into the micro-syringe and was then injected
into the graphite furnace for further ETV-ICP-MS analysis
Tap water and
lake water 4.6 ng L-1 140 ETV-ICP-
MS [218]
A 10µL micro-syringe containing 3.0µL of H2DZ (0.01M)–chloroform
was clamped above the vial containing 5.0mL of sample at pH 8.3 (by
using ammonia) that was stirred at 400rpm. After extracting for 10min,
the micro-drop was retracted into the micro-syringe and injected into the
ETAAS.
Tap water,
river water
and rain water
0.7 ng L-1 - ETAAS [615]
A micro-syringe containing 4.0µL of toluene was clamped above the vial
containing 2.0mL of sample at pH 5.5 and H2DZ (1.2mM) that was
stirred at 400rpm. After extracting for 10min, the micro-drop was
retracted into the micro-syringe and injected into the ETAAS.
Tap water,
spring water,
river water,
pond water
and lake water
2.0 ng L-1 118 ETAAS [616]
A micro-syringe containing 4.0µL of ammonium tetraphenylborate (1.0
% w/v)- nitrobenzene was clamped above the vial containing 2.0mL of
sample at pH 6.0 and 0.5mL of 2-Br-PADAP (0.2% w/v) that was stirred
at 600rpm. After extracting for 15min, the micro-drop was retracted into
the micro-syringe and injected into the ETAAS.
Tap water and
seawater 6.5 ng L-1 390 ETAAS [617]
Sequential injection on-line SDME system where sample (at pH 2.0 and
at 1.5 mL min-1) is complexed (by using DDPA (1.2 % (w/v) at 0.3 mL
min-1) and on-line extracted into 60µL micro-drop of DIBK during
10min. Sampling frequency was 6 h-1.
Tap water,
river water
and seawater
10 ng L-1 10 ETAAS [618]
Table 16. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
After the extraction system has been filled with the sample solution, a
10µL micro-syringe was used to push out smoothly 4.0µL of BZA
(0.1M)-benzene to form drop above the PFA tubing outlet in the
extraction chamber and sample flows around the organic drop
continuously (at 0.2 mL min-1). After extracting for 10min, the micro-
drop was retracted into the micro-syringe and was then injected into the
graphite furnace for further ETV-ICP-MS analysis.
Synthetic
water 0.27 ng L-
1 180 ETV-ICP-
MS [620]
BZA, benzoylacetone; DDPA, ammonium diethyldithiophosphate; DIBK, , 2,6-dimethyl-heptan-4-one; ETAAS, electrothermal atomic absorption
spectrometry; ETV-ICP-MS, electrothermal vaporization-inductively coupled plasma-mass spectrometry; H2DZ, ditizone; 2-Br-PADAP, 2-(bromo-2-
pyridylazo)-5-diethylaminophenol; 8-HQ, 8-hydroxiquinoline.
Table 17. Dispersive liquid-liquid micro-extraction procedures for cadmium pre-concentration from natural waters.
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
500µL methanol containing 34µL carbon tetrachloride and 0.10mg APDC was
rapidly injected by syringe into the water sample (5.0mL) placed in a 10mL
screw cap glass test tube with conic bottom. After the cloudy solution
formation (complex extracted into the fine droplets of tetrachloromethane) and
centrifugation (5000rpm for 2.0min), droplets were sedimented at the bottom
of the test tube. Then a 20µL of sedimented phase was determined by ETAAS.
Tap water,
river water
and seawater
0.6 ng L-1 125 ETAAS [624]
500µL methanol containing 34µL carbon tetrachloride and 0.10mg
Salen(N,N'-bis(salicylidene)ethylenediamine) was rapidly injected by syringe
into the water sample (5.0mL). After the cloudy solution formation and
centrifugation (5000rpm for 2.0min), droplets were sedimented at the bottom
of the test tube. Then a 20µL of sedimented phase was determined by ETAAS.
Tap water,
rain water
and seawater
0.5 ng L-1 122 ETAAS [625]
APDC, ammonium pyrrolidin dithiocarbamate; ETAAS, electrothermal atomic absorption spectrometry.
*
Table 18. Hollow fibre liquid phase micro-extraction procedures for cadmium pre-concentration from natural waters.
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
1.5cm segments of the HF (4.0µL of internal volume) were sonicated in
acetone (for 2.0min) to remove contaminants then, acetone was
evaporated. 4.0µL CCl4 was drawn into the microsyringe. The needle tip
was inserted into the HF and the assembly was immersed in the organic
solvent (for 5.0s). Then, the organic solvent in the syringe was injected
completely into the HF. The HF was then removed from the organic
solvent and immediately immersed into the stirred aqueous sample (at pH
8.0) containing DDC (0.5 % w/v) during 15min at 1000rpm. After
extracting, the extract in the HF was retracted into syringe. The HF was
detached, discarded and the extract was injected into the graphite furnace
for analysis.
Lake
water and
river
water
4.5 ng L-1 29 ETV-ICP-MS [217]
4.5cm segments of the HF was immersed in 1-octanol solution containing
H2DZ (1.6 mg L-1), and then was taken out and washed outside and inside
with water. ~20 µL of HNO3 0.05M (stripping solution) was drawn into
the microsyringe. The needle tip was inserted into the HF and the
stripping solution in the syringe was injected to wash and fill the lumen
of the HF. Then, the two ends of the HF were sealed with heated
tweezers. The extraction was carried out by placing the HF in 100mL of
aqueous solution (at pH 5.2). After stirring for 30min, the HF was taken
out from the solution and one of the sealed ends was cut to allow its
connection with the microsyringe needle; the other sealed end was then
cut to permit the flushing of the stripping solution to a glass vial. The HF
was detached and discarded. The solution was directly analysed.
Seawater 0.8 ng L-1 387 ETAAS [629]
DDC, Diethyldithiocarbamate; ETAAS, Electrothermal atomic absorption spectrometry; ETV-ICP-MS, electrothermal vaporization-inductively coupled
plasma-mass spectrometry; HF, hollow fibre; H2DZ, ditizone.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 124
(A) Organic solvent selection. The final choice of the organic solvent must be based on
several considerations. Firstly, it should have a low solubility in water to prevent its
dissolution into the aqueous phase, and a low volatility, which will restrict solvent
evaporation during extraction. Secondly, it should be strongly immobilized in the
pores of the hollow fiber to prevent leakage. Finally, it should provide high
extraction recoveries.
(B) Sample stirring speed. Magnetic stirring enhances extraction and reduces the time
required to reach the thermodynamic equilibrium because stirring allows continuous
exposure of the extraction surface to fresh aqueous sample. In LPME, the acceptor
solution is confined within the fiber, and it can tolerate very high stirring speeds
(around 1000rpm). However, high stirring rates can cause bubbles which can be
attached on the surface of the hollow fiber and can decrease analyte transfer moreover
promote solvent evaporation. This leads to bad precision in the measurements.
(C) Extraction time. The extraction efficiency of HF-LPME is based on the analyte’s
partitioning between the aqueous sample and the organic solvent. This equilibrium is
attained only after exposing the acceptor solution to the sample for a ‘long’ period of
time for which solvent loss due to dissolution could occur.
(D) Stripping solution concentration. In the three-phase sampling mode (HF-LLLME) the
effect of the stripping solution concentration must be considered. In these cases,
strong acidic solutions are commonly used for stripping of the metal ions from
supported liquid membrane phase.
In general, the extraction efficiency achieved with HF-LPME is higher than with Direct-
SDME, because hydrophobic hollow fibres allow the use of vigorous stirring rates to
accelerate the extraction kinetics. Moreover, the contact area between the aqueous sample and
the extractant phase is higher than in the case of SDME, which invreases the mass transfer
rate. The use of the hollow fibre provides protection of the extractant phase and hence, the
analysis of dirty samples is feasible. The small pore size allows micro-filtration of the sample,
thus yielding very clean extracts. However, it should be noted that this technique suffers from
some drawbacks such as the manipulation of the hollow fibre at the time of placing it at the
tip of the needle of the microsyringe. This operation is carried out before the micro-extraction
process and it could be a source of contamination.
(b) Hollow fibre liquid phase micro-extraction for cadmium pre-concentration
Although HF-LPME has been widely applied to the determination of organic compounds,
especially in biological and environmental samples, the use of this technique for extracting
inorganic species is still scarce [606] HF-LPME [217] and HF-LLLME [629] procedures
applied to Cd extraction are summarized in Table 18.
V.2.10. Solid phase extraction
(a) General aspects
Solid phase extraction (SPE) is based on the partitioning of compounds between a liquid
phase (sample) and a solid phase (ion exchange resin, polymeric resin, activated carbon,
Analytical Chemistry of Cadmium: Sample Pre-treatment… 125
silica, etc.) whereby the intermolecular forces between the phases influences retention and
elution.
There are two SPE modes, the bath and column methods. The bath method consists of
mixing the sample, at a fixed pH, with few grams of the resin, and then, the misture is shaken
or stirred for several minutes to facilitate analyte adsorption onto the solid phase. After
filtration or centrifugation, desorption is achieved by stirring the solid phase with few
milliliters of a dilute acid. Metals are finally determined in this eluate solution. In the column
method, a PTFE or glass column or a syringe is packed with few milligrams of solid phase. A
small amount of glasswool, or a frit made of suitable materials, is placed at both ends to
prevent loss of the particles during sample loading. The use of commercial cartridges, disks or
SEP-PAK devices (various adsorbents within a plastic syringe) has been also used in this pre-
concentration mode. Before using, acid solutions and doubly distilled de-ionized water were
successively passed through the column in order to equilibrate, clean and neutralize it.
Sample, at the desired pH value, is then passed through the column at a fixed flow rate by
using a peristaltic pump. Afterwards, the metal ions retained on column were eluted with the
appropriate solvent and the analytes in the elute solution are determined.
Retention on the solid phases may involve non-polar, polar or ionic interactions and Van
der Waals forces, or hydrophobic interactions, which are not very strong. Other times, strong
interactions can be attained. In this case, the addition of chelating agents to the sample prior
to the extraction or the use of solid phases with immobilized chelating groups are required.
The wide range of sorbents available for trace metal SPE pre-concentration, including
synthetic (divinylbenzene polymers, zeolites, fullerenes and polyurethane foam with or
without chelating ligands, etc.) and natural materials (microbial cells, microoragnisms, dead
biomass, humic and fulvic acids, seaweed, clay, fly ash, moss, iron-oxide-coated sand,
modified wool and cotton, etc.), provides a wide range of interactions. The current researches
in SPE are mainly focused on the development of new sorbents such as nanometer-sized
materials, egg-shell membranes, modified silica beads, ion imprinted polymers, or
mesoporous materials. These new solid supports are mainly prepared to improve the
selectivity.
Optimization of the SPE process involves the study of several factors, such as: (1) the
choice of sorbent; (2) the volume of the sample flushed through the column (dynamic mode);
(3) the chelating agent (batch mode); (4) the mass of the resin and breakthrough volume of
the resin (volume of sample per gram of adsorbent resin which causes the analyte molecules
to migrate from the front of the adsorbent bed to the back of the adsorbent bed); (5) the
elution profile; (6) the sample pH; and, (7) the retention (adsorption) and elution flow rates.
The choice of sorbent is the key point in SPE because it can control parameters such as
selectivity, affinity and capacity. This choice depends strongly on the analyte and the
interactions of the chosen sorbent through the functional groups of the analyte. However, it
also depends on the kind of sample matrix and its interactions with both the sorbent and the
analyte.
Another important consideration is related to the washing and conditioning of the column
before the pre-concentration step. After sorbent material packing into the column or mico-
column (and plugged with a small portion of glass wool or frits at both ends), methanol,
dilute acids or high purity de-ionized water must be passed through the column in sequence to
clean the solid support. Then, the column must be conditioned to the desired pH by passing
trough the column the same buffer solution used for fix the pH of samples.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 126
Table 19. Composition, structure and functional groups of several supports
used for Cd pre-concentration by SPE.
Sorbent
Amberlite XAD Hydrophobic polyaromatic resin. Macroreticular styrene-
divinylbenzene copolymer.
Amberlite IT-120 Cation exchange resin registered trademark.
Amberlist® 36 Cation exchange resin registered trademark.
Capcell-NH2 Chelating resin. Silicone coated silica gel with alkyl amino groups.
Chelamine Chelating resin. Organic polymer containing pentamine ligand
(1,4,7,10,13, pentaazatridecane) immobilised.
Chelex-100 Cation exchange resin. Styrene-divinylbenzene resin containing
iminodiacetic acid groups (HN(CH2CO2H)2).
Cellex P Cation exchange resin. Cellulose sorbent with phosphonic acid
groups (R-PO(O-H+)2).
Chitosan modified
ordered mesoporous
silica
Natural polysaccharide containing hydroxyl and amino groups
immobilized on aminopropyl modified ordered mesoporous silica.
Chromosorb Hydrophilic cross-linked polystyrene resin. Styrene-
divinylbenzene copolymer.
Dowex 50Wx4 Strongly acidic cation exchange resin. Styrene-divinylbenzene
(gel) containing sulfonic acid functional groups (R-SO2O-H+).
Dowex Optipore V-493 Hydrophobic resin. Macroporous styrene-divinylbenzene
copolymer.
Polyorgs VII Chelating resin. Poltacrylonitrile fibrous matrix containing amide
and oxime complex forming groups.
SPE disk 3MTM
EmporeTM Cation exchange resin. Poly-styrene-divinylbenzene copolymer
containing iminodiacetic acid groups (HN(CH2CO2H)2).
Sepabeads Chelating resin. Polyvinyl polymer with alkyl amino groups.
Sephadex Strong anion exchange resin registered trademark.
SiO2-TPP Chelating resin. Aminopropyl silica gel containing porphyrin
ligand (carbonyl phenyl-10,15,20-triphrnyl porphyrin) covalently
attached.
TIOPAN-13 Chelating resin. Fibrous sorbent containing PAN with
mercaptobenzothiazole functional groups.
Toyopearl AF Chelate
650M Cation exchange resin. Macroporous methacrylate backbone
containing iminodiacetic acid groups (HN(CH2CO2H)2).
Zeolite A-4 Cation exchange resin. Highly crystalline aluminosilicate
framework comprising SiO2 and Al2O3 tetrahedral units.
SPE has gradually replaced classical LLE and it becomes the most common sample pre-
treatment technique in environmental areas. SPE overcomes the problems arising from the
use of organic solvents and offers several major advantages that include (1) higher
enrichment factors, (2) simple operation, (3) lower cost and less time, (4) improved
selectivity, specificity and repeatability, (5) safety with respect to hazardous reagents, (6) less
volume requirement and the use of environmentally friendly reagents, (7) absence of foaming
and emulsions, (8) the ability for coupling with different modern detection techniques in on-
line and off-line modes, and (10) ease of automation.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 127
However, SPE offers as main drawback the fact that organic substances presents in
natural waters must be removed previous to the pre-concentration step. Studies undertaken
during the last two decades have shown that organic complexation is important for a number
of trace metals in natural waters. An important fraction (30–99.9%) of metals, including
cadmium, are complexed by natural (humic acids, fulvic acids, glycollic acid, peptides,
proteins, amino acids, lipids, polysaccharides) and anthropogenic (ethylenediaminetetraacetic
acid (EDTA), nitrilotriacetate, phosphonates, citric acid, tartaric acid and surfactants) organic
ligands in aquatic environment. These complexes are not retained on chelating or polymeric
resins. Thus, it is necessary to release the trace metal from the metal–organic complex prior to
the pre-concentration step.
(b) Solid phase extraction for cadmium pre-concentration
The characteristics (composition, structure and functional groups) of the different solid
supports used for cadmium SPE pre-concentration are summarized in Table 19.There
different requirements of solid phase materials to be used as solid-phase extraction
adsorbents. Firstly, these supports must allow the extraction of several metals in a wide pH
range, and they must offer a high sorption capacity, selectivity and enrichment factor. In
addition, a fast and quantitative adsorption of the target metals and easy desorption of the
adsorbed target ions is desirable. Finally, the solid supports should have a long life time so
that repeated adsorption/desorption cycles could be carried out with the same sorbent.
Different substances (Table 20) have been proposed and applied as solid-phase extraction
sorbents for cadmium.
(b.1.) Synthetic sorbents
(A) Ion exchage resins. Cation exchange resins such as Chelex-100 [466,469, 630-637
ethylcellulose with iminodiacetic groups [638], Cellex-P [635, 637,639], Dowex-50 W
[485,635, 637,640], Zeolite [641], Amberlite IT-120 [455], Amberlist-36 [642],Toyopearl
AF-Chelate 650M [543,643] and 3MTM EmporeTM SPE disk [325], have been extensively
used as solid phase extractors for cadmium (and other metals).
(B) Polymeric resins. Polymeric resins such as activated carbon (AC), silica gel (SG),
Amberlite XAD (AXAD), cellulose, Chromosorb, polyurethane foam, silica, etc., have been
also proposed for cadmium pre-concentration. There are two ways for trace metals pre-
concentration by using these solid supports. In the first method, chelates of trace metals must
be formed by the addition to the sample of a specific chelating reagent, and then the sample
solution is passed through the column filled with the resin (column mode) or the sample is
stirred with the resin (bath mode) to retain the chelates. In the second method, chelating
agents are firstly immobilized on the solid phase (chelating resins) and then, the sample
ispassed through the column containing this modified solid phase, or sample is stirred with
the chelating resin.
Therefore, Amberlite XAD (AXAD) [644-648 Sephadex [647,649], nitrocellulose
membrane [650], C-18 [113,215,651-657 co-crystallized NAP adsorbent [658-662
Chromosorb [663-666 Dowex Optipore [499, 667] or polyurethane foam (PUF) [112], have
been used for cadmium sorption on the solid phases previous chelating forming step. The
chelating agents used in these applications were O,O-diethyldithiophosphate (DDTP)
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 128
[112,215], 1-nitroso-2-naphtol (NN) [646], hexamethyleneammonium
hexamethylenedithiocarbamate (HMA-HMDTC) [648], 1-(2-pyridylazo)-2-naphthol (PAN)
[650,662 ,666], or 4-(2-pyridylazo)resorcinol (PAR) [651,649], bis (2-
hydroxyethyl)dithiocarbamate (HEDC) [652], 8-hydroxiquinoline (8-HQ) [653,657], tetra(m-
aminophenyl)porphyrin (Tm-APP) [654], ammonium pyrrolidin dithiocarbamate (APDC)
[113,656,663,665], 2-nitroso-1-naphthol-4-sulfonic acid (Nitroso-S) and
tetradecyldimethylbenzylammonium (TDBA) [658], H2DZ [659], 1-benzylpipera-
zinedithiocarbamate (1-BPzDC) [660], and 1-(2-thiazolylazo)-2-naphthol (TAN) [661].
(C) Chelating resins. The use of chelating resins is especially appealing bcause the high
selectivity offered. Therefore, the use of chelating resins such as Polyorgs VII [668, 669, 670,
671], Polyorgs YIIM [672], TIOPAN-13 [673], SiO2-TPP [637], and Chelamine [654,674],
and sorbents with functional chelating groups introduced into it structure, have extensively
been used for trace metal pre-concentration.
Chelating resins can be obtained after synthesis of the support containing the chelating
groups, although other times, the chelating groups can be bound to conventional materials. In
this last case, the complexing reagents can be chemically introduced into the conventional
supports (functionalized sorbents), or they can be physical bounded to the sorbent by
impregnating the solid matrix with a solution containing specific molecules (loaded sorbents).
Physical binding is the simplest method to be used in practice. However, the chemical binding
approach allows elevated lifetime for the resin due to covalent bonds between the ligand and
the support. This property avoids the possible flush of the ligand molecule from the column
during sample percolation or elution steps. Several complexing agents have been loaded or
immobilized on a variety of solid matrices and they have successfully been used for the pre-
concentration and determination of cadmium. Some examples include Zn-
piperazinedithiocarbamate (ZnPDC) [675], APDC [676], and DDC [677] which can be
loaded or impregnated on AC support. Simmilarly, H2DZ [678] can be loaded on Dowex, 2-
acetylmercaptophenyldiazoaminoazobenzene (AMPDAA) [679], while 2(2-thiazolylazo)-5-
dimethylaminophenol (TAM) [680] and 2,4-dinitrophenyldiazoaminoazobenzene (DNDAA)
[681] have been loaded on AXAD. In addition, a wide range of new sorbents for trace
cadmium extraction have been developed by immobilizing several chelating groups on silica
or ion-exchange resins. Agents such as 8-quinol [682], DDC [683], HMA-HMDTC [683], N-
propylsalicylaldimine (IE11) [684], calcon [685], thioacetamide (TAA) [686], ofloxacin
[687], and tris(2-aminoethyl) amine (TREN) [688] have been immobilized on silica; while
applications of AXAD impregnated with thiosalicylic acid (TSA) [689], PAN [690],
aminothiophenol (AT) [691] or 2,2’-dithiobisaniline[692] have been also reported. Finally, 8-
quinol [693] has been also impregnated on Capcell-NH2 and Sepabeads supports.
Applications of the impregnation of 2-(6’-methyl-2’-benzothiazolylazo)chromotropic
acid (Me-BTANC) on PUF [694], thiourea on chlorosulfonated polymers [79] or
tricaprylmethylammonium chloride (Aliquat 336) on polyvinylidene difluoride (PVDF) films
[316,326] or Aliquat 336 physically included in cellulose triacetate – 2-nitrophenyl octyl
ether (CTA-NPOE) membranes [317], have been also reported.
Chemically modified silica gel (SG) is one of the most successful adsorbents (Table 20),
because the SG supports does not swell or shrink like the other polymeric resins. The
modified SG may be employed for aqueous and organic solvents medium, moreover offer
good thermal stability and appropriate accessibility of ions to the adsorbent groups. In
addition, the modified SG exhibits higher sorption capacities than other polymeric resins
Analytical Chemistry of Cadmium: Sample Pre-treatment… 129
because the number of organic molecules immobilized on the support surface is larger.
However, silica sorbents are unstable in a broader pH range.
(b.2.) Natural/biological sorbents
(A) Microorganisms. Living or non-living microorganisms can accumulate metals from
aqueous solutions by a different chemical and biological mechanism without pre-
concentrating the matrix. This ability is attributed to stable complexes between metals and
any functional groups present in the cell walls of the microorganisms. In addition, the
microbial cell products such as metabolites (polysaccharides) are effective in metal
accumulation. Thus, the use of microorganisms for biosorption of trace metals has been
proposed. In this technique, microorganisms are immobilized on a solid support instead of the
chelating agent. Microorganisms have been used as freely suspended cells or as loaded cells
on a support. The use of loaded cell systems on a support has many advantages over the use
of freely suspended cells. These include better capability of reusing the biomass, better
precision for recovery, easy separation of cells from the reaction mixture, high biomass
loadings, and minimal plugging in continuous flow systems. For cadmium pre-concentration,
freeze-dried Stichococcus bacillaris [695] has been used as suspended cells. In addition,
immobilization of Saccharomyces cerevisiae [696], Aspergillus niger [697], and
Saccharomyces carlsbergensis [698] on materials such as sepiolite [696] and SG [697,698]
has been used to design solid phase extractors.
(B) Vermicompost. Vermicompost is a humic substance recently characterized which has also
been applied to cadmium pre-concentration by SPE [699]. This material offers a high cationic
exchange capacity (CEC) for different species due to the presence of carbonyl groups and
phenolic and alcoholic hydroxyls, among others functional groups, in its structure. This very stable
and complex product is formed when earthworms (Lumbricus rubellus, mainly) metabolise
organic residues from different sources [699]. Fulvic and humic acids are present in vermicompost
are responsible for the high CEC. In this application, humic substances (vermicompost and
purified humic acid), are packed in small columns which can be coupled to FAAS.
The use of natural sorbents for metals has many advantages over the other sorbents.
Some of them are the higher recoveries obtained and the simplicity of the procedure,
economical advantages and environmental protection. However, these sorbents offer poor
selectivity due to the predominance of electrostatic forces among sites (placed on their
surfaces) and analytes. In this sense, unspecific retentions are usually observed. In addition,
these sorbents present low uniformity composition and, obviously, this aspect can damage the
precision and accuracy of the results.
(b.3.) New adsorbents
Conventional sorbents used in typical SPE procedures for both organic compounds and
trace elements pre-concentration can be affected from interfering concomitants from the
matrix which can be co-extracted with the target analytes. Therefore, more selective systems
for separation are required and the development or synthesis of new sorbents has increased in
recent years.
High selectivity can be expected from sorbents such as inmunosorbents (ISs) and
molecularly imprinted polymers (MIPs), although MIPs are advantageous over ISs because the
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 130
fast and less expensive synthesis and the high degree of molecular recognition [700]. Carbon
nanotubes, nanometer-sized materials, mesoporous materials, biomaterials, C60–C70 and their
derivatives have been extensively explored as new adsorbent in SPE technique. Some of then,
have been applied to cadmium pre-concentration and will be commented in this section.
(A) Carbon nanotubes. Since carbon nanotubes (CNTs) were first prepared in 1991 [701],
they have become appealing materials for their novel structure characteristics and unique
properties. Novel mechanical and electronic properties, a large specific surface area and a
high thermal stability indicate the tremendous potential of CNTs for engineering applications
such as hydrogen storage, field emission, quantum nanowires, catalyst supports, chemical
sensors and packing material for gas chromatography [702]. CNTs can be visualized as a
graphite sheet rolled up into a cylinder, and they are classified into single-walled carbon
nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs) according to the carbon
atom layers in the wall of the nanotubes. The hexagonal arrays of carbon atoms in graphite
sheets of CNTs surface have a strong interaction with other molecules or atoms, which make
CNTs a promising adsorbent material substituted for activated carbon in many ways. All of
the facts mentioned above reveal that CNTs may have great analytical potential as a solid
phase extraction adsorbent for metal ions.703, 704 This novel sorbent has been also applied
for cadmium pre-concentration from natural water using a micro-column packed with
MWNTs [703, 705], and also MWNTs packed cartridges [704].
(B) Nanoparticles. Nanometer-sized materials have attracted substantial interest in the
scientific community because of their special properties [706]. The size range of
nanoparticles is from 1.0 to almost 100nm, which falls between the classical fields of
chemistry and solid-state physics. The relatively large surface area and highly active surface
sites of nanoparticles enable them to have a wide range of potential applications, including
shape-selective catalysis, chromatographic separations, enzyme encapsulation, DNA
transfection, drug delivery and sorption of metal ions.
One of nanoparticle’s properties is that most of the atoms are on the surface. The surface
atoms are unsaturated and therefore can bind with other atoms that possess highly chemical
activity. Consequently, the nanometer material can adsorb metal ions with high capacity.
Investigations of the surface chemistry of highly dispersed oxides, e.g. TiO2, Al2O3, ZrO2, CeO2
and ZnO, show that these materials have a very high sorptive capacity and give promising
results for pre-concentrating trace metals from different water samples and solutions of high-
purity alkali salts. The sorption properties of many oxides, including nanometer TiO2, mainly
depend on the characteristics of the solid, e.g. crystal structure, morphology, defects, specific
surface area, hydroxyl coverage, surface impurities, and modifiers. Due to these nanometer-
sized metal oxides are not target-selective, the change of the characteristics of the solid by using
a suitable coating has been proven to be one of the most efficient ways to improve the
selectivity. Thus, TiO2 nanoparticles immobilized on DDC were synthesized and used as an
SPE adsorbent for separating and pre-concentrating cadmium and other metals from natural and
biological samples [707]. Simmilarly, nanometer Al2O3 packed micro-columns coupled to ICP-
MS were described for simultaneous determination of Cd and V, Cr, Mn, Co, Ni, Cu, Zn and Pb
in environmental liquid samples [708].
(C) Ordered mesoporus silica. Mesoporous materials have been quickly developed during
the last decade and they have attracted much attention in various scientific areas of physics,
chemistry and material science. These materials offer performances, such as large surface
Analytical Chemistry of Cadmium: Sample Pre-treatment… 131
area, high pore porosity, well-defined pore size and an ordered pore arrangement, excellent
mechanical resistance properties, none swelling, good chemical stability and well modified
surface properties [709]. Therefore, ordered mesoporous silica has become an ideal adsorbing
material for SPE procedures, although for trace metals pre-concentration modified
mesoporous silica is preferred. Modified mesoporous adsorbents are synthesized in the
presence of surfactants, which used as template and can be later functionalized with different
organic groups. The development of functionalized mesoporous materials for adsorption
applications has generated a considerable interest. Among the variety of adsorption
applications, the preparation of highly effective adsorbents for heavy metal ions trapped by
the grafting or by incorporatind the ligands into mesoporous materials is clearly one of the
most promising methods for environmental clean-up. In particular, materials whose surfaces
have been functionalized with groups containing sulfur and nitrogen as active donor atoms
offer high selectivity. For this purpose, chitosan (TS), a natural polysaccharide, can react with
silica through the–NH2 functional group found on its surface, and it could be used as modifier
for mesoporous silica, support which pre-concentrate cadmium and other trace metals [710].
(D) Ionic imprinted polymers. Addressing molecurlarly imprinted polymerization in the
presence of ions as templates, instead of molecules, new supports for recognizing ions (ionic
imprinted polymers, IIPs) can be synthesized. These polymers offer all the benefits derived
from MIPs and a high capacity for recognizing ions. Moreover bulk polymerization, different
approaches for synthesizing MIPs and IIPs (suspension, emulsion, dispersion and
precipitation methods) have been proposed by several authors. Detailed information on such
methodologies can be found elsewhere [711].
In constrat to MIP for which the organic compound target (template) can react with
different vynilated monomers (4-vinylpyridine, 4-VP, or 2-(diethylamino) ethyl methacrylate,
DEM), when using ions as templates stable ion-vynilated monomer complexes are not
formed. Therefore, only certain complexing agents which can react with the ion and which
can polymerize through vynil groups can be used. As it has recently be reviewed by Rao et al.
[712,713], bifunctional reagents, showing vynil groups and functional groups to interact with
the dissolved ions, can be used following three main approaches: (i) linear chain polymers
carrying metal-binding groups being cross-linked with a bifunctional reagent; (ii) chemical
immobilization by preparation of binary complexes of metal ions with ligands having vinyl
groups, isolation and then polymerization with matrix-forming monomers; and (iii) surface
imprinting conducted on an aqueous-organic interface. Polymerization under these conditions
leads to complexing ligands to be chemically immobilized in the polymeric matrix. However,
the main drawback of these approaches is that vynilated ligands, which act as a complexing
reagent for the target ion and as a monomer for polymerizing, are not commercially available
and they must be previously synthesized.
Additionally, a different approach consisting of trapping a non-vinylated chelating ligand
via imprinting of binary/ternary mixed ligand complexes of metal ions with non-vinylated
chelating agent and vinylated ligand is also possible. In this case, the use of special
synthesized vinylated ligands is avoided. After polymerization, the non-vinylated ligand is
not chemically bonded to the polymer chains, but instead it is trapped inside the polymeric
matrix.
For cadmium, IIPs synthesized by using the dual-ligand reagent (2Z)-N,N'-bis(2-
aminoethylic)but-2-enediamide has been applied for the SPE of cadmium from river water,
lake water and tap water (Table 20) [714]. By mean of the trapping approach, the non-
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 132
vynilated ligand diazoaminobenzene (DAAB) in the presence of 4-VP as a monomer, has
been used for synthesizing IIPs for cadmium pre-concentration from river and tap waters and
acid digests from river sediments (Table 22) [715]. Similarly, the synthesis and evaluation for
cadmium retention of a hierarchically imprinted sol-gel sorbent based on the use of
cetyltrimethyl ammonium bromide (CTAB) as a non-vynilated reagent has carried out using
cadmium aqueous standards. The synthesized imprinted sorbent is promising as SPE support
for cadmium pre-concentration [716].
As it was commented, SPE can lead to low recoveries when water samples containing
high naturally organic matter are treated [543]. In these cases, a previuos stage to release the
trace metal from the metal–organic is needed. For this task, several procedures have been
proposed. Humic substances removal from tap water can be performed by passing the sample
at a pH lower than 2.0 through an AXAD-7 or Bio Beads SM-7 support [645]. Other times,
organic matter can be oxidized by using sulphuric acid combined with ammonium or
potassium peroxodisulphate and heat [673,684]. The organic matter digestion can also be
assisted or not with hydrogen peroxide or UV-photolysis [543,679,681,715]. The use of UV
digestion is highly recommended becuase the addition oxidant reagents can be minimized.
Furthermore, UV digestion is effective and can be readily incorporated in flow injection
manifolds, allowing stand-alone trace metal análisis [543].
Finally, as it was summarized in Table 20, after pre-concentration of target analyte onto
the solid phase, metals are eluted by using an appropriate solvent (usually dilute acid
solutions) in which the target metal is directly determined (in some cases an evaporation step
can be use). However, other applications lead to the direct determination of metals adsorbed
on the solid phase (without elution stage). In these cases, the support material is directly
sampled (slurry sampling or solid sampling techniques) into the detector.
V.2.10.1. Flow injection on-line pre-concentration systems by SPE
As commented in preceding sections, it is evident that the coupling of flow injection on-
line solid phase extraction techniques provides an improvement on the detection limits and
precision of measurements, and it reduces the interference from the matrix sample. In
comparison with the off-line batch modes, these systems offer a number of significant
advantages for trace determination, such as higher efficiency, lower consumption of sample
and reagents, possibility of working in a closed system with a significant reduction of
airborne contamination, and increased sampling frequency.
On-line column pre-concentration systems are based on the retention of the analytes in
micro-column packed with adsorbents that determines the sensitivity and the selectivity of the
analytical method. Since the first paper by Olsen et al. [717] on FI sorbent extraction by using
an ion exchanger (Chelex-100) as column packing material for Cd, Cu, Pb, and Zn pre-
concentration and determination by FAAS, this promising technique has been extensively
applied using several packing materials for the pre-concentration of several trace metals from
environmental and biological samples. For cadmium, methods based on FI on-line pre-
concentration with micro-columns packed with C18 [113,215], Chelex-100 [636], Dowex-50
[640], AXAD [680,691], PUF [112,694], MWNTs [705], ordered mesoporous silica and
Toyopearl AF-Chelate 650M [543,643] have been proposed. The FI systems have been
coupled to both spectrometric and electroanalytical detectors.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 133
FI sorbent extraction consists in two steps. In the first stage, the sample is aspirated by a
pump (A) through a valve (load position) into the column and then to waste (this allow the
sample volume taken to be adjusted according to the analyte concentration). In the second step,
the valve is switched to another position (elute position) and a pump (B) propels the eluting
solvent through the column. Analytes are rapidly released from the column and led to the
detector. Detailed description of FI manifold components has been shown in section IV.11.
V.2.11. Electrochemical solid phase micro-extraction
Electrochemical solid phase micro-extraction (SPME) combines the use of
electrochemistry or electronically conducting polymers with SPME. In this technique, the
analyte extraction/desorption on a conducting polymer coated on an electrode is controlled
simply by changing the potential of an electrode. The main advantage of using conducting
polymers in SPME is that the charge of the coatings can readily be controlled by oxidation
and reduction of the polymers. These potential controlled ion exchangers can therefore be
used to extract ions. Moreover, it should be possible to obtain pre-concentration of ions
present in small sample volumes since electrochemically controlled desorption can be carried
out in small volumes of solutions (~ 100µL). By electrodeposition of a layer of conducting
polymer, the surface area of the electrode can be increased to provide sufficiently high ion
exchange capacities for the use of such electrodes in miniaturized analytical systems. The
properties of the polymer (e.g. the conductivity, pore size and the mechanical characteristics)
can be changed by using different counter ions, surfactants or different solvents during the
electropolymerisation step. If an anion with a high mobility in the polymer film is
incorporated during the electropolymerisation, the film becomes an anion exchanger.
Polymers doped with anions of low mobility, such as large organic anions, operate as cation
exchangers. As a result of the dependence of the polymer morphology on the size of the
dopant ions and interactions between irreversibly incorporated dopants and ions, it is also
possible to control the selectivity in the micro-extraction process. The extraction of Cd using
electrochemical SPME has so far only been described by Liljegren et al. [718] using a
polypyrrole-coated electrode. These authors also determined Co and Zn, as well as chloride,
nitrite, bromide, nitrate, sulfate and phosphate in natural waters.
V.2.12. Low temperature directed crystallization
Low-temperature directed crystallization (LTDC) is based on the crystallization of the
aqueous sample during mixing so that impurities of different nature (organic, inorganic,
particles) are displaced by the crystallization front into the liquid phase. On completion of the
crystallization process, the top of the ice ingot is separated and analyzed by conventional
instrumentation. The mineral composition of water, the chemical forms of elements and the
crystallization conditions offer a great influence on the process. This procedure has also been
applied to Cd pre-concentration from lake waters [590].
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 134
V.3. Sample Pre-treatments for Solid Environmental Samples
As it was commented in previous sections, sample pre-treatments for elemental analysis
in solid materials can be minimized or even omitted when using spectrometric methods based
on laser ablation or electrothermal vaporization coupled to plasma techniques [206-208].
However, these analytical methods can only be applied to limited kinds of samples [719] and
therefore, sample pre-treatment for spectrometric analysis is an active research field in
analytical chemistry. This is especially important after the establishment of the Twelve
Principles of Green Chemistry [720] and the application of such fundamentals to analytical
procedures, which has led to the concept of GAC [449]. Therefore, aspects involving none or
simple sample preparation, or little or no sample destruction, have been also introduced as
principles of GAC. By this way, the sample pre-treatment stage has received special attention
to make it greener [719,721,722] and several sample pre-treatments have been developed and
applied to biological and environmental samples as green and alternative methods to the
conventional acid digestion procedures. These procedures include direct approaches such as
those based on the slurry sampling or solid sampling techniques, lixiviation or acid leaching
procedures and enzymatic hydrolysis [723].
V.3.1. Decomposition sample pre-treatments
Acid digestion procedures are the most used methodologies as sample pre-treatment for
the determination of major and trace elements in biological and environmental materials.
These procedures require the use of mineral/oxidizing acids and an external heat source to
decompose the sample matrix.
The nature of the sample matrix determines the type of acid and also its individual use or
a certain combination of them. Therefore, hydrochloric acid (boiling point at 110°C) can be
used to decompose matrices containing salts of carbonates and phosphates, as well as some
oxides and sulfides. In addition to the strong acidity provided by hydrochloric acid, this
reagent provides complexing properties which facilitates the dissolution of transition
elements. The oxidizing properties of sulfuric acid (boiling point at 338°C) make this acid as
useful reagent for dissolving inorganic matrices such as ores, alloys, and most of oxides and
hydroxides. Nitric acid (boiling point of 122°C) is commonly used to dissolve samples
containing high organic matter contents, such as biological and clinical samples, and biota.
This acid also dissolves carbonates, phosphates, oxides and sulfides that are not well attacked
by hydrochloric acid. Perchloric acid (boiling point of 203°C) is also a strong oxidizing agent
for organic matter, but it must be used with caution because the violent reaction. In such
cases, samples are firstly treated with nitric acid before using perchloric acid. Hydrofluoric
acid (boiling point at 112°C) is the unique agent capable of dissolving matrices containing
silica (geological materials, soils, sediments, atmospheric particulate matter) by forming
SiF62–. This acid is very dangerous (its use is forbidden in some laboratories) and it must be
handled with extreme caution. In addition to the individual use of these mineral acids or
oxidizing agents, certain combinations of acid are useful. The most common used are sulfuric
acid/nitric acid and nitric acid/hydrochloric acid. When using the first mixture (5:20 sulfuric
acid:nitric acid) the nitric acid attacks most of the organic matter but it is not hot enough
(122°C as boiling point) to destroys the last traces. As it boils off during the digestion
process, only sulfuric acid remains at the end of the digestion and it attacks the remaining
Analytical Chemistry of Cadmium: Sample Pre-treatment… 135
organic matter. The nitric acid/hydrochloric acid mixture, at the proportion 1:3, called as aqua
regia, is also quite popular. Nitric acid provides an oxidizing media while hydrochloric acid
acts as a complexing agent and allows the dissolution of noble metals (gold, palladium and
platinum) and many alloys. Sometimes, the addition of bromine or hydrogen peroxide to
mineral acid increases their solvent action, especially for attacking samples with high organic
matter contents.
Instead of mineral acids, certain alkaline solvents, such as tetramethylammonium
hydroxide (TMAH or TMAOH), a quaternary ammonium salt with the molecular formula
(CH3)4NOH, have been used to solubilize samples with high organic matter contents. The use
of this reagent does not totally decompose the sample, and a centrifugation step is needed to
obtain the aqueous digest. TMAH is mainly used when extracting elements which can be lost
in oxidizing medium such as iodine, although it has also be proposed for extracting other
elements [163,174].
In addition to the mineral acid, the other parameter to be controlled in an acid digestion is
the heat source to be applied and the vessels containing the oxidizing mixture and the sample.
The type (material) of the vessels is dependent on the type of heat source and also the nature
of acid. Therefore; PTFE, exhibiting a melting point around 320°C, must not be used when
using sulfuric acid (boiling point of 338°C); or glass must not be used when using
hydrofluoric acid because this material is attacked by this acid. In addition, the source of
heating commonly requires certain properties for the reaction vessels. For example, materials
transparent to microwave radiation, such as PTFE or quartz, must be selected when using
microwave energy to assist the sample decomposition. Either acid decomposition or TMAH
solubilization can be carried out in open glass (sometimes PTFE or quartz) vessels. However,
the use of open vessels offers as main drawbacks the risk of sample contamination and
analyte losses, especially for volatile elements, such as cadmium, which can form volatile
halides or oxyhalides or even hydrides.
Concerning the heat sources, conventional sample decomposition can be carried out by
using hot plate or multi-sampler digestor devices. These heating systems allow the
simultaneous treatment of several samples at once and they can result convenient for several
applications. However, sample pre-treatments involving such heating systems (conductive
heating) lead to time consuming procedures (typically from 1 to 2 hours, but it be much
longer). This long time is because the poor heat conductivity properties of vessels which
require a certain time to be heated and then to transfer the heating energy to the sample-acid
mixture. In addition, the vaporization at the surface of the liquid generates a thermal gradient,
which makes that a small portion of the sample-acid mixture is at the programmed
temperature. Therefore, it can be assumed that most of the acid is above its boiling point. The
poor heat transfer is the responsible of the high volumes of mineral acids required to complete
the digestion. In addition, another drawback associated with these systems is the special
safety considerations because the vapor generation during the digestion process as
consequence of the sample destruction.
V.3.1.1. Microwave assisted sample decomposition
The use of microwave energy for assisting sample preparation procedures is a current
practice in analytical chemistry [724,725]. Regarding elemental analysis, since the first
reported application by Abu-Samra et al. [726] to acid digestion of biological materials,
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 136
microwave assisted acid digestion is now a well-established methodology for treating
biological, environmental and geological samples and it is by far the chosen sample pre-
treatment for routine analysis [727,728].
(a) Microwave energy
Microwave energy is a no ionizing radiation exhibiting frequencies within the 300 –
300000 MHz (wavelengths from 0.005 to 1 m) that does not cause changes in the molecular
structure. When microwave interacts with a material, the energy can be absorbed at a rate
dependent upon de dissipation factor (tan δ)729, parameter defined as a ratio of the sample’s
dielectric loss (ε’’) to its dielectric constant (ε’) as
tan δ = ε’’/ ε’
The dielectric constant (ε’) can be considered as a measure of the material to minimize
the penetration of microwave energy, while the dielectric loss (ε’’) offers an opposite effect
and it is though as a measure of material’s ability to dissipate microwave energy as heat.
Materials which exhibit low dissipation factors, such as quartz (0.6) or PTFE (1.5), can be
considered as transparent to microwave radiation, and they are commonly known as
insulators. However, materials showing high dissipation factors, such as water (1570), offer
absorptive properties to microwave radiation and they are referred as dielectric materials. A
third type of materials, called as conductors, offer reflective properties to the microwave
energy. This is the case of most of metals which reflect microwaves.
The rapid heating achieved by microwave irradiation (microwave energy lost) is caused
by two different mechanisms; the molecular motion by migration of ions (dissolved salts in
the solvent) and rotation of dipoles (molecular solvents). Migration of ions, also referred as
ionic conduction, is the conductive migration of dissolved ions in the applied electromagnetic
field of microwave radiation. Because of the resistance of the solution to this ionic movement,
energy dissipation as heat is produced. Rotation of dipoles is a consequence of the alignment
of molecules (permanent or induced dipole moments) in the electric field supplied by
microwaves. As the electric field increases, it aligns the polarized molecules; however, the
thermally induced disorder is restored when the electric field decreases or disappears and
thermal energy is released. Both mechanisms occur simultaneously and the dominance of one
or another process is highly dependent on the temperature. Therefore, for solvents such as
water, the dielectric loss to a sample due to the dipole rotation decreases as the temperature
increases. In contrast, dielectric loss due to ionic migration increases as sample temperature
increases.
(b) Microwave instrumentation
In general, there are two types of commercially available microwave ovens for laboratory
uses, i.e., an open-focused and closed-vessel type. Open-focused microwave ovens can be
used to treat simultaneously up six samples, and vessels are commonly large test tubes made
of glass or quartz. In order to prevent loss of volatiles, a condenser is fitted to the test tube. In
these systems the microwave radiation is focused to the vessel containing the sample and the
acid, and a controlled and precise heating is obtained.
Table 20. Solid-phase extraction procedures for cadmium pre-concentration from natural waters.
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
180mL of sample at pH 2.5 (10mL of 0.1M citric acid plus 0.1M HCl/0.1M
NaOH, as buffer solution) was mixed with 200mg of chlorosulfonated
polymer functionalized with thiourea. The mixture is stirred for 30min
and after centrifugation, Cd is eluted with 3mL of 1.5M HCl.
Tap water, lake
water, mineral
water
0.021 ng mL-1 20 STAT-FAAS [79]
An injection loop is filled with 2.0mL of sample (pH 2.0 with 10
%(v/v) HCl), and then sample is displaced by a solution containing
0.5%(m/v) DDTP at a flow rate of 2.4 mL min-1. The Cd-DDPT
complex that formed is adsorbed onto 20mg of PUF into a mini-column
for 2.0min. Elution is performed with an etanol/water (80/20) mixture
pumped at a flow rate of 1.0 mL min-1.
Lake water and
mineral water 0.62 µg L-1 5.2 TS-FFAAS [112]
Sample (volume of 5 to 20 mL) at pH 2.0 was pumped to C18 column
functionalized with APDC at flow rate of 15.0 mL min-1. Elution was
carried out by pumping 500 µL of a solution methanol/water (60/40)
containing 0.1 %(m/v) APDC at a flow rate of 0.9 mL min-1.
Well water 0.03 to 0.007
µg L-1 36 to 107 TS-FFAAS [113]
Sample is pumped at a flow rate of 2.3 mL min–1 for 60s and it is on-
line mixed with a solution of 0.6 %(m/v) DDTP (1.1mL). The complex
is then adsorbed onto C18 mini-column previously rinsed with DDPT.
Before elution, 0.5%(v/v) HNO3 is pumped to remove samples solution
retained in the interstitial volume, and the mini-column is washed with
acif¡dified DDPT solution. Elution is performed with 25µL of methanol.
Seawater 0.0002 µg L-1 45 ETV-ICP-MS [215]
190mL of sample acidified with HCl (final HCl concentration of 2.0M)
was contacted with the membrane (PVDF, average pore-size of 0.2mm,
porosity of 75%, and average thickness of 125mm) activated with Aliquat
336 under continuous stirring (800rpm) for 150min. At the end of the
preconcentration, the loaded membranes were rinsed with deionized
water and allowed to dry at room temperature. Finally, the cadmium-
loaded membranes were directly placed into the sample holder device
for XRF analysis
Seawater 0.075 µg L-1 -- HE-P-EDXRF [316]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
190mL of sample acidified with HCl (final HCl concentration of 2.0M)
was contacted with the membrane CTA-NPOE with physical inclusion
of Aliquat 336 under continuous stirring (800rpm) for 150min. At the
end of the preconcentration, the loaded membranes were rinsed with
deionized water and allowed to dry at room temperature. Finally, the
cadmium-loaded membranes were directly placed into the sample
holder device for XRF analysis.
Seawater 0.71 µg L-1 -- HE-P-EDXRF [317]
1000mL of sample at pH 6.0 was passed through a chelating SPE disk
3MTM EmporeTM (cation exchange). Elution with HNO3. Tap water 3.8 ng mL-1 - XRF [325]
Same procedure than reference 316 Seawater 0.17 mg L-1 -- WDXRF [326]
50mL of sample was stirred (5.0min) with 0.5g of Amberlite IT-120.
After drying at 100°C, the resin was grinded and direct analysed. Seawater - - ETAAS [455]
Sample at pH 5.0 was passed through the 50mL syringe filled with
Chelex-100 at 7.0 mL min-1. 1.8 mL of HNO3 (2.0M) was used for
elution step.
Lake water 0.2 ng L-1 25 ICP-MS [469]
Sample at pH 7.0 was passed through the column filled with Dowex
1x2 loaded with 8-HQ-5-sulfonic acid. HNO3 (7.0M) was used for
elution step.
Natural water - - ETAAS [485]
50mL of sample at pH 8.0 (ammonium acetate buffer) containing
0.25mL of HEDC (0.5M) solution was passed at 8.0-10 mL min-1
through the column filled with 1.0g of Dowex Optipore V-493. After
elution with 5.0mL of HNO3 (1.0M) in methanol, the eluate was evapo-
rated to near dryness and made up to 1.0-2.0 mL of HNO3 (1.0 %).
Seawater - 25 ETAAS [499]
Sample was on-line preconcentrated at 1.0 mL min-1 into a Toyopearl
AF-Chelate 650M mini-column using a flow injection system. After
elution with HNO3 at 0.5 mL min-1, eluate was transported directly into
the nebulizer.
Seawater - - ICP-MS [543]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Sample was lowered at a constant rate from the zone of a circular
resistance heater to a cooling thermostat chamber (−15°C) filled with
liquid antifreeze and cooled by means of a compression-type
refrigerator. During this process sample was mixed by reverse rotation
of the vessel about its vertical axis (the period of rotation in one
direction and the duration of a stop being 3 and 1s, respectively). On
completion of crystallization, 1.5mL of hot water was poured onto the
upper part of the ingot where the impurity was located.
Lake water - - ICP-OES [590]
50mL of sample containing 1.0mL of ammonium acetate buffer was
passed through column filled with 5.9mL of Chelex-100 slurry at 1.0-
2.0 mL min-1. 5.0 mL of HNO3 (5.0M) was used for elution step.
Seawater 2.0 ng L-1 10 ETAAS [630]
250mL of sample at pH 4.0 was stirred for 30min with 0.05g of
Chelex-100. After resin filtration and suspended in 5.0mL of water, Cd
was directly analysed in the suspension.
River water 0.1 µg L-1 - ETAAS [632]
500mL of sample at pH 5.4 or 8.0 (20mL of ammonium acetate buffer)
was passed at 4.0 mL min-1 through the column filled with 0.4g of Chelex-
100. Elution with 4 x 2.0 ml of HNO3 2.0M and dilution to 10mL.
Seawater 1.0 ng L-1 50 FAAS [633]
Sample at pH 7.0 was on-line preconcentrated at into a Chelex-100
mini-column using a flow injection system. After elution with HNO3,
the eluate was transported directly into the nebulizer.
Seawater 17 ng L-1 - FAAS [636]
50mL of sample at pH 5.0-6.0 (acetate buffer) was passed at 4.0 mL
min-1 through 0.2g of ethylcellulose with iminodiacetic groups in a
polyethylene funnel at 7.0-8.0 mL min-1. Elution with 2 x 2.5 mL of
HNO3 1.0M and dilution to 10mL.
Tap water 1.0 µg L-1 10 ETAAS [638]
2000mL of sample at pH 7.1 was passed at 8.0 mL min-1 through a
column filled with 2.0g of Chellex-P. 25mL of HNO3 (1.0M) was used
for elution step.
Tap water - 80 ETAAS [639]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
100ml of sample at pH 6.0 was on-line preconcentrated at 1.5 mL min-1
into a column filled with 0.08g Dowex 50W by using a flow cell
connected to a FIA. HCl (3.0M) at 1.5 mL min-1 was used for elution.
Tap water 0.3 µg - EDXRF [640]
800mL of sample at pH 6.0 (diluted sodium hydroxide or nitric acid)
was stirred with 100mg of Zeolite A-4. After filtration (membrane
filter), resin was dissolved in 2.0mL of HNO3 (2.0M).
Natural water
and waste water 20 µg mL-3 400
ETAAS [641]
Sample was passed through the column filled with Amberlyst 36.
5.0mL of HCl (3.0M) at 1.0 mL min-1 was used for elution step. Tap water 0.51 µg L-1 200 FAAS [642]
3.0mL of sample at pH 5.6 was on-line preconcentrated at 1.0 mL min-1
into a column filled with Toyopearl AF-Chelate 650M resin. HNO3
(1.5M) at 1.0 mL min-1 was used for elution.
Estuarine water 1.4 ng L-1 - ICP-MS [643]
100mL of sample at pH 8.0 (dilute HCl or ammonia) was passed at 1.0
mL min-1 through the column filled with 0.5g of AXAD-7. 5.0mL of
HCl (1.0M) at 1.0 mL min-1 was used for elution step.
Lake water and
seawater - - ETAAS [644]
100mL of sample at pH 8.0 (dilute ammonia) was passed at 1.0 mL
min-1 through the column filled with AXAD-7. 5.0mL of HNO3 (1.0 %)
was used for elution step.
Tap water - - ETAAS [645]
500mL of sample at pH 6.0 containing 10mL of NN (10mM) was
passed through 500mg of AXAD-4column at 5.0 mL min-1. After
elution with 7.0-8.0 mL HNO3 (1.0M), eluate was evaporate near to
dryness (hot plate at 35°C) and then dilute to 5.0mL with HNO3 (1.0M)
Tap water 16 ng L-1 - ETAAS [646]
100mL of sample at pH 5.0 (dilute HCl 0.1M) was passed at 2.0 mL
min-1 through a column filled with 1.5g of In-treated AXAD-2. After
column disassembled, desorption (3.0mL of HNO3 (0.5M), ultrasound
(1.0min)), filtration on Nucleopore filter (0.4µm), evaporation to
dryness and dissolution in 1.0mL of HNO3 (0.5M) were carried out.
River water - 100 ETAAS [647]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
100mL of sample at pH 5.0 (dilute HCl0.1 M) was passed at 2.0 mL
min-1 through a DEAE-Sephadex A-25 column. After column
disassembled, desorption (3.0mL of HNO3 (4.0M), ultrasound
(1.0min)), filtration on Nucleopore filter (0.4µm), evaporation to
dryness and dissolution in 1.0mL of HNO3 (0.5M), were carried out.
River water - 100 ETAAS [647]
Sample at pH 9.0 (NH3/NH4Cl buffer) containing HMA-HMDTC was
stirred with AXAD-16. Seawater - 150 FAAS [648]
Sample at pH 8.0 containing PAR (50-fold excess) was passed through
50mg of C-18 column at 3.0 mL min-1. After elution with 3.0mL HNO3
(1.0M) at 0.1 mL min-1, eluate was directly determined.
Natural water 30 µg L-1 20 FAAS [651]
600ml of sample at pH 8.5 (maleic acid/ammonium hydroxide)
containing HEDC (1.2mM) was passed through C-18 cartridge at 10
mL min-1. After elution with 3.0mL HNO3 (1.0M) in methanol at 10
mL min-1, eluate was evaporated to near dryness and then dilute to
1.0mL with HNO3 (0.1M).
Seawater - 600 ICP-MS [652]
500mL of sample containing 0.5mL of 8-HQ (0.5M) was passed
through a column filled with 1.0g of C18 at 4.0mL min-1. 10mL HNO3
(2.0M), was used for elution step.
Seawater - 50 ICP-OES [653]
100-1000 µL of sample at pH 6.0-8.0 containing 40µL APDC (5.0 %
m/v) was passed through a mini-column filled 5mg of C-18 at 0.1 mL
min-1. 20µL of methanol was used for elution.
Seawater 0.178 ng L-1 5-50 ETAAS [656]
100mL of sample at pH 8.0 containing 8-HQ (5.0 10–4 M) was passed
through C-18 cartridges at 6.0 mL min-1. 2.5mL of nitric acid (2.0M)
was used for elution at a flow rate of 4.5 mL min-1.
Seawater 0.04 µg L-1 40 ICP-OES [657]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
150mL of sample at pH 10 (2.0mL of ammonia/ammonium acetate)
containing 2.0mL of nitroso-S (0.1 % w/v) and 2.0mL of TBDA was
treated with 2.0mL of naphthalene in acetone (20 %) under continuous
shaking. After filtration, the solid was shaken with 9.0mL of HCl
(1.0M) and 1.0mL KCl (1.0M) and directly measured.
Tap water 90 ng mL-1 15 DPASV [658]
200mL of sample at pH 8.5 was mechanical stirred (15min) with 1.5g
of H2DZ co-crystalline with NAP. After filtration with paper filter,
solid was washed with 15mL of HNO3 (8.0M).
Saline water 5.0 µg L-1 13 ICP-OES [659]
Sample containing 1-BPzDC was extracted with microcrystalline NAP.
After eluting with KCN solution, Cd was directly determined. Natural water 34 µg L-1 50 FAAS [660]
Sample at pH 7.0-10.5 containing TAN-TPB was stirred with NAP
adsorbent. After filtration, the solid mass was shaken with 8.0mL of
HCl (2.0M) and 2.0mL of NaBH4 (4.0%).
Natural water 66 ng mL-1 - DPP [661]
Sample at pH 8.2 -11 was passed through a PAN co-crystalline with
NAP packed column. After eluting with 9.0mL of HCl (1.0M), Cd was
directly determined.
Natural water 70 µg L-1 - DPP [662]
600mL of sample pH 6.0 containing 3.0mL of APDC (0.05 % w/v) was
passed through a mini-column filled with 500mg of Chromosorb-102 at
2.0 mL min-1. After elution wit 10mL of acetone, elute was evaporated
to near dryness and dilute to 2.0-10 mL with HNO3 (1.0M).
Tap water and
seawater 0.10 µg L-1 - FAAS [663]
250mL of sample pH 9.0-9.5 (NH3/NH4Cl buffer) was passed through a
column filled with 500mg of Chromosorb-102 at 5.0 mL min-1. After
elution with10mL HNO3 (1.0M), the eluate was evaporate to dryness
(hot plate at 30-40°C) and then dissolved in 2.0mL of HNO3 (1.0M).
Tap water and
seawater 0.8 µg L-1 - FAAS [664]
5 x 5 mL of sample at pH 6.0 (HCl 0.1M or NaOH 0.1M) treated with
1.0mL APDC (0.1 % w/v) was drawn at 20 mL min-1 into a syringe
filled with 0.3g Chromosorb-107. Elution was carried out with 2.5mL
HNO3 (3.0M) at 20 mL min-1.
Seawater 1.2 µg L-1 10 ETAAS [665]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
500mL of sample at pH 9.0 (ammonia/ammonium chloride buffer)
containing 3.0mL of PAN (0.01M) solution was passed at 5.0 mL min-1
through the column filled with 600mg of Chromosorb-106. After
elution with 8.0-10 mL of HNO3 (1.0M) in acetone at 5.0 mL min-1, the
eluate was evaporated to near dryness and made up to 2.0mL with
HNO3 (1.0M).
Tap water, river
water, mineral
water and
seawater
0.19 µg L-1 - FAAS [666]
250mL of sample at pH 2.0 containing 5.0mL of DBDTC (0.01M) was
passed at 4.0 mL min-1 through the column filled with 600mg of
Dowex Optipore V-493. After elution with 5.0-10 mL of HNO3 (1.0M)
at 4.0 mL min-1, eluate was evaporated to 0.5-1.0 mL and dilute to
2.5mL with HNO3 (1.0M).
Tap water, river
water, mineral
water and
seawater
0.43 µg L-1 100 FAAS [667]
1.0L of sample at pH 7.0 (sodium citrate 0.7 mM and HCl) was stirred
for 1.0h with 0.5g of Polyorgs VII M sorbent. After filtration, Cd was
eluted with 8.0-9.0 mL of HNO3 (1.0M).
Mineral water
and spring water
- - ETAAS [668]
0.5L of sample at pH 7.0 (sodium citrate 0.7 mM and HCl) was stirred
for 1.0h with 0.2g of Polyorgs VII M sorbent. After resin filtration and
suspended in 5.0-10 mL of water, Cd was directly analysed in the
suspension.
River water,
groundwater
and seawater
1.0 ng mL-1 - ETAAS [669]
10mL of sample at pH 6.0-8.0 was stirred ultrasonically for 2.0min
with 20mg of Polyorgs VII M sorbent. After centrifugation and suspend-
ded in 1.0mL of water, Cd was directly analysed in the suspension.
Seawater - - ETAAS [670]
10-20 mL of sample at pH 6.0-8.0 was ultrasonically stirred (5.0min)
with 0.02g of Polyorgs YIIM. After centrifugation (6000rpm) and rinse
with 10mL of water, the solid mass, as slurry, was directly analysed.
Seawater 10 ng L-1 - ETAAS [672]
500mL of sample at pH 7.0 (ammonia) was stirred for 45min with 0.5g
of TIOPAN-13 sorbent. After elution with 2 x 5 mL of HNO3 (1.0M),
the combined extract was evaporated to 5.0mL and analysed.
River water 8.0 ng L-1 - ETAAS [673]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
400mL of sample at pH 8.0 (Tris 1.0M and ammonia 2.0M) was passed
through the Chelamine column at 0.3 mL min-1. 4.0mL of HNO3
(2.0M) was used for elution.
Seawater 2.3 ng L-1 100 ETAAS [674]
1000mL of sample at pH 4.5-5.0 (20mL of acetate buffer) was shaken
with 0.5g of AC loaded with ZnPDC for 10min. After filtration, the solid
phase was treated with 2.0-5.0 mL of Hg(II) (0.1M) and stirred 10min.
Tap water and
seawater 11.8 ng mL-1 - FAAS [675]
50mL of sample at pH 5.0 was passed at 1.0 mL min-1 through the
column filled with 500mg of APDC-AC. HNO3 (1.0M) in acetone at
0.6 mL min-1 was used for elution step.
Tap water and
pom water 20 ng L-1 - FAAS [676]
Sample at pH 4.5 was passed through the column filled with AC
impregnated with DDC. After elution at 1.5 mL min-1 with 10mL of
HNO3 (1.0M) in acetone, the eluate was direct analyzed.
Natural water - - FAAS [677]
Sample was on-line preconcentrated at 1.1 mL min-1 into a Dowex 1 X
10 column loaded with dithizone using a flow injection system. After
elution with NaOH (1.0mM) at 1.0 mL min-1, eluate was measured at
544nm. Sampling frequency of 11.4 h-1.
Surface water,
well water and
tap water
5.4 µg L-1 6.5 UV-Vis [678]
800mL of sample at pH 7.0 was passed at 2.0 mL min-1 through the
column filled with 500mg of AXAD-4 modified with AMPDAA.
8.0mL of HCl (1.0M) – NaCl (1.0M) and 2.0mL of water was used for
elution step.
Tap water and
river water 28 ng L-1 - FAAS [679]
Sample was on-line pre-concentrated at 3.0 mL min-1 into a AXAD-2
column loaded with TAM using a flow injection system. After elution
with HCl (1.0M) at 1.0 mL min-1, eluate was transported directly into
the nebulizer.
Fresh water with
low saline conc. 22 ng L-1 548 FAAS [680]
1000mL of sample at pH 8.0 was passed at 5.0 mL min-1 through the
mini-column filled with 200mg of AXAD-2 loaded with DNDAA.
After elution with 10mL of HCl (0.1M), the eluate was direct analyzed.
Tap water and
river water 62 ng L-1 100 FAAS [681]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
50mL of sample at pH 5.0 (ammonium hydroxide) was passed at 1.0
mL min-1 through a micro-column packed with 80mg of 8-quinol
immobilized on SG. 1.7mL of HCl (2.0M)-HNO3 (0.1M) was used for
elution step.
Seawater 42 ng L-1 - ETAAS [682]
20mL of sample at pH 5.0 (acetate buffer) was passed through a SG
modified with DDC or HMA-HMDTC column at 1.0 mL min-1. 2.0mL
IBMK at 1.0 mL min-1 was used for elution.
River water 0.04 µg L-1 - ETAAS [683]
Sample at pH 9.0 was stirred during 30min with 20mg of porous SiO2
modified with IE11. HNO3 (2.0M) was used for elution. River water, tap
water and
seawater
- - FAAS [684]
Sample at pH 5.0-10 was passed at 2.0 mL min-1 through the column
filled with 1.0g of TAA immobilized on SG. 10.0mL of HCl/HNO3
(0.1/0.5 M) at 2.0 mL min-1 was used for elution step.
Tap water and
river water 0.96 µg L-1 200 FAAS [686]
Sample was passed through a column filled with of functionalised SG-
ofloxacin. After elution with HCl (0.5M), the eluate was directly
measured.
River water and
seawater 0.29 ng mL-1 100 ICP-OES [687]
500mL of sample at pH 8.0 was passed at 4.0 mL min-1 through the
column filled with 300mg of SG-TREN. After elution with 5.0ml of
HCl (0.1M), the eluate was direct analyzed.
River water 0.14 ng mL-1 100 ICP-OES [688]
1000mL of sample pH 4.0-4.5 was passed through a column filled with
1.0g of TSA-AXAD-2 at 2.0–5.0 mL min-1. 5.0mL of HNO3 (2.0M) at
1.5-2.5 mL min-1 was used for elution.
Tap water and
river water 0.48 µg L-1 200 FAAS [689]
1000mL of sample at pH 9.0 containing 5.0mL of PAN (0.75mM) was
passed at 5.0-10 mL min-1 through a column filled with 0.5g AXAD-4.
After elution with 8.0-10 mL of HNO3 (1.0M) in acetone, eluate was
evaporated to near dryness and dissolved in 5.0mL of HNO3 (1.0M).
River water, tap
water and lake
water
0.16 µg L-1 200 FAAS [690]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Sample at pH 8.0 (borate buffer) was on-line preconcentrated at 5.0 mL
min-1 into a AXAD-2 mini-column functionalized with AT using a flow
injection system. After elution with HCl (0.5M) at 5.0 mL min-1, eluate
was transported directly into the nebulizer. Sampling frequency of 51 h-
1.
River water,
well water, tap
water and
mineral water
0.89 µg L-1 28 FAAS [691]
Sample at pH 5.0 was passed through a 2,2’-dithiobisaniline modified
AXAD-2 column. After elution with HCl (2.0M), eluate was
transported directly into the nebulizer.
Tap water and
river water 0.1 µg L-1 80-200 FAAS [692]
Sample at pH 7.5 (borate buffer) was on-line pre-concentrated at 7.0
mL min-1 into a PUF column loaded with Me-BTANC using a flow
injection system. After elution with HCl (1.0M) at 5.0 mL min-1, eluate
was transported directly into the nebulizer. Sampling frequency of 19 h-1.
River water, tap
water and well
water
0.8 µg L-1 37 FAAS [694]
100mL of sample at pH 7.0 (sodium citrate 0.7mM and HCl) was
mixed with 10mg of freeze-dried Stichococcus bacillaris. After
centrifugation and suspended in 1.0mL of water, Cd was directly
analysed in the suspension.
River water and
seawater - - ETAAS[ [695]
100mL of sample at pH 7.5 (dilute HCl or ammonia) was passed at 3.0
mL min-1 through the column filled with 0.2g of sepiolite immobilized
with Saccharomyces cerevisiae. 10mL of HCl (1.0M) was used for
elution step.
River water and
seawater 44 ng mL-1 75 FAAS [696]
500mL of sample at pH 6.0 (dilute HCl or ammonia) was passed at 3.0
mL min-1 through the column filled with 0.3g of SG 60 loaded with
Aspergillus niger. 10mL of HCl (1.0M) at 1.0 mL min-1 was used for
elution step.
Tap water and
lake water 1.4 ng mL-1 50 FAAS [697]
Sample was passed through a column filled with SG-60 loaded with
Saccharomyces carlsbergensis. HCl was used for elution step. Dam water, lake
water and tap
water
1.48 ng mL-1 705 FAAS [698]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
Sample was on-line pre-concentrated at 4.0 mL min-1 into a mini-
column packed with humic acids or vermicompost using a flow
injection system. After elution with HNO3 (1.0M) at 4.0 mL min-1,
eluate was transported directly into the nebulizer.
Mineral water
and tap water 0.8 µg L-1
(0.4 µg L-1)a 27 (46)a FAAS [699]
50mL of sample at pH 8.0 was passed at 2.0 mL min-1 through a
column filled with 30mg MWCNT. 2.0mL of HNO3 (0.5M) was used
for elution step.
Lake water 48 ng L-1 25 ICP-OES [703]
200ml of sample at pH 9.0 was passed through a 0.2g MWCNT packed
cartridge. 10ml of HNO3 (1.0M) at 1.5 ml min-1 was used for elution. Tap water,
reservoir and
stream water
- - ETAAS [704]
Sample at pH 4.9 was on-line preconcentrated at 5.0 ml min-1 into a
microcolumn filled with 30mg of MWNTs. HNO3 (0.5M) was used for
elution. Sampling frequency of 20 h-1.
Mineral water,
tap water and
river water
11.4 ng L-1 51 TS-FFAAS [705]
Sample at pH 5.0 was passed through column filled with DDC-TiO2
nanoparticles. After elution, eluate was transported directly into the
nebulizer.
Natural water 0.52 ng mL-1 - ICP-OES [707]
Sample at pH 9.0 (ammonia 0.01M) was flow injection on-line
preconcentrated at 2.0 ml min-1 into a column filled with Al2O3
nanoparticles. 0.4ml HNO3 (1.0M) at was used for elution. Sampling
frequency of 15 h−1.
Lake water 79 ng L-1 5 ICP-MS [708]
6.0ml of sample at pH 6.5 was on-line preconcentrated at 2.0 ml min-1
into a PTFE micro-column filled with 50mg of chitosan modified
ordered mesoporous silica. 0.3ml of HCl (1.0M) was used for elution.
Sampling frequency of 10 h-1.
Lake water and
river water 50 ng L-1 20 ICP-OES [710]
100mL of sample adjusted at pH 6.0 was passed through the column (a
glass tube of 100mm in length and 10mm in diameter) containing
300mg of IIP at a flow rate of 3 mL min-1. Elution was performed with
5mL of 0.2M HCl (or HNO3) at a flow rate of 1 mL min-1.
River water,
lake water, tap
water
0.14 µg L–1 20 ICP-OES [714]
Table 20. (Continued)
Brief description Sample LOD Concentration
factor Analytical
technique Ref.
100mL of sample adjusted at pH 5.0 was passed through the column (a
glass tube of 100mm in length and 10mm in diameter) containing
500mg of IIP at a flow rate of 0.5 mL min-1. Elution was performed
with 2mL of 0.2M HCl at a flow rate of 0.5 mL min-1.
River water, tap
water, river
sediment
0.093 µg L–1 50 FAAS [715]
25mL of sample was stirred at constant potential (20.5V) for 1000s for
extraction of Cd on polypyrrole-coated electrode. After the washing
step, desorption was made into a vial containing 200µL of HNO3
(0.01M) by scanning the potential to +0.8V, at a scan rate of 100 mV s-
1. The latter potential was maintained for 2min.
Natural water - 125 ICP-MS [718]
a value in brackets corresponds to vermicompost
AC, activated carbon; AMPDAA, 2-acetylmercaptophenyldiazoaminoazobenzene; APDC, ammonium pyrrolidin dithiocarbamate; AT, aminothiophenol;
AXAD, Amberlite XAD; CTA, cellulose triacetate; CZE, capillary zone electrophoresis; DBDTC, dibenzyldithiocarbamate; DDC, diethyldithiocarbamate;
DDTP, O,O-diethyldithiophosphate; DNDAA, 2,4-dinitrophenyldiazoaminoazobenzene; DPP, Differential pulse polarography; EDXRF, energy dispersive
x-ray fluorescence; ETAAS, electrothermal atomic absorption spectrometry; FAAS, flame atomic absorption spectrometry; FIA, flow injection analysis;
HEDC; bis (2-hydroxyethyl)dithiocarbamate; HE-P-EDXRF, high-energy polarized- energy dispersive x-ray fluorescence; HMA-HMDTC,
hexamethyleneammonium hexamethylenedithiocarbamate; IBMK, 4-methyl-2-pentanone; ICP-MS, inductively coupled plasma-mass spectrometry; ICP-
OES, inductively coupled plasma-optical emission spectrometry; IE11, N-propylsalicylaldimine; Me-BTANC, 2-(6’-methyl-2’-
benzothiazolylazo)chromotropic acid; MWCNT, multi-walled carbon nanotubes; NAP, microcrystalline naphthalene; NN, 1-nitroso-2-naphtho; Nitroso-S,
2-nitroso-1-naphthol-4-sulfonic acid; NPOE, 2-nitrophenyl octyl ether; PAN, 1-(2-pyridylazo)-2-naphthol; PAR, 4-(2-pyridylazo)resorcinol; PUF,
polyurethane foam; PVDF, polyvinylidene difluoride; SG, functionalized silica gel; STAT-FAAS, slotted tube atom trap - flame atomic absorption
spectrometry; TAA, thioacetamide; TAM, 2(2-thiazolylazo)-5-dimethylaminophenol; TAN, 1-(2-thiazolylazo)-2-naphthol, TDBA,
tetradecyldimethylbenzylammonium; THF, tetrahydroforan; Tm-APP, tetra(m-aminophenyl)porphyrin; TPB, tetraphenylborate; TREN, tris(2-aminoethyl)
amine; TSA, thiosalicylic acid; TS-FFAAS, thermospray flame furnace atomic absorption spectrometry; UV-Vis, ultraviolet- visible; WDXRF, wavelength
dispersive x-ray fluorescence spectrometry; XRF, x-ray fluorescence spectrometry; ZnPDC, Zn-piperazinedithiocarbamate; 1-BPzDC, 1-
benzylpiperazinedithiocarbamate; 8-HQ, 8-hydroxiquinoline.
Table 21 Selected applications of acid digestion procedures.
Sample Processor Acid composition Analytical
technique Ref.
Mollusks / fish Microwave-assisted HNO3/H2O2 ETAAS [149]
Mollusks Microwave-assisted HNO3/H2O2 STAT-FAAS [72]
Mollusks Heating block HNO3/H2O2 ICP-MS [733]
Mollusks Microwave-assisted HNO3 ETAAS [734]
Mollusks Heating block 2-stage digestion with HNO3 and H2O2 ICP-OES [735]
Mollusks Microwave-assisted HNO3/H2O2/HF ICP-MS [736]
Mollusks / fish /
plants / sediments Microwave-assisted HNO3/H2O2
ICP-OES [737]
Mollusks / fish Microwave-assisted HNO3/H2O2 ICP-MS [738]
Lake fish
lake sediment Microwave-assisted HNO3/H2O2
HNO3/HCl ETAAS [155]
Fish Ultrasounds-assisted
Heating block HNO3/H2O2
HNO3/H2O2 ETAAS [730]
Fish Ultrasounds-assisted
Heating block HNO3/H2O2
HNO3/H2O2 ETAAS [731]
Fish Microwave-assisted HNO3/H2O2 ETAAS [739]
Fish Heating at 100C for 4 h + reflux for 2 h
Heating at 100C for 4 h + reflux for 2 h
Heating at room temperature overnight, 100C
for 4 h + reflux for 2 h
Heating at 100C for 4 h + reflux for 2
Heating at 50C for 2 h + heating 70C for 2 h
HNO3 (1.0M) + HCl
HNO3 + HCl
HNO3 + HCl
HNO3 + HClO4
H2O2 + HCl
ASV [740]
Antarctic krill Microwave-assisted HNO3/H2O2
HNO3/HF ETAAS [151]
Plants / soils Heating block HNO3/HF STAT-FAAS [83]
Table 21. (Continued)
Sample Processor Acid composition Analytical
technique Ref.
Plants Dry ashing / microwave-assisted HNO3/H2O2 STAT-FAAS [84]
Plants Pressurized digestion system HNO3 TSFF-AAS [117]
Plants Microwave-assisted HNO3/H2O2 TSFF-AAS [119]
Plants Microwave-assisted HNO3/H2O2 TSFF-AAS [120]
Plants Heating block HNO3 ETAAS [142]
Plants Heating block HNO3/H2O2 ETAAS [143]
Plants Soil/sediment/
coal fly ashes Microwave-assisted HNO3/H2O2
HNO3/HCl/HF ETAAS [176]
Plants / lichens /
sediments Microwave-assisted performed at
atmospheric pressure by a multi-samples
rotor and screw-capped disposable
polystyrene line
----- ICP-MS [741]
Plants Microwave-assisted HNO3/H2O2
HNO3/H2O2/HF/H3BO3
HNO3/H2O2/HBF4
ICP-MS [742]
Lichens Oven closed vessel acid digestion
Hot plate open vessel acid digestion
Microwave-assisted
HNO3/H2O2/HF
2-stage HNO3/H2O2 and HF
HNO3/H2O2/HF
ICP-OES [743]
Lichens Digestion, room temperature overnight
Microwave-assisted HNO3/HClO4
HNO3/HClO4 + HF + H3BO3
ICP-MS [744]
Soil / sediment Microwave-assisted HNO3/HCl/HF ETAAS [146]
Soil / sediment Microwave-assisted HNO3/HCl/HF ETAAS [167]
Soil Microwave-assisted HNO3/HCl/HF IDA-ICP-MS [745]
Soil Microwave-assisted
DigiPrep system HCl/HNO3
HCl/HNO3 ICP-MS [746]
Table 21. (Continued)
Sample Processor Acid composition Analytical
technique Ref.
Soil Microwave-assisted
Microwave-assisted
Microwave-assisted
Microwave-assisted
HNO3/HCl/HF/H2O2
HNO3/HCl/H2O2
HNO3/HCl/HF
HNO3/HCl
ICP-OES [747]
Soil Microwave-asisted HCl/HNO3 ICP-OES [748]
Soil Microwave-assisted +
Heating block (HF removal) HNO3/HCl/HF
H2SO4 ICP-MS [749]
Soil Microwave-assisted HF/HCl/HNO3 ICP-MS [750]
Soil / dust Microwave-assisted HNO3/HCl ETAAS [751]
Sediments Heating block HNO3/HClO4/HF TSFF-AAS [114]
Sediment / fly ash Microwave-assisted +
Heating block (HF removal) HNO3/HCl/HF
H2SO4 ETAAS [169]
Soil / sediment Microwave-assisted HNO3/HCl/HF ETAAS [170]
Fish Sediment Microwave-assisted HNO3
HNO3/HCl/HF ETAAS [172]
Fish /mollusk
Sediment Microwave-assisted HNO3
HNO3/HCl/HF ETAAS [173]
Sediment Ultrasound-assisted
Microwave-assisted HNO3/H2O2/HF
HNO3/H2O2/HF ICP-MS [732]
Sediment Open-focused microwave-assisted HCl/HNO3 ICP-OES [752]
Sediment Microwave-assisted
Heating block HNO3/HCl/H2SO4
HNO3/HCl FAAS [753]
Sediment Hot plate acid digestion
evaporation to dryness
ultrasounds-assisted + oven heating
evaporation to dryness
dissolution in 2.0M HCl
H2O2/HNO3
+ HF/HNO3
Anion exchange
chromatography –
IDA-ICP-MS
[754]
Table 21. (Continued)
Sample Processor Acid composition Analytical
technique Ref.
Sediment Microwave-assisted 2-stage microwave-assisted acid digestion
procedure using HNO3 and HCl ETAAS [755]
Sewage sludge Heating block HNO3/H2O2 STAT-FAAS [81]
Fly ashes Open system digestion
Microwave-assisted
Microwave-assisted
HF/HNO3/HClO4
HF/HNO3
HF/HCl/HNO3
ICP-OES [756]
Atmospheric
aerosol Oven heating acid digestion
Oven heating acid digestion
Oven heating acid digestion
Oven heating acid digestion
Microwave-assisted
HF/HNO3/HClO4
HF/HNO3/HCl
HF/HNO3/H2O2
HF/HNO3
HF/HNO3
ETAAS [757]
Atmospheric
aerosol Microwave-assisted HNO3/HF/H2O2 ICP-MS [758]
Atmospheric
particulate matter Microwave-assisted HNO3/HClO4 ICP-OES [759]
Atmospheric
particulate matter Microwave-assisted 2-stage microwave-assisted acid digestion
procedure using HNO3/HF, and H3BO3 ICP-MS [760]
Atmospheric
particulate matter Microwave-assisted HNO3/HF/H2O2 ICP-MS [761]
Dust Acid digestion HNO3/HCl FAAS [762]
Suspended
particulate matter in
seawater
Microwave-assisted ---- FI-ICP-MS [763]
ASV, anodic stripping voltametry; BIFF-AAS, beam injection flame furnace – atomic absorption spectrometry; ETAAS, electrothermal atomic absorption
spectrometry; FAAS, flame atomic absorption spectrometry; FI-ICP-MS, flow injection - inductively couple plasma – mass spectrometry; ICP-OES,
inductively couple plasma – optical emission spectrometry; ICP-MS, inductively couple plasma – mass spectrometry; IDA-ICP-MS, isotopic dilution
analysis - inductively couple plasma – mass spectrometry; STAT-FAAS, slotted tube atom trap - flame atomic absorption spectrometry; TSFF-AAS,
thermospray flame furnace – atomic absorption spectrometry
Table 22. Applications of ultrasound assisted leaching methods (ultrasonic bath and probe).
Sample Processor Leaching solution (atomic spectrometric technique) Temperature Leaching time Ref.
Aquatic plant, mussel Probe 3.0%(v/v) HNO3 (ETAAS) Room temperature 1.0 min [148]
Mussels, clams, tuna Bath (0.5–4.5)M HNO3 + (2.0–4.0)M HCl + 1.5M H2O2 (ETAAS) Room temperature 10.0 min [150]
Sludge, ash Bath 6.0M HCl (CVAAS) Room temperature 20.0 min [294]
Mollusks Bath (0.5–4.5)M HNO3 + (2.0–4.0)M HCl + 1.5M H2O2 (ETAAS) Room temperature 10.0 min [764]
Seaweed Bath Step1: 6.0M HCl
Step 2: 3.7M HNO3 + 3.0M HCl + 3.0M H2O2 (ICP-OES) Room temperature
65 °C 10.0 min
35.0 min [779]
Seafood (TORT-2) Bath 80% MeOH
2.0 %(v/v) HNO3 (ICP-MS) Room temperature 60.0 min [780]
Plants Bath Diluted HNO3 and HCl (ICP-OES)a Room temperature
40.0 min [781]
Plants Bath 0.5M HNO3 (ETAAS) Room temperature 5.0 min [782]
Plants Bath 1.0M HCl + surfactants (ICP-OES) Room temperature
40.0 min [783]
Plants, aquatic moss Probe 0.1M EDTA pH 10 (ETAAS) Room temperature 3.0 min [784]
Animal tissues Probeb 15.0 %(v/v) HNO3 (ETAAS) Room temperature 10.0 min [785]
Plankton, algae, oyster Probe 1.0M HNO3 (ETAAS) Room temperature 1.0 – 5.0 min [786]
Mussel, fish Bath
Probe 2.8M HNO3 (BIFF-AAS)
0.7M HNO3 (BIFF-AAS) Room temperature
Room temperature 10 min
5 min [111]
Fish, algae,
sediments Probe 0.5 – 5.0 %(v/v) HNO3 (ETAAS) Room temperature 2.0 – 5.0 min [173]
Mussel Probe 3.0%(v/v) HNO3 (ETAAS) Room temperature 15 s [787]
Mussels Bath 1.6M HNO3 + 1.2M HCl + 1.5M H2O2 (ETAAS) Room temperature 120.0 min [788]
Mollusks Bath (0.5–4.5)M HNO3 + (2.0–4.0)M HCl + 1.5M H2O2 (ETAAS) Room temperature 10.0 min [789]
Fish and mussels Bath 4.0M HNO3 + 4.0M HCl + 0.5M H2O2 (ETAAS) 56 °C 30.0 min [790]
(a) Concentrations not available
(b) Sample into the extraction chamber is immersed into a water bath at room temperature and the ultrasound probe is placed 1 mm from the top surface of the
extraction cell
BIFF-AAS, beam injection flame furnace – atomic absorption spectrometry; CVAAS, cold vapor atomic absorption spectrometry; ETAAS, electrothermal
atomic absorption spectrometry; ICP-OES, inductively couple plasma – optical emission spectrometry
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 154
A closed-vessel microwave oven system also allows the simultaneous treatment of several
samples but their main characteristic is that closed-vessels must be equipped with a safety relief
valve to control the internal pressure. A closed-vessel microwave instrument for laboratory purposes
is formed by the microwave generator (magnetron), the wave guide, the microwave cavity, the mode
stirrer, a circulator and a turntable. The magnetron generates the microwave radiation which is
propagated down the wave guide, and injected directly into the microwave cavity where the mode
stirrer distributes the incoming energy. Most of the commercial microwave ovens have a fixed-tuned
magnetron, giving an output frequency of 2450 MHz. Such instruments use approximately 1200W
of electrical line power that is transformed to 600W of electromagnetic energy. The turntable is
needed for such microwave instruments which use more than one digestion vessels so that all vessels
can uniformly be exposed to the microwave energy. The turntable (carousel) is available that rotate
360° continuously, or that alternate back and forth 180°. In addition, other features include a function
for monitoring both pressure and temperature. Typically, the pressure (up to 800psi) is continuously
measured at a rate of 200 s–1, while temperature (up to 300°C) is monitored every 7s.
Digestion vessels must be made of materials transparent to microwave radiation (low dissipation
factors) so that the energy will not be absorbed by the vessel but will pass through the vessel to the
solution inside. As commented, PTFE (dissipation factor of 1.5), polystyrene (3.3) and quartz (0.6)
are adequate materials for microwave accessories. It must be taken into account that although quartz
offer good transparency to microwave irradiation, this material have not adequate chemical
resistance when using certain acids such as hydrofluoric acid, and vessels made of PTFE are
preferred (good thermal and chemical resistance). Nowadays, most of the digestion vessels are
closed-vessels, which give as main advantage the higher temperatures reached because the boiling
point of the acid is raised by the pressure produced in the vessel. This implies a shorter digestion
time. In addition, closed-vessels avoid loss of volatile elements during digestion and also minimize
the possibility of airborne contamination. Less volume of acids are commonly required when
working with closed-vessels since no acid evaporation occurs. Typically, closed-vessels used for
microwave digestion are designed to operate at internal pressures up to 120psi, and in addition to the
vessel body and the vessel cap, closed vessels have a safety relief valve which is open when the
internal pressure exceeds the target pressure. In these cases, the safety relief valve works by lifting of
the vessel cap to release excess pressure and then immediately releasing to prevent loss of sample.
V.3.1.2. Sample decomposition for cadmium determination
Nowadays, acid digestion procedures based on the use of microwave energy are the most
used methods to decompose biological and geological materials. Nevertheless, recent
applications of ultrasound energy for digesting biological samples [730,731] and sediments
[732] have been also proposed. However, because total sample decomposition is not reached,
these methods are commonly referred as ultrasonic-assisted acid pseudodigestion procedures
[730,731]. Table 21 summarises selected applications of acid digestion procedures for several
environmental materials; including biological samples, such as biota, mainly mollusks
[72,149,733-738 and fish [149,730,731, 737--740 soft tissues, Antarctic krill [151], plants
[83,84,117,119,120,142,143,166,176,737,741,742], lichens [741,743,744], soils
[146,167,745-751 sediments [146, 155, 167, 169, 170,172,173,732,737,752-755 sewage
sludge [81], fly ashes [169,756], atmospheric aerosol [757,758], atmospheric particulate
matter [759-761 dust [751,762] and suspended particulate matter in seawater [763].
Analytical Chemistry of Cadmium: Sample Pre-treatment… 155
V.3.2. Leaching (extraction) procedures
Leaching (extraction or lixiviation) methods consist of using diluted reagents (commonly
acids) to extract or leach the target trace elements from solid samples. These procedures do
not involve the total matrix sample decomposition but the breakdown of certain chemical
bonds between the trace elements and the matrix sample constituents [764]. Nevertheless,
partial dissolution of the solid sample is unavoidable especially when working with samples
containing high organic matter and with diluted oxidizing acids such as nitric acid. In
addition, the sample dissolution ratio is expected to be increased when assisting the leaching
process with an external source of energy (ultrasounds, microwaves or pressurization) [723].
Therefore, as proposed by Lorentzen and Kingston [765], the mechanisms through which the
targets reach the solution can be considered as partial sample decomposition. This fact can
lead to non quantitative extractions, mainly for elements associated to silicate fractions in
certain environmental matrices because common diluted (or even concentrated) mineral acids
do not partially decompose silica [723].
In general, since total sample matrix destruction is not required, leaching methods are
considered as environmentally friendly procedures and they have received especial attention.
From the first applications on using a leaching method for total metal extraction based on
concentrated acids (9/1 HCl/HNO3) and heating [766], different procedures using diluted
mineral or organic acids and chelating agents and different sources of energy to speed up the
leaching process (microwaves, ultrasounds and pressurization) have been developed as
alternative sample pre-treatments to conventional microwave assisted acid digestion
procedures.
V.3.2.1. Ultrasounds assisted acid leaching procedures
Ultrasound energy has been commonly applied for assisting solid-liquid extractions
[767]. Concerning trace elements, most of these procedures have involved the use of diluted
reagents (mineral acids), although some applications involving concentrated mineral acids
have been performed to assist total sample decomposition process [768-771]
(a) Ultrasound energy
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of
human hearing. Although this limit varies from person to person, it is approximately 20kHz
in healthy, young adults and thus, 20kHz serves as a useful lower limit in describing
ultrasound. Because of the cyclic pressure variations when ultrasound passes through a
material (liquid), successive expansion and compression cycles occur, and the expansion
cycles are responsible of producing negative pressures in the liquid. These negative pressures
generate bubble nucleation because molecules of vapor and gases can migrate to the cavities.
In contract, a fraction of gases and vapor molecules can be expelled from the bubbles during
the compression cycles. The successive expansion and compression cycles originate the
cavitation phenomenon which leads to bubbles collapses and the generation of high local
temperatures and pressure inside the liquid [772]. When a solid is suspended in a liquid
subjected to ultrasound irradiation, the cavitation process occurs by asymmetric collapses,
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 156
which originates high-speed microjets toward the solid surface, leading to the solid erosion
and cleavages [767,773]. This fact increases the solid surface area and under these conditions
analyte transport from the solid particles to the liquid phase is more efficient and quantitative
extractions in short times can be expected. In addition, due to a best interaction between the
solid sample and the extracting solution, chemical bonds between the analytes and the matrix
components can be broken by the action of diluted reagents. This is especially advantageous
when extracting inorganic species (major and trace elements) because ultrasound energy does
not modify/decompose them during the extraction process. However, partial or complete
mineralization of organic compounds, as well as oxidation process, can be occurred and
certain limitations have been found when extracting organic substances and organometallic
species [767].
(b) Ultrasound instrumentation
As recently reviewed by Santos Júnior et al. [767], two ultrasound generator devices are
available: ultrasounds water-baths (ultrasonic baths, ultrasonic cleaning devices) and
ultrasonic probes.
Ultrasounds water-baths consist of a tank, with or without temperature control, in which the
ultrasound transducer (normally placed at the bottom of the tank) transfers the ultrasound
energy to the liquid, normally ultrapure water, contained in the tank. In these devices, the
ultrasound energy is partially dissipated into both the reaction vessels placed in the bath and
also through the water contained into the tank. Therefore, the ultrasound energy which is
dissipated into the reaction vessel (where the sample and the extracting solution are placed) is
lower than the ultrasound intensity supplied by the transducer (commonly between 1 and 5
Wcm−2). In addition, it has been established that the ultrasound energy transfer depends on
several factors, such as the position in which the sample vessel is situated inside the bath
(vertical and horizontal position) and the number of vessels as well as wall thickness and
material, the bath dimensions, the water volume contained in the tank and the transducer
position. Therefore, the intensity of the ultrasound energy is not constant in all points of the bath
neither inside all reaction vessels [774]. This fact together with the different ultrasounds
frequencies and powers depending on the specific transducers used in the different commercial
ultrasonic baths makes the attempts to compare experimental conditions difficult [767].
Ultrasounds probes consist of an ultrasound transducer coupled to a detachable metal
probe or tip, commonly made of a titanium alloy (Ti-6Al-4V). The tip transmits the
ultrasound energy to the sample-extracting liquid mixture, and there are different tip sizes in
function of the volume of the sample-extracting liquid mixture to be sonicated. Although
titanium alloys tips are the most used probes, silica glass or pure titanium tips are available to
minimize metal contamination [775,776]. Conventional ultrasonic probes generate higher
ultrasound intensities than ultrasonic water-baths, within the 50–750 Wcm–2. In addition,
because of the tip is directly immersed into the reaction vessels the ultrasound energy is
completely absorbed by the reaction medium, increasing the sonication efficiency. Therefore,
lower extraction times are attained and better extraction efficiencies are obtained when using
ultrasounds probes. Another important consideration is related to the dimension and shape of
the sample vessel to be irradiated. Conical-type vessels with the diameter as small as possible
in order to rise up the liquid level are recommended to avoid the formation of aerosols and
foams [775]
Analytical Chemistry of Cadmium: Sample Pre-treatment… 157
(c) Ultrasound assisted leaching methods for cadmium extraction from
environmental samples
From the early work by Kumina et al.[777] using ultrasounds (water-bath device) and a
diluted acid as an extracting solution, several applications involving the use of diluted
acids/reagents, and leaching at room temperature or at moderate high temperatures have been
carried out for biological, clinical and environmental samples. Different reported methods for
cadmium extraction from environmental samples are summarized in Table 22. Quantitative
cadmium recoveries were achieved by using either ultrasounds water-baths or probes working
at room temperature; although sonication at 65°C was needed to quantitatively extract
cadmium from seaweed [778,779], and a temperature of 56°C was also found necessary when
pre-treating TORT-2 certified reference material [780]. Terrestrial plants [781-,783 moss
[784], animal tissues [785], plankton [786], aquatic plants [148,784], algae [173,786[,
mollusks [111,150,764,786,787-790 fish [111,150,173,790], sediments [173], sludge and ash
[294] have been subjected to ultrasound irradiation to extract cadmium.
Ultrasound assisted acid leaching methods have shown great potential for extracting trace
elements from samples containing high organic matter content. However, these
methodologies lead to non quantitative recoveries when treating samples containing silica
(geological materials and sediments and soils). In such cases, concentrated acids are required
in order to digest or partially digest the sample [791].
(d) Continuous on line ultrasound assisted leaching
As commented before, implementation of flow injection analysis systems (FIA) in the
analytical laboratories is a current practice, leading to on-line sample pre-treatment just
before analyte determination. This fact considerably shortens the pre-treatment time and
mainly decreases analyte losses and sample contamination. Most of the on line ultrasound
assisted extraction methods by means of a FIA imply the use of a single leaching solution to
extract a certain number of analytes, but applications using different extracting solutions
which are sequentially passed through the solid sample enclosed in a chamber (column or
cell) have been also described [792].
Both ultrasound water bath and ultrasound probe devices have been used for continuous
ultrasound assisted leaching procedures. In the first case, the solid sample enclosed in the
extraction cell is immersed in the ultrasonic water bath and after loading the leaching solution
in the circuit the system is subjected to ultrasound irradiation for a certain time. When using
an ultrasonic probe, similar operations regarding extraction chamber filling and leaching
solution loading are applied but the extraction chamber is immersed in a water bath and the
ultrasound irradiation is applied with the probe placed 1mm from the top surface of the
extraction cell [723]. Extraction cells (chambers) containing the solid sample consist of glass
mini-columns (50mm×3mm i.d.) or of stainless-steel cylinders (10cm×10mm i.d.), closed
with screws at either end. The screw caps are plugged with filter paper or cellulose filters to
ensure that the sample remained in the extraction unit [793,794].
Two main approaches for the continuous on line ultrasound assisted acid leaching of
trace elements from solid samples are described in literature. In the first one, after immersing
the extraction cell containing the sample into the ultrasonic bath, the system is loaded with a
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 158
certain volume of the leaching solution, the circuit is then closed and this volume of leaching
solution is pumped through the extraction cell for a certain time under sonication conditions.
After this extraction time, the leachate passes to a mixing coil and a small volume (around
250µL) is injected by means of an injection valve into a carrier (ultrapure water) stream
through the detection system [793]. In the second operation mode, after the extraction
chamber containing the sample is immersed in the water bath, the leaching solution is
continuously circulated through the solid sample for a certain time under ultrasound
irradiation. Finally, after extraction was completed, the extract is collected and conveniently
diluted before analyte determination [794]. For either operation modes, the direction of the
flow of the leaching solution during the extraction is changed after a fixed period of time
(commonly between 30 and 80s) to avoid compactness of the sample at the extraction
chamber ends that could cause overpressure in the system.
Several applications of continuous ultrasound acid leaching coupled to atomic
spectrometric detectors have been recently reviewed [723]. Most of these applications use
ultrasounds water-baths and sonication at room temperature. Concerning cadmium continuous
extraction, 3.0M nitric acid has been used as an extracting solution at room temperature and
for 2.0 min to pre-treat mollusks [795]. However, sonication at 50°C was needed for the
continuous leaching of cadmium from welding fumes [796].
V.3.2.2. Microwave assisted acid leaching procedures
As commented in preceding sections, the use of microwave energy for sample preparation
has been widely accepted for assessing inorganic and organic compounds in different biological
and environmental samples. Although microwave ovens are mainly used for total sample
decomposition when assessing metals, there are several applications in which diluted mineral
acids are used, and metals are leached from the solid sample without implying the total sample
decomposition. Similarly to ultrasounds assisted acid leaching procedures, the mechanisms
through which the elements reach the solution must be considered as partial sample
decomposition due to diluted oxidizing and relatively high temperature and pressure are used.
Both off-line and on-line microwave assisted acid leaching procedures have been
described in literature [723]. Diluted nitric acid, hydrochloric acid and oxygen peroxide have
commonly been used as extracting solutions. In few cases assisted acid leaching have been
carried out in household microwave ovens, and high pressure microwave instruments have
mainly been used and recommended.
Table 23 summarizes the application of microwave assisted acid leaching procedures for
extracting cadmium from environmental samples. Mollusks [149,780,797], fish [149] and
plant [798,799] samples have been efficiently treated to release cadmium before an atomic
spectrometric determination. In addition, Table 23 also lists an application involving a
continuous microwave assisted acid leaching procedure [799]. In this approach, the sample
(plant material) is placed into an extraction cell connected to an extraction loop which is filled
with the acid leaching solution (2.0 mL 1%(v/v) HNO3). The extraction cell is then placed in
the vessel of a focused microwave oven which contains water and it is irradiated at 300W for
10–15 min.
Table 23. Applications of microwave assisted leaching methods.
Sample Microwave oven Leaching solution (atomic
spectrometric technique) Power / Temperature Leaching time Ref.
Mollusks, fish Household microwave
oven 4.5M HNO3 + 2.8M HCl + 0.5M
H2O2 (ETAAS) 64 W 2.0 min [149]
Seafood (TORT-2) High pressure
microwave oven 2.0 %(v/v) HNO3
(ICP-MS) From room temperature
to 75 °C 6.0 min [780]
Mussels High pressure
microwave oven 2.5M HNO3 + 3.0M HCl +
0.5%(m/v) H2O2
(ICP-OES)
From room temperature
to 65 °C 2.5 min [797]
Plants High pressure
microwave oven 0.02M EDTA
or
1.0M HCl
(ICP-OES)
250 W + 400 W + 550
W + 250 W 5.0 min + 10.0 min +
5.0 min + 5.0 min [798]
Plants Focused microwave
ovena 1%(v/v) HNO3
(ETAAS) 300 W 10-15 min [799]
Continuous microwave assisted acid leaching
ETAAS, electrothermal atomic absorption spectrometry; ICP-OES, inductively couple plasma – optical emission spectrometry; ICP-MS, inductively couple
plasma – mass spectrometry
Table 24. Applications of pressurized hot water extraction and pressurized liquid extraction.
Sample Pressurized
device PHWE/PLE conditions Atomic spectrometry
technique Leaching time Ref.
Plants Laboratory-made
extractor Acidified water (1.0%(v/v) HNO3),
100-250°C ETAAS 5.0 min [808]
Industrial oils Laboratory-made
extractor Acidified water (4.0%(v/v) HNO3 +
0.1M KCl), 150°C ETAAS [809]
Table 24. (Continued)
Sample Pressurized
device PHWE/PLE conditions Atomic spectrometry
technique Leaching time Ref.
Particulate
matter Laboratory-made
extractor Acidified water (0.1%(v/v) HNO3),
150°C, 1500psi ICP-OES/ICP-MS 15.0 min (30.0 min)a [810]
Squid waste Laboratory-made
extractor Water, 170-380°C, 0.79-30MPa FAAS 1.0-40.0 min [811]
Sediments Supercritical fluid
extractor Water or water+CO2, 80°C, 27MPa ICP-OES/ICP-MS 5.0 h [812]
Particulate
matter Dionex ASE-200 40.0mM EDTAb, 100°C, 1000psi, 1
cycle ICP-OES 10.0 min [814]
Plants Dionex ASE-200 0.01M CDTAc, 75°C, 1500psi, 1
cycle ETAAS 5.0 min [815]
Seafood Dionex ASE-200 1.0M formic acid, 125°C, 500psi, 1
cycle ICP-OES 10.0 min [816]
Marine
sediments,
soils
Dionex ASE-200 8.0M acetic acid, 100°C, 1500psi, 2
cycles ICP-OES 15.0 min [817]
Seaweed Dionex ASE-200 0.75M acetic acid, room
temperature, 10.3MPa, 1 cycle ICP-OES 5.0 min [818]
(a) 30 min dynamic extraction time
(b) EDTA, ethylendiaminotetraacetic acid
(c) CDTA, 1,2-diaminocyclohexane-N,N,N’N’-tetraacetic acid
FAAS, flame atomic absorption spectrometry; ETAAS, electrothermal atomic absorption spectrometry; ICP-OES, inductively couple plasma – optical emission
spectrometry; ICP-MS, inductively couple plasma – mass spectrometry
Table 25. Applications of enzymatic hydrolysis for cadmium extraction from environmental materials.
Sample Enzyme Enzymatic hydrolysis conditions Atomic spectrometry
technique Hydrolysis time Ref.
Mussels Pronase E 37°C, buffer 0.1M/0.1M TRISa/HCl, pH 7.4 FAAS/ETAAS 5.0 h [158]
Mussels Trypsin 37°C, buffer 0.1M/0.1M PDHP/PHPb, pH 9.0 ICP-OES 24 h [819]
Seaweed Pepsin 37°C, 1.0%(m/v) NaCl, pH 1.0 (with HCl) ICP-OES 6.0 h [823]
Mussels Trypsinc
Pancreatinc
37°C, buffer 0.2M/0.2M PDHP/PHP, pH 8.0
37°C, buffer 0.5M/0.5M
ICP-OES 30 min [826]
Seaweed Pepsinc 37°C, 1.0%(m/v) NaCl, pH 1.0 (with HCl) ICP-OES 30 min [823]
Mussel,
plants Pepsind 50°C, 1500 psi, 3 PLE cycles, Milli-Q water, pH 1.0
(with HCl) ICP-OES 6.0 min [824]
TRIS, Tris(hydroxymethyl)aminomethane
PDHP/PHP, potassium dihydrogen phosphate / potassium hydrogen phosphate
Ultrasound bath assisted enzymatic hydrolysis
Pressurized assisted enzymatic hydrolysis
FAAS, flame atomic absorption spectrometry; ETAAS, electrothermal atomic absorption spectrometry; ICP-OES, inductively couple plasma – optical emission
spectrometry
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 162
V.3.3. Pressurized liquid extraction. Pressurized hot water extraction
Pressurized liquid extraction (PLE), also called pressurized fluid extraction (PFE),
pressurized solvent extraction (PSE) or accelerated solvent extraction (ASE), is a reference
extraction methodology for organic compounds, mainly organic pollutants [800,801] and it
has also been popular for extracting organometallic compounds [802]. PLE consists of using
solvents at a high pressure and/or high temperature without reaching the critical point.
Dielectric constant and the surface tension of a certain solvent subjected to high pressure and
temperatures are changed, and the solubility of analytes in the extracting solution has been
found to be increased. When using water as a solvent, the technique is commonly called as
pressurized hot water extraction (PHWE) [803], but the instrumentation as well as critical
parameters controlling the extraction process is similar to PLE.
PLE takes place into stainless-steel extraction cell, in which the sample, previously
dispersed in an inert support, is placed between two cellulose filters, and which is subjected to
controlled temperature and pressure for a certain static extraction time, or for several
extraction cycles. Dispersion of the sample in an inert support (functionalized C18,
diatomaceous earth, alumina, Florisil, among other) is required to assure good solvent sample
contact within the extraction cell.
High efficiencies for analyte transfer from the solid sample to the extracting solution are
obtained because the high temperature and pressure used. Therefore, extraction can be
completed using small amounts of solvents. When extracting inorganic species, the enhanced
contact between sample and the extracting solution attributed to high pressure and
temperature allow the use of diluted reagents (acids), and diluted organic carboxylic acids can
be used to breakdown bounds between trace elements and sample matrix constituents. This
makes the technique environmentally friendly [723].
V.3.3.1. Pressurized hot water extraction / pressurized liquid extraction methods for
cadmium extraction from environmental samples
First applications for extracting metals can be attributed to Luque de Castro et al., who
used sub-critical water [804] or acidified sub-critical water [805,806] in a prototype extractor
made of stainless steel [807]. Similar to the extraction of several major and trace elements
from coal [806,807], acidified sub-critical water extraction (4.0%(v/v) nitric acid) was firstly
used to extract cadmium from plants [808] and industrial oils [809] (Table 24). Morales-Riffo
and Richter [810] have also used a laboratory-made extractor to extract cadmium from
atmospheric particulate matter using 0.1%(v/v) nitric acid as a extracting solution; while a
similar prototype was also used by Tavakoli and Yoshida [811] to release cadmium from
solid (squid) wastes. Finally, cadmium has been also extracted from sediments by
pressurization using a commercial supercritical extractor (supercritical carbon dioxide
extraction, SFE) [812] which involved three sequential steps: a first supercritical carbon
dioxide extraction, followed by a second sub-critical water extraction, and a final sub-critical
water (90%) and carbon dioxide (10%) extraction stage.
However, the first application of commercial PLE piece of equipment (Dionex ASE-200
system) for leaching metals was developed by Wanekaya et al. [813] who investigated the use
of EDTA aqueous solution as extracting under pressurized conditions. Concerning cadmium
Analytical Chemistry of Cadmium: Sample Pre-treatment… 163
extraction by using commercial Dionex ASE-200 system and chelating agents as extracting
solutions, atmospheric particulate matter (PM10), previously collected on quartz fiber filters
[814], and plant samples [815] (Table 24) have successfully pre-treated. Table 5 also lists
other applications of PLE using diluted formic acid to extract cadmium from mollusks [816],
and diluted acetic acid for pre-treating sediments and soils [817] and seaweed [818].
V.3.4. Enzymatic hydrolysis methods
Enzymatic hydrolysis or enzymic hydrolysis is a group of procedures commonly
considered as environmentally friendly methods because digest biological samples under mild
temperature and pH conditions and in absence of polluting or toxic reagents [723]. These
procedures consist of hydrolyzing biomolecules by breaking down certain bonds, allowing the
selective release of metals (and/or organometallic species) from the sample matrix. Enzymes
are mainly of proteolytic nature (proteases), and they attack the peptidic bonds of proteins and
peptides (Pronase E, pepsin, pancreatin or trypsin). Other enzymes, such as lipases, are able
to hydrolyze fats into long-chain fatty acids and glycerol, while amylases hydrolyze starch
and glycogen to maltose and to residual polysaccharides. The enzyme must be carefully
selected in base of the nature of sample (high protein, fat or carbohydrate content). In some
cases, a mixture of different enzymes can be simultaneously used but this approach is not
always possible because the optimum pH value for a certain enzyme can not be same for the
other one (i.e., pepsin normally operates at pH around 2 while lipase or α-amylase work at a
pH around 7) [819]. Complete information about enzymatic methods for the extraction of
metals and organometals species can be found in the review by Bermejo et al. [820].
The first work involving the use of enzymes to extract metals (cadmium, copper, lead and
thallium) was carried out in 1981 by Carpenter [821], who used subtilisin Carlsberg
proteolytic enzyme for hydrolyzing human liver and kidney tissues. Then, the use of enzymes
for extracting metals and organometallic compounds (mainly arsenic, selenium and tin
species) from biological and environmental samples was quite popular [820,822]. Concerning
cadmium extraction from environmental materials (Table 25), proteolytic enzymes such as
Pronase E [158], trypsin [819] and pepsin [823] have been successfully used to digest mussel
soft tissue [819,158] and seaweed samples [823]. As reported, temperature, pH and ionic
strength were the most important parameters affecting the enzymatic hydrolysis of mussel
soft tissue. However, for seaweed material the high ionic strength in enzyme/sample mixture
controlled the enzymatic hydrolysis process. This can be explained by assuming the high salts
content in the seaweed material [823].
Although enzymatic hydrolysis procedures are appealing methodologies especially in
organometallic speciation studies, their general application is limited because the long time
required for completing the hydrolysis process, around 5-24 h [820]. Several attempts based
on the use of ultrasound energy (both ultrasonic water-baths and ultrasonic probes) [820], and
more recently pressurization [824] or microwave energy [825], and have been carried out to
speed up the enzymatic hydrolysis process. The reduction on the enzymatic hydrolysis time
when using ultrasounds can be attributed to cell membranes disruptions by the action of
ultrasounds and the direct cytosolic content attack by enzymes [826]. By this way, enzymatic
hydrolysis time can be reduced from hours to 30 min when treating mussel soft tissue with
trypsin and pancreatin [826] or seaweed with pepsin [823] in ultrasonic water baths.
Similarly, pressurization also disrupts cell membranes before enzymatic digestion of marine
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 164
biota (mussel), fatty biological tissues and plants [824]. In this case, the pre-treatment time
for the pressurized enzymatic hydrolysis procedure was shortened to 6 min (3 PLE cycles of 2
min each one at 50°C and 1500 psi) using pepsin or pancreatin as proteases.
V.3.5. Slurry sampling technique
The slurry sampling technique is a well-established methodology for direct determination
of trace metals in solid samples by atomic spectrometry. As reported in several reviews on
this topic, the sampling of slurries or suspensions in conventional sample introduction
systems for atomic spectrometry offers several advantages. Firstly, the time for sample
prepratation is shortened; the possibility of analyte losses and/or sample contamination is
reduced because the minimum sample manipulation; dangerous or corrosive reagents
commonly required for other sample pre-treatment are avoided, and this technique can be
considered as an environmentally friendly approach; and the same atomizers and autosampler
devices than those used for liquid sampling can be employed. When comparing with the solid
sampling technique, mainly used for ETV coupled to ICP-OES/MS (section III.2.5.7(a)), the
slurry sampling approach overcomes some of the problems associated with direct solid
sampling in most of conventional atomizers.
Most of the applications of slurry sampling technique make use of ETAAS. In this way,
several reviews have been published reporting application of the slurry sampling and solid
sampling in atomic spectrometry [827] and in ETAAS [828,829,830].
Among the different factors affecting the slurry sampling, one of the most critical
variables are the particle size distribution and the need for maintaining stable the slurry during
the time required for sample introduction. The first factor is mainly important when
nebulizing slurries in FAAS and ICP-OES/MS, while the second one widely affects
determinations by ETAAS. Therefore, different approaches have been adopted for
homogenizing the slurry just before sampling. These techniques include manual shaking,
mechanical agitation by magnetic stirring or vortex mixing, gas bubbling, ultrasonic
homogenization by ultrasonic baths or ultrasonic probes either hand-held or assembled to the
autosampler tray (first reported by Carnrick et al.[831]) [827,828,829,830]. Concerning
particle size, large sizes can lead to rapid sedimentation of the solid particles, fact which is
associated to errors when sampling high density materials [832,833]. In these cases,
sedimentation problems are avoided by an effective grinding (particle size reduction of the
sample) and mainly with the addition of special agents (thixotropic or surfactant agents) to the
liquid medium. In low-density materials the main problem is the flotation of finely ground
material on the surface of the slurry. Therefore, wetting and dispersing agents must be used
and ethanol, Triton X-100 and glycerol are the most commonly reported [829,830]. It is well-
established that a minimum number of particles must be introduced into the graphite tube to
ensure the representativity of the aliquot of sample. In this sense, Miller-Ihli [834] developed
an equation to calculate the representative sample mass. In this equation the percentage of
analyte extracted from the solid particles to the liquid media during the slurry preparation is
taken into account.
Another important factor is the liquid media used to obtain the slurry, which acts as a
suspension media but also as an extracting medium, improving the accuracy and the
precision. For several applications, the use of H2O was good enough to obtain satisfactory
results [829]. Other researchers have used concentrated acids, including HNO3, HF, or
Analytical Chemistry of Cadmium: Sample Pre-treatment… 165
combination of HNO3 and H2O2 [835] or H2O2 [829,830]. As commented in section V.2.1
regarding ultrasound probes, metal extraction efficiencies from the solid particles can be
increased when using ultrasound irradiation, and under these conditions dilute acids can be
used. As most of the autosampler devices for ETAAS are equipped with ultrasonic probes,
adequate slurry homogenization but also analyte extraction is achieved just before sample
introduction into the graphite furnace.
Continuous slurry nebulization for FAAS and ICP-OES/MS is also a well-established
practice. In these cases, the slurry introduction is more limited because of the restriction of
the particle size. The developed methods require an acid medium to prepare the slurry and
some times the use of ultrasound energy for increasing metal releasing from the solid
particles. Because higher sensitivities for cadmium determination by ICP-OES/MS, there are
several applications of slurry sampling in plasma detectors, as firstly reviewed by Ebdon et
al.[836], and recently by Santos and Nóbrega [837]. A critical factor in direct analysis of
slurries by ICP-OES/MS is the stability and homogeneity of the slurry. High stable and
homogeneous slurry is necessary in order to achieve a homogeneous and reliable aerosol for
introduction into the plasma and to obtain precise and accurate results [837]. Therefore,
different suspending or dispersing agents such as Triton X-100, poly(acrylate amine), NH3,
HNO3, HCl have been used for several samples [837]. Usually, the slurry is stirred before the
introduction in the plasma using the same systems reported above [837]; however, the use of
surfactant agent does not avoid problems related to particle size distribution, parameter which
affects the efficiency on transport and atomization-excitation. In general, a mean particle size
of 5-10 µm is essential to ensure that the slurry has similar transport properties than an
aqueous solution and therefore, an efficient grinding process is needed.
Several approaches have been developed for metal vapour generation, mainly for hydride
forming elements, from solid particles (slurries). As reviewed by Matusiewicz [838], the main
problem associated to this technique is the low vapour generation efficiency from the metals
trapped in the solid particles. To avoid this problem, several authors have prepared the slurry
sample in acid medium in combination with a mechanical agitation or sonication process. The
use of these acids improved the vapour generation efficiency due to the analyte´s mobilization
from the solid particle to the liquid phase. The disadvantages of these procedures are the time
required to complete the acid slurry preparation (10 min-hours) and in some cases, the high
acid concentrations used, which are not environmentally friendly. For certain matrices such as
coal fly ash and urban dust, it was possible the determination of these elements using aqueous
slurry sampling [839,840]. For cadmium, volatile cadmium species formation (HCl and
NaBH4) has been obtained from seawage sludge and Antartic krill slurries under sonication
conditions [294].
VI. CADMIUM SPECIATION
According to the International Union for Pure and Applied Chemistry (IUPAC) chemical
species of an element is referred to a specific form (isotopic composition, electronic or
oxidation state, and/or complex or molecular structure) of that element [841]. By this way,
speciation analysis is understood as all those analytical activities of identifying and/or
measuring the concentrations of one or more individual chemical species in a sample. In some
cases, there are not specific molecular structures or oxidation states which can define a certain
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 166
compound or ion as a chemical species of an element. In such cases, the term fractionation is
preferred than speciation. Fractionation procedures consist of classifying an element in a
sample according to physical (size, solubility) or chemical (bonding reactivity) properties.
In contrast to other heavy metals such as mercury, arsenic, antimony, selenium, lead or
tin, there are few identified chemical species for cadmium in function of the oxidation state or
defined simple organometallic structures. DPASV studies have demonstrated that cadmium is
mainly complexed with organic chelates (around 80%) in estuarine water [842], while most of
the cadmium is present as free ion (Cd2+) in lake waters [843]. Electrochemical methods
have also revealed the presence of monomethyl-cadmium ion (CH3Cd+) in seawater
(concentration around 0.5 µg L–1) [844] which is biosynthesized by the plankton by a
biomethylation process [844,845].
However, most of the speciation studies concerning cadmium are related to the fact that
this element is one of the most important inductor for metallothioneins (MTs) biosynthesis in
animals. Therefore, cadmium is present in the structures of these metal-binding-proteins. In
addition, cadmium is also complexed to the phytochelatin peptides in plants cultivated in
highly cadmium-contaminated soils.
Therefore, cadmium speciation studies discussed in this section will be focused to
different sequential extraction methods for cadmium fractionation in environmental samples
(soils, sediments, sludge, atmospheric particulate matter); and different hyphenated
chromatography/electrophoresis and atomic spectrometric methods for the determination of
cadmium bound to MTs.
VI.1. Sequential Extraction Schemes for Metal Fractionation
In general, metal fractionation studies can be carried out by using single or sequential
extractions (different extractants with increasing extraction capacity), and both often address
operationally defined fractions which identify certain groups of elements without clear
identification. Although in same cases a particular single extraction can be related to certain
chemical form of an element, these extraction procedures are mainly developed for isolating
of a particular metal-containing matrix phase (e.g. water soluble fraction or carbonate bound
fraction). This information is useful to characterize a sample, commonly soils and marine and
river sediments, in base of metals mobility or availability. For soil and sediment samples, as
well as for other similar environmental solid materials (sludge, atmospheric particulate
matter) is possible identify different defined phases (fractions) as follows:
(1) Water-soluble, phase which contains the most mobile and potentially available
metals species.
(2) Exchangeable fraction, phase which contains weakly bound metals species that can
be released by ion-exchange (mainly NH4+, Ca2+ and Mg2+). Solutions containing
ammonium (NH4+), especially ammonium acetate, are used as extractants.
(3) Carbonate bound, phase which contains metals easily released at acidic pHs (sodium
acetate or acetic acid, pHs around 5).
(4) Oxides of manganese and iron, phase which contains metals easily released (easily
reducible) after minimal attack of iron/manganese oxide phases. Extractants based of
hydroxylamine hydrochloride at acid pH are commonly used.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 167
(5) Organically bound, phase which contains metals bound to easily extractable organic
matter, mainly humic substances. Extractants commonly consist of ammonium
acetate solution after an organic matter oxidation stage hydrogen peroxide or sodium
hypochlorite solutions.
(6) Moderately reducible oxides, phase which contains metals bound to moderately
reducible oxides. Extractions are mainly based on hydroxylamine
hydrochloride/acetic acid mixtures.
(7) Oxide and sulfide fraction, phase which contains elements bound to oxides and
mainly sulfides. Hydrogen peroxide and ammonium acetate are commonly used as
extractants.
(8) Silicate bound (residual fraction), phase which mainly contains crystalline bound
trace elements. This fraction is released after silicate decomposition with
concentrated acid, specially hydrofluoric acid, perchloric acid and nitric acid.
VI.1.1. BCR sequential extraction scheme
The lack of uniformity of the different sequential extraction schemes to assess the
fraction of elements belonging to some of the defined phases listed above, offers as a main
disadvantage that the significance of the analytical results is dependent to the extraction
scheme used. Therefore, comparison between different laboratories in assessing metal
mobility or availability is difficult. This fact has led to the development of standardized single
and sequential extraction schemes by the Standard, Measurements and Testing Programme
(SM&T, formerly BCR – The Community Bureau of Reference) of the European Union in
order to harmonize sequential extraction methods for metals in sediments [846]. In addition,
efforts have been made to prepare soil and sediment certified reference materials with
certified extractable single or sequential extraction fractions [847]. The BCR protocol
proposes a standardized three-stage extraction procedure (BCR EUR 14763 EN), originally
developed for sediments [848], which consists of the following steps:
Step 1. Extraction with 0.11M acetic acid, 40mL for each gram of dry sample (sediment).
The sample and the extractant are shacked in a sealed container for 16 hours at room
temperature (20°C) on an end-over-end mechanical shaker at a speed of 30rpm. Elements
extracted during this stage are those related to water-soluble, exchangeable, and carbonate
bound phases, above coded as (1), (2) and (3), respectively. Therefore, elements weakly
absorbed on the sample surface, elements that can be released by ion-exchange processes and
elements co-precipitated with carbonates are extracted during this step.
Step 2. Extraction with 0.11M hydroxylammonium chloride (adjusted pH 2 with nitric
acid), 40mL for each gram of dry sample (sediment). After centrifugation at 2500rpm for 25
min and removal of the supernatant (0.11M acetic acid), the residue is washed with 20mL of
ultrapure water and after centrifugation and washing solution discarding, the
hydroxylammonium chloride solution is added and the mixture is again shacked in a sealed
container for 16 hours at room temperature (20°C) on an end-over-end mechanical shaker at a
speed of 30rpm. Because of the reducing properties of hydroxylammonium chloride, elements
bound to iron/manganese oxides are released. Therefore elements extracted during this stage
are those related to easily or moderately reducible phases, above coded as (4) and (6-7),
respectively.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 168
Step 3. Organic matter decomposition (oxidation) with 8.8M hydrogen peroxide, 10mL
plus 10mL for each gram of sample (sediment); and extraction with 1.0M ammonium acetate
(adjusted pH 2 with nitric acid), 50mL for each gram of dry sample (sediment). After
separation of the solution containing the reducible fraction and washing stages, the hydrogen
peroxide (10mL) is added to residue and it is digested at room temperature for 1 hour with
occasional manual stirring. Organic matter oxidation is then continued by heating the
container at 85°C on a hot plate for 1 hour. Then, the volume of the liquid is reduced to 2-3
mL by further heating. After this, another 10mL of hydrogen peroxide is added, and the
mixture is again heated at 85°C for 1 hour. The liquid is also reduced to a few mL by heating,
and after cooling the ammonium acetate solution is added. The mixture is then shacked for 16
hours at room temperature (20°C) on an end-over-end mechanical shaker at a speed of 30rpm.
Because of the oxidation of the organic matter, elements bound to organic matter are released.
In addition, metals bound to sulfides must be also extracted during this stage. Therefore
elements extracted are those related to organically bound and sulfide fraction phases, above
coded as (5) and (7), respectively.
The simplicity of the BCR sequential extraction scheme based on three stages has made
that this protocol overcome the early sequential extraction scheme based on five sequential
stages proposed by Tessier et al. [849]. This scheme uses a mechanical stirring of the sample
with 1.0M magnesium chloride solution (pH 7.0) for 1 hour to isolate the water-soluble and
exchangeable fractions (first step), followed to an extraction with 1.0M sodium acetate (pH
5.0) for 5 hours to release metals bound to carbonates (second step). The third extraction is
carried out with 0.04M hydroxylammonium chloride and acetic acid (25%(v/v)) for 6 hours at
96°C in order to release metals bound to the iron/manganese oxides fraction (third step).
8.8M hydrogen peroxide (adjusted pH 2.0 with nitric acid) is also used to decompose the
organic matter fraction, an metals associated with it are then released with 3.2M ammonium
acetate in 20% nitric acid (mechanical shacking for 30 min). During this fourth stage,
elements bound to organic matter and sulfides are extracted. The fifth stage consists of an
acid digestion with hydrofluoric acid and perchloric acid to decompose the silicate fraction.
Therefore, metals bound to silicate are extracted.
VI.1.2. Assisted – BCR sequential extraction
Because the long time required for BCR sequential extraction methods, several attempts
have been adopted to speed up the whole process. Ultrasound energy (ultrasonic water baths
and ultrasonic probes) and microwave energy have been commonly used for assisting the
three stages of the BCR scheme. According with recent reviews [850,851], good agreement
between extractability of certain fractions, mainly exchangeable and carbonate-bound) after
ultrasound assistance and conventional BCR procedures has been obtained [852]. However,
acceleration with microwave energy has been reported to give worse performances. This can
be attributed to the efficient heating caused by microwaves which increases the extractability
of certain fractions.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 169
VI.1.3. Cadmium fractionation in environmental samples by BCR sequential
extraction scheme
BCR scheme has been used to assess cadmium fractionation in several solid
environmental samples. Some examples include soils [157,165,853-862 sediments [847,863-
874 sewage sludge [165,853,875-882 and atmospheric particulate matter [154,883-886.
Assisted BCR sequential extraction scheme for cadmium fractionation in sediments has been
also proposed. Both ultrasound and microwave sequential extraction energy have given
extractable cadmium contents for the three stages of the BCR procedure (excluding the H2O2
digestion in step 3) statistically similar than conventional BCR; but the over-all procedure
could be completed in 15-30 min using ultrasounds, or after 60-120 s for microwave
assistance [887,888]. Ultrasound-assisted BCR extraction procedures were also optimised for
the evaluation of cadmium mobility in bottom sediments [887,889].
VI.2. Metallothioneins / Metallothioneins-like Proteins
There are certain products biosynthesised in an organism as consequence of a toxic event
that can be used to measure exposure to contaminants and to study its possible chronic toxic
effects. These molecular features, named biomarkers, can be defined as biological responses
to an environmental chemical which may be found at the molecular level but also at the
cellular or even the whole organism level. Metallothioneins (MTs) are considered as
biomarkers of heavy metal impregnation [890-894 MTs are non–enzymic metal–binding
proteins characterized by low molecular weight, high content of cysteine residues, lack of
aromatic amino acids and high resistance to heat. Their sulphydrryl–rich primary structure
confers them with a high capacity for metal binding, mainly copper, cadmium and zinc [895].
This type of metal–binding proteins has been also isolated from microorganisms and marine
invertebrates, but they usually contain less cysteine residues and lower metal concentrations
[896]. Therefore, MTs isolated from invertebrates are preferably called as metallothioneins–
like proteins (MLPs). As biomarkers, both MTs and MLPs have been described as
physiologically multifunctional and they are involved in the transport, storage and
detoxification of metals [897].
Isolation procedures based on ion exchange properties and electrophoresis has led to the
recognition of a variety of forms of MTs which were subsequently shown to differ in amino
acid composition and sequence [896]. In mammalians there are at least two
isometallothioneins (metallothioneins isoforms) numbered according to the order in which
they elute from anion exchanger as metallothionein–1 (MT–1) and metallothionein–2 (MT–2)
[898]. Advances in HPLC combined with sensitive spectrometric detectors have aided the
resolution of not only differently charged MT isoforms but also other polymorphic variants
not separated by classical chromatographic procedures. Thus, several MT subisoforms have
been reported, mainly MTs subisoforms isolated from mammalians [899-902 Electrospray
mass spectrometry (ESI-MS) and electrospray tamdem mass spectrometry (ESI-MS-MS)
have shown excellent capabilities for this characterization. These detection methods have
allowed the determination of the molecular mass for the different MTs and their subisorforms,
as well as structural information [899-901,903-907
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 170
In plants, MTs have been also found and they play major roles in the hyperaccumulation
of toxic metals. In fact, different studies have pointed out that certain plants can be useful in
the cleanup of metal-contaminated soils and waters [908]. Plant MTs differ from animal MTs
by a peculiar sequence organization consisting of two short cysteine-rich terminal domains
linked by a long cysteine-devoid spacer. The role of the plant MT domains in the protein
structure and functionality is largely unknown [909]. Other studies have proved the existence
of other plants metallo-proteins (metallohistins), such as multimeric histidine-rich proteins,
coded as AgNt84 and Ag164, which are encoded by two nodule-specific cDNAs isolated
from the actinorhizal host plant Alnus glutinosa. Characterization studies have revealed that
these polypeptides are capable of binding metals such as cadmium [910].
VI.2.1. Sample preparation for MTs/MLPs extraction
Sample preparation for the extraction of MTs and MLPs (commonly known as cytosol
preparation) are based on modifications of the procedure proposed by Roesijadi and Fowler
[911]. This method consists of triturating (cutting)/homogenizing the wet sample with an
extracting TRIS/HCl buffer solution at a physiological pH of 7.4. Different substances such
as phenylmethylsulfonyl fluoride (PMSF) and 2–mercaptoethanol (2–MEC) are commonly
added to the buffer solution to prevent protein hydrolysis and/or protein oxidation during the
extraction process [911].
Cutting mixers with cutting edges made of steel [911], are commonly used to prepare the
cytosol. In order to avoid or minimize metal contamination, metal free cutting devices have
been also applied in the recent literature [912]. However, cutting mixers offer other
drawbacks such as the strong air uptake (the vessel containing the wet sample and the
extracting solution is open) and a large amount of sample needed to perform the
cutting/homogenization. To overcome these problems, some authors have proposed the use of
rotating pestles or sonication in closed vessels under inert Ar or N2 atmosphere [912].
Contamination, as well as oxidation by air, is minimized by using these methods, which also
allow the treatment of small samples. Other methods for cytosol preparation have consisted of
freeze thaw cycles in liquid nitrogen [913], or blending the sample and the extractant inside a
free-metals disposable closure bag [152]. The last procedure has been demonstrated to be free
of metal contamination and it has been found adequate for small samples (lower than 5 g)
[152,914]. Recently PLE has also been proposed for mussel cytosolic preparation [914].
VI.2.2. Separation/determination of MTs/MLPs
Although electroanalytical techniques such as DPASV [915,916] are used for monitoring
complexation of metals such as cadmium by α- and β-domains of MTs, and also for MTs
quantification, most of the research efforts have been led to separating the different
MTs/MLPs isoforms. For this purpose, poweful separation techniques such as HPLC and
capillary electrophoresis (CE) have been widely tested and optimized in combination to
sensitive element-specific or molecule-specific detectors, such as ICP-MS or ESI-MS [917],
respectively. These detection systems offer more adequate capabilities that have overcome
those offerred by early UV detectors. By this way, hyphenate techniques (HPLC or CE) with
ICP-MS detection are nowadays used for the detection and quantification of metals bound to
MTs/MLPs, while detection by ESI-MS is used to characterize and quantify the different
Analytical Chemistry of Cadmium: Sample Pre-treatment… 171
isoforms and subisoforms [918,919]. CE offers great potential for the separation of the
different MTs/MLPs isoforms and sub-isoforms, as well as allows the use of small volumes
of sample [920]. Advances on the interfaces for hyphenating CE to ICP-MS have led to
promissing developments [918]. HPLC techniques also offer excellent abilities to separate
MTs/MLPs isoforms, with the advantage that HPLC and ICP-MS coupling can be easy
realized. HPLC separation based on size exclusion, anion-exchange and reversed-phase have
been widely applied. As reported, size exclusion does not allow the separation of MT-1 and
MT-2 isoforms although the isolation of MT-3 from cytosols from human tissues is possible.
Despite of this limitation, size exclusion is commonly used for isolating MTs from other
metallo-proteins as a preliminary step (preparative size-exclusion chromatography) [921,922].
Anion exchange allows the resolution of the two main MTs/MLPs isoforms (MT-1 and MT-
2), and recent advances have shown capabilities for distinguishing between sub-isoforms
[922]. Finally, reversed-phase chromatography with C4, C8, or C18 columns and also
capillary C8 reversed-phase liquid chromatography (cLC) [923] can be used for separating
MT isoforms, although the long chromatogragrahic time for reversed-phase MTs separation is
a drawback of this type chromatography.
Recently, multidimensional nuclear magnetic resonance spectroscopy (NMR) has been
also used for characterizing the metal-thiolate connectivity of recombinant Cd7-MT10
metallothionein from the sea mussel Mytilus galloprovincialis. The 113Cd NMR spectrum of
mussel MT10 shows unique features, with a remarkably wide dispersion (210 ppm) of 113Cd
NMR signals [924].
VI.2.3. Separation/determination of cadmium bound to MTs/MLPs
Table 26 lists the different developments for MTs/MLPs separation by size-exclusion
HPLC hyphenated with, mainly, ICP-MS detection. The methods have been applied for
isolating MTs from mammalians tissues (bovine liver [925], rat liver [907,926], porcine liver
[927], human liver [928], human thyroid [929], human tissues [930] and human brain [931]),
fish tissues (eel liver [932,933], catfish liver [934], pearl cichlid liver [934], carp liver [
933,935-937), mollusks soft tissues (mussel hepatopancreas [921,922,938]), and plant
extracts [939]. Similarly, HPLC separations based on anion exchange are also given in Table
26. MTs/MLPs separation in cytosols prepared from rabbit liver [940], rat liver [926,941],
human liver [928], mussels [922,938], carp liver [935,942], eel liver [943], were developed to
assess cadmium associated with. Reversed-phase HPLC methods are also included in Table
26. In this case, most of the developments have been performed for MTs from rabbit liver
[899,900,937,944-946 although some applications for isolating MTS from porcine liver [927]
and rat liver [907] can be also found. Studies on fish [937,945], and mussel [938] soft tissues
have been also carried out. Applications by CE are listed in Table 27. Electrophoretic MTs
separations from rabbit liver [947-958 horse kidney [959], rat tissue [904], and human tissues
[953,960] have been successfully addressed. Separations in cytosols from fish tissues [961-
963 and mussels [964] have been also reported.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 172
Table 26. HPLC MTs/MLPs isoforms separation for the determination
of cadmium associated with.
Size-exclusion HPLC
Sample type Column / conditions Detection Reference
Bovine liver TSK-GEL G3000 SWXL / 50mM
Tris/HCl, pH 7.3 ICP-MS [925]
Rat liver Superdex-75, Sephadex G-75 /
30mM Tris/HCl, 10mM
Tris/HCl, pH 8.5
SE preparative for RP-
HPLC [907]
Rat liver TSK-GEL G3000 SW / 25mM
Tris/HCl, pH 8.0 SE preparative for
anion exchange HPLC [926]
Porcine liver TSK HW 55S / 10mM
ammonium acetate + 10mM
sodium chloride, pH 7.5
ICP-MS / ESI-MS [927]
Human liver Superdex-75 HR (16/60) /
20mM Tris/HCl, pH 7.4 ICP-OES [928]
Human thyroid BioSEp-SEC-S3000 / 20mM
Tris/HCl, pH 7.4 ICP-MS [929]
Human tissues Superdex 75 PG / 20mM
Tris/HCl, pH 7.4 ICP-MS [930]
Human brain Superdex 75 PG / 20mM
Tris/HCl, pH 7.4 ICP-MS [931]
Eel liver TSK-GEL G3000 PWXL / 30mM
Tris/HCl, pH 7.4 ICP-TOFMS [932]
Eel liver TSK-GEL G3000 PWXL / 30mM
Tris/HCl, pH 7.4 ICP-TOFMS [933]
Catfish liver Superdex HR 10/30 / 10mM
Tris/HCl, pH 7.4 ICP-MS [934]
Pearl cichlid liver Superdex HR 10/30 / 10mM
Tris/HCl, pH 7.4 ICP-MS [934]
Carp liver TSK-GEL G3000 PWXL / 30mM
Tris/HCl, pH 7.4 ICP-TOFMS [933]
Carp liver TSK-GEL G3000 PWXL / 30mM
Tris/HCl, pH 7.4 ICP-TOFMS [935]
Carp liver TSK-GEL G3000 PWXL / 30mM
Tris/HCl, pH 7.4 ICP-TOFMS [936]
Carp liver TSK-GEL G3000 PWXL / 30mM
Tris/HCl, pH 7.4 ICP-TOFMS [937]
Mussel
hepatopancreas Sephadex G-75 / 10mM
Tris/HCl, pH 7.4 ICP-MS [921]
Mussel
hepatopancreas Sephadex G-75 / 10mM
Tris/HCl, pH 7.4 ICP-TOFMS [922]
Mussel
hepatopancreas Sephadex G-75 / 10mM
Tris/HCl, pH 7.4 SE preparative for
anion exchange HPLC [938]
Plants extracts Superdex peptide HR / 30mM
Tris/HCl, pH 7.5 ICP-MS [939]
Analytical Chemistry of Cadmium: Sample Pre-treatment… 173
Table 26. (Continued)
Anion exchange HPLC
Sample type Column / conditions Detection Reference
Rabbit liver DEAE-5PN / gradient (A: 2mM
Tris/HCl, pH 7.4 and B: 200mM
Tris/HCl, pH 7.4)
HG-ICP-MS [940]
Rat liver Shodex Asahipak ES-502N7C /
gradient (A: 2mM Tris/HCl, pH
7.2 and B: 50mM Tris/HCl, pH
7.2)
ICP-MS [926]
Rat liver Fractogel EMD DEAE-650 /
gradient (A: 20mM Tris/HCl, pH
7.4 and B: 120mM Tris/HCl, pH
7.4)
AAS [941]
Human liver Fractogel EMD DEAE / gradient
(A: 20mM Tris/HCl, pH 7.4 and
B: 250mM Tris/HCl, pH 7.4)
ICP-OES [928]
Catfish liver Mono Q (HR 5/5) / gradient (A:
20mM Tris/HCl, pH 7.4 and B:
20mM Tris/HCl + 0.4M
ammonium acetate, pH 7.4)
ICP-MS [934]
Pearl cichlid liver Mono Q (HR 5/5) / gradient (A:
20mM Tris/HCl, pH 7.4 and B:
20mM Tris/HCl + 0.4M
ammonium acetate, pH 7.4)
ICP-MS [934]
Carp liver ProteinPak DEAE-5PW /
gradient (A: 2mM Tris/HCl, pH
7.4 and B: 200mM Tris/HCl, pH
7.4)
ICP-TOFMS [935]
Carp liver ProteinPak DEAE-5PW / gradient
(A: 2mM Tris/HCl, pH 7.4 and
B: 200mM Tris/HCl, pH 7.4)
ICP-TOFMS [936]
Carp liver ProteinPak DEAE-5PW /
gradient (A: 2mM Tris/HCl, pH
7.4 and B: 200mM Tris/HCl, pH
7.4)
ICP-TOFMS [942]
Eel liver Mono Q (HR 5/5) / gradient (A:
4mM Tris/HCl, pH 7.4 and B:
10mM Tris/HCl + 0.25M
ammonium acetate, pH 7.4)
ICP-MS [943]
Mussel
hepatopancreas Mono Q (HR 5/5) / gradient (A:
4mM Tris/HCl and B: 10mM
Tris/HCl + 250mM ammonium
acetate)
ICP-TOFMS [922]
Mussel
hepatopancreas Mono Q (HR 5/5) / gradient (A:
4mM Tris/HCl and B: 10mM Tris/
HCl + 250mM ammonium acetate)
ICP-MS [938]
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 174
Table 26. (Continued)
Reversed-phase HPLC
Sample type Column / conditions Detection Reference
Rabbit liver Vydac C8 / gradient (A: 5mM
ammonium acetate, pH 6.0 and
B: 5mM ammonium acetate, pH
6.0 + 50% acetonitrile)
ICP-MS / ESI-MS [906]
Rabbit liver Microbore C8 / gradient (A:
5mM ammonium acetate, pH 6.0
and B: 5mM ammonium acetate,
pH 6.0 + 50% methanol)
UV / ICP-MS / ESI-MS [944]
Rabbit liver Sperisorb C18 modified with
DDAB / gradient (A: 2mM
Tris/HCl, pH 7.4 and B: 200mM
Tris/HCl, pH 7.4)
HG-ICP-MS [945]
Rabbit liver Vydac C8 / gradient (A: 10mM
Tris/HCl in water, pH 7.4 and B:
10mM Tris/HCl in 50%
methanol, pH 7.4)
ICP-TOFMS [946]
Porcine liver HyPurity C4 / gradient (A: 10mM
ammonium acetate in water, pH
7.5 and B: 10mM ammonium
acetate in methanol, pH 7.5)
ICP-MS / ESI-MS [927]
Rat liver Vydac C8 / gradient (A: 5mM
ammonium acetate, pH 6.0 and
B: 5mM ammonium acetate, pH
6.0 + 50% acetonitrile)
ICP-MS / ESI-MS [907]
Carp liver Spherisorb ODS 2 / gradient (A:
30mM ammonium acetate in 1%
methanol, pH 7.4 and B: 30mM
ammonium acetate in methanol,
pH 7.4)
ICP-TOFMS [937]
Eel liver Sperisorb C18 modified with
DDAB / gradient (A: 2mM
Tris/HCl, pH 7.4 and B: 200mM
Tris/HCl, pH 7.4)
HG-ICP-MS [945]
Eel kidney Sperisorb C18 modified with
DDAB / gradient (A: 2mM
Tris/HCl, pH 7.4 and B: 200mM
Tris/HCl, pH 7.4)
HG-ICP-MS [945]
Mussel
hepatopancreas Vydac C8 / gradient (A: 10mM
Tris/HCl in water, pH 7.4 and B:
10mM Tris/HCl in 50%
methanol, pH 7.4)
IDA-ICP-MS [938]
AAS, atomic absorption spectrometry; ESI-MS, electrospray ionization - mass spectrometry; HG-ICP-
MS, hydride generation - inductively couple plasma – mass spectrometry; ICP-OES, inductively
couple plasma – optical emission spectrometry; ICP-MS, inductively couple plasma – mass
spectrometry; IDA-ICP-MS, isotopic dilution analysis - inductively couple plasma – mass
spectrometry; ICP-TOFMS, inductively couple plasma – time of flight mass spectrometry; RP-
HPLC, reverse-phase high performance liquid chromatography; SE, size exclusion; UV, ultraviolet
Analytical Chemistry of Cadmium: Sample Pre-treatment… 175
Table 27. Capillary electrophoresis MTs/MLPs isoforms separation for
the determination of cadmium associated with.
Sample type Conditions Detection Reference
Rabbit liver Capillary ID, 75µm; length 100cm /
voltage, 20V / buffer, 5mM
ammonium acetate, pH 6.0
ICP-MS [901]
Rabbit liver Capillary ID, 75µm; length 100cm /
voltage, 20V / buffer, 12mM
Tris/HCl, pH 7.5
ICP-MS [903]
Rabbit liver Capillary ID, 75µm; length 100cm /
voltage, 20V / buffer, 5mM
ammonium acetate, pH 6.0
ICP-MS [906]
Rabbit liver Capillary ID, 75µm; length 150cm /
voltage, 30.1V / buffer, 20mM
Tris/HCl, pH 7.1
ICP-MS [947]
Rabbit liver Capillary ID, 75µm; length 120cm /
voltage, 30.1V / buffer, 20mM
Tris/HCl, pH 7.8
ICP-MS [948]
Rabbit liver Capillary ID, 75µm; length 85cm /
voltage, 18-25V / buffer, 15mM
Tris/HCl, pH 6.8
ICP-MS [949]
Rabbit liver Capillary ID, 50µm; length 77cm /
voltage, 25V / buffer, 20mM
Tris/HCl, pH 7.8
ICP-MS [950]
Rabbit liver Capillary ID, 50µm; length 110cm /
voltage, 25V / buffer, 50mM
Tris/HCl, pH 9.1
ICP-MS [951]
Rabbit liver Capillary ID, 50µm; length 10cm /
voltage, 25V / buffer, 50mM
Tris/HCl, pH 9.1
ICP-MS [952]
Rabbit liver Capillary ID, 75µm; length 70cm /
voltage, 30V / buffer, 20mM
Tris/HCl, pH 7.4
ICP-MS [953]
Rabbit liver Capillary ID, 75µm; length 70cm /
voltage, 30V / buffer, 20mM
Tris/HCl, pH 7.4
ICP-MS [954]
Rabbit liver Capillary ID, 75µm; length 120cm /
voltage, 30V / buffer, 20mM
Tris/HCl, pH 7.4
ICP-MS [955]
Rabbit liver Capillary ID, 75µm; length 68-72cm
/ voltage, 20kV / buffer, 70mM
Tris/HCl in 5% methanol, pH 7.4
HG-ICP-
MS [956]
Rabbit liver Capillary ID, 75µm; length 74-79cm
/ voltage, 20kV / buffer, 70mM
Tris/HCl in 5% methanol, pH 7.4
ICP-MS [957]
Rabbit liver [958]
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 176
Table 27. (Continued)
Sample type Conditions Detection Reference
Rat liver Capillary ID, 75µm; length 70cm /
voltage, 30V / buffer, 20mM
Tris/HCl, pH 7.4
ICP-MS [904]
Horse kidney Capillary ID, 50µm; length 100cm /
voltage, 25V / buffer, 50mM
Tris/HCl, pH 9.0
ICP-MS [959]
Human tissue Capillary ID, 75µm; length 70cm /
voltage, 30kV / buffer, 20mM
Tris/HNO3, pH 7.4
ICP-MS [960]
Bramer liver Capillary ID, 75µm; length 70cm /
voltage, 30kV / buffer, 10mM
ammonia/HNO3, pH 7.4
ICP-MS [961]
Bramer liver Capillary ID, 75µm; length 70cm /
voltage, 30kV / buffer, 20mM
Tris/HNO3, pH 7.0- 7.4
ICP-MS [962]
Eel liver Capillary ID, 75µm; length 74-79cm
/ voltage, 20kV / buffer, 70mM
Tris/HCl in 5% methanol, pH 7.4
ICP-MS [963]
Mussel
hepatopancreas Capillary ID, 75µm; length 81cm /
voltage, 20kV / buffer, 70mM
Tris/HCl in 5% methanol, pH 7.4
ICP-MS [964]
HG-ICP-MS, hydride generation – inductively couple plasma – mass spectrometry; ICP-MS,
inductively couple plasma – mass spectrometry
VII. OFFICIAL METHODS FOR CADMIUM DETERMINATION IN
ENVIRONMENTAL SAMPLES
A great number of different standard and official analytical methods are available for the
determination of cadmium in environmental samples. Although standard and official
guidelines for sampling, preservation and handling of natural and waste water, sediment and
soil, and air (PM10 and exhaust gases) have been commented in sections II and III, those
guidelines are summarized in Table 28. Finally, Tables 29-31 show a brief description of
several standard methods for environmental sample treatment and cadmium determination.
ABBREVIATIONS AND ACRONYMS
The following abbreviations and acronyms are used in this chapter:
AAS atomic absorption spectrometry
AC activated carbon
ADC analog to digital converter
AdSV adsorptive striping voltammentry
Analytical Chemistry of Cadmium: Sample Pre-treatment… 177
AE anion exchange
AES atomic emission spectrometry
AFS atomic fluorescence spectrometry
AMPDAA 2-acetylmercaptophenyldiazoaminoazobenzene
APDC ammonium pyrrolidin dithiocarbamate
ASE accelerated solvent extraction
ASV anodic stripping voltammetry
AT aminothiophenol
AXAD Amberlite XAD
BiBDD bismuth boron doped diamond
BiFE bismuth film electrode
BIFF-AAS beam injection flame furnace – atomic absorption spectrometry
Br-PADAP 2-(5-bromo-2-pyridylazo)-5-(diethylamino)-phenol
BTADAP 2-[2-benzothiazolylazo)-5-dimethylaminophenol
BZA benzoylacetone
CCE N,N'-bis(2-hydroxy-5-nitrobenzyl)-4,13-diazadibenzo-18-crown-6
CCD charge coupled device
CDTA 1,2-diaminocyclohexane-N,N,N’N’-tetraacetic acid
CE capillary electrophoresis
CEC cationic exchange capacity
CFA continuous flow analysis
CFME continuous flow micro-extraction
CID charge injection device
cLC capillary liquid chromatography
CMC critical micellar concentration
CNT carbon nanotubes
CNAA cyclic neutron activation analysis
CPE cloud point extraction
CRM certified reference material
CS continuous source
CSV cathodic stripping voltammetry
CTAB cetyltrimethyl ammonium bromide
CV-AAS cold vapor - atomic absorption spectrometry
CV-AFS cold vapor - atomic fluorescence spectrometry
CV-ICP-OES cold vapor - inductively coupled plasma - optical emission
spectrometry
CV-ICP-MS cold vapor - inductively coupled plasma - mass spectrometry
CVG chemical vapor generation
CVG-AAS chemical vapor generation - atomic absorption spectrometry
CVG-AFS chemical vapor generation - atomic fluorescence spectrometry
CVG-ICP-OES chemical vapor generation - inductively coupled plasma - optical
emission spectrometry
CVG-ICP-MS chemical vapor generation - inductively coupled plasma - mass
spectrometry
DAAB diazoaminobenzene
DAD1 N,N’-bis((1R)-1-ethyl-2-hydroxyethyl)ethanediamide
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 178
DAD2 N,N’-bis((1S)-1-benzyl-2-hydroxyethyl)-ethanediamide
DAFS dispersive atomic fluorescence spectrometry
DC direct current
DDAB didodecyldimethylammonium-bromide
DDC diethyldithiocarbamate
DDDE dibenzylammonium dibenzyldithiocarbamate
DDTP ammonium O,O-diethyl-dithiophosphate
DEM (4-vinylpyridine, 4-VP, or 2-(diethylamino) ethyl methacrylate,
DGNAA delayed gamma ray neutron activation analysis
DIBK 2,6-dimethyl-heptan-4-one
DIHN direct injection high efficiency nebulizer
DLLME dispersive liquid - liquid micro-extraction
DMF dimethylformamide
DNDAA 2,4-dinitrophenyldiazoaminoazobenzene
DPASV differential pulse anodic striping voltammentry
DPV differential pulse voltammentry
DQQH 2,2’diquinolyl ketone 2-quinolylhidrazone
DTC bis(carboxymethyl)dithiocarbamate
EcHG electrochemical hydride generation
EDL electrodeless discharged lamp
EDTA ethylendiaminotetraacetic acid
EDXRF energy dispersive X-ray fluorescence spectrometry
EDS energy dispersive system
ENAA epithermal neutron activation analysis
EOF electroosmotic flow
ESI-MS electrospray ionization - mass spectrometry
ESI-MS-MS electrospray ionization tandem - mass spectrometry
ETAAS electrothermal atomic absorption spectrometry
ETV electrothermal vaporization
FAAS flame atomic absorption spectrometry
FAES flame atomic emission spectrometry
FI flow injection
FIA flow injection analysis
FIT flame in-tube atomizer
Freon-TF 1,1,2 trichlorofluoroethane
GAC green analytical chemistry
GBHA glyoxal-bis (2-hydroxyanil)
GC green chemistry
GCE glassy carbon electrode
GECE graphite-epoxy composite electrode
HCL hollow cathode lamp
HDAA o-hydroxybenzenediazoaminoazobenzene
HDPBA N1-hydroxy-N1,N2-diphenylbenzamidine
HEDC bis (2-hydroxyethyl)dithiocarbamate
HEPA high efficiency particulate
HMDE hanging mercury drop electrode
Analytical Chemistry of Cadmium: Sample Pre-treatment… 179
HDz dithizone (diphenylthiocarbazone)
HEP-XRF high-energy polarized X-ray fluorescence spectrometry
HF hollow fibre
HF-LPME hollow fibre – liquid phase micro-extraction
HF-LLLME hollow fibre – liquid-liquid-liquid micro-extraction
HG-AAS hydride generation - atomic absorption spectrometry
HG-AFS hydride generation - atomic fluorescence spectrometry
HG-ICP-OES hydride generation - inductively coupled plasma - optical emission
spectrometry
HG-ICP-MS hydride generation - inductively coupled plasma - mass spectrometry
HHPN hydraulic high pressure pneumatic nebulizer
HI-HCL high intensity - hollow cathode lamp
HMA-HMDTC hexamethyleneammonium hexamethylenedithiocarbamate
HPLC high-performance liquid chromatography
HPN high pneumatic nebulizer
HpDTC heptyldithiocarbamate
HR-CS AAS high resolution - continuous source atomic absorption spectrometry
HS-SDME head space single-drop micro-extraction
H2DZ ditizone
IAT integrated atom trap
IBMK 4-Methyl-2-pentanone
ICP inductively coupled plasma
ICP-OES inductively coupled plasma-optical emission spectrometry
ICP-MS inductively coupled plasma-mass spectrometry
IDA isotopic dilution analysis
IE11 N-propylsalicylaldimine
IIP ionic imprinted polymer
IS inmunosorbent
ISE ion selective electrode
ISO International Organization for Standardization
JIN jet impact nebulization
JIV jet impact vaporization
KR knotted reactor
LEAFS laser-excited atomic fluorescence spectrometry
LIF laser-induced atomic fluorescence spectrometry
LLE liquid - liquid extraction
LLME liquid – liquid micro-extraction
LLLME liquid-liquid-liquid micro-extraction
LPME liquid phase micro-extraction
LPAT long-path absorption tube
LS line source
LSASV linear sweep anodic stripping voltammetry
LSV linear sweep voltammetry
LTDC low-temperature directed crystallization
MCA multichannel analyzer
MDE mercury drop electrode
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 180
MIP moleculary imprinted polymer
MLP metallothionein-like protein
MPT microwave plasma torch
MSIS multimode sample introduction system
MT metallothionein
MWCNT multi-walled carbon nanotubes
NAA neutron activation analysis
NDAFS non-dispersive atomic fluorescence spectrometry
NIST national institute for standardization and testing
NMR nuclear magnetic resonance spectroscopy
NPV normal pulse voltammetry
OES optical emission spectrometry
PAN 1-(2-pyridylazo)-2-naphthol
PAR 4-(2-pyridylazo)resorcinol
PDHP potassium dihydrogen phosphate
PGNAA prompt gamma ray neutron activation analysis
PHP potassium hydrogen phosphate
PHWE pressurized hot water extraction
PLE pressurized liquid extraction
PMBP 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone
PN pneumatic nebulizer
PNIPAAm poly (N-isopropylacrylamide)
PONPE 7.5 polyethyleneglycolmonop-nonylphenylether
PPT phenanthraquinone monophenylthiosemicarbazone
PSE pressurized solvent extraction
PTFE polytetrafluoroethylene
PUF polyurethane foam
PVC polyvinylchloride
PVME poly(vinyl methyl ether)
PVP polyvinylpyrrolidinone
QADP 2-[2-(4-Methylquinolyl)azo]-5-diethylaminophenol
QTA quartz tube atomizer
RF radiofrequency
RP reverse-phase
SCA single channel analyzer
SCE saturated calomel electrode
SDME single – drop micro-extraction
SE size-exclusion
SFA segmented-flow analysis
SG functionalized silica gel
SFE supercritical fluid extraction
SMDE static mercury drop electrode
SM&T standard, measurements and testing programme
SPE solid phase extraction
S-PE screen-printed electrode
SPME solid phase microextraction
Analytical Chemistry of Cadmium: Sample Pre-treatment… 181
SR synchrotron radiation
SR-XRF synchrotron radiation induced X-ray fluorescence spectrometry
SRTrfCCP single ring electrode radio frequency capacitavely coupled plasma
torch
STAT slotted tube atom trap
STPF stabilized temperature platform furnace
STWAT slotted tube water cooled atom trap
SWASV squared wavelength anodic striping voltammentry
SWNT single-walled carbon nanotubes
SWV squared wavelength voltammentry
TAA thioacetamide
TAM 2(2-thiazolylazo)-5-dimethylaminophenol
TAN 1-(2-thiazolylazo)-2-naphthol
TDBA tetradecyldimethylbenzylammonium
THF tetrahydroforan
THGA transversal heated graphite atomizer
TMAH tetramethylammonium hydroxide
TMAOH tetramethylammonium hydroxide
Tm-APP tetra(m-aminophenyl)porphyrin
TN thermospray nebulizer
TOF time of flight
TOPO 1,1,1-trifluoro-4-mercapto-4-(2-thienyl)but-3-en-2-one
TREN tris(2-aminoethyl) amine
TRIS Tris(hydroxymethyl)aminomethane
TSA thiosalicylic acid
TSC thermostated spray chamber
TSFF-AAS thermospray flame furnace – atomic absorption spectrometry
TSP total suspended particles
TTA 1-(2-thenoyl)-3,3,3-trifluoraceton
TUDS two-unit desolvatation system
Tween 80 polyethylene glycol sorbitan monooleate
TXRF total reflection X-ray fluorescence spectrometry
UN ultrasonic nebulizer
UV ultraviolet
VIS visible
WCAT water cooled atom trap
WDXRF wavelength dispersive X-ray fluorescence spectrometry
WDS wavelength dispersive system
XRF X-ray fluorescence spectrometry
ZnPDC Zn-piperazinedithiocarbamate
1-BPzDC 1-benzylpiperazinedithiocarbamate
5-Br-PADAP 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol
8-HQ 8-hydroxiquinoline
Table 28. Selected standard methods for environmental sampling, preservation and handling of environmental samples.
Details of method Ref.
Natural and waste water
ISO 5667-1: 2006. Water quality. Sampling Part 1:
Guidance on the design of sampling programmes and
sampling techniques
This norm sets out the general principles for, and provides guidance on, the
design of sampling programmes and sampling techniques for all aspects of
sampling of water (including waste waters, sludges, effluents and bottom
deposits).
[9]
ISO 5667-4. Water quality. Sampling Part 4: Guidance
on sampling from lakes, natural and man-made
This norm presents detailed principles to be applied to the design of
programmes, techniques and the handling and preservation of samples of water. [11]
ISO 5667-5. Water quality. Sampling Part 5: Guidance
on sampling of drinking water from treatment works
and piped distribution systems
This norm establishes principles to be applied to the techniques of sampling
water intended for human consumption (all water either in its original state or
after treatment, intended for drinking, cooking, food preparation, or other
domestic purposes, regardless of its origin, plus all water used in any production
undertaking for the manufacture, processing, preservation or marketing of
products or substances intended for human consumption).
[12]
ISO 5667-6. Water quality. Sampling Part 6: Guidance
on sampling of rivers and streams This norm sets out the principles to be applied to the design of sampling
programmes, sampling techniques and the handling of water samples from rivers
and streams for physical and chemical assessment. It is not applicable to the
sampling of estuarine or coastal waters and has limited applicability to
microbiological sampling.
[13]
ISO 5667-8. Water quality. Sampling Part 8: Guidance
on the sampling of wet deposition This norm provides guidance on the design of sampling programmes and the
choice of instrumentation and techniques for the sampling of the quality of wet
deposition. Does not cover measurement of the quantity of rain, dry deposition
or other types of wet deposition such as mist, fog and cloud-waters.
[31]
ISO 5667-9. Water quality. Sampling Part 9: Guidance
on sampling from marine waters
This norm provides guidance on the principles to be applied to the design of
sampling programmes, sampling techniques and the handling and preservation of
samples of sea water from tidal waters. The main objectives are quality
characterization measurement, quality control measurement, measurements for
specific reasons, and examination of the effects of man-made structures.
[15]
Table 28. (Continued)
Details of method Ref.
ISO 5667-10. Water quality. Sampling Part 10:
Guidance on sampling of waste waters
This norm contains details on the sampling of domestic and industrial waste
water, i.e. the design of sampling programmes and techniques for collection of
samples including safety aspects. Covers waste water in all its forms. Sampling
of accidental spillages is not included, although the methods described in certain
cases may also be applicable to spillages.
[14]
ISO 5667-11. Water quality. Sampling Part 11:
Guidance on sampling of groundwaters This norm provides guidance on the sampling of groundwaters. It informs the
user of the necessary considerations when planning and undertaking
groundwater sampling to survey the quality of groundwater supply, to detect and
assess groundwater contamination and to assist in groundwater resource
management, protection and remediation.
[16]
ISO 5667-3: 2003. Water quality. Sampling Part 3:
Guidance on the preservation and handling of water
samples
This norm gives general guidelines on the precautions to be taken to preserve
and transport all water samples. [33]
1060. Collection and preservation of samples This section addresses the collection and preservation of water and wastewater
samples; the general principles also apply to the sampling of solid or semisolid
matrices. Topics covered include collection of samples and sample storage and
preservation.
[34]
Soil, sediment and sludge
ISO 5667-12. Water quality. Sampling Part 12:
Guidance on sampling of bottom sediments
Provides guidance on the sampling of sediments from rivers, streams, lakes and
similar standing waters and estuarines. Sampling of industrial and sewage works
sludges and ocean sediments are excluded.
[18]
ISO 5667-19. Water quality. Sampling Part 19:
Guidance on sampling of marine sediments This norm provides guidance for the sampling of sediments in marine areas for
analyses of their physical and chemical properties for monitoring purposes and
environmental assessments. It encompasses sampling strategy, requirements for
sampling devices, observations made and information obtained during sampling,
handling sediment samples, and packaging and storage of sediment samples.
[19]
Table 28. (Continued)
Details of method Ref.
ISO 10381-1. Soil quality. Sampling Part 1: Guidance
on the design of sampling programmes
This norm sets out the general principles to be applied in the design of sampling
programmes for the purpose of characterizing and controlling soil quality and
identifying sources and effects of contamination of soil and related material,
with emphasis on procedures required to locate points from which samples may
be taken for examination or at which instruments may be installed for in situ
measurement including statistical implications, procedures for determining how
much sample to collect and whether to combine samples, methods of collecting
samples, methods for containing, storing and transporting samples to prevent
deterioration/contamination.
[20]
ISO 10381-2. Soil quality. Sampling Part 2: Guidance
on sampling techniques
This norm gives guidance on techniques for taking and storing soil samples,
information on typical equipment and selection of the equipment and the
techniques to use to enable both disturbed and undisturbed samples to be
correctly taken at different depths that is applicable in particular sampling
situations.
[21]
ISO 5667-15. Water quality. Sampling Part 15:
Guidance on preservation and handling of sludge and
sediment samples
This norm gives general guidelines on the precautions to be taken to preserve
and handling sludge and sediment samples. [35]
ISO 23909. Soil quality. Preparation of laboratory
samples from large samples This norm specifies a method for the preparation of laboratory samples from
large samples. [36]
ISO 11464. Soil quality. Pretreatment of samples for
physico-chemical analysis
This norm specifies the pretreatments required for soil samples that are to be
subjected to physico-chemical analyses of stable and non-volatile parameters and
describes the following five types of pretreatment of samples: drying, crushing,
sieving, dividing and milling.
[37]
ISO 16720. Soil quality. Pretreatment of samples by
freeze-drying for subsequent analysis This norm specifies a method for pretreatment of soil, sludges and sediments
samples by freeze-drying for subsequent analysis. [38]
Table 28. (Continued)
Details of method Ref.
3030. Preliminary treatment of samples This section describes general pretreatment for samples in which metals are to be
determined. Section provides tips for preventing contamination to samples
during pretreatment. Methods of pretreatment described are filtration for
dissolved and suspended metals, treatment for acid-extractable metals, digestion
for metals, nitric acid digestion, nitric acid-hydrochloric acid digestion, nitric
acid-sulfuric acid digestion, nitric-acid-perchloric acid digestion, nitric acid-
perchloric acid-hydrofluoric acid digestion, and microwave-assisted digestion.
[39]
Air
EN 12341. Air quality. Determination of the PM10
fraction of suspended particulate matter. Reference
method and field test procedure to demonstrate
reference equivalence of measurement methods
The norm describes a reference method for the determination of the PM10
fraction of suspended particulate matter as well as a field test procedure which
helps to state the equivalence of commercial PM10 measuring devices compared
to the reference method.
[29]
Table 29. Selected standard methods for natural waters treatment and cadmium determination in natural waters.
Details of method Analytical characteristics Ref.
Dissolved Cd and
total Cd
determination in
natural and waste
water by FAAS.
Dissolved Cd and / or total Cd is measured by FAAS
aspirating the sample into the air/C2H2 flame and
measuring the absorbance at 228.8nm.
Dissolved Cd; sample must be filtered as soon as
possible after sampling through a membrane filter of
0.45µm and then acidified with HNO3 (pH < 2).
Total Cd (after mineralization for water with high
organic matter content); to 100mL of sample at pH < 2
was added 1.0mL of conc. HNO3 and 1.0mL of conc.
H2O2, the mixture is heated in a hot plate (near to
dryness ∼ 0.5mL) and then dilute to 100mL.
Applicable to Cd concentrations between 0.05 – 1.0
mg L-1 in natural and waste water. High
concentration may be determined before sample
dilution. Low concentration may determine after
careful boiling previous sample acidification by
nitric acid. Applicable to sediment and sludge after
acid digestion. SO42-, Cl-, PO43-, Na, K, Mg, Ca, Fe,
Ni, Co, Pb, Si and Ti may interfere.
[965]
Table 29. (Continued)
Details of method Analytical characteristics Ref.
Dissolved Cd
determination by
FAAS after LLE
Cd is measured by FAAS aspirating the sample
directly into the air/C2H2 flame and measuring the
absorbance at 228.8nm after chelation with APDC and
extraction in IBMK or after chelation with HMA-
HMDTC and extraction in DIBK-xylene.
Applicable to natural waters. [966]
Dissolved Cd and
total Cd
determination in
natural and waste
water by ETAAS.
Dissolved Cd and / or total Cd is measured by ETAAS
at 228.8nm using an injection volume of 10µL. Pd (15
and 50µg for natural water of low and high salinity,
respectively) and NH4NO3 (150 and 500µg for natural
water of low and high salinity, respectively) was used
as chemical modifiers. Pyrolytic coated graphite tube
with L’vov platform is recommended.
Applicable to Cd concentrations between 0.3 – 3.0
µg L-1 in natural and waste water. High
concentration may be determined before sample
dilution or by using injection volume <10 µL.
Applicable to sediment and sludge after acid
digestion. Fe, Cu, Ni, Co, Pb, Na, K, Ca, Mg, Cl- and
SO42- may interfere.
[965]
Dissolved Cd and
total Cd
determination in
natural and waste
water by ETAAS.
Dissolved Cd and / or total Cd (ISO-15587-1 and ISO
15587-2) is measured by ETAAS at 228.8nm using an
injection volume of 20µL. The mixtures Pd (15µg) +
Mg(NO3)2 (10µg) or NH4H2PO4 (200µg) + Mg(NO3)2
(10µg) were used as chemical modifiers. Pyrolytic
coated graphite tube with L’vov platform is
recommended.
Applicable to Cd concentrations between 0.4 – 4.0
µg L-1 in surface, ground, tap and waste water. A
high Cl- concentration interferes. To minimize the
matrix effect, the chemical modification, standard
addition method and background correction systems
may be used.
[967]
Water digestion by
aqua regia Water digestion for total metal analysis by using aqua
regia (ISO-15587-1): 6.0mL of HCl (12.0M) and
2.0mL of HNO3 (16.8M) is added to 25mL of sample;
the mixture is heated at 103°C for 2h to 175°C for 8h,
in a open or close digestion vessel. N-dodecane may
add if foam formation occurs. After filtration (if it is
necessary) sample is dilute to a fixed volume.
Microwave energy can also be used for heating.
Applicable to waters with < 20 g L-1 of suspended
solid and < 5 g L-1 of COT (expressed as C). [968]
Table 29. (Continued)
Details of method Analytical characteristics Ref.
Water digestion by
nitric acid Water digestion for total metal analysis by using nitric
acid (ISO-15587-2): 6.25mL of HNO3 (16.8M) is
added to 25mL of sample; the mixture is heated at
103°C for 2h to 175°C for 8h, in a open or close
digestion vessel. N-dodecane may add if foam
formation occurs. After filtration (if it is necessary)
sample is dilute to a fixed volume. Microwave energy
can also be used for heating.
Applicable to waters with < 20 g L-1 of suspended
solid and < 5 g L-1 of COT (expressed as C). [969]
Dissolved Cd and
total Cd
determination in
natural and waste
water by ICP-OES.
Dissolved Cd and / or suspended Cd and / or total Cd
is measured by ICP-OES at 214,438, 226.502 or
228.802nm using the manufacturer’s recommended
conditions.
Dissolved Cd; sample must be filtered as soon as
possible after sampling through a membrane filter of
0.45µm and then acidified with HNO3 (pH < 2).
Suspended Cd; 4.0mL of conc. HNO3 is added to the
filter and the mixture heated in a hot plate near to
dryness, the procedure is repeated by using 3.0mL of
conc HNO3. Then 10mL of HCl (0.2M) and 15mL of
water is added and heated for 15min. Finally, the
residue is removed by filtration or centrifugation and
dilute to 100mL.
Total Cd; to 100mL of un-filtered sample (pH < 2)
was added 0.5mL of cocn. HNO3, the mixture is
evaporated near to dryness in a hot plate, the residue is
dissolved in 1.0mL of conc HNO3 and then the
mixture is dilute 100mL.
Applicable to tap and waste water. Fe, As and Co
may interfere. [970]
Table 29. (Continued)
Details of method Analytical characteristics Ref.
Dissolved Cd and
total Cd
determination in
natural and waste
water by ICP-MS.
Dissolved Cd and / or total Cd (ISO-15587-1 and ISO
15587-2 is measured by ICP-MS using the manufacturer’s
recommended conditions (111Cd or 114Cd).
Dissolved Cd determination; sample must be filtered
as soon as possible after sampling through a
membrane filter of 0.45µm and then acidified with
HNO3 (pH < 2).
Applicable to 0.1 – 1.0 µg L-1 in tap, surface, ground
and waste water. Sn+, MoO, MoOH and ZrOH may
interfere.
[971],[972]
Dissolved Cd and
total Cd
determination in
natural waste water
by FAAS
Dissolved Cd and / or suspended Cd and / or total Cd
is measured by FAAS aspirating the sample into the
air/C2H2 flame and measuring the absorbance at 328.1nm.
Dissolved Cd; sample must be filtered as soon as
possible after sampling through a membrane filter of
0.45µm and then acidified with 3.0mL of HNO3.
Suspended Cd is determined after dissolution of
membrane filtration (addition of 3.0mL of conc. HNO3
to the filter and heat near to dryness the mixture; cool,
and add 3.0mL of conc HNO3 and heat until digestion
is complete; add 2.0mL of HCl (1:1) and heat gently
to dissolve the residue; finally dilute water.
Total Cd; add of 3.0mL of conc. HNO3 to an un-
filtered aliquot and heat and evaporate near to dryness
(do not boil); cool, and add 3.0mL of conc HNO3 and
heat until digestion is complete; add 2.0mL of HCl
(1:1) and heat gently to dissolve the residue; finally
dilute water.
Low Cd concentrations are measured after chelation
with 2.5mL of APDC (1.0 % w/v) to 100mL of
sample at pH = 2.5 and extraction with 10mL of
IBMK after 1.0min of shaking.
Applicable to surface, saline and waste waters.
Ionization interferences are generally removed by
the addition of LaCl3.
[973]
Table 29. (Continued)
Details of method Analytical characteristics Ref.
Cd determination in
natural and waste
water by FAAS.
Cd is determined by FAAS aspirating the sample
directly into the air/C2H2 flame. Applicable to natural waters. At low Cd
concentrations a previous extraction is required. [974]
Cd determination in
natural and waste
water by ETAAS.
Cd is measured by ETAAS using the manufacturer’s
recommended conditions. This method is suitable for determination of micro
quantities of cadmium. [975]
Cd determination in
natural and waste
water by ICP-OES.
Cd is measured by ICP-OES using the manufacturer’s
recommended conditions. Applicable to tap and waste water. [976]
Cd determination in
natural and waste
water by ICP-MS.
Cd is measured by ICP-MS using the manufacturer’s
recommended conditions. Applicable to 0.1 – 1.0 µg L-1 in tap, surface, ground
and waste water. It may also be suitable for
wastewater, soils, sediments, sludge, and biological
samples after suitable digestion followed by dilution
and/or cleanup.
[977]
Cd determination in
natural and waste
water by ASV.
Cd is measured by ASV using the manufacturer’s
recommended conditions. The method requires no sample extraction or pre-
concentration. Several interferences and potential
trace background contamination may occur.
[978]
APDC, ammonium pyrrolidin dithiocarbamate; ASV, anodic stripping voltammetry; COT, total organic carbon; DIBK, 2,6-dimethyl-heptan-4-one; ETAAS,
electrothermal atomic absorption spectrometry; FAAS, flame atomic absorption spectrometry; HMA-HMDTC, hexamethyleneammonium
hexamethylenedithiocarbamate; IBMK, 4-methyl-2-pentanone; ICP-MS, inductively coupled plasma-mass spectrometry; ICP-OES, inductively coupled
plasma-optical emission spectrometry; LLE, liquid-liquid extraction.
Table 30. Selected standard methods for soil, sediment and sludge treatment and Cd determination in soil, sediment and sludge.
Details of method Analytical characteristics Ref.
Cd extraction form
sediment and sludge by
nitric acid
Nitric acid extraction: to a maximum of 0.1000g dry sediment
or equivalent of wet sample, 20mL of HNO3 (7.0M) was added
and the mixture heated at 120°C for 1 hour in close vessel.
Heating must be done using microwave energy. After cooling,
the acid liquid phase was decanted, filtrated or centrifuged.
The solution was made up to 100mL.
Aqua regia extraction: to a maximum of 0.1000g dry sediment
or equivalent of wet sample, 5.0 mL of HNO3 (7.0M) and
15mL of HCl (7.0M) were added and the mixture heated at
120°C for 1 hour in close vessel. Heating must be done using
microwave energy. After cooling, the acid liquid phase was
decanted, filtrated or centrifuged. The solution was made up to
100mL.
After extraction, Cd is determined by FAAS (ISO 5961,
reference 965) or ETAAS (ISO 15586, reference 967). [967]
Cd extraction from soil by
aqua regia Samples (3.0g) were digested at room temperature with 21mL
HCl (12.0M) and 7.0mL of HNO3 (16.8M) mixture (28mL per
3g of sample) for 16h. After this, the suspension was digested
at 130°C, for 2h under reflux conditions. The suspension was
then filtered and diluted to 100mL with HNO3 0.5M.
Applicable to soils and similar materials containing < 20
% (m/m) of organic carbon. For samples containing > 20
% an additional treatment with HNO3 is required.
[979]
Cd extraction from soil by
hydrofluoric acid and
perchloric acid
This procedure involves two step; first organic matter removal
and then silicate matrix decomposition.
Organic matter removal by calcination (0.250g of sample
placed in a Pt crisol is heated up 450°C (1h) and then remain at
450°C for 3h, after cooling at room temperature, few mL of
water is added to fly ash obtained) or HNO3 treatment (to
0.250g of sample 5.0mL of HNO3 (15.2M) was added and the
mixture heated and evaporate at 150°C (hot plate) up ∼ 1.0mL,
then the mixture is cool at room temperature.
Silicate matrix decomposition by addition of 5.0mL of HF
(27.8M) and 1.5mL of HClO4 (11.6M) and heating (hot plate)
until vapor releasing finish, add 1.0mL of HNO3 (15.2M),
1.0mL of HCl (12.1M) and 5.0mL of water and heat gently.
Finally the mixture was made up to 50mL.
Applicable for total dissolution of soils and similar
materials by silicate matrix decomposition in an open
vessel system by a mixture of HF and HClO4 after
organic matter removal by calcination or by nitric acid
treatment.
[980]
Table 30. (Continued)
Details of method Analytical characteristics Ref.
Cd determination by FAAS
in aqua regia extracts of
soil
Extracted Cd is measured by FAAS aspirating the sample into
the air/C2H2 flame and measuring the absorbance at 228.8nm.
Deuterium lamp may used as background corrector.
Applicable to Cd concentrations > 2.0 mg Kg-1 in dry
soil. Fe may interfere. [981]
Cd determination by
ETAAS in aqua regia
extracts of soil
Extracted Cd is measured by ETAAS at 228.8nm using
manufacturer’s recommended conditions. Pyrolytic coated
graphite tube with L’vov platform is recommended. Deuterium
lamp and Zeeman effect background correctors may use.
Applicable to Cd concentrations < 2.0 mg Kg-1 in dry
soil. [981]
Cd determination by ICP-
OES in aqua regia extracts
of soil
Extracted Cd is measured by ICP-OES using the
manufacturer’s recommended conditions. Applicable to soil extracts obtained with aqua regia in
accordance with ISO 11466 (reference 971), with DTPA
in accordance with ISO 14870 (reference 974) or other
weak extractants, or soil extracts for the determination
of total element contents using the acid digestion method
of ISO 14869-1 (reference 972).
[982]
ETAAS, electrothermal atomic absorption spectrometry; FAAS, flame atomic absorption spectrometry; ICP-OES, inductively coupled plasma-optical emission spectrometry.
Table 31. Selected standard methods for cadmium determination in air sample (PM10 fraction) and exhaust gases.
Details of method Analytical characteristics Ref.
Sampling and Cd
determination by
AAS or AES in
exhaust gases
A fixed volume of effluent from a gas pipe or chimney is
isokinetic extracted for a certain period of time at a flow rate
controlled (EN 13284-1, reference 961). Particles from the
sampled gas are captured on a filter. Subsequently the gas is
passed through a series of impingers containing a mixture of
HNO3 (3.3 % m/v) and H2O2 (1.5 % m/v) where metals are
captured. Then, these solutions are analyzed by AAS or AES.
Filter is digested in a close PTFE vessel after analysis.
This method is applicable to the determination of metals in exhaust
gases from hazardous and municipal waste incinerators in the
concentration range of 0.005 mg m-3 to 0.5 mg m-3. Unless otherwise
stated, concentrations are expressed at volumes under dry conditions,
normalised to 273 K, 101.3 kPa, and oxygen content with a volume
fraction of 11 %.
[983]
Sampling and Cd
determination by
AAS or AES in
PM10 fraction
Cd-containing particles are captured by aspiration of a
measured volume of air (for 24h) through a filter mounted on a
system designed to capture the PM10 fraction of suspended
particulate matter (EN 12341, reference 962). After sampling,
filter is dissolved in closed vessel by using microwave energy
(180°C for 20min and 220°C for 20min) and HNO3 and H2O2.
This method is applicable to the determination of Cd in PM10 by
ETAAS or ICP-MS in the concentration range of 0.1 ng m-3 to 50 ng m-
3 after microwave extraction of samples.
[984]
AAS, atomic absorption spectrometry; AES, atomic emission spectrometry; ETAAS, electrothermal atomic absorption spectrometry; ICP-MS, inductively coupled plasma-mass
spectrometry; PTFE, polytetrafluoroethylene.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 192
REFERENCES
[1] Davis, A. C., Wu, P., Zhang, X., Hou, X. & Jones, B. T. (2006). Determination of
cadmium in biological samples. Appl Spectrosc Rev., 41, 35-75.
[2] Watanabe, T., Shimbo, S., Yasumoto, M., lmai, Y., Kimura, K., Yamamoto, K.,
Kawamura, S. & lkeda, M. (1994). Reduction to one half in dietary intake of cadmium
and lead among Japanese populations. Bull Environ Contam Toxicol., 52, 196-202.
[3] World Health Organisation (WHO). (1992). Environmental Health Criteria 134 -
Cadmium International Programme on Chemical Safety (IPCS) Monograph.
[4] Elinder, C. G. (1985). In Cadmium:Uses, occurrence, and intake; Cadmium and
health: A toxicological and epidemiological appraisal; CRC Press: Boca Raton, FL.
[5] American Conference of Governmental Industrial Hygienists (ACGIH). (1996). TLVs
and BEls, threshold limit values and biological exposure indices for chemical
substances and physical agents.
[6] Jensen, A. & Bro-Rasmussen, F. (1992). Environmental contamination in Europe. Rev
Environ Cont Toxicol., 125, 101-181.
[7] Cook, M. E. & Morrow, H. (1995). Anthropogenic sources of cadmium in canada,
National Workshop on Cadmium Transport Into Plants, Canadian Network of
Toxicology Centres, Ottawa, Ontario, Canada, June 20-21.
[8] Keith, L. H. (1991). Environmental sampling and analysis: A practical guide; Lewis
Publishers: Boca Raton, FL.
[9] International Standard Organization. (2006). Water quality. Sampling Part 1: Guidance
on the design of sampling programmes and sampling techniques. ISO 5667-1.
International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20
Switzerland
[10] Krajča, J. M. (1989). Water sampling; Ellis Horwood Limited: Chichester.
[11] International Standard Organization. (1987). Water quality. Sampling Part 1: Water
quality. Sampling Part 4: Guidance on sampling from lakes, natural and man-made.
ISO 5667-4. International Organization for Standardization, Case Postale 56, CH-
1211, Geneva 20 Switzerland.
[12] International Standard Organization. (2006). Water quality. Sampling Part 5: Guidance
on sampling of drinking water from treatment works and piped distribution systems.
ISO 5667-5. International Organization for Standardization, Case Postale 56, CH-
1211, Geneva 20 Switzerland
[13] International Standard Organization. (2005). Water quality. Sampling Part 6: Guidance
on sampling of rivers and streams. ISO 5667-6. International Organization for
Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[14] International Standard Organization. (1992). Water quality. Sampling Part 10:
Guidance on sampling of waste waters. ISO 5667-10. International Organization for
Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[15] International Standard Organization. (1992). Water quality. Sampling Part 9: Guidance
on sampling from marine waters. ISO 5667-9. International Organization for
Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 193
[16] International Standard Organization. (2009). Water quality. Sampling Part 11:
Guidance on sampling of groundwaters. ISO 5667-11. International Organization for
Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[17] Ruda, T. & Farrar, J. (2006). Environmental drilling for soil sampling, rock coring,
borehole logging and monitoring well installation, in Nielsen, D. (ed.) Practical
handbook of environmental site characterization and groundwater monitoring. CRC
Press: Boca Raton, FL; 297-344.
[18] International Standard Organization. (1995). Water quality. Sampling Part 12:
Guidance on sampling of bottom sediments. ISO 5667-12. International Organization
for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[19] International Standard Organization. (2004). Water quality. Sampling Part 19:
Guidance on sampling of marine sediments. ISO 5667-19. International Organization
for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[20] International Standard Organization. (2002). Soil quality. Sampling Part 1: Guidance
on the design of sampling programmes. ISO 10381-1. International Organization for
Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[21] International Standard Organization. (2002). Soil quality. Sampling Part 2: Guidance
on sampling techniques. ISO 10381-2. International Organization for Standardization,
Case Postale 56, CH-1211, Geneva 20 Switzerland.
[22] Garner, F. C., Stapanian, M. A. & Williams, L. R. (1988). Composite sampling for
environmental monitoring, in L.H. Keith (ed.) Principles of environmental sampling.
ACS: Washington, D.C; 363-374.
[23] Albert, R. & Horwitz, W. (1988). Coping with sampling variability in biota.
Percentiles and other strategies, in L. H. Keith (ed.) Principles of environmental
sampling. ACS: Washington, D.C; 337-353.
[24] Gupta, A., Kumar, R., Kumari, K. M. & Srivastava, S. S. (2003). Measurement of
NO2, HNO3, NH3 and SO2 and related particulate matter in rural site in Rampur, India.
Atmos Environ., 37, 4837-4846.
[25] EPA Fourth External Review draft of Air Quality Criteria for Particulate Matter Office
of Research and Development, Washington, DC; June 2003.
[26] Pope, C. A., Burnett, R. T., Thun, M. J., Calle, E. E., Krewski, D., Ito, K. & Thurston,
G. D. (2002). Lung cancer, cardiopulmonary mortality, and long-term exposure to fine
particulate air pollution. J Am Med Assen., 287, 1132-1141.
[27] Rizzio, E., Giaveri, G. & Gallorini, M. (2000). Some analytical problems encountered
for trace elements determination in the airborne particulate matter of urban and rural
areas. Sci Total Environ., 256, 11-22.
[28] McCormac, B. H. (1971). Introduction to the scientific study of atmospheric pollution;
Reide: Dordrect.
[29] European Standard (1998). Air quality. Determination of the PM10 fraction of
suspended particulate matter. Reference method and field test procedure to
demonstrate reference equivalence of measurement methods. EN 12341. European
Committee for Standardization, Brussels, Belgium.
[30] European Community, Council Decision 2004/470/EC of 29 April 2004 concerning
guidance on a provisional reference method for the sampling and measurement of
PM2.5. Off. J. L160 (2004) 53.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 194
[31] International Standard Organization. (1993). Water quality. Sampling Part 8: Guidance
on the sampling of wet deposition. ISO 5667-8. International Organization for
Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[32] ASTM D 5012-89 (1994). Guide for preparation of material used for the collection
and preservation of atmospheric wet deposition.
[33] International Standard Organization. (2003). Water quality. Sampling Part 3: Guidance
on the preservation and handling of water samples. ISO 5667-3. International
Organization for Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[34] APHA. Standard Methods for the Examination of Water and Wastewater. 1060
Collection and Preservation of Samples. American Public Health Association
(APHA), American Water Works Association (AWWA), Water Environment
Federation publication (WPCF). APHA, Washington, DC.
[35] International Standard Organization. (1999). Water quality. Sampling Part 15:
Guidance on preservation and handling of sludge and sediment samples. ISO 5667-15.
International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20
Switzerland.
[36] International Standard Organization. (2008). Soil quality. Preparation of laboratory
samples from large samples. ISO 23909. International Organization for
Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[37] International Standard Organization. (2006). Soil quality. Pretreatment of samples for
physico-chemical analysis. ISO 11464. International Organization for Standardization,
Case Postale 56, CH-1211, Geneva 20 Switzerland.
[38] International Standard Organization. (2005). Soil quality. Pretreatment of samples by
freeze-drying for subsequent analysis. ISO 16720. International Organization for
Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[39] APHA. (2004). Standard Methods for the Examination of Water and Wastewater.
3030 Preliminary Treatment of Samples. American Public Health Association
(APHA), American Water Works Association (AWWA), Water Environment
Federation publication (WPCF). APHA, Washington, DC.
[40] Heinz-Helmut, P. (1992). UV-VIS Spectroscopy and it applications; Springer-Verlag:
Berlin.
[41] Harris, D. A. & Bashford, C. L. (1988). Spectrophotometry and spectrofluorimetry: a
practical approach; Oxford University Press: Oxford.
[42] Sandell, E. B. & Onishi, H. (1978). Photometric determination of trace metals.
General aspects; John Wiley and Sons: New York.
[43] Singh, A. K. & Ratnam, B. K. (1989). Spectrophotometric determination of cadmium
with dithizone in water analysis-improvement in sensitivity by surfactant-induced
sensitization. Microchem J., 39, 241-243.
[44] Otomo, M. & Singh, R. B. (1985). 2,2'-Diquinolyl ketone 2-quinolylhydrazone as a
highly sensitive reagent for the extractive spectrophotometric determination of
cadmium(II). Anal Sci., 1, 165-168.
[45] López García, I., Navarro, P. & Hernández Córdoba, M. (1988). Manual and FIA
methods for the determination of cadmium with Malachite Green and iodide. Talanta,
35, 885-889.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 195
[46] Gomes Neto, J. A., Oliveira, A. P., Freshi, G. P. G., Dakuzaku, C. S. & de Moraes, M.
(2000). Minimization of lead and copper interferences on spectrophotometric
determination of cadmium using electrolytic deposition and ion-exchange in multi-
commutation flow system. Talanta, 53, 497-503.
[47] Agrawal, Y. K. & Desai, T. A. (1986). Extraction-spectrophotometric determination of
cadmium. Analyst, 111, 305-307.
[48] Grudpan, K. & Taylor, C. G. (1989). Some azo-dye reagents for the spectro-
photometric determination of cadmium. Talanta, 36, 1005-1009.
[49] Salim, R. & Shraydeh, B. (1986). Spectrophotometric determination of cadmium(II)
using 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol. Microchem J., 34, 251-253.
[50] Raman, B. & Shinde, V. M. (1990). Extraction, separation and spectrophotometric
determination of cadmium and mercury using triphenylphosphine oxide and its
application to environmental samples. Analyst, 115, 93-98.
[51] Chakravarty, S., Deb, M. K. & Mishra, R. K. (1993). Hydroxyamidines as new
extracting reagents for spectrophotometric determination of cadmium with 4-(2-
pyridylazo)naphthol in industrial effluents, coal, and fly ash. J AOAC Int., 76,
604-608.
[52] Mathew, L., Rao, T. P., Iyer, C. S. P. & Damodaran, A. D. (1993). Liquid-liquid
extraction and spectrophotometric determination of cadmium with 1,10-
phenanthroline and thymol blue. Mikrochim Acta, 111, 231-237.
[53] Ishizuki, T., Matsumoto, K., Yuchi, A., Ozawa, T., Yamada, H. & Wada, H. (1994).
Flow-injection spectrophotometric determination of cadmium with quinolylazo
compound after online separation using a silica gel column. Talanta, 41, 799-803.
[54] Hashem, E. Y. (2002). Spectrophotometric studies on the simultaneous determination
of cadmium and mercury with 4-(2-pyridylazo)-resorcinol. Spectrochim Acta A, 58,
1401-1410.
[55] Akl, M. A., Khalifa, M. E., Ghazy, S. E. & Hassanien, M. M. (2002). Selective
flotation-separation and spectrophotometric determination of cadmium using
phenanthraquinone monophenylthiosemicarbazone. Anal Sci., 18, 1235-1240.
[56] Bulgariu, L., Bulgariu, D. & Sarghie, I. (2005). Spectrophotometric determination of
cadmium(II) using p,p'-dinitro-SYM-diphenylcarbazid in aqueous solutions. Anal
Lett., 38, 2365-2375.
[57] Vaidya, B., Porter, M. D., Utterback, M. D. & Bartsch, R. A. (1997). Selective
determination of cadmium in water using a chromogenic crown ether in a mixed
micellar solution. Anal Chem., 69, 2688-2693.
[58] Hsu, C., Wang, W., Yang, L., Pan, J. & Wang, Y. (1989). Spectrophotometric and
first-derivative spectrophotometric determination of cadmium with o-
hydroxybenzenediazoaminoazobenzene (HDAA). Mikrochim Acta, 1, 313-320.
[59] Zhu, Y.-r., Wang, C.-c., Chen, J., Jiang, W.-q., Jin, G. Spectrophotometric
determination of cadmium by the chromogenic reagent 2-pyridinedia-
zoaminoazobenzene in the presence of Triton X-100. Analyst 1995, 120, 2853-2856.
[60] Amin, A. S. (2001). Spectrophotometric determination of cadmium using thiazolylazo
chromogenic reagents in the presence of Triton X-100: Application in environmental
samples. Anal Lett., 34, 163-176.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 196
[61] Welz, B. & Sperling, M. (1999). Atomic Absorption Spectrometry; Wiley–VCH
Verlag: Weinheim.
[62] Ebdon, L., Evans, E. H., Fisher, A. & Hill, S. J. (1998). An introduction to analytical
atomic spectrometry; Wiley and Sons: West Sussex.
[63] Welz, B., Becker-Ross, H., Florek, S. & Heitmann, U. (2005). High resolution
continuum source AAS; Wiley-VCH Verlag: Weinheim.
[64] Ivanova, E., Schaldach, G. & Berndt, H. (1992). Hydraulic high-pressure nebulization
sample introduction for direct analysis or on-line matrix separation and trace
preconcentration in flame AAS. Fresenius J Anal Chem., 342, 47-50.
[65] Todolí, J. L., Canals, A. & Hernandis, V. (1993). Characterization of a new single-
bore high-pressure pneumatic nebulizer for atomic spectrometry. I. Drop size
distribution, transport variables and analytical signal in flame atomic absorption
spectrometry. Spectrochim Acta B, 48, 373-386.
[66] Todolí, J. L., Canals, A. & Hernandis, V. (1993). Characterization of a new single-
bore high-pressure pneumatic nebulizer for atomic spectrometry-II. Discrete sample
introduction in flame atomic absorption spectrometry. Spectrochim Acta B, 48, 1461-
1470.
[67] Matusiewicz, H. (1997). Atom trapping and in situ pre-concentration techniques for
flame atomic absorption spectrometry. Spectrochim Acta B, 52, 1711-1736.
[68] Delves, H. T. (1970). A micro-sampling method for the rapid determination of lead in
blood by atomic-absorption spectrophotometry. Analyst, 95, 431-438.
[69] Watling, R. J. (1977). The use of a slotted quartz tube for the determination of arsenic,
antimony, selenium and mercury. Anal Chim Acta, 94, 181-186.
[70] Watling, R. J. (1978). The use of a slotted quartz tube for the determination of lead,
zinc, cadmium, bismuth, cobalt, manganese and silver by atomic absorption
spectrometry. Anal Chim Acta, 97, 395-398.
[71] Yaman, M. (2005). The improvement of sensitivity in lead and cadmium
determinations using flame atomic absorption spectrometry. Anal Biochem., 339, 1-8.
[72] Santiago-Rivas, S., Moreda-Piñeiro, A., Bermejo-Barrera, A. & Bermejo-Barrera, P.
(2005). Sensitive STAT-FAAS determination of cadmium in seafood products. At
Spectrosc., 26, 165-172.
[73] Korkmaz, D., Mahmut, M., Helles, R., Ertas, N. & Ataman, O. Y. (2003). Interference
studies in slotted silica tube trap technique. J Anal At Spectrom., 18, 99-104.
[74] Yaman, M. (1999). Determination of cadmium and lead in human urine by STAT-
FAAS after enrichment on activated carbon. J Anal At Spectrom., 14, 275-278.
[75] Gáspár, A. & Berndt, H. (2000). Beam injection flame furnace atomic absorption
spectrometry: A new flame method. Anal Chem., 72, 240-246.
[76] Küllmer, G. & Milner, B. A. (1984). Enhancement procedure for flame-AAS. Labor
Praxis, 8, 50-55.
[77] Brown, A. A., Milner, B. A. & Taylor, A. (1985). Use of a slotted quartz tube to
enhance the sensitivity of conventional flame atomic-absorption spectrometry.
Analyst, 110, 501-505.
[78] Yaman, M. (2001). Simultaneous enrichment of Cd, Pb, Ni, and Al and their
determination in water by STAT-FAAS. Spectrosc Lett., 34, 763-773.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 197
[79] Senkal, B. F., Ince, M., Yavuz, E. & Yaman, M. (2007). The synthesis of new
polymeric sorbent and its application in preconcentration of cadmium and lead in
water samples. Talanta, 72, 962-967.
[80] Ertas, N., Korkmaz, D. K., Kumser, S. & Ataman, O. Y. (2002). Novel traps and
atomization techniques for flame AAS. J Anal At Spectrom., 17, 1415-1420.
[81] Yaman, M. & Bakirdere, S. (2003). Identification of chemical forms of lead, cadmium
and nickel in sewage sludge of waste water treatment facilities. Mikrochim Acta, 141,
47-54.
[82] Yaman, M. & Dilgin, Y. (2002). AAS determination of cadmium in fruits and soils.
At Spectrosc., 23, 59-64.
[83] Bakirdere, S. & Yaman, M. (2008). Determination of lead, cadmium and copper in
roadside soil and plants in Elazig, Turkey. Environ Monit Assess., 136, 401-410.
[84] Kaya, G. & Yaman, M. (2008). Online preconcentration for the determination of lead,
cadmium and copper by slotted tube atom trap (STAT)-flame atomic absorption
spectrometry. Talanta, 75, 1127-1133.
[85] Khalighie, J., Ure, A. M. & West, T. S. (1982). Atom-trapping absorption
spectrometry with water-cooled metal collector tubes. Anal Chim Acta, 134, 271-281.
[86] Lau, C. M., Ure, A. M. & West, T. S. (1982). The determination of selenium by atom-
trapping atomic absorption spectrometry. Anal Chim Acta, 141, 213-224.
[87] Korkmaz, D., Kumser, S., Ertas, N., Mahmut, M. & Ataman, O. Y. (2002).
Investigations on nature of re-volatilization from atom trap surfaces in flame AAS.
J Anal At Spectrom., 17, 1610-1614.
[88] Khalighie, J., Ure, A. M. & West, T. S. (1980). Some observations on the mechanisms
of atomization in atomic absorption spectrometry with atom-trapping and
electrothermal techniques. Anal Chim Acta, 117, 257-266.
[89] Khalighie, J., Ure, A. M. & West, T. S. (1981). Atom-trapping atomic absorption
spectrometry of arsenic, cadmium, lead, selenium and zinc in air-acetylene and air-
propane flames. Anal Chim Acta, 131, 27-36.
[90] Brown, A. A., Roberts, D. J. & Kahokola, K. V. (1987). Methods for improving the
sensitivity in flame atomic absorption spectrometry. J Anal At Spectrom, 2, 201-204.
[91] Roberts, D. J. & Kahokola, K. V. (1989). Improving sensitivity and precision by
automated dual silica tube atom trapping. J Anal At Spectrom, 4, 185-189.
[92] Turner, A. D., Roberts, D. J. & Le Cor, Y. (1995). Improving the design of a water-
cooled atom trap to increase sensitivity and precision. J Anal At Spectrom, 10,
721-725.
[93] Ellis, L. A. & Roberts, D. J. (1996). Further development of the “bent” tube water
cooled atom trap for arsenic, antimony copper and manganese. J Anal At Spectrom,
11, 259-263.
[94] Hallam, C. & Thompson, K. C. (1985). Determination of lead and cadmium in potable
waters by atom-trapping atomic-absorption spectrophotometry. Analyst, 110, 497-500.
[95] Lau, C. M., Ure, A. M. & West, T. S. (1988). Atom trapping atomic absorption
spectrometric determination of some trace elements in soils, natural waters, seawater,
and bovine liver. Bull Chem Soc Jpn, 61, 79-85.
[96] Lau, C. M., Ure, A. M. & West, T. S. (1983). The determination of lead and cadmium
in soils by atom-trapping atomic absorption spectrometry. Anal Chim Acta, 146, 171-179.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 198
[97] Fraser, S. M., Ure, A. M., Mitchell, M. C. & West, T. S. (1986). Determination of
cadmium in calcium chloride extracts of soils by atom-trapping atomic absorption
spectrometry. J Anal At Spectrom, 1, 19-21.
[98] Turner, A. D. & Roberts, D. J. (1996). Metal determinations with a novel slotted-tube
water-cooled atom trap. J Anal At Spectrom, 11, 231-234.
[99] Matusiewicz, H. & Kopras, M. (1997). Methods for improving the sensitivity in atom
trapping flame atomic absorption spectrometry: analytical scheme for the direct
determination of trace elements in beer. J Anal At Spectrom, 12, 1287-1291.
[100] Matusiewicz, H. & Krawczyk, M. (2006). On-line hyphenation of hydride generation
with in situ trapping flame atomic absorption spectrometry for arsenic and selenium
determination. Anal Sci., 22, 249-253.
[101] Matusiewicz, H. & Krawczyk, M. (2008). Determination of total antimony and
inorganic antimony species by hydride generation in situ trapping flame atomic
absorption spectrometry: a new way to (ultra)trace speciation analysis. J Anal At
Spectrom, 23, 43-53.
[102] Gáspár, A. & Berndt, H. (2000). Thermospray flame furnace atomic absorption
spectrometry (TS-FF-AAS) - a simple method for trace element determination with
microsamples in the µg/l concentration range. Spectrochim Acta B, 55, 587-597.
[103] Petrucelli, G. A., Stocco, P. K., Bueno, M. I. M. S. & Pereira-Filho, E. (2006). Tube
atomizers in thermospray flame furnace atomic absorption spectrometry:
characterization using x-ray fluorescence, scanning electron microscopy and
chemometrics. J Anal At Spectrom, 21, 1298-1304.
[104] Doherty, M. P. & Hieftje, G. M. (1984). Jet-impact nebulization for sample
introduction in inductively coupled plasma spectrometry. Appl Spectrosc, 38, 405-412.
[105] Gáspár, A. & Berndt, H. (2002). Beam-injection flame-furnace atomic-absorption
spectrometry (BIFF-AAS) with low-pressure sample-jet generation. Anal Bioanal
Chem., 372, 695-699.
[106] Ratka, A. & Berndt, H. (2004). Beam-injection flame furnace AAS: comparison of
different nozzle types for beam generation and application of sub-critical liquid carbon
dioxide as carrier and gas pressure pump. Anal Bioanal Chem., 378, 416-422.
[107] Gáspár, A., Szeles, E. & Berndt, H. (2002). Analysis of submicroliter samples using
micro thermospray flame furnace atomic absorption spectrometry. Anal Bioanal
Chem., 372, 136-140.
[108] Brancalion, M. L., Sabadini, E. & Arruda, M. A. Z. (2007). Description of the
thermospray formed at low flow rate in thermospray flame furnace atomic absorption
spectrometry based on high-speed images. Anal Chem., 79, 6527-6533.
[109] Davies, J. & Berndt, H. (2003). Improvements in thermospray flame furnace atomic
absorption spectrometry. Anal Chim Acta, 479, 215-223.
[110] Brancalion, M. L., Sabadini, E. & Arruda, M. A. Z. (2009). Thermospray nebulization
for flame furnace atomic absorption spectrometry - Correlations between spray
formation and cadmium analytical sensitivity. Spectrochim Acta B, 64, 89-94.
[111] Aleixo, P. C., Santos Júnior, D., Tomazelli, A. C., Rufini, I. A., Berndt, H. & Krug,
F. J. (2004). Cadmium and lead determination in foods by beam injection flame
furnace atomic absorption spectrometry after ultrasound-assisted sample preparation.
Anal Chim Acta, 512, 329-337.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 199
[112] Teixeira Tarley, C. R., Zezzi Arruda, M. A. (2004). A sensitive method for cadmium
determination using an on-line polyurethane foam preconcentration system and
thermospray flame furnace atomic absorption spectrometry. Anal Sci., 20, 961-966.
[113] Ivanova, E., Berndt, H. & Pulvermacher, E. (2004). Air driven on-line separation and
preconcentration on a C18 column coupled with thermospray flame furnace AAS for
the determination of cadmium and lead at µg l-1 levels. J Anal At Spectrom, 19, 1507-
1509.
[114] González, E., Ahumada, R., Medina, V., Neira, J. & González, U. (2004).
Thermospray flame furnace atomic absorption spectrometry: Application for total
determination of Cd, Pb and Zn in fresh waters, seawater and marine sediments.
Quimica Nova, 27, 873-877.
[115] Wu, P., Zhang, Y., Lv, Y. & Hou, X. (2006). Cloud point extraction-thermospray
flame quartz furnace atomic absorption spectrometry for determination of ultratrace
cadmium in water and urine. Spectrochim Acta B, 61, 1310-1314.
[116] Coelho, L. M., Bezerra, M. A., Arruda, M. A. Z., Bruns, R. E. & Ferreira, S. L. C.
(2008). Determination of Cd, Cu, and Pb after cloud point extraction using
multielemental sequential determination by thermospray flame furnace atomic
absorption spectrometry (TS-FF-AAS). Separation Sci Technol., 43, 815-827.
[117] Lemos, V. A., Bezerra, M. A. & Amorim, F. A. C. (2008). On-line preconcentration
using a resin functionalized with 3,4-dihydroxybenzoic acid for the determination of
trace elements in biological samples by thermospray flame furnace atomic absorption
spectrometry. J Hazar Mat., 157, 613-619.
[118] Pereira-Filho, E. R., Berndt, H. & Zezzi Arruda, M. A. (2002). Simultaneous sample
digestion and determination of Cd, Cu and Pb in biological samples using thermospray
flame furnace atomic absorption spectrometry (TS-FF-AAS) with slurry sample
introduction. J Anal At Spectrom, 17, 1308-1315.
[119] Pereira, M. G., Pereira-Filho, E. R., Berndt, H. & Arruda, M. A. Z. (2004).
Determination of cadmium and lead at low levels by using preconcentration at
fullerene coupled to thermospray flame furnace atomic absorption spectrometry.
Spectrochim Acta B, 59, 515-521.
[120] Brancalion, M. L. & Arruda, M. A. Z. (2005). Evaluation of medicinal plant
decomposition efficiency using microwave ovens and mini-vials for Cd determination
by TS-FF-AAS. Microchim Acta, 150, 283-290.
[121] Amorim, F. A. C. & Bezerra, M. A. (2007). Online preconcentration system for
determining ultratrace amounts of Cd in vegetal samples using thermospray flame
furnace atomic absorption spectrometry. Microchim Acta, 159, 183-189.
[122] Petrucelli, G. A., Poppi, R. J., Mincato, R. L. & Pereira-Filho, E. R. (2007). TS-FF-
AAS and multivariate calibration: A proposition for sewage sludge slurry sample
analyses. Talanta, 71, 620-626.
[123] L´vov, B. V. (1978). Electrothermal atomization – way toward absolute methods of
atomic absorption analysis. Spectrochim Acta B, 33, 153-193.
[124] Ediger, R. D. (1975). Atomic absorption analysis with the graphite furnace using
matrix modification. At Absorpt Newsl., 14, 127-130.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 200
[125] Ebdon, L. & Lechotycki, A. (1987). The determination of cadmium in environmental
samples by slurry atomization graphite furnace atomic absorption spectrometry using
platforms and matrix modification. Microchem J., 36, 207-215.
[126] Viñas, P., Campillo, N., López-García, I. & Hernández-Córdoba, M. (1995). Slurry-
electrothermal atomic absorption spectrometric determination of aluminum and
chromium in vegetables using hydrogen peroxide as a matrix modifier. Talanta, 42,
527-533.
[127] Hassell, D. C., Majidi, V. & Holcombe, J. A. (1991). Temperature programmed static
secondary ion mass spectrometric study of phosphate chemical modifiers in
electrothermal atomizers. J Anal At Spectrom, 6, 105-108.
[128] Schlemmer, G. & Welz, B. (1986). Palladium and magnesium nitrates, a more
universal modifier for graphite furnace atomic absorption spectrometry. Spectrochim
Acta B, 41, 1157-1165.
[129] Hoenig, M., Puskaric, E., Choisy, P. & Wartel, M. (1991). Direct determination of
high- and mid-volatile metals (cadmium, lead, manganese) in seawater by
electrothermal atomic absorption spectrometry: existing approaches and critical
parameters. Analusis, 19, 285-291.
[130] Bermejo-Barrera, P., Moreda-Piñeiro, J., Moreda-Piñeiro, A. & Bermejo-Barrera, A.
(1998). Usefulness of the chemical modification and the multi-injection technique
approaches in the electrothermal atomic absorption spectrometric determination of
silver, arsenic, cadmium, chromium, mercury, nickel and lead in sea-water. J Anal At
Spectrom, 13, 777-786.
[131] Bermejo-Barrera, P., Moreda-Piñeiro, A. & Bermejo-Barrera, A. (2002). Study of
ammonium molybdate to minimize the phosphate interference in the selenium
determination by electrothermal atomic absorption spectrometry with deuterium
background correction. Spectrochim Acta B, 57, 327-337.
[132] Voth-Beach, L. M. & Shrader, D. E. (1987). Investigation of a reduced palladium
chemical modifier for graphite furnace atomic absorption. J Anal At Spectrom, 2,
45-50.
[133] Cimadevilla Álvarez-Cabal, E., Wróbel, K. & Sanz-Medel, A. (1995). Capabilities and
limitations of different techniques in electrothermal atomic absorption spectrometry
for direct monitoring of arsenic, cadmium and lead contamination of sea-water. J Anal
At Spectrom, 10, 149-54.
[134] Arpadjan, S. & Krivan, V. (1988). Behavior of chromium in the graphite furnace
during the performance of the flameless absorption spectrometry. Fresenius’J Anal
Chem., 329, 745-749.
[135] Slavin, W., Manning, D. C. & Carnrick, G. R. (1981). The stabilized temperature
platform furnace. At Spectrosc, 2, 137-145.
[136] Ohlsson, K. E. A. & Frech, W. (1989). Photographic observation of molecular spectra
in inverse Zeeman-effect graphite furnace atomic absorption spectrometry. J Anal At
Spectrom, 4, 379-385.
[137] Yin, X. f., Schlemmer, G. & Welz, B. (1987). Cadmium determination in biological
materials using graphite furnace atomic absorption spectrometry with palladium
nitrate-ammonium nitrate modifier. Anal Chem., 59, 1462-1466.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 201
[138] L’vov, B. V. & Yatsenko, L. F. (1984). Carbothermal reduction of zinc, cadmium,
lead and bismuth oxides in graphite furnaces for atomic absorption analysis in the
presence of organic substances. Zh Anal Khim, 39, 1773-1780.
[139] Hulanicki, A., Bulska, E. & Wróbel, K. (1985). Effect of inorganic matrixes on the
determination of cadmium by atomic-absorption spectrometry with electrothermal
atomization. Analyst., 110, 1141-1145.
[140] Slavin, W. & Manning, D. C. (1980). The L'vov platform for furnace atomic
absorption analysis. Spectrochim Acta A, 35, 701-714.
[141] Hernández Caraballo, E. A., Domínguez, J. R. & Alvarado, D. J. (2001).
Determination of Cd in river sediment reference material by GF-ETAAS using lithium
tetraborate as chemical modifier. At Spectrosc, 22, 295-298.
[142] Feo, J. C., Castro, M. A., Lumbreras, J. M., de Celis, B. & Aller, A. J. (2003). Nickel
as a chemical modifier for sensitivity enhancement and fast atomization processes in
electrothermal atomic absorption spectrometric determination of cadmium in
biological and environmental samples. Anal Sci., 19, 1631-1636.
[143] Acar, O. (2001). Determination of cadmium and lead in biological samples by Zeeman
ETAAS using various chemical modifiers. Talanta, 55, 613-622.
[144] Arpadjan, S. & Karadjova, I. (2003). Behavior of volatile elements in the graphite
furnace in the presence of silver as matrix modifier. Spectrosc Lett, 36, 441-447.
[145] Fuyi, W., Zucheng, J., Bin, H. & Tianyou, P. (1999). Comparative studies on chemical
modification of polytetrafluoroethylene slurry in ETV-ICP-AES and ETAAS. J Anal
At Spectrom, 14, 1619-1624.
[146] Acar, O. (2005). Determination of cadmium, copper and lead in soils, sediments and
sea water samples by ETAAS using a Sc + Pd + NH4NO3 chemical modifier. Talanta,
65, 672-677.
[147] Imai, S., Kanematsu, Y., Satoh, A. & Yonetani, A. (2004). Pyrolytic and non-pyrolytic
boron nitride platforms in electrothermal AAS for the determination of cadmium in
sea and estuarine water without chemical modification. Anal Sci., 20, 1755-1758.
[148] Capelo, J. L., Lavilla, I. & Bendicho, C. (1998). Ultrasound-assisted extraction of
cadmium from slurried biological samples for electrothermal atomic absorption
spectrometry. J Anal At Spectrom, 13, 1285-1290.
[149] Bermejo-Barrera, P., Moreda-Piñeiro, A., Muñiz-Naveiro, O., Gómez-Fernández,
A. M. J. & Bermejo-Barrera, A. (2000). Optimization of a microwave-pseudo-
digestion procedure by experimental designs for the determination of trace elements in
seafood by atomic absorption spectrometry. Spectrochim Acta B, 55, 1351-1371.
[150] Bermejo-Barrera, P., Muñiz-Naveiro, O., Moreda-Piñeiro, A. & Bermejo-Barrera, A.
(2001). The multivariate optimization of ultrasonic bath-induced acid leaching for the
determination of trace elements in seafood by atomic absorption spectrometry. Anal
Chim Acta, 439, 211-227.
[151] Smichowski, P., Valiente, L. & Piccinna, M. (2001). ETAAS determination of
essential and potentially toxic trace elements in Antarctic Krill certified reference
material: evaluation of two microwave digestion procedures. At Spectrosc, 22, 350-
355.
[152] Santamaría-Fernández, R., Santiago-Rivas, S., Moreda-Piñeiro, A., Bermejo-Barrera,
A. & Bermejo-Barrera, P. (2004). Blending procedure for the cytosolic preparation of
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 202
mussel samples for AAS determination of Cd, Cr, Cu, Pb and Zn bound to low
molecular weight compounds. At Spectrosc, 25, 37-43.
[153] Aranda, P. R., Gil, R. A., Moyano, S., de Vito, I. & Martínez, L. D. (2008). Cloud
point extraction for ultra-trace Cd determination in microwave-digested biological
samples by ETAAS. Talanta, 77, 663-666.
[154] Bikkes, M., Polyak, K. & Hlavay, J. (2001). Fractionation of elements by particle size
and chemical bonding from aerosols followed by ETAAS determination. J Anal At
Spectrom, 16, 74-81.
[155] Arain, M. B., Kazi, T. G., Jamali, M. K., Jalbani, N., Afridi, H. I. & Shah, A. (2008).
Total dissolved and bioavailable elements in water and sediment samples and their
accumulation in Oreochromis mossambicus of polluted Manchar Lake. Chemosphere,
70, 1845-1856.
[156] Pancras, J. P., Ondov, J. M. & Zeisler, R. (2005). Multi-element electrothermal AAS
determination of 11 marker elements in fine ambient aerosol slurry samples collected
with SEAS-II. Anal Chim Acta, 538, 303-312.
[157] Zemberyova, M., Bartekova, J. & Hagarova, I. (2006). The utilization of modified
BCR three-step sequential extraction procedure for the fractionation of Cd, Cr, Cu, Ni,
Pb and Zn in soil reference materials of different origins. Talanta, 70, 973-978.
[158] Bermejo-Barrera, P., Fernández-Nocelo, S., Moreda-Piñeiro, A. & Bermejo-Barrera,
A. (1999). Usefulness of enzymatic hydrolysis procedures based on the use of pronase
E as sample pre-treatment for multi-element determination in biological samples.
J Anal At Spectrom, 14, 1893-1900.
[159] Bermejo-Barrera, P., Moreda-Pineiro, A., Moreda-Pineiro, J., Kauppila, T. &
Bermejo-Barrera, A. (2000). Slurry sampling for electrothermal AAS determination of
cadmium in seafood products. At Spectrosc, 21, 5-9.
[160] Vaeisaenen, A. & Suontamo, R. (2002). Comparison of ultrasound-assisted extraction,
microwave-assisted acid leaching and reflux for the determination of arsenic,
cadmium and copper in contaminated soil samples by electrothermal atomic
absorption spectrometry. J Anal At Spectrom, 17, 739-742
[161] Imai, S., Ishikawa, S., Kikuchi, Y. & Yonetani, A. (2004). Large-volume injection
electrothermal AAS combined with tungsten-treated pyrolytic graphite furnace:
Determination of cadmium in tap, snow and river water samples with phosphate
modifier. Anal Sci., 20, 575-577
[162] Barciela-Alonso, M. C., Pazos-Capeáns, P., Regueira-Míguens, M. E., Bermejo-
Barrera, A. & Bermejo-Barrera, P. (2004). Study of cadmium, lead and tin distribution
in surface marine sediment samples from Ría de Arousa (NW of Spain). Anal Chim
Acta, 524, 115-120.
[163] Silva, R. G. L., Santelli, R. E., Willie, S. N., Sturgeon, R. E. & Sella, S. M. (1999).
Alkaline solubilization of biological materials for trace element analysis by
electrothermal atomic absorption spectrometry. Analyst, 124, 1843-1846.
[164] Scriver, C., Kan, M., Willie, S., Soo, C. & Birnboim, H. (2005). Formic acid
solubilization of marine biological tissues for multi-element determination by ETAAS
and ICP-AES. Anal Bioanal Chem., 381, 1460-1466.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 203
[165] Žemberyová, M., Barteková, J., Závadská, M. & Šišoláková, M. (2007).
Determination of bioavailable fractions of Zn, Cu, Ni, Pb and Cd in soils and sludges
by atomic absorption spectrometry. Talanta, 71, 1661-1668.
[166] Vassileva, E., Baeten, H. & Hoenig, M. (2001). Advantages of the iridium permanent
modifier in fast programs applied to trace-element analysis of plant samples by
electrothermal atomic absorption spectrometry. Fresenius' J Anal Chem., 369,
491-495.
[167] Acar, O. (2005). Molybdenum, Mo-Ir and Mo-Ru coatings as permanent chemical
modifiers for the determination of cadmium and lead in sediments and soil samples by
electrothermal atomic absorption spectrometry. Anal Chim Acta, 542, 280-286.
[168] Acar, O. (2005). Evaluation of V, Ir, Ru, V-Ir, V-Ru, and W-V as permanent chemical
modifiers for the determination of cadmium, lead, and zinc in botanic and biological
slurries by electrothermal atomic absorption spectrometry. Anal Chim.Acta, 545,
244-251.
[169] Lima, E. C., Barbosa, R. V., Vaghetti, J. C. P. & Ferreira, L. S. (2002). Evaluation of
different permanent modifiers for the determination of cadmium in environmental
samples by electrothermal AAS. At Spectrosc, 23, 135-142.
[170] Lima, E. C., Barbosa, R. V., Brasil, J. L. & Santos, A. H. D. P. (2002). Evaluation of
different permanent modifiers for the determination of arsenic, cadmium and lead in
environmental samples by electrothermal atomic absorption spectrometry. J Anal At
Spectrom, 17, 1523-1529.
[171] Lima, E. C., Krug, F. J. & Jackson, K. W. (1998). Evaluation of tungsten-rhodium
coating on an integrated platform as a permanent chemical modifier for cadmium,
lead, and selenium determination by electrothermal atomic absorption spectrometry.
Spectrochim Acta B, 53, 1791-1804.
[172] Lima, E. C., Barbosa, F. & Drug, F. J. (2000). The use of tungsten-rhodium permanent
chemical modifier for cadmium determination in decomposed samples of biological
materials and sediments by electrothermal atomic absorption spectrometry. Anal Chim
Acta, 409, 267-274.
[173] Lima, E. C., Barbosa, F. Jr., Krug, F. J., Silva, M. M. & Vale, M. G. R. (2000).
Comparison of ultrasound-assisted extraction, slurry sampling and microwave-assisted
digestion for cadmium, copper and lead determination in biological and sediment
samples by electrothermal atomic absorption spectrometry. J Anal At Spectrom, 15,
995-1000.
[174] Borba da Silva, J. B., Borges, D. L. G., Silva da Veiga, M. A. M., Curtius, A. J. &
Welz, B. (2003). Determination of cadmium in biological samples solubilized with
tetramethylammonium hydroxide by electrothermal atomic absorption spectrometry,
using ruthenium as permanent modifier. Talanta, 60, 977-982.
[175] Brasil, J. L., Lima, E. C., Veses, R. C. & Tisott, M. M. (2004). Ultrasound-assisted
extraction of cadmium in environmental samples for ETAAS determination. At
Spectrosc, 25, 94-101.
[176] Acar, O. (2004). Electrothermal atomic absorption spectrometric determination of
cadmium and lead in environmental, botanic and biological samples by different
permanent modifiers. J Anal At Spectrom, 19, 709-711.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 204
[177] de Almeida Pereira, L., Gonçalves de Amorim, I. & Borba da Silva, J. B. (2004).
Development of methodologies to determine aluminum, cadmium, chromium and lead
in drinking water by ET AAS using permanent modifiers. Talanta, 64, 395-400.
[178] Piascik, M. & Bulska, E. (2001). Performance of electrodeposited noble metals as
permanent modifiers for the determination of cadmium in the presence of mineral
acids by electrothermal atomic absorption spectrometry. Spectrochim Acta B, 56,
1615-1623.
[179] Nölte, J. (2003). ICP emission spectrometry. A practical guide; Wiley-VCH Verlag:
Weinheim.
[180] Montaser, A., Ohls, K. D. & Golightly, D. W. (1998). Inductively coupled plasmas in
gases other than argon; A. Montaser, & D. W. Golightly (Eds.), Inductively coupled
plasmas in analytical atomic spectrometry; Wiley-VCH: New York.
[181] Dean, J. R. (2005). Practical inductively coupled plasma spectroscopy; Wiley:
Hoboken, NJ.
[182] Todolí, J. L. & Mermet, J. M. (2005). Elemental analysis of liquid microsamples
through inductively coupled plasma spectrochemistry. Trends Anal Chem., 24,
107-116.
[183] Maestre, S. E., Todolí, J. L. & Mermet, J. M. (2004). Evaluation of several pneumatic
micronebulizers with different designs for use in ICP-AES and ICP-MS. Future
directions for further improvements. Anal Bioanal Chem., 379, 888-899.
[184] Almagro, B., Ganan-Calvo, A. M., Hidalgo, M. & Canals, A. (2006). Flow focusing
pneumatic nebulizers in comparison with several micronebulizers in inductively
coupled plasma atomic emission spectrometry. J Anal At Spectrom, 21, 770-777.
[185] McLaughlin, R. L. J. & Brindle, I. D. (2005). Multimode sample introduction system.
United States Patent US 6,891,605 B2, May, 10.
[186] Ding, L., Liang, F., Huan, Y., Cao, Y., Zhang, H. & Jin, Q. (2000). A low-powered
microwave thermospray nebulizer for inductively coupled plasma atomic emission
spectrometry. J Anal At Spectrom, 15,293-296.
[187] Mora, J., Todolí, J. L., Rico, I. & Canals, A. (1999). Aerosol desolvation studies with
a thermospray nebulizer coupled to inductively coupled plasma atomic emission
spectrometry. Analyst, 123, 1229-1234.
[188] Beres, S. A., Bruckner, P. H. & Denoyer, E. R. (1994). Performance evaluation of a
cyclonic spray chamber for ICP-MS. At Spectrosc, 15, 96-99.
[189] Tanner, S. D., Baranov, B. I. & Bandura, D. R. (2002). Reaction cells and collision
cells for ICP-MS: a tutorial review. Spectrochim Acta B, 57, 1361-1452.
[190] Koppenaal, D. W., Eden, G. C. & Barinaga, C. J. (2004). Collision and reaction cells
in atomic mass spectrometry: development, status, and applications. J Anal At
Spectrom, 19, 561-570.
[191] Broekaert, J. A. C. (2005). Analytical atomic spectrometry with flames and plasmas;
Wiley-VCH: Weinheim.
[192] Sabe, R. & Rauret, G. (2004). Challenges for achieving trazability of analytical
measurements of heavy metals in environmental samples by isotopic dilution mass
spectrometry. Trends Anal Chem., 23, 273-280.
[193] Xu, S., Sturgeon, R. E., Guo, Y., Zang, W. & Zhao, H. (2005). Chemical vapor
generation of copper: optimization of generation media. Ann Chim, 95, 491-499.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 205
[194] Peña-Vázquez, E., Villanueva-Alonso, J. & Bermejo-Barrera, P. (2007). Optimization
of a vapor generation method for metal determination using ICP-OES. J Anal At
Spectrom, 22, 642-649.
[195] Feng, J. L., Lam, J. W. & Sturgeon, R. E. (2001). Expanding the scope of chemical
vapor generation for noble and transition metals. Analyst, 126, 1833-1837.
[196] Feng, J. L., Sturgeon, R. E., Lam, J. W. & D’Ulivo, A. (2005). Insights into the
mechanisms of chemical vapor generation of transition and noble metals. J Anal At
Spectrom, 20, 255-265.
[197] Pohl, P. & Zyrnicki, W. (2001). Study of chemical vapor generation of Au, Pd and Pt
by inductively coupled plasma atomic emission spectrometry. J Anal At Spectrom, 16,
1442-1445.
[198] Pohl, P. & Zyrnicki, W. (2001). On the transport of some metals into the inductively
coupled plasma during hydride generation process. Anal Chim Acta, 429, 135-143.
[199] Pohl, P. & Zyrnicki, W. (2002). Study of reaction of Ir, Os, Rh and Ru ions with
NaBH4 in the acid medium by inductively coupled plasma atomic emission
spectrometry. J Anal At Spectrom, 17, 746-749.
[200] Pohl, P. & Zyrnicki, W. (2003). Analytical features of Au, Pd and Pt chemical vapor
generation inductively coupled plasma atomic emission spectrometry. J Anal At
Spectrom, 18, 798-801.
[201] Asfaw, A. & Wibetoe, G. (2005). Simultaneous determination of hydride (Se) and
non-hydride forming (Ca, Mg, K, P, S and Zn) elements in various beverages (beer,
coffee, and milk), with minimum sample preparation, by ICP-AES and use of a dual-
mode sample introduction system. Anal Bioanal Chem., 382, 173-179.
[202] Ritschdorff, E. T., Fitzgerald, N., Mclaughlin, R. L. J. & Brindle, I. D. (2005). The use
of a modified Multimode Sample Introduction System for the simple and rapid
determination of cadmium by chemical vapor generation atomic absorption
spectrometry. Spectrochim Acta B, 60, 139-143.
[203] Grabi, G., Cavalli, P., Achilli, M., Rossi, G. & Omenetto, N. (1982). Use of the HGA-
500 graphite furnace as a sampling unit for ICP emission spectrometry. At Spectrosc,
3, 81-88.
[204] Kántor, T. & Záray, G. (1995). Improved design and optimization of an electrothermal
vaporization system for inductively coupled plasma atomic emission spectrometry.
Microchem J, 51, 266-277.
[205] Peng Tianyou & Jiang Zucheng. (1998). Fluorination assisted slurry electrothermal
vaporization in ICP-AES for the direct analysis of silicon dioxide powder. Fresenius’J
Anal Chem., 360, 43-46.
[206] Resano, M., Vanhaecke, F. & de Loos-Vollebregt, M. T. C. (2008). Electrothermal
vaporization for sample introduction in atomic absorption, atomic emission and
plasma mass spectrometry - A critical review with focus on solid sampling and slurry
analysis. J Anal At Spectrom, 23, 1450-1475.
[207] Martin-Esteban, A. & Slowikowski, B. (2003). Electrothermal vaporization -
inductively coupled plasma-mass spectrometry (ETV-ICP-MS): A valuable tool for
direct multielement determination in solid samples. Critical Rev Anal Chem., 33,
43-55.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 206
[208] Belarra, M. A., Resano, M., Vanhaecke, F. & Moens, L. (2002). Direct solid sampling
with electrothermal vaporization/atomization: what for and how?. Trends Anal Chem.,
21, 828-839.
[209] Todolí, J. L. & Mermet, J. M. (2005). Elemental analysis of liquid microsamples
through inductively coupled plasma spectrochemistry. Trends Anal Chem., 24, 107-
116.
[210] Vanhaecke, F., Resano, M. & Moens, L. (2002). Electrothermal vaporization ICP-
mass spectrometry (ETV-ICP-MS) for the determination and speciation of trace
elements in solid samples - A review of real-life applications from the author's lab.
Anal Bioanal Chem., 374, 188-195.
[211] Hu, B., Li, S., Xiang, G., He, M. & Jiang, Z. (2007). Recent progress in electrothermal
vaporization-inductively coupled plasma atomic emission spectrometry and
inductively coupled plasma mass spectrometry. Appl Spectrosc Rev., 42, 203-234.
[212] Vanderpool, R. A. & Buckley, W. T. (1999). Liquid–liquid extraction of cadmium by
sodium diethyldithiocarbamate from biological matrixes for isotope dilution
inductively coupled plasma mass spectrometry. Anal Chem., 71. 652-659.
[213] Jarrett, J. M., Xiao, G., Caldwell, K. L., Henahan, Shakirova, G. & Jones, R. L.
(2008). Eliminating molybdenum oxide interference in urine cadmium biomonitoring
using ICP-DRC-MS. J Anal At Spectrom, 23, 962-967.
[214] Rosland, E. & Lund, W. (1999). Direct determination of trace metals in sea-water by
inductively coupled plasma mass spectrometry. J Anal At Spectrom, 13, 1239-1244.
[215] Pozebon, D., Dressler, V. L. & Curtius, A. J. (1998). Determination of copper,
cadmium, lead, bismuth and selenium(IV) in sea-water by electrothermal vaporization
inductively coupled plasma mass spectrometry after online separation. J Anal At
Spectrom, 13, 363-369.
[216] Liu, H. W., Jiang, S. J. & Liu, S. H. (1999). Determination of cadmium, mercury and
lead in seawater by electrothermal vaporization isotope dilution inductively coupled
plasma mass spectrometry. Spectrochim Acta B, 54, 1367-1375.
[217] Xia, L., Wu, Y. & Hu, B. (2007). Hollow-fiber liquid-phase microextraction prior to
low-temperature electrothermal vaporization ICP-MS for trace element analysis in
environmental and biological samples. J Mass Spectrom, 42, 803-810.
[218] Li, L., Hu, B., Xia, L. & Jiang, Z. (2006). Determination of trace Cd and Pb in
environmental and biological samples by ETV-ICP-MS after single-drop
microextraction. Talanta, 70, 468-473.
[219] Pu, X., Jiang, Z. & Hu, B. (2006). Zirconia-coated graphite adsorption bar micro-
extraction combined with ETV-ICP-MS for the determination of trace amounts of Cd,
Hg and Pb in environmental and biological samples. J Mass Spectrom, 41, 887-893.
[220] Li, Y. C. & Jiang, S. J. (1998). Determination of Cu, Zn, Cd and Pb in fish samples by
slurry sampling electrothermal vaporization inductively coupled plasma mass
spectrometry. Anal Chim Acta, 359, 205-212.
[221] Liao, H. C. & Jiang, S. J. (1999). Determination of cadmium, mercury and lead in coal
fly ash by slurry sampling electrothermal vaporization inductively coupled plasma
mass spectrometry. Spectrochim Acta B, 54, 1233-1242.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 207
[222] Liao, H. C. & Jiang, S. J. (1999). EDTA as the modifier for the determination of Cd,
Hg and Pb in fish by slurry sampling electrothermal vaporization inductively coupled
plasma mass spectrometry. J Anal At Spectrom, 14, 1583-1588.
[223] Lu, H. H. & Jiang, S. J. (2001). Organic acids as the modifier to determine Zn, Cd, Tl
and Pb in soil by slurry sampling electrothermal vaporization inductively-coupled
plasma mass spectrometry. Anal Chim Acta, 429, 247-255.
[224] Rybak, M. E. & Salin, E. D. (2001). Direct determination of metals in soils and
sediments by induction heating-electrothermal vaporization (IH-ETV) inductively
coupled plasma-optical emission spectrometry (ICP-OES). Appl Spectrosc, 55, 816-
821.
[225] Maia, S. M., Pozebon, D. & Curtius, A. J. (2003). Determination of Cd, Hg, Pb and Tl
in coal and coal fly ash slurries using electrothermal vaporization inductively coupled
plasma mass spectrometry and isotopic dilution. J Anal At Spectrom, 18, 330-337.
[226] Dias, L. F., Miranda, G. R., Saint'Pierre, T. D., Maia, S. M., Frescura, V. L. A. &
Curtius, A. J. (2005). Method development for the determination of cadmium, copper,
lead, selenium and thallium in sediments by slurry sampling electrothermal
vaporization inductively coupled plasma mass spectrometry and isotopic dilution
calibration. Spectrochim Acta B, 60, 117-124.
[227] Vieira, M. A., Ribeiro, A. S., Dias, L. F. & Curtius, A. J. (2005). Determination of Cd,
Hg, Pb and Se in sediments slurries by isotopic dilution calibration ICP-MS after
chemical vapor generation using an on-line system or retention in an electrothermal
vaporizer treated with iridium. Spectrochim Acta B, 60, 643-652.
[228] Chen, S. F. & Jiang, S. J. (1998). Determination of cadmium, mercury and lead in soil
samples by slurry sampling electrothermal vaporization inductively coupled plasma
mass spectrometry. J Anal At Spectrom, 13, 1113-1117.
[229] Galbacs, G., Vanhaecke, F., Moens, L. & Dams, R. (1996). Determination of cadmium
in certified reference materials using solid sampling electrothermal vaporization
inductively coupled plasma mass spectrometry supplemented with thermogravimetric
studies. Microchem J, 54, 272-286.
[230] Bitterli, B. A., Cousin, H. & Magyar, B. (1997). Determination of metals in airborne
particles by electrothermal vaporization inductively coupled plasma mass
spectrometry after accumulation by electrostatic precipitation. J Anal At Spectrom, 12,
957-961.
[231] Ludke, C., Hoffmann, E., Skole, J. & Kriews, M. (1999). Determination of trace
metals in size fractionated particles from arctic air by electrothermal vaporization
inductively coupled plasma mass spectrometry. J Anal At Spectrom, 14, 1685-1690.
[232] Masson, P. (2005). Determination of trace elements in plants by electrothermal
vaporization-high frequency plasma emission spectrometry coupling. Canadian J Anal
Sci Spectrosc, 50, 268-279.
[233] Masson, P., Dauthieu, M., Trolard, F. & Denaix, L. (2007). Application of direct solid
analysis of plant samples by electrothermal vaporization-inductively coupled plasma
atomic emission spectrometry: Determination of Cd and Si for environmental
purposes. Spectrochim Acta B, 62, 224-230.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 208
[234] Luedke, C., Hoffmann, E. & Skole, J. (1994). Comparative studies on metal
determination in airborne particulates by LA-ICP-MS and furnace atomization non-
thermal excitation spectrometry. Fresenius' J Anal Chem., 350, 272-276.
[235] Tanaka, S., Yasushi, N., Sato, N., Fukasawa, T., Santosa, S. J., Yamanaka, K. &
Ootoshi, T. (1998). Rapid and simultaneous multi-element analysis of atmospheric
particulate matter using inductively coupled plasma mass spectrometry with laser
ablation sample introduction. J Anal At Spectrom, 13, 135-140.
[236] Chin, C. J., Wang, C. F. & Jeng, S. L. (1999). Multi-element analysis of airborne
particulate matter collected on PTFE-membrane filters by laser ablation inductively
coupled plasma mass spectrometry. J Anal At Spectrom, 14, 663-668.
[237] Okuda, T., Kato, J., Mori, J., Tenmoku, M., Suda, Y., Tanaka, S., He, K., Ma, Y.,
Yang, F., Yu, X., Duan, F. & Lei, Y. (2004). Daily concentrations of trace metals in
aerosols in Beijing, China, determined by using inductively coupled plasma mass
spectrometry equipped with laser ablation analysis, and source identification of
aerosols. Sci Total Environ., 330, 145-158.
[238] Hu, Z., Liu, Y., Gao, S., Hu, S., Dietiker, R. & Guenther, D. (2008). A local aerosol
extraction strategy for the determination of the aerosol composition in laser ablation
inductively coupled plasma mass spectrometry. J Anal At Spectrom, 23, 1192-1203.
[239] Coedo, A. G., Padilla, I. & Dorado, M. T. (2005). Determination of minor elements in
steelmaking flue dusts using laser ablation inductively coupled plasma mass
spectrometry. Talanta, 67, 136-143.
[240] Guo, X. & Lichte, F. E. (1995). Analysis of rocks, soils and sediments for the
chalcophile elements by laser ablation-inductively coupled plasma mass spectrometry.
Analyst, 120, 2707-2711.
[241] Lee, Y. L., Chang, C. C. & Jiang, S. J. (2003). Laser ablation inductively coupled
plasma mass spectrometry for the determination of trace elements in soil. Spectrochim
Acta B, 58, 523-530.
[242] Bellotto, V. R. & Miekeley, N. (2000). Improvements in calibration procedures for the
quantitative determination of trace elements in carbonate material (mussel shells) by
laser ablation ICP-MS. Fresenius' J Anal Chem., 367, 635-640.
[243] Barats, A., Pecheyran, C., Amouroux, D., Dubascoux, S., Chauvaud, L. & Donard,
O. F. X. (2007). Matrix-matched quantitative analysis of trace-elements in calcium
carbonate shells by laser-ablation ICP-MS: application to the determination of daily
scale profiles in scallop shell (Pecten maximus). Anal Bioanal Chem., 387, 1131-1140.
[244] Bellotto, V. R. & Miekeley, N. (2007). Trace metals in mussel shells and
corresponding soft tissue samples: A validation experiment for the use of Perna perna
shells in pollution monitoring. Anal Bioanal Chem., 389, 769-776.
[245] Morrison, G., Fatoki, O. S., Linder, S. & Lundehn, C. (2004). Determination of heavy
metal concentrations and metal fingerprints of sewage sludge from Eastern Cape
Province, South Africa by inductively coupled plasma - mass spectrometry (ICP-MS)
and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS).
Water Air Soil Pollut., 152, 111-127.
[246] Reinhardt, H., Kriews, M., Miller, H., Schrems, O., Ludke, C., Hoffmann, E. & Skole,
J. (2001). Laser ablation inductively coupled plasma mass spectrometry: a new tool for
trace element analysis in ice cores. Fresenius' J Anal Chem., 370, 629-636.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 209
[247] Jiménez, M. S., Gómez, M. T. & Castillo, J. R. (2007). Multi-element analysis of
compost by laser ablation-inductively coupled plasma mass spectrometry. Talanta, 72,
1141-1148.
[248] Watmough, S. A., Hutchinson, T. C. & Evans, R. D. (1998). Development of solid
calibration standards for trace elemental analyses of tree rings by laser ablation
inductively coupled plasma-mass spectrometry. Environ Sci Technol., 32, 2185-2190.
[249] Prohaska, T., Stadlbauer, C., Wimmer, R., Stingeder, G., Latkoczy, Ch., Hoffmann, E.
& Stephanowitz, H. (1998). Investigation of element variability in tree rings of young
Norway spruce by laser-ablation-ICPMS. Sci Total Environ., 219, 29-39.
[250] Monticelli, D., Iorio, A., Ciceri, E., Castelletti, A. & Dossi, C. (2009). Tree ring
microanalysis by LA-ICP-MS for environmental monitoring: validation or refutation?
Two case histories. Microchim Acta, 164, 139-148.
[251] Binet, M. R. B., Ma, R., McLeod, C. W. & Poole, R. K. (2003). Detection and
characterization of zinc- and cadmium-binding proteins in Escherichia coli by gel
electrophoresis and laser ablation-inductively coupled plasma-mass spectrometry.
Anal Biochem., 318, 30-38.
[252] Polatajko, A., Azzolini, M., Feldmann, I., Stuezel, T. & Jakubowski, N. (2007). Laser
ablation-ICP-MS assay development for detecting Cd- and Zn-binding proteins in Cd-
exposed Spinacia oleracea L. J Anal At Spectrom, 22, 878-887.
[253] Yang, L., Sturgeon, R. E. & Mester, Z. (2005). Quantitation of trace metals in liquid
samples by dried-droplet laser ablation inductively coupled plasma mass spectrometry.
Anal Chem., 77, 2971-2977.
[254] Bol'shov, M. A., Boutron, C. F., Ducroz, F. M., Gorlach, U., Kompanets, O. N.,
Rudnev, S. N. & Hutch, B. (1991). Direct ultratrace determination of cadmium in
Antarctic and Greenland snow and ice by laser atomic fluorescence spectrometry. Anal
Chim Acta, 251, 169-75.
[255] Bol'shov, M. A., Rudnev, S. N. & Huetsch, B. (1992). Determination of trace amounts
of cadmium by laser excited atomic fluorescence spectrometry. J Anal A. Spectrom, 7,
1-6.
[256] Bol’shov, M. A., Koloshnikov, V. G., Rudnev, S. N., Boutron, C. F., Gorlach, U. &
Patterson, C. C. (1992). Detection of trace amounts of toxic metals in environmental
samples by laser-excited atomic fluorescence spectrometry. J Anal At Spectrom, 7,
99-104.
[257] Bol’shov, M. A. & Boutron, C. F. (1994). Determination of heavy metals in polar
snow and ice by laser-excited atomic fluorescence spectrometry. Analusis, 22, M44-
M46.
[258] Cheam, V., Lawson, G., Lechner, J., Desrosiers, R. & Nriagu, J. (1996). Thallium and
cadmium in recent snow and firn layers in the Canadian Arctic by atomic fluorescence
and absorption spectrometries. Fresenius' J Anal Chem., 355, 332-335.
[259] Zhou, J. X., Hou, X., Yang, K. X. & Michel, R. G. (1998). Laser-excited atomic
fluorescence spectrometry in a graphite furnace with an optical parametric oscillator
laser for sequential multi-element determination of cadmium, cobalt, lead, manganese
and thallium in Buffalo river sediment. J Anal At Spectrom, 13, 41-47.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 210
[260] Duan, Y., Du, X., Li, Y. & Jin, Q. (1995). Characterization of a modified, low-power
argon microwave plasma torch (MPT) as an atomization cell for atomic fluorescence
spectrometry. Appl Spectrosc, 49, 1079-1085.
[261] Frentiu, T., Darvasi, E., Senila, M., Ponta, M. & Cordos, E. (2008). Preliminary
investigation of a medium power argon radiofrequency capacitively coupled plasma as
atomization cell in atomic fluorescence spectrometry of cadmium. Talanta, 76, 1170-
1176.
[262] Dědina J. & Tsalev, D. L. (1995). Hydride Generation Atomic Absorption
Spectrometry. Wiley and Sons: Surrey, U.K..
[263] Sturgeon, R. E., Guo, X. & Mester, Z. (2005). Chemical vapor generation: are further
advances yet possible?. Anal Bioanal Chem., 382, 881-883.
[264] Luna, A. S., Borges Pereira, H., Takase, I., Araujo Goncalves, R., Sturgeon, R. E. &
Calixto de Campos, R. (2002). Chemical vapor generation-electrothermal atomic
absorption spectrometry: new perspectives. Spectrochim Acta B, 57, 2047-2056.
[265] Guo, X., Sturgeon, R. E., Mester, Z. & Gardner, G. J. (2004). Vapor generation by UV
irradiation for sample introduction with atomic spectrometry. Anal Chem., 76, 2401-
2405.
[266] Pohl, P. (2004). Recent advances in chemical vapor generation via reaction with
sodium tetrahydroborate. Trends Anal Chem., 23, 21-27.
[267] Pohl, P. (2004). Hydride generation – recent advances in atomic emission
spectrometry. Trends Anal Chem., 23, 87-101.
[268] Narsito, Agterdenbos, J. & Santosa, S. J. (1990). Study of processes in the hydride
generation atomic absorption spectrometry of antimony, arsenic and selenium. Anal.
Chim. Acta, 237, 189-199.
[269] D’Ulivo, A. (2004). Chemical vapor generation by tetrahydroborate (III) and other
borane complexes in aqueous media – A critical discussion of fundamental processes
and mechanisms involved in reagent decomposition and hydride formation.
Spectrochim Acta B, 59, 793-825.
[270] Willie, S. N. (1996). First order speciation of As using flow injection hydride
generation atomic absorption spectrometry with in-situ trapping of the arsine in a
graphite furnace. Spectrochim. Acta B, 51, 1781-1790.
[271] Laborda, F., Bolea, E. & Castillo, J. R. (2007). Electrochemical hydride generation as
a sample introduction technique in atomic spectrometry: fundamentals, interferences
and applications. Anal Bioanal Chem., 388, 743-775.
[272] Chaudhry, M. M., Ure, A. M., Cooksey, B. G., Littlejohn, D. & Halls, D. J. (1991).
Investigation of in situ concentration of hydride forming elements in a graphite
furnace atomizer. Anal. Proc., 28, 44-46.
[273] Sturgeon, R. E., Willie, S. N., Sproule, G. I. & Berman, S. S. (1987). Sorption and
atomization of metallic hydrides in a graphite furnace. J Anal At Spectrom, 2, 719-722.
[274] Dědina, J. (1988). Evaluation of hydride generation and atomization for atomic
absorption spectrometry. Prog Analyt Spectrosc, 11, 251-360.
[275] D’Ulivo, A. & Chen, Y. W. (1989). Determination of cadmium in aqueous samples by
vapor generation with sodium tetraethylborate(III) reagent. J Anal At Spectrom, 4,
319-322.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 211
[276] Cacho, J., Beltrán, I. & Nerín, C. (1989). Generation of a volatile cadmium species in
an organic medium. J Anal At Spectrom, 4, 661-663.
[277] Lampugnani, L., Salvetti, C. & Tsalev, D. L. (2003). Hydride generation atomic
absorption spectrometry with different flow systems and in-atomizer trapping for
determination of cadmium in water and urine - Overview of existing data on cadmium
vapour generation and evaluation of critical parameters. Talanta, 61, 683-698.
[278] Valdés-Hevia y Temprano, M. C., Fernández de la Campa, M. R. & Sanz-Medel, A.
(1994). Sensitive inductively coupled plasma atomic emission spectrometric
determination of cadmium by continuous alkylation with sodium tetraethylborate. J
Anal At Spectrom, 9, 231-236.
[279] Sanz-Medel, A., Valdés-Hevia y Temprano, M. C., Bordel García, N. & Fernández de
la Campa, M. R. (1995). The use of surfactants to obtain cadmium atoms at room
temperature and its application for the cold vapor AAS determination of the metal.
Anal Proc., 32, 49-52.
[280] Sanz-Medel, A., Valdés-Hevia y Temprano, M. C., Bordel García, N. & Fernández de
la Campa, M. R. (1995). Generation of cadmium atoms at room temperature using
vesicles and its application to cadmium determination by cold vapor atomic
spectrometry. Anal Chem., 67, 2216-2223.
[281] Guo, X. W. & Guo, X. M. (1995). Studies on the reaction between cadmium and
potassium tetrahydroborate in aqueous solution and its application in atomic
fluorescence spectrometry. Anal Chim Acta, 310, 377-385.
[282] Guo, X. W. & Guo, X. M. (1995). Determination of cadmium at ultratrace levels by
cold vapour atomic absorption spectrometry. J Anal At Spectrom, 10, 987-991.
[283] Goenaga Infante, H., Fernández Sánchez, M. L. & Sanz-Medel, A. (1996). Ultratrace
determination of cadmium by atomic absorption spectrometry using hydride
generation with in situ preconcentration in a palladium-coated graphite atomizer.
J Anal At Spectrom, 11, 571-575.
[284] Goenaga Infante, H., Fernández Sánchez, M. L. & Sanz-Medel, A. (1997). Vesicular
hydride generation-in situ preconcentration-electrothermal atomic absorption
spectrometry determination of sub-parts-per-billion levels of cadmium. J Anal At
Spectrom, 12, 1333-1336.
[285] Bermejo-Barrera, P., Moreda-Piñeiro, J., Moreda-Piñeiro, A. & Bermejo-Barrera, A.
(1996). Use of flow injection cold vapor generation and preconcentration on coated
graphite tubes for the determination of cadmium in sea-water by electrothermal atomic
absorption spectrometry. J Anal At Spectrom, 11, 1081-1086.
[286] Bermejo-Barrera, P., Moreda-Piñeiro, J., Moreda-Piñeiro, A. & Bermejo-Barrera, A.
(1998). Iridium-coated graphite tubes for the direct determination of As, Cd, Hg and
Pb in seawater by vapor generation ETAAS. At Spectrosc, 19, 100-106.
[287] Hwang, T. J. & Jiang, H. J. (1997). Determination of cadmium by flow injection
isotope dilution inductively coupled plasma mass spectrometry with vapor generation
sample introduction. J Anal At Spectrom, 12, 579-584.
[288] Liu, M. & Xu, S. (1997). The determination of trace cadmium by flow injection cold
vapor generation AAS. At Spectrosc, 18, 195-201.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 212
[289] Matusiewicz, H., Kopras, M. & Sturgeon, R. E. (1997). Determination of cadmium in
environmental samples by hydride generation with in situ concentration and atomic
absorption detection. Analyst 1997, 122, 331-336.
[290] Liva Garrido, M., Muñoz-Olivas, R. & Cámara, C. (1998). Determination of cadmium
in aqueous media by flow injection cold vapor atomic absorption spectrometry.
Application to natural water samples. J Anal At Spectrom, 13, 295-300.
[291] Liva Garrido, M., Muñoz-Olivas, R. & Cámara, C. (1998). Interference removal for
cadmium determination in waste water and sewage sludge by flow injection cold
vapor generation atomic absorption spectrometry. J Anal At Spectrom, 13, 1145-1149.
[292] Tyson, J. F. (1999). High-performance, flow-based, sample pre-treatment and
introduction procedures for analytical atomic spectrometry. J Anal At Spectrom, 14,
169-178.
[293] Vargas-Razo, C. & Tyson, J. F. (2000). Determination of cadmium by flow injection-
chemical vapor generation-atomic absorption spectrometry. Fresenius J Anal Chem.,
366, 182-190.
[294] Liva, M., Muñoz-Olivas, R. & Cámara, C. (2000). Determination of Cd in sonicated
slurries and leachates of biological and environmental materials by FI-CV-AAS.
Talanta, 51, 381-387.
[295] Liang, J., Wang, Q. & Huang, B. (2004). Concentrations of hazardous heavy metals in
environmental samples collected in Xiamen, China, as determined by vapor generation
non-dispersive atomic fluorescence spectrometry. Anal Sci., 20, 85-88.
[296] Sun, H. W. & Ran, S. (2004). Enhancement reagents for simultaneous vapor
generation of zinc and cadmium with intermittent flow system coupled to atomic
fluorescence spectrometry. Anal Chim Acta, 509, 71-76.
[297] Li, G., Wu, L., Xin, J. & Hou, X. (2004). Chemical vapor generation by reaction of
cadmium withpotassium tetrahydroborate and sodium iodate in acidic aqueous
solution for atomic fluorescence spectrometric application. J Anal At Spectrom, 19,
1010-1013.
[298] Korkmaz, D., Demir, C., Aydin, F. & Ataman, O. Y. (2005). Cold vapor generation
and on-line trapping of cadmium species on quartz surface prior to detection by atomic
absorption spectrometry. J Anal At Spectrom, 20, 46-52.
[299] Chuachuad, W. & Tyson, J. F. (2005). Determination of cadmium by flow injection
atomic absorption spectrometry with cold vapor generation by a tetrahydroborate-form
anion-exchanger. J Anal At Spectrom, 20, 273-281.
[300] Vieira, M. A., Ribeiro, A. S., Dias, L. F. & Curtius, A. J. (2005). Determination of Cd,
Hg, Pb and Se in sediments slurries by isotopic dilution calibration ICP-MS after
chemical vapor generation using an on-line system or retention in an electrothermal
vaporizer treated with iridium. Spectrochim Acta B, 60, 643-652.
[301] Chuachuad, W. & Tyson, J. F. (2004). Determination of cadmium by electrothermal
atomic absorption spectrometry with flow injection chemical vapor generation from a
tetrahydroborate-form anion-exchanger and in-atomizer trapping. Canadian J Anal Sci
Spectrosc, 49, 362-373.
[302] Ke, Y., Sun, Q., Yang, Z., Xin, J., Chen, L. & Hou, X. (2006). Determination of trace
cadmium and zinc in corn kernels and related soil samples by atomic absorption and
Analytical Chemistry of Cadmium: Sample Pre-treatment… 213
chemical vapor generation atomic fluorescence after microwave-assisted digestion.
Spectrosc Lett, 39, 29-43.
[303] Manzoori, J. L., Abdolmohammad-Zadeh, H. & Amjadi, M. (2007). Ultratrace
determination of cadmium by cold vapor atomic absorption spectrometry after
preconcentration with a simplified cloud point extraction methodology. Talanta, 71,
582-587.
[304] Fu, Q., Yang, L. & Wang, Q. (2007). Determination of cadmium in seawater by vapor
generation atomic fluorescence spectrometry after online preconcentration with a
novel alkyl phosphinic acid resin. Spectrosc Lett, 40, 547-557.
[305] Cankur, O. & Yavuz Ataman, O. (2007). Chemical vapor generation of Cd and on-line
preconcentration on a resistively heated W-coil prior to determination by atomic
absorption spectrometry using an unheated quartz absorption cell. J Anal At Spectrom,
22, 791-799.
[306] Zhang, S. J. & Gan, W. E. (2007). Determination of cadmium using thermo-nebulous
phase chemical vapor generation with atomic fluorescence spectrometry. At Spectrosc,
28, 108-112.
[307] Chen, M. L., Tian, Y. & Wang, J. H. (2008). Integrating preconcentration,
tetrahydroborate immobilization, elution and chemical vapor generation onto a
cellulose surface for the determination of cadmium with atomic fluorescence
spectrometry. J Anal At Spectrom, 23, 876-880.
[308] Ebdon, L., Goodall, P., Hill, S. J., Stockwell, P. B. & Thompson, K. C. (1993). Ultra-
trace determination of cadmium by vapor generation atomic fluorescence
spectrometry. J Anal At Spectrom, 8, 723-729.
[309] Tao, S. & Kumamaru, T. (1995). Inductively coupled plasma atomic emission
spectrometric determination of cadmium in biological and environmental materials
using electrothermal vaporization after in situ alkylation. Anal Chim Acta, 310,
369-375.
[310] Valles Mota, J. P., Fernández de la Campa, M. R., García Alonso, J. I. & Sanz-Medel,
A. (1999). Determination of cadmium in biological and environmental materials by
isotope dilution inductively coupled plasma mass spectrometry: effect of flow sample
introduction methods. J Anal At Spectrom, 14, 113-120.
[311] Arbab-Zavar, M. H., Chamsaz, M., Youssefi, A. & Aliakbari, M. (2006). Mechanistic
aspects of electrochemical hydride generation for cadmium. Anal Chim Acta, 576,
215-220.
[312] Klockenkamper, R. (1997). Total reflection X-ray fluorescence analysis; John Wiley:
New York.
[313] Jenkins, R., Gould, R. W. & Gedcke, D. (1995). Quantitative X-ray spectrometry;
Marcel Dekker: New York.
[314] Van Grieken, R. E. & Marckowicz, A. A. (1993). Handbook of X-ray Spectrometry:
Methods and techniques; Marcel Dekker: New York.
[315] Knoth, J., Prange, A., Schneider, H. & Schwenke, H. (1997). Variable X-ray
excitation for total reflection X-ray fluorescence spectrometry using an Mo/W alloy
anode and a tunable double multilayer monochromator. Spectrochimica Acta B, 52,
907-913.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 214
[316] Van Meel, K., Fontàs, C., Van Grieken, R., Queralt, I., Hidalgo, M. & Marguí, E.
(2008). Application of high-energy polarised beam energy dispersive X-ray
fluorescence spectrometry to cadmium determination in saline solutions. J Anal At
Spectrom, 23, 1034-1037.
[317] Marguí, E., Fontàs, C., Van Meel, K., Van Grieken, R., Queralt, I. & Hidalgo, M.
(2008). High-energy polarized-beam energy-dispersive x-ray fluorescence analysis
combined with activated thin layers for cadmium determination at trace levels in
complex environmental liquid samples. Anal Chem., 80, 2357-2364.
[318] Potts, P. J., Ellis, A. T., Kregsamer, P., Streli, C., Vanhoof, C., West, M. &
Wobrauschek, P. (2005). Atomic spectrometry update. X-Ray fluorescence
spectrometry. J Anal At Spectrom, 20, 1124-1154.
[319] Krivan, V., Franek, M., Baumann, H. & Pavel, J. (1990). Sequential multielement
analysis of a single aerosol filter by different instrumental and wet-chemical methods.
Fresenius' J Anal Chem., 338, 583-587.
[320] Steinhoff, G., Haupt, O. & Dannecker, W. (2000). Fast determination of trace
elements on aerosol-loaded filters by X-ray fluorescence analysis considering the
inhomogeneous elemental distribution. Fresenius' J Anal Chem., 366, 174-177.
[321] Kyotani, T. & Iwatsuki, M. (1998). Multi-element analysis of environmental samples
by X-ray fluorescence spectrometry using a simple thin-layer sample preparation
technique. Analyst, 123, 1813-1816.
[322] Sinem Atgin, R., El-Agha, O., Zararsiz, A., Kocatas, A., Parlak, H. & Tuncel, G.
(2000). Investigation of the sediment pollution in Izmir Bay: trace elements.
Spectrochim Acta B, 55, 1151-1164.
[323] Krishna, A. K., Murthy, N. N. & Govil, P. K. (2007). Multielement analysis of soils by
wavelength-dispersive X-ray fluorescence spectrometry. At Spectrosc, 28, 202-214.
[324] Stephens, W. E. & Calder, A. (2004). Analysis of non-organic elements in plant
foliage using polarized X-ray fluorescence spectrometry. Anal Chim Acta, 527, 89-96.
[325] Hou, X., Peters, H. L., Yang, Z., Wagner, K. A., Batchelor, J. D., Daniel, M. M. &
Jones, B. T. (2003). Determination of trace metals in drinking water using solid-phase
extraction disks and X-ray fluorescence spectrometry. Appl Spectrosc, 57, 338-342.
[326] Marguí, E., Fontàs, C., Hidalgo, M. & Queralt, I. (2008). Improved instrumental
sensitivity for Cd determination in aqueous solutions using wavelength dispersive x-
ray fluorescence spectrometry, Rh-target tube instrumentation. Spectrochim Acta B,
63, 1329-1332.
[327] Fontàs, C., Marguí, E., Hidalgo, M. & Queralt, I. (2009). Improvement approaches for
the determination of Cr(VI), Cd(II), Pd(II) and Pt(IV) contained in aqueous samples
by conventional XRF instrumentation. X-Ray Spectrom, 38, 9-17.
[328] Eksperiandova, L. P., Blank, A. B. & Makarovskaya, Y. N. (2002). Analysis of waste
water by X-ray fluorescence spectrometry. X-Ray Spectrom, 31, 259-263.
[329] Heckel, J., Brumme, M., Weinert, A. & Irmer, K. (1991). Multi-element trace analysis
of rocks and soils by EDXRF using polarized radiation. X-Ray Spectrom, 20, 287-292.
[330] Hettipathirana, T. D. (2004). Simultaneous determination of parts-per-million level Cr,
As, Cd and Pb, and major elements in low level contaminated soils using borate fusion
and energy dispersive X-ray fluorescence spectrometry with polarized excitation.
Spectrochim Acta B, 59, 223-229.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 215
[331] Akyuez, T., Bassari, A., Saltoglu, T. & Kurtcebe, T. (1995). Determination of the
concentration of some elements in the Black Sea sediment samples using energy
dispersive X-ray fluorescence analyzer. Toxicol Environ Chem., 48, 125-133.
[332] Akyuz, T., Akyuz, S. & Bassari, A. (2001). Radioisotope excited EDXRF analysis of
sediment core samples from the southern part of the Black Sea. J Radioanal Nucl Ch,
250, 129-137.
[333] Manso, M., Carvalho, M. L. & Nunes, M. L. (2007). Characterization of essential and
toxic elements in cephalopod tissues by EDXRF and AAS. X-Ray Spectrom, 36,
413-418.
[334] Russell, P. A. & James, R. (1997). Determination of toxic elements in liquid
hazardous waste using high-resolution energy-dispersive x-ray fluorescence
spectrometry. J Anal At Spectrom, 12, 25-32.
[335] Nkono, N. A. & Asubiojo, O. I. (1998). Elemental composition of drinking water
supplies in three states in Southeastern Nigeria. J Radioanal Nucl Ch, 227, 117-119.
[336] Pepelnik, R., Prange, A. & Niedergesaess, R. (1994). Comparative study of multi-
element determination using inductively coupled plasma mass spectrometry, total
reflection x-ray fluorescence spectrometry and neutron activation analysis. J Anal At
Spectrom, 9, 1071-1074.
[337] Gasparics, T., Csato, I. & Gy Zaray. (1997). Analysis of Antarctic marine sediment by
inductively coupled plasma atomic emission and total reflection x-ray fluorescence
spectrometry. Microchem J, 55, 56-63.
[338] Pepelnik, R., Erbsloeh, B., Michaelis, W. & Prange, A. (1993). Determination of trace
element deposition into a forest ecosystem using total-reflection X-ray fluorescence.
Spectrochim Acta B, 48, 223-229.
[339] Theisen, M. & Niessner, R. (1999). Elemental analysis of airborne dust samples with
TXRF. Comparison of oxygen-plasma ashing on sapphire carriers and acid digestion
for sample preparation. Fresenius' J Anal Chem., 365, 332-337.
[340] Markert, B., Reus, U. & Herpin, U. (1994). The application of TXRF in instrumental
multielement analysis of plants, demonstrated with species of moss. Sci Total
Environ., 152, 213-220.
[341] Mages, M., Bandow, N., Kuester, E., Brack, W. & von Tuempling, W. (2008). Zinc
and cadmium accumulation in single zebrafish (Danio rerio) embryos - A total
reflection X-ray fluorescence spectrometry application. Spectrochim Acta B, 63, 1443-
1449.
[342] Prange, A., Boeddeker, H. & Kramer, K. (1993). Determination of trace elements in
river-water using total-reflection X-ray fluorescence. Spectrochim Acta B, 48, 207-215.
[343] Reus, U., Markert, B., Hoffmeister, C., Spott, D. & Guhr, H. (1993). Determination of
trace metals in river water and suspended solids by TXRF spectrometry. A methodical
study on analytical performance and sample homogeneity. Fresenius' J Anal Chem.,
347, 430-435.
[344] Vértes, A., Nagy, S. & Süvegh, K. (1998). Nuclear methods in mineralogy and
geology; Plenum Press: New York.
[345] Alfassi, Z. (1990). Activation analysis; CRC Press: Boca Raton, FL.
[346] Simonits, A., De Corte, F. & Hoste, J. (1975). Single-comparator methods in reactor
neutron activation analysis. J Radioanal Chem., 24, 31–46.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 216
[347] Acharya, R. N., Nair, A. G. C., Reddy, A. V. R. & Manohar, S. B. (2002). Validation
of a neutron activation analysis method using k0-standardization. Appl Radiat Isot, 57,
391-398.
[348] Dinescu, L. C., Steinnes, E., Duliu, O. G., Ciortea, C., Sjobakk, T. E., Dumitriu, D. E.,
Gugiu, M. M. & Haralambie, M. (2004). Distribution of some major and trace
elements in Danube Delta lacustrine sediments and soil. J Radioanal Nucl Ch, 262,
345-354.
[349] Coskun, M., Steinnes, E., Frontasyeva, M. V., Sjobakk, T. E. & Demkina, S. (2006).
Heavy Metal Pollution of Surface Soil in the Thrace Region, Turkey. Environ Monit
Assess, 119, 545-556.
[350] Grosheva, E., Zaichick, D. & Zaichick, V. (2007). Application of INAA in the
assessment of chemical elements in soils of the Khamar-Daban mountain range.
J Radioanal Nucl Ch, 271, 565-572.
[351] Akyuz, S., Akyuz, T., Algan, A. O., Mukhamedshina, N. M. & Mirsagatova, A. A.
(2002). Energy dispersive X-ray fluorescence and neutron activation analysis of
surficial sediments of the Sea of Marmara and the Black Sea around Istanbul.
J Radioanal Nucl Ch, 254, 569-575.
[352] Tanaka, Y., Kuno, A. & Matsumo, M. (2003). A study of environmental analysis of
urban river sediments using activation analysis. J Radioanal Nucl Ch, 255, 239-243.
[353] Kataoka, M., Kuno, A. & Matsumo, M. (2003). A study on vertical distribution of
elements and their chemical states in Yatsu tideland sediments. J Radioanal Nucl Ch,
255, 283-286.
[354] Yusof, A. M., Thanapalasingham, V. & Wood, A. K. H. (2007). Assessment of the
health of a river ecosystem due to the impact of pollution from industrial discharge
using instrumental neutron activation analysis and inductively coupled plasma-mass
spectrometry. J Radioanal Nucl Ch, 273, 525-531.
[355] Bin Saion, E., Wood, A. K. H., Sulaiman, A. Z. A., Alzahrany, A. A., Elias, Md S. &
Siong, W. B. (2007). Determination of heavy metal pollution in depth profile of
marine sediment samples from the Strait of Malacca. Fresenius Environ Bull, 16,
1279-1287.
[356] Adomako, D., Nyarko, B. J. B., Dampare, S. B., Serfor-Armah, Y., Osae, S., Fianko,
J. R. & Akaho, E. H. K. (2008). Determination of toxic elements in waters and
sediments from River Subin in the Ashanti Region of Ghana. Environ Monit Assess,
141, 165-175.
[357] Bem, H., Gallorini, M., Rizzio, E. & Krzeminska, M. (2003). Comparative studies on
the concentrations of some elements in the urban air particulate matter in Lodz City of
Poland and in Milan, Italy. Environ Int, 29, 423-428.
[358] Karakas, D., Oelmez, I., Tosun, S. & Tuncel, G. (2004). Trace and major element
compositions of Black Sea aerosol. J Radioanal Nucl Ch, 259, 187-192.
[359] Athari, M., Sohrabpour, M., Shahriari, M. & Rostami, S. (2004). Elemental
characterization of TSP and two size fractions of airborne particulate matter from
Tehran by INAA and AAS. J Radioanal Nucl Ch, 260, 351-356.
[360] Bergamaschi, L., Rizzio, E., Profumo, A. & Gallorini, M. (2005). Determination of
trace elements by INAA in urban air particulate matter and transplanted lichens.
J Radioanal Nucl Ch, 263, 745-750.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 217
[361] Giaveri, G., Bergamaschi, L., Rizzio, E., Verza, G., Zambelli, G., Brandone, A.,
Profumo, A., Baudo, R., Tartari, G. & Gallorini, M. (2005). INAA at the top of the
world: elemental characterization and analysis of airborne particulate matter collected
in the Himalayas at 5100 m high. J Radioanal Nucl Ch, 263, 725-732.
[362] Waheed, S., Rahman, A., Daud, M., Rahman, S., Islam, Z. & Ahmad, S. (2005). Air
quality evaluation of some industrial cities of Pakistan using INAA and AAS.
Radiochim Acta, 93, 487-495.
[363] Freitas, M. C. & Pacheco, A. M. G. (2007). Elemental concentrations of aerosols near
Portuguese power plants by INAA and PIXE. J Radioanal Nucl Ch, 271, 185-189.
[364] Avino, P., Capannesi, G. & Rosada, A. (2008). Heavy metal determination in
atmospheric particulate matter by Instrumental Neutron Activation Analysis.
Microchem J, 88, 97-106.
[365] Landsberger, S., Massicotte, A., Braisted, J. & Gong, S. (2008). Determination of
cadmium in Arctic air filters by epithermal neutron activation analysis and Compton
suppression. J Radioanal Nucl Ch, 276, 193-197.
[366] Ni, B., Tian, W., Wang, P., Zhang, L., Zhang, G., Liu, C., Huang, D. & Li, D. (2008).
Study on air pollution in Beijing's major industrial areas using multielements in
biomonitors and NAA techniques. Int J Environ Pollut., 32, 456-466.
[367] Marrero, J., Polla, G., Jiménez Rebagliati, R., Pla, R., Gómez, D. & Smichowski, P.
(2007). Characterization and determination of 28 elements in fly ashes collected in a
thermal power plant in Argentina using different instrumental techniques. Spectrochim
Acta B, 62, 101-108.
[368] Papaefthymiou, H., Symeopoulos, B. D. & Soupioni, M. (2007). Neutron activation
analysis and natural radioactivity measurements of lignite and ashes from Megalopolis
basin, Greece. J Radioanal Nucl Ch, 274, 123-130.
[369] Yusof, A. M., Yanta, N. F. & Wood, A. K. H. (2004). The use of bivalves as bio-
indicators in the assessment of marine pollution along a coastal area. J Radioanal Nucl
Ch, 259, 119-127.
[370] Serfor-Armah, Y., Carboo, D., Akuamoah, R. K. & Chatt, A. (2006). Determination of
selected elements in red, brown and green seaweed species for monitoring pollution in
the coastal environment of Ghana. J Radioanal Nucl Ch, 269, 711-718.
[371] Bergamaschi, L., Rizzio, E., Giaveri, G., Giordani, L., Profumo, A. & Gallorini, M.
(2005). INAA for the determination of trace elements and evaluation of their
enrichment factors in lichens of high altitude areas. J Radioanal Nucl Ch, 263,
721-724.
[372] Smodis, B. & Bleise, A. (2007). IAEA quality control study on determining trace
elements in biological matrices for air pollution research. J Radioanal Nucl Ch, 271,
269-274.
[373] Bergamaschi, L., Rizzio, E., Giaveri, G., Loppi, S. & Gallorini, M. (2007).
Comparison between the accumulation capacity of four lichen species transplanted to
a urban site. Environ Pollut., 148, 468-476.
[374] Saiki, M., Alves, E. R. & Marcelli, M. P. (2007). Analysis of lichen species for
atmospheric pollution biomonitoring in the Santo Andre municipality, Sao Paulo,
Brazil. J Radioanal Nucl Ch, 273, 543-547.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 218
[375] Ayrault, S., Galsomies, L., Amblard, G., Sciarretta, M. D., Bonhomme, P. & Gaudry,
A. (2002). Instrumental neutron activation analysis (INAA) and inductively coupled
plasma/mass spectrometry (ICP-MS) for trace element biomonitoring using mosses.
Int J Environ Anal Chem., 82, 463-473.
[376] Frontasyeva, M. V., Galinskaya, T. Y., Krmar, M., Matavuly, M., Pavlov, S. S.,
Povtoreyko, E. A., Radnovic, D. & Steinnes, E. (2004). Atmospheric deposition of
heavy metals in northern Serbia and Bosnia-Herzegovina studied by the moss
biomonitoring, neutron activation analysis and GIS technology. J Radioanal Nucl Ch,
259, 141-144.
[377] Cucu-Man, S., Mocanu, R., Culicov, O., Steinnes, E. & Frontasyeva, M. (2004).
Atmospheric deposition of metals in Romania studied by biomonitoring using the
epiphytic moss Hypnum cupressiforme. Int J Environ Anal Chem, 84, 845-854.
[378] Ermakova, E. V., Frontasyeva, M. V., Pavlov, S. S., Povtoreiko, E. A., Steinnes, E. &
Cheremisina, Y. N. (2004). Air pollution studies in Central Russia (Tver and Yaroslavl
regions) using the moss biomonitoring technique and neutron activation analysis.
J Atmos Chem, 49, 549-561.
[379] Culicov, O. A., Mocanu, R., Frontasyeva, M. V., Yurukova, L. & Steinnes, E. (2005).
Active moss biomonitoring applied to an industrial site in romania: relative
accumulation of 36 elements in moss-bags. 2005. Environ Monit Assess, 108, 229-240.
[380] Barandovski, L., Cekova, M., Frontasyeva, M. V., Pavlov, S. S., Stafilov, T., Steinnes,
E. & Urumov, V. (2008). Atmospheric deposition of trace element pollutants in
Macedonia studied by the moss biomonitoring technique. Environ Monit Assess, 138,
107-118.
[381] Bard, A. J. & Faulkner, L. R. (2001). Electrochemical methods. Fundamentals and
applications; John Wiley & Sons: New York.
[382] Wang, J. (1985). Stripping analysis. Principles, instrumentation and applications;
VCH: weinheim.
[383] Brainina, K. H. & Neyman, E. (1993). Electroanalytical stripping method; John Wiley
& Sons: New York.
[384] Economou, A. (2005). Bismuth-film electrodes: Recent developments and
potentialities for electroanalysis. Trends Anal Chem, 24, 334-340.
[385] Prego, R. & Cobelo-García, A. (2004). Cadmium, copper and lead contamination of
the seawater column on the Prestige shipwreck (NE Atlantic Ocean). Anal Chim Acta,
524, 23-26.
[386] Nedeltcheva, T., Atanassova, M., Dimitrov, J. & Stanislavova, L. (2005).
Determination of mobile form contents of Zn, Cd, Pb and Cu in soil extracts by
combined stripping voltammetry. Anal Chim Acta, 528, 143-146.
[387] Vargas Mamani, M. C., Aleixo, L. M., Ferreira de Abreu, M. & Rath, S. (2005).
Simultaneous determination of cadmium and lead in medicinal plants by anodic
stripping voltammetry. J Pharmaceutical Biomed Anal, 37, 709-713.
[388] Afzali, D., Mostafavi, A., Taher, M. A., Rezaeipour, E. & Khayatzadeh Mahani, M.
(2005). Natural analcime zeolite modified with 5-Br-PADAP for the preconcentration
and anodic stripping voltammetric determination of trace amount of cadmium. Anal
Sci., 21, 383-386.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 219
[389] Buzica, D., Gerboles, M., Borowiak, A., Trincherini, P., Passarella, R. & Pedroni, V.
(2006). Comparison of voltammetry and inductively coupled plasma-mass
spectrometry for the determination of heavy metals in PM10 airborne particulate
matter. Atmos Environ., 40, 4703-4710.
[390] Vieira dos Santos, A. C. & Masini, J. C. (2006). Development of a sequential injection
anodic stripping voltammetry (SI-ASV) method for determination of Cd(II), Pb(II)
and Cu(II) in wastewater samples from coatings industry. Anal Bioanal Chem., 385,
1538-1544.
[391] Carvalho, L. M., do Nascimento, P. C., Koschinsky, A., Bau, M., Stefanello, R. F.,
Spengler, C., Bohrer, D. & Jost, C. (2007). Simultaneous determination of cadmium,
lead, copper, and thallium in highly saline samples by anodic stripping voltammetry
(ASV) using mercury-film and bismuth-film electrodes. Electroanal, 19, 1719-1726.
[392] Honary, S., Ebrahimi, P., Naghibi, F., Mosaddegh, M. & Shahhoseini, S. (2007).
Study on the simultaneous determination of Pb and Cd in some commercial medicinal
plants by both atomic absorption and voltametry methods. Anal Lett, 40, 2405-2414.
[393] Monterroso, S. C. C., Carapuca, H. M. & Duarte, A. C. (2006). Mixed polyelectrolyte
coatings on glassy carbon electrodes: Ion-exchange, permselectivity properties and
analytical application of poly-L-lysine-poly(sodium 4-styrenesulfonate)-coated
mercury film electrodes for the detection of trace metals. Talanta, 68, 1655-1662.
[394] McGaw, E. A. & Swain, G. M. (2006). A comparison of boron-doped diamond thin-
film and Hg-coated glassy carbon electrodes for anodic stripping voltammetric
determination of heavy metal ions in aqueous media. Anal Chim Acta, 575, 180-189.
[395] Truzzi, C., Annibaldi, A., Illuminati, S., Bassotti, E. & Scarponi, G. (2008). Square-
wave anodic-stripping voltammetric determination of Cd, Pb, and Cu in a hydrofluoric
acid solution of siliceous spicules of marine sponges (from the Ligurian Sea, Italy, and
the Ross Sea, Antarctica). Anal Bioanal Chem, 392, 247-262.
[396] Hutton, E. A., Van Elteren, J. T., Ogorevc, B. & Smyth, M. R. (2004). Validation of
bismuth film electrode for determination of cobalt and cadmium in soil extracts using
ICP-MS. Talanta, 63, 849-855.
[397] Kefala, G., Economou, A. & Voulgaropoulos, A. (2004). A study of Nafion-coated
bismuth-film electrodes for the determination of trace metals by anodic stripping
voltammetry. Analyst, 129, 1082-1090.
[398] Kirgoez, U. A., Marín, S., Pumera, M., Merkoci, A. & Alegret, S. (2005). Stripping
voltammetry with bismuth modified graphite-epoxy composite electrodes.
Electroanal., 17, 881-886.
[399] Legeai, S., Soropogui, K., Cretinon, M., Vittori, O., Heeren de Oliveira, A., Barbier, F.
& Grenier-Loustalot, M. L. (2005). Economic bismuth-film microsensor for anodic
stripping analysis of trace heavy metals using differential pulse voltammetry. Anal
Bioanal Chem, 383, 839-847.
[400] Svancara, I., Baldrianova, L., Tesarova, E., Hocevar, S. B., Elsuccary, S. A. A.,
Economou, A., Sotiropoulos, S., Ogorevc, B. & Vytras, K. (2006). Recent advances in
anodic stripping voltammetry with bismuth-modified carbon paste electrodes.
Electroanal, 18, 177-185.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 220
[401] Legeai, S. & Vittori, O. (2006). A Cu/Nafion/Bi electrode for on-site monitoring of
trace heavy metals in natural waters using anodic stripping voltammetry: an alternative
to mercury-based electrodes. Anal Chim Acta, 560, 184-190.
[402] Kefala, G. & Economou, A. (2006). Polymer-coated bismuth film electrodes for the
determination of trace metals by sequential-injection analysis/anodic stripping
voltammetry. Anal Chim Acta, 576, 283-289.
[403] Zhu, W. W., Li, N. B. & Luo, H. Q. (2006). Anodic stripping voltammetry
determination of Pb(II) and Cd(II) at a bismuth/poly(aniline) film electrode. Anal Lett,
39, 2273-2284.
[404] Baldrianova, L., Svancara, I., Economou, A. & Sotiropoulos, S. (2006). Anodic
stripping voltammetry at in situ bismuth-plated carbon and gold microdisc electrodes
in variable electrolyte content unstirred solutions. Anal Chim Acta, 580, 24-31.
[405] Economou, A. & Voulgaropoulos, A. (2007). On-line stripping voltammetry of trace
metals at a flow-through bismuth-film electrode by means of a hybrid flow-
injection/sequential-injection system. Talanta, 71, 758-765.
[406] Kachoosangi, R. T., Banks, C. E., Ji, X. & Compton, R. G. (2007). Electroanalytical
determination of cadmium(II) and lead(II) using an in-situ bismuth film modified edge
plane pyrolytic graphite electrode. Anal Sci., 23, 283-289.
[407] Baldrianova, L., Svancara, I. & Sotiropoulos, S. (2007). Anodic stripping voltammetry
at a new type of disposable bismuth-plated carbon paste mini-electrodes. Anal Chim
Acta, 599, 249-255.
[408] Adraoui, I., Rhazi, M. E. & Amine, A. (2007). Fibrinogen-coated bismuth film
electrodes for voltammetric analysis of lead and cadmium using the batch injection
analysis. Anal Lett, 40, 349-368.
[409] Jia, J., Cao, L. & Wang, Z. (2007). Nafion/poly(sodium 4-styrenesulfonate) mixed
coating modified bismuth film electrode for the determination of trace metals by
anodic stripping voltammetry. Electroanal, 19, 1845-1849.
[410] Hwang, G. H., Han, W. K., Park, J. S. & Kang, S. G. (2008). Determination of trace
metals by anodic stripping voltammetry using a bismuth-modified carbon nanotube
electrode. Talanta, 76, 301-308.
[411] Wang, N. & Dong, X. (2008). Stripping voltammetric determination of Pb(II) and
Cd(II) based on the multiwalled carbon nanotubes-nafion-bismuth modified glassy
carbon electrodes. Anal Lett, 41, 1267-1278.
[412] Hočevar, S. B., Švancara, I., Vytřas, K. & Ogorevc, B. (2005). Novel electrode for
electrochemical stripping analysis based on carbon paste modified with bismuth
powder. Electrochim Acta, 51, 706-710.
[413] Hoyer, B. & Jensen, N. (2004). Use of sodium dodecyl sulfate as an antifouling and
homogenizing agent in the direct determination of heavy metals by anodic stripping
voltammetry. Analyst, 129, 751-754.
[414] Farghaly, O. A. & Ghandour, M. A. (2005). Square-wave stripping voltammetry for
direct determination of eight heavy metals in soil and indoor-airborne particulate
matter. Environ Res., 97, 229-235.
[415] Monterroso, S. C. C., Carapuca, H. M. & Duarte, A. C. (2005). Ion-exchange and
permselectivity properties of poly(sodium 4-styrenesulfonate) coatings on glassy
Analytical Chemistry of Cadmium: Sample Pre-treatment… 221
carbon: application in the modification of mercury film electrodes for the direct
voltammetric analysis of trace metals in estuarine waters. Talanta, 65, 644-653.
[416] Heitzmann, M., Basaez, L., Brovelli, F., Bucher, C., Limosin, D., Pereira, E., Rivas,
B. L., Royal, G., Saint-Aman, E. & Moutet, J. C. (2005). Voltammetric sensing of
trace metals at a poly(pyrrole-malonic acid) film modified carbon electrode.
Electroanal, 17, 1970-1976.
[417] Shams, E. & Torabi, R. (2006). Determination of nanomolar concentrations of
cadmium by anodic-stripping voltammetry at a carbon paste electrode modified with
zirconium phosphated amorphous silica. Sensors Actuators B, 117, 86-92.
[418] Carregalo, S., Merkoci, A. & Alegret, S. (2004). Application of graphite-epoxy
composite electrodes in differential pulse anodic stripping voltammetry of heavy
metals. Microchim Acta, 147, 245-251.
[419] Garnier, C., Lesven, L., Billon, G., Magnier, A., Mikkelsen, O. & Pizeta, I. (2006).
Voltammetric procedure for trace metal analysis in polluted natural waters using
homemade bare gold-disk microelectrodes. Anal Bioanal Chem, 386, 313-323.
[420] Zhu, W. W., Li, N. B. & Luo, H. Q. (2007). Simultaneous determination of
chromium(III) and cadmium(II) by differential pulse anodic stripping voltammetry on
a stannum film electrode. Talanta, 72, 1733-1737.
[421] Toghill, K. E., Wildgoose, G. G., Moshar, A., Mulcahy, C. & Compton, R. G. (2008).
The fabrication and characterization of a bismuth nanoparticle modified boron doped
diamond electrode and its application to the simultaneous determination of
cadmium(II) and lead(II). Electroanal, 20, 1731-1737.
[422] Parat, C., Betelu, S., Authier, L. & Potin-Gautier, M. (2006). Determination of labile
trace metals with screen-printed electrode modified by a crown-ether based
membrane. Anal Chim Acta, 573+574, 14-19.
[423] Cooper, J., Bolbot, J. A., Saini, S. & Setford, S. J. (2007). Electrochemical method for
the rapid on site screening of cadmium and lead in soil and water samples. Soil Water
Air Soil Pollut., 179, 183-195.
[424] Guell, R., Aragay, G., Fontas, C., Antico, E. & Merkoci, A. (2008). Sensitive and
stable monitoring of lead and cadmium in seawater using screen-printed electrode and
electrochemical stripping analysis. Anal Chim Acta, 627, 219-224.
[425] Meucci, V., Laschi, S., Minunni, M., Pretti, C., Intorre, L., Soldani, G. & Mascini, M.
(2009). An optimized digestion method coupled to electrochemical sensor for the
determination of Cd, Cu, Pb and Hg in fish by square wave anodic stripping
voltammetry. Talanta, 77, 1143-1148.
[426] Evans, A. (1987). Potentiometry and ion selective electrodes; John Wiley & Sons:
New York.
[427] Bakker, E. & Pretsch, E. (2005). Potentiometric sensors for trace-level analysis. Trend
Anal Chem, 24, 199-207.
[428] Kloock, J. P., Moreno, L., Bratov, A., Huachupoma, S., Xu, J., Wagner, T.,
Yoshinobu, T., Ermolenko, Y., Vlasov, Y. G. & Schoening, M. J. (2006). PLD-
prepared cadmium sensors based on chalcogenide glasses-ISFET, LAPS and mISE
semiconductor structures. Sensors and Actuators B Chemical, 118, 149-155.
[429] Bizerea-Spiridon, O. & Bizerea, M. (2002). Cd-ion selective membrane electrode
based microprobe for trace elements detection. J Trace Microprobe Tech, 20, 473-479.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 222
[430] Javanbakht, M., Shabani-Kia, A., Darvich, M. R., Ganjali, M. R. & Shamsipur, M.
(2000). Cadmium(II)-selective membrane electrode based on a synthesized tetrol
compound. Anal Chim Acta, 408, 75-81.
[431] Gupta, V. K., Jain, A. K., Ludwig, R. & Maheshwari, G. (2008). Electroanalytical
studies on cadmium(II) selective potentiometric sensors based on t-butyl
thiacalix[4]arene and thiacalix[4]arene in poly(vinyl chloride). Electrochim Acta, 53,
2362-2368.
[432] Panwar, A., Baniwal, S., Sharma, C. L. & Singh, A. K. (2002). A polystyrene based
membrane electrode for cadmium(II) ions. Fresenius' J Anal Chem, 368, 768-772.
[433] Abbas, M. N. & Zahran, E. (2005). Novel solid-state cadmium ion-selective electrodes
based on its tetraiodo- and tetrabromo-ion pairs with cetylpyridinium. J Electroanal
Chem, 576, 205-213.
[434] Khan, A. A. & Alam, M. M. (2003). Synthesis, characterization and analytical
applications of a new and novel organic-inorganic' composite material as a cation
exchanger and Cd(II) ion-selective membrane electrode: polyaniline Sn(IV)
tungstoarsenate. Reactive Functional Polymers, 55, 277-290.
[435] Nabi, S. A. & Alam, Z. Inamuddin. (2008). A cadmium ion-selective membrane
electrode based on strong acidic organic-inorganic composite cation-exchanger:
polyaniline Ce(IV) molybdate. Sensors Transducers J, 92, 87-98.
[436] Mashhadizadeh, M. H., Sheikhshoaie, I. & Saeid-Nia, S. (2005). Asymmetrical Schiff
bases as carriers in PVC membrane electrodes for cadmium(II) ions. Electroanalysis,
17, 648-654.
[437] Gupta, V. K., Singh, A. K. & Gupta, B. (2007). Schiff bases as cadmium(II) selective
ionophores in polymeric membrane electrodes. Anal Chim Acta, 583, 340-348.
[438] Zamani, H. A., Ganjali, M. R. & Adib, M. (2006). Cd(II) PVC-based membrane
sensor based on N'-[1-(2-furyl)methylidene]-2-furohydrazide. Sensor Letters, 4,
345-350.
[439] Rezaei, B., Meghdadi, S. & Sarandi, R. F. (2008). A fast response cadmium-selective
polymeric membrane electrode based on N,N'-(4-methyl-1,2-phenylene)diquinoline-2-
carboxamide as a new neutral carrier. J Hazard Mat, 153, 179-186.
[440] Ion, A. C., Bakker, E. & Pretsch, E. (2001). Potentiometric Cd2+-selective electrode
with a detection limit in the low ppt range. Anal Chim Acta, 440, 71-79.
[441] Gupta, K. C. & D'Arc, M. J. (2000). Effect of concentration of ion exchanger,
plasticizer and molecular weight of cyanocopolymers on selectivity and sensitivity of
Cd(II) ion selective electrode. Talanta, 52, 1087-1103.
[442] Gupta, K. C. & D'Arc, M. J. (2000). Cadmium ion-selective electrode based on
cyanocopolymer. Electroanalysis, 12, 1408-1413.
[443] Shamsipur, M. & Mashhadizadeh, M. H. (2001). Cadmium ion-selective electrode
based on tetrathia-12-crown-4. Talanta, 53, 1065-1071.
[444] Gupta, V. K., Chandra, S. & Mangla, R. (2002). Dicyclohexano-18-crown-6 as active
material in PVC matrix membrane for the fabrication of cadmium selective
potentiometric sensor. Electrochim Acta, 47, 1579-1586.
[445] Gupta, V. K., Jain, A. K. & Kumar, P. (2006). PVC-based membranes of
dicyclohexano-24-crown-8 as Cd(II) selective sensor. Electrochim Acta, 52, 736-741.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 223
[446] Singh, A. K., Mehtab, S., Singh, U. P. & Aggarwal, V. (2007). Comparative studies of
tridentate sulfur and nitrogen-containing ligands as ionophores for construction of
cadmium ionselective membrane sensors. Electroanalysis, 19, 1213-1221.
[447] Valcárcel, M. & Luque de Castro, M. D. (1988). Automatic methods of analysis;
Elsevier: New York.
[448] Namieśnik, J. (2001). Green analytical chemistry - some remarks. J Sep Sci., 24,
151-153.
[449] Anastas, P. T. (1999). Green Chemistry and the role of analytical methodology
development. Crit Rev Anal Chem, 29, 167-175.
[450] Thomson, G. Ocean chemical processes. www.water encyclopedia.com/Mi-Oc/Ocean-
Chemical-Processes.html.
[451] Boutron, C. & Martin, S. (1979). Preconcentration of dilute solutions at the 10-12 g/g
level by nonboiling evaporation with variable variance calibration curves. Anal Chem,
51, 140-145.
[452] Nishikawa, M., Ambe, Y. & Mizoguchi, T. (1985). Evaporation preconcentration of
trace elements in rainwater for inductively coupled plasma emission spectrometry.
Bunseki Kagaku, 34, 659-664.
[453] Gorlach, U. & Boutron, C. F. (1990). Preconcentration of lead, cadmium, copper and
zinc in water at the pg g-1 level by non-boiling evaporation. Anal Chim Acta, 236,
391-398.
[454] Qian, X. & Li, X. (1986). Comparison of two preconcentration techniques for neutron
activation analysis of natural water. Huanjing Huaxue, 5, 60-64.
[455] Dekersabiec, A. M., Blanc, G. & Pinta, M. (1985). Water analysis by Zeeman Atomic-
Absorption Spectrometry. Fresenius´Z Anal Chem, 322, 731-735.
[456] Harrison, S. H., LaFleur, P. D. & Zoller, W. H. (1975). Evaluation of lyophilization
for the preconcentration of natural water samples prior to neutron activation analysis.
Anal Chem, 47, 1685-1688.
[457] Burba, P., Lieser, K. H., Neitzert, V. & Roeber, H. M. (1978). Preconcentration and
determination of trace elements in fresh water and sea water. Comparison of results
obtained by different methods (x-ray fluorescence, neutron activation and atomic
absorption). Fresenius' Z Anal Chem, 291, 273-277.
[458] Kusaka, Y., Tsuji, H., Imai, S. & Ohmori, S. (1979). Neutron activation analysis of
trace elements in sea water samples. Radioisotopes, 28, 139-144.
[459] Suzuki, S., Okada, Y. & Hirai, S. (1997). Determination of trace elements in certified
river water reference materials by instrumental NAA using a freeze-dried
preconcentration. Bunseki Kagaku, 46, 223-227.
[460] Hiraide, M., Ito, T., Baba, M., Kawaguchi, H. & Mizuike, A. (1980). Multielement
preconcentration of trace heavy metals in water by coprecipitation and flotation with
indium hydroxide for inductively coupled plasma-atomic emission spectrometry. Anal
Chem, 52, 804-807.
[461] Mizuike, A., Hiraide, M. & Mizuno, K. (1983). Preconcentration of trace heavy metals
in large aqueous samples by coprecipitation-flotation in a flow system. Anal Chim
Acta, 148, 305-309.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 224
[462] Hiraide, M., Chen, Z. S. & Kawaguchi, H. (1991). Co-precipitation of traces of heavy
metals with indium hydroxide for graphite-furnace atomic-absorption spectrometry.
Anal Sci., 7, 65-68.
[463] Hiraide, M., Usami, T. & Kawaguchi, H. (1992). Minimization of the amount of
indium carrier in co-precipitation for the determination of cadmium by graphite-
furnace atomic-absorption spectrometry. Anal Sci, 8, 31-34.
[464] Kyung-Hee, P. & Yong-Nam, P. (1995). A new online coprecipitation
preconcentration technique for trace metal analysis by ICP-AES. Bull Korean Chem
Soc., 16, 422-427.
[465] Ueda, J. & Yamazaki, N. (1986). Flameless atomic-absorption spectrometric and
differential pulse polarographic determination of cadmium after co-precipitation with
hafnium hydroxide. Bull Chem Soc Jpn, 59, 1845-1848.
[466] Quigley, M. N. & Vernon, F. (1991). Comparison of coprecipitation and chelating ion
exchange for the preconcentration of selected heavy metals from seawater. Anal Proc,
28, 175-176.
[467] Hiraide, M., Chen, Z. S., Sugimoto, K. & Kawaguchi, H. (1995). Coprecipitation with
tin(IV) hydroxide followed by removal of tin carrier for the determination of trace
heavy metals by graphite-furnace atomic-absorption spectrometry. Anal Chim Acta,
302, 103-107.
[468] Kim, Y. S. & Kim, K. C. (1995). Simultaneous preconcentration and determination of
trace elements in water samples by coprecipitation-flotation with lanthanum hydroxide
[La(OH)3]. Bull Korean Chem Soc, 16, 582-587.
[469] Rahmi, D., Zhu, Y., Umemura, T., Haraguchi, H., Itoh, A. & Chiba, K. (2008).
Determination of 56 elements in Lake Baikal water by high-resolution ICP-MS with
the aid of a tandem preconcentration method. Anal Sci, 24, 1513-1517.
[470] Nakamura, T., Oka, H., Ishii, M. & Sato, J. (1994). Direct atomization atomic-
absorption-spectrometric determination of beryllium, chromium, iron, cobalt, nickel,
copper, cadmium and lead in water with zirconium hydroxide co-precipitation.
Analyst, 119, 1397-1401.
[471] Wu, J. & Boyle, E. A. (1997). Low blank preconcentration technique for the
determination of lead, copper and cadmium in small-volume sea water samples by
isotope-dilution ICP MS. Anal Chem, 69, 2464-2470.
[472] Peker, D. S. K., Turkoglu, O. & Soylak, M. (2007). Dysprosium(III) hydroxide
coprecipitation system for the separation and preconcentration of heavy metal contents
of table salts and natural waters. J Hazard Mater, 143, 555-560.
[473] Kagaya, S., Miwa, S., Mizuno, T. & Tohda, K. (2007). Rapid coprecipitation
technique using yttrium hydroxide for the preconcentration and separation of trace
elements in saline water prior to their ICP-AES determination. Anal Sci, 23, 1021-
1024.
[474] Soylak, M., Kars, A. & Narin, I. (2008). Coprecipitation of Ni2+, Cd2+ and Pb2+ for
preconcentration in environmental samples prior to flame atomic absorption
spectrometric determinations. J Hazard Mater, 159, 435-439.
[475] Umashankar, V., Radhamani, R., Ramadoss, K. & Murty, D. S. R. (2002).
Simultaneous separation and preconcentration of trace elements in water samples by
Analytical Chemistry of Cadmium: Sample Pre-treatment… 225
coprecipitation on manganese dioxide using D-glucose as reductant for KMnO4.
Talanta, 57, 1029-1038.
[476] Nakashima, S., Sturgeon, R. E., Willie, S. N. & Berman, S. S. (1988). Determination
of trace elements in sea-water by graphite-furnace atomic-absorption spectrometry
after pre-concentration by tetrahydroborate reductive precipitation. Anal Chim Acta,
207, 291-299.
[477] Sella, S., Sturgeon, R. E., Willie, S. N. & Campos, R. C. (1997). Flow Injection On-
line Reductive Precipitation Preconcentration With Magnetic Collection for
Electrothermal Atomic Absorption Spectrometry. J Anal At Spectrom, 12, 1281-1285.
[478] Zhuang, Z. X., Yang, C. L., Wang, X. R., Yang, P. Y. & Huang, B. L. (1996).
Preconcentration of trace elements from natural water with palladium precipitation.
Fresenius' J Anal Chem, 355, 277-280.
[479] Shan, X., Tie, J. & Xie, G. (1988). Determination of trace elements in water, sea-water
and biological samples by inductively coupled plasma atomic-emission spectrometry
after pre-concentration with ammonium pyrrolidine-1-carbodithioate precipitation. J
Anal At Spectom, 3, 259-263.
[480] Vircavs, M., Pelne, A., Rone, V. & Vircava, D. (1992). Oxidation product of
ammonium pyrrolidin-1-yldithioformate as a coprecipitator for the preconcentration of
vanadium, cobalt, zinc, arsenic, iron, cadmium, selenium and mercury from aqueous
solution. Analyst, 117, 1013-1017.
[481] Rao, R. R. & Chatt, A. (1993). Preconcentration neutron activation analysis for trace
elements in seawater [and biological material] by coprecipitation with 1-(2-
thiazolylazo)-2-naphthol, pyrrolidinedithiocarbamate, and N-nitrosophenylhydroxy-
lamine. J Radioanal Nucl Ch, 168, 439-448.
[482] Souza, A. S., dos Santos, W. N. L. & Ferreira, S. L. C. (2005). Application of Box–
Behnken design in the optimisation of an on-line pre-concentration system using
knotted reactor for cadmium determination by flame atomic absorption spectrometry.
Spectrochim Acta B, 60, 737 -742.
[483] Delayette-Mills, M., Karm, L., Janauer, G. E., Chan, P. K. & Bernier, W. E. (1981).
Selective separations by reactive ion exchange. IV. Pre-concentration of cadmium and
zinc by in situ precipitation as hexacyanoferrate(II) salts on gel and macro-porous ion-
exchange resins. Anal Chim Acta, 124, 365-372.
[484] Moore, R. V. (1982). Dibenzylammonium and sodium dibenzyldithiocarbamates as
prescipitants for pre-concentration of trace elements in water for analysis by energy-
dispersive X-ray fluorescence. Anal Chem, 54, 895-897.
[485] Zmijewska, W., Polkowska-Motrenko, H. & Stokowska, H. (1984). Preconcentration
of trace elements from water by coprecipitation and ion exchange. J Radioanal Nucl
Ch, 84, 319-328.
[486] Akatsuka, K. & Atsuya, I. (1987). Direct analysis of solid samples by atomic-
absorption spectrometry, following pre-concentration of trace elements from sea-water
with quinolin-8-ol. Fresenius´J Anal Chem, 329, 453-456.
[487] Akatsuka, K., Nobuyema, N. & Atsuya, I. (1988). Atomic-absorption spectrometry of
nanogram amounts of cadmium, lead and zinc after precipitation with quinolin-8-ol.
Anal Sci, 4, 281-285.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 226
[488] Beinrohr, E., Hofbauerova, H. Preconcentration of trace metals from acidic solutions
by coprecipitation with dithizone. Mikrochim Acta 1989, 98, 119-128.
[489] Garrido, I., Soto, R. M., Carlosena, A., López-Mahía, P., Muniategui, S. & Padra, D.
(2001). Anal Lett, 34, 1763-1779.
[490] Krishnamurty, K. V. & Reddy, M. M. (1977). Tris(pyrrolidine dithiocarbamato)-
cobalt(III) chelate matrix for trace metal preconcentration from aqueous solution by
coprecipitation. Anal Chem, 49, 222-226.
[491] Zhuang, Z., Wang, X., Yang, P., Yang, C. & Huang, B. (1994). Online flow injection
cobalt-ammonium pyrrolidin-1-yldithioformate coprecipitation for preconcentration of
trace amounts of metals in waters with simultaneous determination by inductively
coupled plasma atomic emission spectrometry. J Anal At Spectrom, 9, 779-784.
[492] Laumond, F., Copin-Montegut, G., Courav, P. & Nicolas, G. (1984). Cadmium,
copper and lead in the western Mediterranean Sea. Mar Chem, 15, 251-261.
[493] Beazley, P. I., Rao, R. R. & Chatt, A. (1994). Preconcentration neutron activation
analysis of trace elements in surface waters by coprecipitation with
pyrrolidinedithiocarbamate in the presence of Bi(III). J Radioanal Nucl Ch, 179, 267-
276.
[494] Min, R. W. & Hansen, E. H. (1995). Flow injection - flame atomic absorption
spectrometric determination of trace levels of cadmium with online preconcentration
by means of coprecipitation. Chem Anal (Warsaw), 40, 361-371.
[495] Stafilov, T., Pavlovska, G., Cundeva, K., Zendelovska, D. & Paneva, V. Separation,
preconcentration, and determination of cadmium in drinking waters. J Environ Sci
Heal A, 36, 735-746.
[496] Pavlovska, G., Stafilov, T. & Cundeva, K. (2001). Preconcentration and separation of
cadmium by use of cobalt(III) hexamethylenedithiocarbamate as a collector prior to its
determination by atomic-absorption spectrometry. Fresenius´J Anal Chem, 369, 670-
673.
[497] Chen, Z. S., Hiraide, M. & Kawaguchi, H. (1996). Preconcentration of trace heavy
metals in water by coprecipitation with magnesium oxinate for electrothermal atomic
absorption spectrometry. Mikrochim Acta, 124, 27-34.
[498] Chen, H., Jin, J. & Wang, Y. (1997). Flow injection on-line coprecipitation-
preconcentration system using copper(II) diethyldithiocarbamate as carrier for flame
atomic absorption spectrometric determination of cadmium, lead and nickel in
environmental samples. Anal Chim Acta, 353, 181-188.
[499] Komjarova, I. & Blust, R. (2006). Comparison of liquid-liquid extraction, solid-phase
extraction and co-precipitation preconcentration methods for the determination of
cadmium, copper, nickel, lead and zinc in seawater. Anal Chim Acta, 576, 221-228.
[500] Elçi, L., Sahin, U. & Oztas, S. (2007). Determination of trace amounts of some metals
in samples with high salt content by atomic-absorption spectrometry after cobalt /
diethyldithiocarbamate coprecipitation. Talanta, 44, 1017-1023.
[501] Liu, J., Chen, H., Mao, X. & Jin, X. (2000). Determination of trace copper, lead,
cadmium, and iron in environmental and biological samples by flame atomic
absorption spectrometry coupled to flow injection on-line coprecipitation
preconcentration using DDTC-nickel as coprecipitate carrier. Int J Environ An Ch, 76,
267-282.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 227
[502] Sato, H. & Ueda, J. (2000). Electrothermal atomic-absorption-spectrometric deter-
mination of cadmium after coprecipitation with nickel diethyldithiocarbamate. Anal
Sci, 16, 99-301.
[503] Kim, Y. S., Kim, K. C. & Lee, C. W. (1999). Preconcentration and determination of
trace Cd(II) and Pb(II) in a water sample by organic precipitate flotation with 8-
hydroxyquinoline. Bull Korean Chem Soc, 20, 431-435.
[504] Cundeva, K., Stafilov, T., Pavlovska, G., Karadjova, I. & Arpadjan, S. (2003).
Preconcentration procedures for trace cadmium determination in natural aqueous
systems prior to Zeeman ETAAS. Int J Environ An Ch, 83, 1009-1019.
[505] Soylak, M. & Erdogan, N. D. (2006). Copper(II)-rubeanic acid coprecipitation system
for separation-preconcentration of trace metal ions in environmental samples for their
flame atomic absorption spectrometric determinations. J Hazard Mater, 137, 1035-
1041.
[506] Fang, Z., Sperling, M. & Welz, B. (1991). Flame atomic-absorption spectrometric
determination of lead in biological samples using a flow-injection system with online
pre-concentration by co-precipitation without filtration. J Anal At Spectrom, 6, 301-306.
[507] Matsubara, C., Izumi, S., Takamura, K., Yoshioka, H. & Mori, Y. (1993).
Determination of trace amounts of phosphate in water after pre-concentration using a
thermally reversible polymer. Analyst, 118, 553-556.
[508] Ohyama, T., Arai, K., Nakagawa, T., Matsubara, C. & Takamura, K. (1997).
Collection of chemical substances in water with use of a thermally reversible polymer.
Bunseki Kagaku, 46, 59-62.
[509] Saitoh, T., Yoshida, Y., Matsudo, T., Fujiwara, S., Dobashi, A., Iwaki, K., Suzuki, Y.
& Matsubara, C. (1999). Concentration of hydrophobic organic compounds by
polymer-mediated extraction. Anal Chem, 71, 4506-4512.
[510] Saitoh, T., Yoshida, Y. & Matsubara, C. (2000). Concentration of nonylphenol and its
polyethoxylated derivatives by polymer-mediate extraction. J Chromatogr A, 891,
69-74.
[511] Matsubara, C., Kikuchi, N. & Takamura, K. (1995). Preconcentration on poly(methyl
vinyl ether) for the determination of polycyclic aromatic hydrocarbons in air. Bunseki
Kagaku, 44, 311-312.
[512] Hiraide, M. & Morishima, A. (1997). Collection of traces of heavy metals on a
thermally reversible polymer for the analysis of river water and sea water. Anal Sci,
13, 829-831.
[513] Saitoh, T., Satoh, F. & Hiraide, M. (2003). Concentration of heavy metal ions in water
using thermoresponsive chelating polymer. Talanta, 61, 811-817.
[514] Bakircioğlu, Y., Şeren, G. & Akman, S. (2000). Concentration of cadmium, copper
and zinc using water soluble polyacrylic acid polymer. Spectrochim Acta B, 55, 1129-
1133.
[515] Lund, W. & Larsen, B. V. (1974). The application of electrodeposition techniques to
flameless atomic absorption spectrometry: Part I. The determination of cadmium with
a tungsten filament. Anal Chim Acta, 70, 299-310.
[516] Lund, W. & Larsen, B. V. (1974). The application of electrodeposition techniques to
flameless atomic absorption spectrometry: Part II. Determination of cadmium in sea
water. Anal Chim Acta, 72, 57-62.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 228
[517] Jensen, F. O., Dolezal, J. & Langmyhr, F. J. (1974). Atomic-absorption spectrometric
determination of cadmium, lead and zinc in salts or salt solutions by hanging mercury
drop electrodeposition and atomization in a graphite furnace. Anal Chim Acta, 72,
245-250.
[518] Lund, W., Thomassen, Y. & Doevle, P. (1977). Flame-atomic absorption analysis for
trace metals after electrochemical preconcentration on a wire filament. Anal Chim
Acta, 93, 53-60.
[519] Long, S. E. & Snook, R. D. (1983). Electrochemical preconcentration technique for
use with inductively coupled plasma atomic-emission spectrometry. Part I. Analyst,
108, 1331-1338.
[520] Veber, M., Gomi ek, S. & Stre ko, V. (1987). Electrothermal atomic absorption
spectrometry of elements after electrochemical deposition on graphite electrodes. Anal
Chim Acta, 193, 157-167.
[521] Vrana, A. & Komarek, J. (1996). Determination of cadmium and copper with ET AAS
after electrochemical deposition on a graphite electrode. Fresenius' J Anal Chem, 355,
321-323.
[522] Zhang, G., Li, J., Fu, D., Hao, D. & Xiang, P. (1993). Atomic absorption
determination of traces of cadmium in urine after electrodeposition onto a tungsten
wire. Talanta, 40, 409-413.
[523] Komárek, J. & Holý, J. (1999). Determination of heavy metals by electrothermal
atomic absorption spectrometry after electrodeposition on a graphite probe.
Spectrochim Acta B., 54, 733-738.
[524] Abdullin, I. F., Turova, E. N. & Budnikov, G. K. (2000). Determination of copper and
cadmium by Atomic Absorption Spectrometry with Electrochemical and Sorption
Preconcentration. J Anal Chem, 55, 567-569.
[525] Bulska, E., Wałcerz, M., Jędral, W. & Hulanicki, A. (1997). On-line preconcentration
of lead and cadmium for flame atomic absorption spectrometry using a flow-through
electrochemical microcell. Anal Chim Acta, 357, 133-140.
[526] Knápek, J., Komárek, J. & Krásenský, P. (2005). Determination of cadmium by
electrothermal atomic absorption spectrometry using electrochemical separation in a
microcell. Spectrochim Acta B, 60, 393-398.
[527] Riddick, J. A., Bunger, W. B. & Saikano, T. K. (1986). Organic Solvents. Physical
Properties and Methods of Purification, vol. II; Willey: New York.
[528] Brooks, R. R., Presley, B. J. & Kaplan, I. R. (1967). APDC-MIBK extraction system
for the determination of trace metals in saline waters by atomic-absorption
spectrometry. Talanta, 14, 809-816.
[529] Danielsson, L. G., Magnusson, B., Westerlund, S. & Zhang, K. (1982). Trace-metal
determinations in estuarine waters by electrothermal atomic-absorption spectrometry
after extraction of dithiocarbamate complexes into Freon [1,1,2-trichlorotri-
fluoroethane]. Anal Chim Acta, 144, 183-188.
[530] Statham, P. J. (1985). The determination of dissolved manganese and cadmium in sea
water at low nmol 1−1 concentrations by chelation and extraction followed by
electrothermal atomic absorption spectrometry. Anal Chim Acta, 169, 149-159.
[531] Chakraborti, D., Adams, F., Van Mol, W. & Irgolic, K. J. (1987). Determination of
trace metals in natural waters at nanogram per litre levels by electrothermal atomic-
Analytical Chemistry of Cadmium: Sample Pre-treatment… 229
absorption spectrometry after extraction with sodium diethyldithiocarbamate. Anal
Chim Acta, 196, 23-31.
[532] Jung, W. T., Kim, S. K., Lee, K. J. & Sohn, D. H. (1997). Multi-element determination
of heavy metals in the Han River of South Korea using liquid-liquid extraction-flame
atomic absorption spectrometry. Jpn. J Toxicol Environ Health, 43, 243-250.
[533] Jan, T. K. & Young, D. R. (1978). Determination of microgram amounts of some
transition metals in sea water by methyl isobuthyl ketone-nitric acid successive
extraction and flameless atomic absorption spectrometry. Anal Chem, 50, 1250-1253.
[534] Apte, S. C. & Gunn, A. M. (1987). Rapid determination of copper, nickel, lead and
cadmium in small samples of estuarine and coastal waters by liquid - liquid extraction
and electrothermal atomic-absorption spectrometry. Anal Chim Acta, 193, 147-156.
[535] Kinrade, J. D. & Van Loon, J. C. (1974). Solvent extraction for use with flame atomic
absorption spectrometry. Anal Chem, 46, 1894-1898.
[536] Danielsson, L. G., Magnusson, B. & Westerlund, S. (1978). An improved metal
extraction procedure for the determination of trace metals in sea water by atomic
absorption spectrometry with electrothermal atomization. Anal Chim Acta, 98, 47-57.
[537] Bone, K. M. & Hibbert, W. D. (1979). Solvent extraction with ammonium
pyrrolidinedithiocarbamate and 2,6-dimethyl-4-heptanone for the determination of
trace metals in effluents and natural waters. Anal Chim Acta, 107, 219-229.
[538] Sturgeon, R. E., Berman, S. S., Desaulniers, A. & Russell, D. S. (1980). Pre-
concentration of trace metals from sea-water for determination by graphite-furnace
atomic-absorption spectrometry. Talanta, 27, 85-94.
[539] Magnusson, B. & Westerlund, S. (1981). Solvent extraction procedures combined with
back-extraction for trace metal determinations by atomic absorption spectrometry.
Anal Chim Acta, 131, 63-72.
[540] Lo, M. J., Yu, J. C., Hutchison, F. I. & Wai, C. M. (1982). Solvent extraction of
dithiocarbamates complexes and back-extraction with mercury (II) for determination
of trace metals in sea water by atomic absorption spectrometry. Anal Chem, 54, 2536-
2539.
[541] Batterham, G. J. & Parry, D. L. (1996). Improved dithiocarbamate-oxine solvent-
extraction method for the preconcentration of trace metals from sea water using metal-
exchange back-extraction. Mar Chem, 55, 381-388.
[542] Batterham, G. J., Munksgaard, N. C. & Parry, D. L. (1997). Determination of Trace
Metals in Sea-water by Inductively Coupled Plasma Mass Spectrometry After Off-line
Dithiocarbamate Solvent Extraction. J Anal At Spectrom, 12, 1277-1280.
[543] Ndung’u, K., Franks, R. P., Bruland, K. W. & Flegal, A. R. (2003). Organic
complexation and total dissolved trace metal analysis in estuarine waters: comparison
of solvent-extraction graphite furnace atomic absorption spectrometric and chelating
resin flow injection inductively coupled plasma-mass spectrometric analysis. Anal
Chim Acta, 481, 127-138.
[544] Itoh, Y., Kawamoto, H. & Akaiwa, H. (1987). Pre-concentration of cadmium(II) by
synergic extraction with dithizone and tributylphosphine oxide. Bunseki Kagaku, 36,
T119-T122.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 230
[545] Facchin, I. & Pasquín, C. (1995). Two-phase liquid-liquid extraction in
monosegmented flow analysis. Determination of cadmium with 1-(2'-pyridylazo)
naphthol. Anal Chim Acta, 308, 231-237.
[546] Suzuki, T., Kobayashi, H. & Sawada, K. (1982). The determination of cadmium in
sea-water by solvent-extraction using capriquat and carbon furnace atomic-absorption
spectrometry. Nippon Kagaku Kaishi, 7, 1167-1170.
[547] Ueda, K., Kitahara, S., Kubo, K. & Yamamoto, Y. (1987). Pre-concentration and
graphite-furnace AAS determination of ultra-trace elements in natural waters by
utilizing synergic extraction. Bunseki Kagaku, 36, 728-734.
[548] Ueda, K., Kubo, K., Yoshimura, O. & Yamamoto, Y. (1988). Determination of trace
metals in sea-water by graphite-furnace atomic-absorption spectrometry after pre-
concentration using synergic extraction. Bull Chem Soc Jpn, 61, 2791-2795.
[549] Granzhan, A.V., Kuchuk, G. M. & Charykov, A. K. (1990). Solvent extraction -
atomic-absorption spectrometry determination of beryllium and cadmium in some
natural samples. Zh Anal Khim, 45, 2015-2018.
[550] Nord, L. & Karlberg, B. (1981). Automated extraction system for flame atomic-
absorption spectrometry. Anal Chim Acta, 125, 199-202.
[551] Nord, L. & Karlberg, B. (1983). Sample pre-concentration by continuous-flow
extraction with a flow-injection atomic-absorption detection system. Anal Chim Acta
983, 145, 151-158.
[552] Fang, Z. (1995). Flow Injection Atomic Absorption Spectrometry; Wiley: West Sussex.
[553] Backstrom, K. & Danielsson, L. G. (1990). Design of a continuous-flow two-step
extraction sample work-up system for graphite-furnace atomic-absorption
spectrometry. Anal Chim Acta, 232, 301-315.
[554] Anthemidis, A. N., Zachariadis, G. A., Farastelis, C. G. & Stratis, J. A. (2004). On-
line liquid-liquid extraction system using a new phase separator form flame atomic
absorption spectrometric determination of ultra-trace cadmium in natural waters.
Talanta, 62, 437-443.
[555] Miro, M., Estela, J. M. & Cerda, V. (2005). Recent advances in on-line solvent
extraction exploiting flow injection/sequential injection analysis. Curr Anal Chem, 1,
329-343.
[556] Anthemidis, A. N. & Miro, M. (2009). Recent Developments in Flow
Injection/Sequential Injection Liquid-Liquid Extraction for Atomic Spectrometric
Determination of Metals and Metalloids. Appl Spectros Rev, 44, 140-167.
[557] Wang, J. & Hansen, E. H. (2002). Development of an automated sequential injection
on-line solvent extraction-back extraction procedure as demonstrated for the
determination of cadmium with detection by electrothermal atomic absorption
spectrometry. Anal Chim Acta, 456, 283-292.
[558] Majors, R. E. (1996). Liquid extraction techniques for sample preparation. LC-GC, 14,
936-938.
[559] Murray, D. A. J. (1979). Rapid micro extraction procedure for analyses of trace
amounts of organic compounds in water by gas choromatography and comparisons
with macro extraction methods. J Chromatogr A, 177, 135-140.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 231
[560] Zapf, A., Heyer, R. & Stan, H. (1995). Rapid micro liquid-liquid extraction method for
trace analysis of organic contaminants in drinking water. J Chromatogr A, 694,
453-461.
[561] Barrio, C. S., Melgosa, E. R., Asensio, J. S. & Bernal, J. G. (1996). Extraction of
pesticides from aqueous samples: A comparative study. Mikrochim Acta, 122, 267-
277.
[562] Carasek, E. (2000). A low-cost flame atomic absorption spectrometry method for
determination of trace metals in aqueous samples. Talanta, 51, 173-178.
[563] Luque-Pérez, E., Rios, A. & Valcárcel, M. (1997). Development of a liquid-liquid
(micro) extraction method for online monitoring of lead. Quim Anal, 16, 107-112.
[564] Sachsenberg, S., Klenke, T., Krumbein, W. E. & Zeeck, E. (1992). Back-extraction
procedure for the dithiocarbamate solvent extraction method. Rapid determination of
metals in sea-water matrices. Fresenius' J Anal Chem, 342, 163-166.
[565] Carasek, E., Tonjes, J. W. & Scharf, M. (2002). A new method of microvolume back-
extraction procedure for enrichment of Pb and Cd and determination by flame atomic
absorption spectrometry. Talanta, 56, 185-191.
[566] Carasek, E., Tonjes, J. W. & Scharf, M. (2002). A liquid-liquid microextraction
system for Pb and Cd enrichment and determination by flame atomic absorption
spectrometry. Quim Nova, 25, 748-752.
[567] Miura, J., Ishii, H. & Watanabe, H. (1976). Extraction and separation of nickel chelate
of 1-(2-thiazolylazo)-2-naphthol in nonionic surfactant solution. Bunseki Kagaku, 25,
808-809.
[568] Watanabe, H. (1982). Solution Behavior of Surfactants; K. L. Mittal, & E. J. Fendler
(Eds.), Plenum Press: New York.
[569] Pelizzetti, E. & Pramauro, E. (1985). Analytical applications of organized molecular
assemblies. Anal Chim Acta, 169, 1-29.
[570] Hinze, W. L. & Pramauro, E. (1993). A critical-review of surfactant-mediated phase
separations (cloud-point extractions) - theory and applications. Critical Rev Anal
Chem, 24, 133-177.
[571] Stalikas, C. D. (2002). Micelle-mediated extraction as a tool for separation and
preconcentration in metal analysis. Trend Anal Chem, 21, 343-355.
[572] Burguera, J. L. & Burguera, M. (2004). Analytical applications of organized
assemblies for on-line spectrometric determinations: present and future. Talanta, 64,
1099-1108.
[573] Bezerra, M. D., Arruda, M. A. Z. & Ferreira, S. L. C. (2005). Cloud point extraction as
a procedure of separation and pre-concentration for metal determination using
spectroanalytical techniques: A review. Appl Spectros Rev, 40, 269-299.
[574] Silva, M. F., Cerutti, E. S. & Martinez, L. D. (2006). Coupling cloud point extraction
to instrumental detection systems for Metal Analysis. Microchim Acta, 155, 349-364.
[575] Carabias-Martínez, R., Rodríguez-Gonzalo, E., Moreno-Cordero, B., Pérez-Pavón,
J. L., García-Pinto, C. & Fernández Laespada, E. (2000). Surfactant cloud point
extraction and preconcentration of organic compounds prior to chromatography and
capillary electrophoresis. J Chromatogra A, 902, 251-265.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 232
[576] Sosa-Ferrera, Z., Padrón-Sanz, C., Mahugo-Santana, C. & Santana-Rodríguez, J. J.
(2004). The use of micellar systems in the extraction and pre-concentration of organic
pollutants in environmental samples. Trend Anal Chem, 23, 469-479.
[577] Paleólogos, E. K., Giokas, D. L. & Karayannis, M. I. (2005). Micelle-mediated
separation and cloud-point extraction. Trend Anal Chem, 24, 426-436.
[578] Baghdadi, M. & Shemirani, F. (2008). Cold-induced aggregation microextraction: A
novel sample preparation technique based on ionic liquids. Anal Chim Acta, 613,
56-63.
[579] Attwood, D. & Florence, A. T. (1985). Surfactants Systems-Their Chemistry
Pharmacy and Biology; Chapman and Hall: London.
[580] Gullickson, N. D., Scamehom, J. F. & Harwell, J. H. (1989). Surfactant-Based
Separation Processes; Schamehom, J.F., Harwell, J.H., Ed., Marcel Dekker: New
York.
[581] García Pinto, C., Pérez Pavón, J. L., Moreno Cordero, B., Romero Beato, E. & García
Sánchez, S. (1996). Cloud point preconcentration and flame atomic absorption
spectrometry: application to the determination of cadmium. J Anal At Spectrom, 11,
37-41.
[582] Zhu, X., Zhu, X. & Wang, B. (2006). Determination of trace cadmium in water
samples by graphite furnace atomic absorption spectrometry after cloud point
extraction. Microchim Acta, 154, 95-100.
[583] Borkowska-Burnecka, J., Jakubiel, M. & Zyrnicki, W. (2008). Cloud point extraction
as sample preparation procedure prior to multielemental analysis by inductively
coupled plasma-optical emission spectrometry. Chem Anal (Warsaw), 53, 335-346.
[584] da Silva, M. A. M., Frescura, V. L. A. & Curtius, A. J. (2000). Determination of trace
elements in water samples by ultrasonic nebulization inductively coupled plasma mass
spectrometry after cloud point extraction. Spectrochim Acta B, 55, 803-813.
[585] Coelho, L. M. & Arruda, M. A. Z. (2005). Preconcentration procedure using cloud
point extraction in the presence of electrolyte for cadmium determination by flame
atomic absorption spectrometry. Spetrochim Acta B, 60, 743-748.
[586] Chen, J. & Teo, K. C. (2001). Determination of cadmium, copper, lead and zinc in
water samples by flame atomic absorption spectrometry after cloud point extraction.
Anal Chim Acta, 450, 215-222.
[587] Bezerra, M. A., Maêda, S. M. N., Oliveira, E. P., Batista de Carvalho, M. F. &
Santelli, R. E. (2007). Internal standardization for the determination of cadmium,
cobalt, chromium and manganese in saline produced water from petroleum industry by
inductively coupled plasma optical emission spectrometry after cloud point extraction.
Spectrochim Acta B, 62, 985-991.
[588] Manzoori, J. L. & Karim-Nezhad, G. (2004). Development of a cloud point extraction
and preconcentration method for Cd and Ni prior to flame atomic absorption
spectrometric determination. Anal Chim Acta, 521, 173-177.
[589] Yuan, C. G., Jiang, G. B., Cai, Y. Q., He, B. & Liu, J. F. (2004). Determination of
cadmium at the nanogram per liter level in seawater by graphite furnace AAS using
cloud point extraction, Atom Spectrom, 25, 170-176.
[590] Giokas, D. L., Eksperiandova, L. P., Blank, A. B. & Karayannis, M. I. (2004).
Comparison and evaluation of cloud point extraction and low-temperature directed
Analytical Chemistry of Cadmium: Sample Pre-treatment… 233
crystallization as preconcentration tools for the determination of trace elements in
environmental samples. Anal Chim Acta, 505, 51-58.
[591] Liang, P., Li, J. & Yang, X. (2005). Cloud Point Extraction Preconcentration of Trace
Cadmium as 1-Phenyl-3-methyl-4-benzoyl-5-pyrazolone Complex and Determination
by Flame Atomic Absorption Spectrometry. Microchim Acta, 152, 47-51.
[592] Shemirani, F., Abkenar, S. D. & Jamali, M. R. (2005). Determination of cadmium(II),
copper(II) and zinc(II) in water samples by flame atomic absorption spectrometry after
cloud point extraction Indian J Chem Sect A, 44, 1211-1214.
[593] Farajzadeh, M. A. & Fallahi, M. R. (2006). Simultaneous cloud-point extraction of
nine cations from water samples and their determination by flame atomic absorption
spectrometry. Anal Sci, 22, 635-639.
[594] Bezerra, M. A., Bruns, R. E. & Ferreira, S. L. C. (2006). Statistical design-principal
component analysis optimization of a multiple response procedure using cloud point
extraction and simultaneous determination of metals by ICP OES. Anal Chim Acta,
580, 251-257.
[595] Yamini, Y., Faraji, M., Shariati, S., Hassani, R. & Ghambarian, M. (2008). On-line
metals preconcentration and simultaneous determination using cloud point extraction
and inductively coupled plasma optical emission spectrometry in water samples. Anal
Chim Acta, 612, 144-151.
[596] Kilinc, E., Cetin, A., Togrul, M. & Hosgoren, H. (2008). Synthesis of bis(amino
alcohol)oxalamides and their usage for the preconcentration of trace metals by cloud
point extraction. Anal Sci, 24, 763-768.
[597] Filik, H., Dondurmacioglu, F. & Apak, R. (2008). Micelle mediated extraction of
cadmium from water and tobacco samples with glyoxal-bis(2-hydroxyanil) and
determination by electrothermal atomic absorption spectrometry. Int J Environ Anal
Chem, 88, 637-648.
[598] Afkhami, A., Madrakian, T. & Siampour, H. (2006). Flame atomic absorption
spectrometric determination of trace quantities of cadmium in water samples after
cloud point extraction in Triton X-114 without added chelating agents. J Hazard
Mater, 138, 269-272.
[599] Manzoori, J. L., Abdolmohammad-Zadeh, H. & Amjadi, M. (2007). Ultratrace
determination of cadmium by cold vapor atomic absorption spectrometry after
preconcentration with a simplified cloud point extraction methodology. Talanta, 71,
582-587.
[600] Candir, S., Narin, I. & Soylak, M. (2008). Ligandless cloud point extraction of Cr(III),
Pb(II), Cu(II), Ni(II), Bi(III), and Cd(II) ions in environmental samples with Tween 80
and flame atomic absorption spectrometric determination. Talanta, 77, 289-293.
[601] Gu, T. & Galera-Gómez, P. A. (1995). Clouding of Triton X-114: the effect of added
electrolytes on the cloud point of Triton X-114 in the presence of ionic surfactants.
Colloid Surf A, 104, 307-312.
[602] Komaromy-Hiller, G., Calkins, N. & Wandruszka, R. (1996). Changes in polarity and
aggregation number upon clouding of a nonionic detergent: effect of ionic surfactant
and sodium chloride. Langmuir, 12, 919-920.
[603] Armstrong, J. K., Chowdhry, B. Z., Snowden, M. J. & Leharne, S. A. (1998). Effect of
sodium chloride upon micellization and phase separation transitions in aqueous
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 234
solutions of triblock copolymers: a highsensitivity differential scanning calorimetry
study. Langmuir, 14, 2004-2010.
[604] Fernández-Laespada, M. E., Pérez-Pavón, J. L. & Moreno-Cordero, B. (1993).
Micelle-mediated methodology for the pre-concentration of uranium prior to its
determination by flow injection. Analyst, 118, 209-212.
[605] Fang, Q., Du, M. & Huie, C. W. (2001). Online incorporation of cloud point extraction
to flow-injection analysis. Anal Chem, 73, 3502-3505.
[606] Pena-Pereira, F., Lavilla, I. & Bendicho, C. (2009). Miniaturized preconcentration
methods based on liquid–liquid extraction and their application in inorganic ultratrace
analysis and speciation: A review. Spectrochim Acta B, 64, 1-15.
[607] Liu, S. & Dasgupta, P. K. (1995). Liquid droplet. A renewable gas sampling interface.
Anal Chem, 67, 2042-2049.
[608] Liu, H. & Dasgupta, P. K. (1996). Analytical chemistry in a drop. Solvent extraction
in a microdrop. Anal Chem, 68, 1817-1821.
[609] Jeannot, M. A. & Cantwell, F. F. (1996). Solvent microextraction into a single drop.
Anal Chem, 68, 2236-2240.
[610] Jeannot, M. A. & Cantwell, F. F. (1997). Mass transfer characteristics of solvent
extraction into a single drop at the tip of a syringe needle. Anal Chem, 69, 235-239.
[611] Dietz, C., Sanz, J. & Cámara, C. (2006). Recent developments in solid-phase
microextraction coatings and related techniques. J Chromatog A, 1103, 183-192.
[612] Chamsaz, M., Arbab-Zavar, M. H. & Nazari, S. (2003). Determination of arsenic by
electrothermal atomic absorption spectrometry using headspace liquid phase
microextraction after in situ hydride generation. J Anal At Spectrom, 18, 1279-1282.
[613] Xu, L., Basheer, C. & Lee, H. K. (2007). Developments in single-drop
microextraction. J Chromatog A, 1152, 184-192.
[614] Psillakis, E. & Kalogerakis, N. (2002). Developments in single-drop microextraction.
Trend Anal Chem, 21, 53-63.
[615] Fan, Z. & Zhou, W. (2006). Dithizone-chloroform single drop microextraction system
combined with electrothermal atomic absorption spectrometry using Ir as permanent
modifier for the determination of Cd in water and biological samples. Spectrochim
Acta B, 61, 870-874.
[616] Jiang, H. & Hu, B. (2008). Determination of trace Cd and Pb in natural waters by
direct single drop microextraction combined with electrothermal atomic absorption
spectrometry. Microchim Acta, 161, 101-107.
[617] Nazari, S. (2008). Determination of trace amounts of cadmium by modified graphite
furnace atomic absorption spectrometry after liquid phase microextraction. Microchem
J, 90, 107-112.
[618] Anthemidis Aristidis, N. & Adam Ibrahim, S. I. (2009). Development of on-line
single-drop micro-extraction sequential injection system for electrothermal atomic
absorption spectrometric determination of trace metals. Anal Chim Acta, 632, 216-220.
[619] Cantwell, F. F. & Losier, M. (2002). Liquid–liquid extraction; Pawliszyn J., Ed.,
Sampling and Sample Preparation for Field and Laboratory; Elsevier: Amsterdam.
[620] Xia, L., Hu, B., Jiang, Z., Wu, Y. & Liang, Y. (2004). Single-Drop Microextraction
Combined with Low-Temperature Electrothermal Vaporization ICPMS for the
Analytical Chemistry of Cadmium: Sample Pre-treatment… 235
Determination of Trace Be, Co, Pd, and Cd in Biological Samples. Anal Chem, 76,
2910-2915.
[621] Rezaee, M., Assadi, Y., Milani Hosseini, M. R., Aghaee, E., Ahmadi, F. & Berijani, S.
(2006). Determination of organic compounds in water using dispersive liquid–liquid
microextraction. J Chromatogr A, 1116, 1-9.
[622] Schramm, L. L. (2005). Emulsions, Foams, and Suspensions. Fundamentals and
Applications; Willey-VCH: Weinheim.
[623] Takahiko, B., Fumio, K., Susumu, N. & Katsuroku, T. (2000). Study of drop
coalescence behavior for liquid–liquid extraction operation. Chem Eng Sci, 55, 5385-
5391.
[624] Elham, Z. J., Araz, B., Yaghoub, A., Mohammad-Reza, M. H. & Mohammad-Reza, J.
(2007). Dispersive liquid-liquid microextraction combined with graphite furnace
atomic absorption spectrometry: ultra trace determination of cadmium in water
samples. Anal Chim Acta, 585, 305-311.
[625] Moghimi, A. (2008). Preconcentration ultra trace of Cd(II) in water samples using
dispersive liquid-liquid microextraction with salen(N,N'-bis(salicylidene)-
ethylenediamine) and determination graphite furnace atomic absorption spectrometry.
J Chin Chem Soc (Taipei), 55, 369-376.
[626] Pedersen-Bjergaard, S. & Rasmussen, K. E. (1999). Liquid-liquid-liquid
microextraction for sample preparation of biological fluids prior to capillary
electrophoresis. Anal Chem, 71, 2650-2656.
[627] Psillakis, E. & Kalogerakis, N. (2003). Developments in liquid-phase microextraction.
Trend Anal Chem, 22, 565-574.
[628] Rasmussen, K. E. & Pedersen-Bjergaard, S. (2004). Developments in hollow fibre-
based, liquid-phase microextraction, Trend Anal Chem, 23, 1-10.
[629] Peng, F., Liu, R., Liu, J. F., He, B., Hu, X. L. & Jiang, G. B. (2007). Ultrasensitive
determination of cadmium in seawater by hollow fiber supported liquid membrane
extraction coupled with graphite furnace atomic absorption spectrometry. Spectrochim
Acta B, 62, 499-503.
[630] Danielsson, L. G., Magnusson, B. & Zhang, K. (1982). Matrix interference in
determination of trace metals by graphite-furnace A.A.S. after Chelex-100 pre-
concentration. At Spectrosc, 2, 39-40.
[631] Baffi, F., Frache, R. & Dadone, A. (1984). Comparison between pre-concentration
methods for determination of the chemical forms of cadmium in inshore sea-water by
atomic-absorption spectrophotometry with electrothermal atomization. Ann Chim, 74,
385-397.
[632] Isozaki, A., Veki, K., Sasaki, H. & Utsumi, S. (1987). Simultaneous determination of
cadmium and lead by a graphite furnace-dual channel AAS using direct heating of
metal-adsorbed chelating resin. Bunseki Kagaku, 36, 672-677.
[633] Pai, S. C. (1988). Pre-concentration efficiency of chelex-100 resin for heavy metals in
seawater: Part 2. Distribution of Heavy Metals on a Chelex-100 Column and
Optimization of the Column Efficiency by a Plate Simulation Method. Anal Chim
Acta, 211, 271-280.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 236
[634] Pai, S. C., Fang, T. H., Chem, C. T. & Jeng, K. L. (1990). A low contamination
chelex-100 technique for shipboard pre-concentration of heavy metals in seawater.
Mar. Chem., 29, 295-306.
[635] Pyrzynska, K., Calatayud, J. M. & Mateo, J. V. G. (2001). Preconcentration of
cadmium on different sorbents in flow-injection system. Chem Anal (Warsaw), 46,
539-546.
[636] Soylak, M. & Narin, I. (2005). On-line preconcentration system for cadmium
determination in environmental samples by flame atomic absorption spectrometry.
Chem Anal (Warsaw), 50, 705-715.
[637] Pyrzynska, K. & Jonca, Z. (2000). Multielement preconcentration and removal of trace
metals by solid-phase extraction. Anal Lett, 33, 1441-1450.
[638] Horvath, Z., Lasztity, A. & Szakacs; Bozsai, G. (1985). Iminodiacetic
acid/ethylcellulose as a chelating ion exchanger: Part 1. Determination of trace metals
by atomic absorption spectrometry and collection of uranium. Anal Chim Acta, 173,
273-280.
[639] Brajter, K. & Slonawka, K. (1986). The efficiency of cellix-p for the preconcentration
of lead and other trace metals from waters. Anal Chim Acta, 185, 271-277.
[640] Pérez-Serradilla, J. A. & Luque de Castro, M. D. (2007). Integrated sorption-energy-
dispersive X-ray fluorescence detection for automatic determination of lead and
cadmium in low-concentration solutions. Anal Bioanal Chem, 389, 1541-1547.
[641] Minamisawa, H., Okunugi, R., Minamisawa, M., Tanaka, S., Saitoh, K., Arai, N. &
Shibukawa, M. (2006). Preconcentration and determination of cadmium by GFAAS
after solid phase extraction with synthetic zeolite. Anal Sci, 22, 709-713.
[642] Kenduzler, E. (2006). Determination of cadmium(II) in water and soil samples after
preconcentration with a new solid phase extractor. Sep Sci Technol, 41, 1645-1659.
[643] Beck, N. G., Franks, R. P. & Bruland, K. W. (2002). Analysis for Cd, Cu, Ni, Zn, and
Mn in estuarine water by inductively coupled plasma mass spectrometry coupled with
an automated flow injection system. Anal Chim Acta, 455, 11-22.
[644] Wan, C. C., Chiang, S. & Corsini, A. (1985). Two-column method for
preconcentration of trace metals in natural waters on acrylate resin. Anal Chem, 57,
719-723.
[645] Subramanian, K. S., Meranger, J. C., Wan, C. C. & Corsini, A. (1985). Int J Environ
Anal Chem, 19, 261-272.
[646] Soylak, M. & Elci, L. (2000). Solid phase extraction of trace metal ions in drinking
water samples from Kayseri-Turkey. J Trace Microprobe Tech, 18, 397-403.
[647] Hiraide, M., Arima, Y. & Mizuike, A. (1987). Separation and determination of traces
of heavy metals complexed with humic substances in river waters by sorption on
indium-treated Amberlite XAD-2 resin. Anal Chim Acta, 200, 171-179.
[648] Tokalioglu, S., Kartal, S. & Elci, L. (2002). Determination of trace metals in waters by
FAAS after enrichment as metal-HMDTC complexes using solid phase extraction.
Bull Korean Chem Soc, 23, 693-698.
[649] Nechar, M., Molina, M. F. & Bosque-Sendra, J. M. (1999). Application of Doehlert
optimization and factorial designs in developing and validating a solid-phase
spectrophotometric determination of trace levels of cadmium Anal Chim Acta, 382,
117-130.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 237
[650] Yaguchi, S., Yanazaki, S., Yamamoto, H., Urayana, Y., Hata, W., Kasahara, Y. &
Goto, K. (1988). Acid-soluble membrane filter for the pre-concentration and
electrothermal atomisation atomic absorption spectrometric determination of trace
levels of cadmium in water. Analyst, 113, 1695-1698.
[651] Leepipatpiboon, V. (1995). Trace enrichment by solid-phase extraction for the
analysis of heavy metals in water. J Chromatogr A, 697, 137-43
[652] Wells, M. L. & Bruland, K. W. (1998). An improved method for rapid
preconcentration and determination of bioactive trace metals in seawater using solid
phase extraction and high resolution inductively coupled plasma mass spectrometry.
Mar Chem, 63, 145-153.
[653] Abbasse, G., Ouddane, B. & Fischer, J. C. (2002). Determination of total and labile
fraction of metals in seawater using solid phase extraction and inductively coupled
plasma atomic emission spectrometry (ICP-AES). J Anal At Spectrom, 17, 1354-1358.
[654] Hu, Q., Yang, G., Zhao, Y. & Yin, J. (2003). Determination of copper, nickel, cobalt,
silver, lead, cadmium, and mercury ions in water by solid-phase extraction and RP-
HPLC with UV-Vis detection. Anal Bioanal Chem, 375, 831-835.
[655] Ouddane, B., Abbasse, G., Halwani, J. & Fischer, J. C. (2004). Determination of metal
partitioning in porewater extracted from the Seine River Estuary sediment (France).
J Environ Monitor, 6, 243-253.
[656] Liu, Z. S. & Huang, S. D. (1992). Determination of copper and cadmium in sea water
by preconcentration and electrothermal atomic absorption spectrometry. Anal Chim
Acta, 267, 31-37.
[657] Otero-Romaní, J., Moreda-Piñeiro, A., Bermejo-Barrera, A. & Bermejo-Barrera, P.
(2005). Evaluation of commercial C18 cartridges for trace elements solid phase
extraction from seawater followed by inductively coupled plasma – optical emission
spectrometry determination. Anal Chim Acta, 536, 213-218.
[658] Taher, M. A., Puri, B. K. & Bansal, R. K. (1998). Simultaneous determination of
cadmium and lead in real and environmental samples by differential pulse
polarography after adsorption of their 2-nitroso-1-naphthol-4-sulfonic acid-
tetradecyldimethylbenzylammonium ion-associated complex on microcrystalline
naphthalene. Microchem J, 58, 21-30.
[659] Costa, A.C.S., Lopes, L., Korn, M. G. A. & Portela, J. G. (2002). Separation and
preconcentration of cadmium, copper, lead, nickel and zinc by solid-liquid extraction
of their cocrystallized naphthalene dithizone chelate in saline matrices. J Brazil Chem
Soc, 13, 674-678.
[660] Cesur, H. & Bati, B. (2002). Determination of cadmium by FAAS after solid-phase
extraction of its 1-benzylpiperazinedithiocarbamate complex on microcrystalline
naphthalene. Turk J Chem, 26, 29-35.
[661] Bhalotra, A. & Atamiyot; Puri, B. K. (2002). Simultaneous determination of cadmium
and lead in a standard alloy, various biological and environmental samples by
differential pulse polarography after preconcentration by solid phase extraction of their
1-(2-thiazolylazo)-2-naphthol-tetraphenylborate complexes on microcrystalline
naphthalene. Quim Anal, 20, 229-236.
[662] Mohammad Ali, T. (2003). Differential pulse polarographic determination of cadmium
after solid liquid extraction and preconcentration using PAN. J Chem, 27, 529-537.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 238
[663] Saracoglu, S. & Elci, L. (2002). Column solid-phase extraction with Chromosorb-102
resin and determination of trace elements in water and sediment samples by flame
atomic absorption spectrometry. Anal Chim Acta, 452, 77-83.
[664] Saracoglu, S., Soylak, M., Elci, L. & Dogan, M. (2002). Determination of Cu, Fe, Ni,
Co, Pb, Cd, Mn, and Cr in natural water samples after solid phase extraction on
Chromosorb 102. Anal Lett, 35, 2603-2616.
[665] Tokman, N. & Akman, S. (2004). Determination of bismuth and cadmium after solid-
phase extraction with Chromosorb-107 in a syringe. Anal Chim Acta, 519, 87-91.
[666] Tuzen, M., Parlar, K. & Soylak, M. (2005). Enrichment/separation of cadmium(II) and
lead(II) in environmental samples by solid phase extraction. J. Hazard. Mater, 121,
79-87.
[667] Melek, E., Tuzen, M. & Soylak, M. (2006). Flame atomic absorption spectrometric
determination of cadmium(II) and lead(II) after their solid phase extraction as
dibenzyldithiocarbamate chelates on Dowex Optipore V-493. Anal Chim Acta, 578,
213-219.
[668] Myasoedova, G. V., Shcherbinina, N. I., Svanidze, Z. S., Varshal, G. M. &
Myasoedova, B. F. (1986). Atomic-absorption determination of cadmium in mineral
waters after its pre-concentration by sorption. Zh Anal Khim, 41, 477-480.
[669] Svanidze, Z. S., Sedykh, E. M., Bannykh, L. N. & Myasoedov, B. F. (1987). Atomic-
absorption determination of cadmium after direct introduction of an organic sorbent
into the atomizer. Zh Anal Khim, 42, 1989-1994.
[670] Ishmiyarova, G. R., Shcherbinina, N. I., Sedykh, E. M., Myasoedova, G. V., Vul’Fron,
E. K. & Vernadskii, V. I. (1988). Sorption pre-concentration of copper, lead, cobalt,
nickel and cadmium from sea-water and their electrothermal atomic-absorption
determination in sorbent suspension. Zh Anal Khim, 43, 1981-1986.
[671] Sedykh, E. M., Myasoedova, G. V., Ishmiyarova, G. R. & Kasimova, O. G. (1990).
Direct analysis of sorbent-concentrate in a graphite furnace. Zh Anal Khim, 45, 1895-
1903.
[672] Sedykh, E. M., Yu, G., Ishmiyarova, G. R. & Ostronova, M. M. (1994). Methods for
the analysis of sorbent concentrate in graphite furnace AAS. At Spectrosc, 15,
245-249.
[673] Samchuk, A. I., Kazakevich; Yu. E., Danilova, E., Khabazova, T. A., Emets, L. V. &
Kokot, T. K. (1988). Atomic-absorption determination of heavy metals in natural
waters. Zh Anal Khim, 43, 629-631.
[674] Blain, S., Appriov, P. & Handel, H. (1993). Preconcentration of trace metals from sea
water with the chelating resin Chelamine. Anal Chim Acta, 272, 91-97.
[675] Cesur, H., Macit, M. & Bati, B. (2000). Determination of copper, nickel and cadmium
by FAAS after preconcentration with zinc-piperazinedithiocarbamate loaded on
activated carbon by solid-phase extraction Anal Lett, 33, 1991-2004.
[676] Daorattanachai, P., Unob, F. & Imyim, A. (2005). Multi-element preconcentration of
heavy metal ions from aqueous solution by APDC impregnated activated carbon.
Talanta, 67, 59-64.
[677] Kiran, K. & Janardhanam, K. (2007). Determination of trace metals in water samples
by flame atomic absorption spectrometry using column solid-phase extraction. Asian J
Chem, 19, 3468-3474.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 239
[678] Di Nezio, M. S., Palomeque, M. E. & Band, B. S. F. (2005). Automated flow injection
method for cadmium determination with preconcentration and on-line reagent
preparation. Quim Nova, 28, 145-148.
[679] Liu, Y., Guo, Y., Chang, X., Meng, S., Yang, D. & Din, B. (2005). Column solid-
phase extraction with 2-acetylmercaptophenyldiazoaminoazobenzene (AMPDAA)
impregnated Amberlite XAD-4 and determination of trace heavy metals in natural
waters by flame atomic absorption spectrometry. Microchim Acta, 149, 95-101.
[680] dos Santos, W. N. L., dos Santos, C. M. & Ferreira, S. L. C. (2005). Field sampling
system for determination of cadmium and nickel in fresh water by flame atomic
absorption spectrometry. J Brazil Chem Soc, 16, 727-732.
[681] Liu, Y., Guo, Y., Meng, S., Feng, F. & Chang, X. (2007). Determination of trace
heavy metals in waters by flame atomic absorption spectrometry after
preconcentration with 2,4-dinitrophenyldiazoaminoazobenzene on Amberlite XAD-2.
Microchim Acta, 157, 209-214.
[682] Nakashima, S., Sturgeon, R. E., Willie, S. N. & Bernan, S. S. (1988). Determination of
trace metals in sea-water by graphite-furnace atomic-absorption spectrometry with
pre-concentration on silica-immobilized quinolin-8-ol in a flow system. Fresenius’Z
Anal Chem, 330, 592-595.
[683] Karadjova, I. (1999). Determination of Cd, Co, Cr, Cu, Fe, Mn, Ni and Pb in natural
waters, alkali and alkaline earth salts by electrothermal atomic absorption
spectrometry after preconcentration by column solid phase extraction. Microchim
Acta, 130, 185-190.
[684] Abou-El-Sherbini, K. S., Kenawy, I. M. M., Hamed, M. A., Issa, R. M. & Elmorsi, R.
(2002). Separation and preconcentration in a batch mode of Cd(II), Cr(III, VI), Cu(II),
Mn(II, VII) and Pb(II) by solid-phase extraction by using of silica modified with N-
propylsalicylaldimine. Talanta, 58, 289-300.
[685] Kocjan, R., Blazewicz, A. & Matosiuk, D. (2004). Properties and Application of a
Chelating Sorbent Prepared by Modification of LiChroprep-NH2 with Calcon.
Microchim Acta, 144, 221-226.
[686] Xie, Z. H., Xie, F. Z., Guo, L. Q., Lin, X. C. & Chen, G. N. (2005). Thioacetamide
chemically Immobilized on silica gel as a solid phase extractant for the extraction and
preconcentration of copper(II), lead(II), and cadmium(II). J Sep Sci, 28, 462-470.
[687] Cui, Y., Chang, X., Zhu, X. & Zou, X. (2008). Selective solid phase extraction of trace
cadmium(II) and lead(II) from biological and natural water samples by ofloxacin-
modified-silica gel. Int J Environ Anal Chem, 88, 857-868.
[688] Huang, X., Chang, X., He, Q., Cui, Y., Zhai, Y. & Jiang, N. (2008). Tris(2-
aminoethyl) amine functionalized silica gel for solid-phase extraction and
preconcentration of Cr(III), Cd(II) and Pb(II) from waters. J Hazard Mat, 157, 154-
160.
[689] Tewari, P. K. & Singh, A. K. (2000). Thiosalicylic acid-immobilized Amberlite XAD-
2: metal sorption behaviour and applications in estimation of metal ions by flame
atomic absorption spectrometry. Analyst, 125, 2350-2355.
[690] Tuzen, M., Narin, I., Soylak, M. & Elci, L. (2004). XAD-4/PAN Solid Phase
Extraction System for Atomic Absorption Spectrometric Determinations of Some
Trace Metals in Environmental Samples. Anal Lett, 37, 473-489.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 240
[691] Lemos, V. A. & Baliza, P. X. (2005). Amberlite XAD-2 functionalized with 2-
aminothiophenol as a new sorbent for on-line preconcentration of cadmium and
copper. Talanta, 67, 564-570.
[692] Liu, Y., Chang, X., Guo, Y., Ding, B. & Meng, S. (2005). Solid phase extraction and
preconcentration of trace heavy metal ions in natural water with 2,2'-dithiobisaniline
modified Amberlite XAD-2. Solvent Extr Ion Exc, 23, 725-740.
[693] Kasahara, Y., Willie, S. N., Sturgeon, R. E., Bernan, S. S., Taguchi, S. & Goto, K.
(1993). Preparation of quinolin-8-ol immobilized adsorbents with minimum
contamination for the pre-concentration of trace metals in water. Bunseki Kagaku, 42,
107-110.
[694] Gama, E. M., Lima, A. S. & Lemos, V. A. (2006). Preconcentration system for
cadmium and lead determination in environmental samples using polyurethane
foam/Me-BTANC. J Hazard Mater, 136, 757-762.
[695] Magidi, V. & Holcombe, J. A. (1989). Pre-concentration of cadmium from
environmental samples by an alga and analysis by graphite furnace atomic absorption
spectrometry. J Anal At Spectrom, 4, 439-442.
[696] Bag, H., Lale, M. & Türker, A. R. (1999). Determination of Cu, Zn and Cd in water by
FAAS after preconcentration by baker’s yeast (Saccharomyces cerevisiae)immobilized
on sepiolite. Fresenius J Anal Chem, 363, 224-230.
[697] Baytak, S., Türker, A. R. & Cevrimli, B. S. (2005). Application of silica gel 60 loaded
with Aspergillus niger as a solid phase extractor for the separation/preconcentration of
chromium(III), copper(II), zinc(II), and cadmium(II). J Sep Sci, 28, 2482-2488.
[698] Baytak, S., Kenduzler, E. & Türker, A. R. (2006). Separation/preconcentration of
Zn(II), Cu(II), and Cd(II) by Saccharomyces carlsbergensis immobilized on silica gel
60 in various samples. Sep Sci Tech, 41, 3449-3465.
[699] Pereira, M. G. & Arruda, M. A. Z. (2004). Preconcentration of Cd(II) and Pb(II) using
humic substances and flow systems coupled to flame atomic absorption spectrometry.
Microchim Acta, 146, 215-222.
[700] Martin-Esteban, A. (2001). Molecularly imprinted polymers: new molecular
recognition materials for selective solid-phase extraction of organic compounds.
Fresenius J Anal Chem, 370, 795-802.
[701] Iijima, S. (1991). Helical microtubules of graphitic carbon. Nature, 354, 56-58.
[702] Iijima, S. & Ichihashi, T. (1993). Single-shell carbon nanotubes of 1-nm diameter.
Nature, 363, 603-605.
[703] Liang, P., Liu, Y., Guo, L., Zeng, J. & Lu, H. (2004). Multiwalled carbon nanotubes as
solid-phase extraction adsorbent for the preconcentration of trace metal ions and their
determination by inductively coupled plasma atomic emission spectrometry. J Anal At
Spectrom, 19, 1489-1492.
[704] El-Sheikh, A. H., Sweileh, J. A. & Al-Degs, Y. S. (2007). Effect of dimensions of
multi-walled carbon nanotubes on its enrichment efficiency of metal ions from
environmental waters. Anal Chim Acta, 604, 119-126.
[705] Teixeira Tarley, C. R., Barbosa, A. F., Gava Segatelli, M., Costa Figueiredo, E. &
Orival Luccas, P. (2006). Highly improved sensitivity of TS-FF-AAS for Cd(II)
determination at ng L−1 levels using a simple flow injection minicolumn
Analytical Chemistry of Cadmium: Sample Pre-treatment… 241
preconcentration system with multiwall carbon nanotubes. J Anal At Spectrom, 21,
1305-1313.
[706] Henglein, A. (1989). Small-particle research: physicochemicalproperties of extremely
small colloidal metal and semiconductor particles. Chem Rev, 89, 1861-1873
[707] Zheng, H., Chang, X. J., Lian, N., Wang, S., Cui, Y. M. & Zhai, Y. H. (2006). A pre-
enrichment procedure using diethyldithiocarbamate-modified TiO2 nanoparticles for
the analysis of biological and natural water samples by ICP-AES. Int J Environ Anal
Chem, 86, 431-441.
[708] Yin, J., Jiang, Z., Chang, G. & Hu, B. (2005). Simultaneous on-line preconcentration
and determination of trace metals in environmental samples by flow injection
combined with inductively coupled plasma mass spectrometry using a nanometer-
sized alumina packed micro-column. Anal Chim Acta, 540, 333-339.
[709] Kresage, C. T., Leonowicz, M. E., Roth, W. J., Vartuli, J. C. & Beck, J. S. (1992).
Ordered mesoporous molecular sieves synthesized by a liquid-crystal template
mechanism. Nature, 359, 710-712.
[710] Chen, D. H., Hu, B. & Huang, C. Z. (2009). Chitosan modified ordered mesoporous
silica as micro-column packing materials for on-line flow injection-inductively
coupled plasma optical emission spectrometry determination of trace heavy metals in
environmental water samples. Talanta, 78, 491-497.
[711] Arshady, R. (1992). Suspension, emulsion, and dispersion polymerization – A
methodological survey. Colloid Polym Sci, 270, 717-732.
[712] Rao, T. P., Daniel, S. & Gladis, J. M. (2004). Tailored materials for preconcentration
or separation of metals by ion-imprinted polymers for solid-phase extraction (IIP-
SPE). Trends Anal Chem, 23, 28-35.
[713] Rao, T. P., Kala, R. & Daniel, S. (2006). Metal ion-imprinted polymers-Novel
materials for selective recognition of inorganics. Anal Chim Acta, 578, 105-116.
[714] Zhai, Y., Liu, Y., Chang, X., Chen, S. & Huang, X. (2007). Selective solid-phase
extraction of trace cadmium(II) with an ionic imprinted polymer prepared from a dual-
ligand monomer. Anal Chim Acta, 593, 123-128.
[715] Liu, Y., Chang, X., Wang, S., Guo, Y., Din, B. & Meng, S. (2004). Solid-phase
extraction and preconcentration of cadmium(II) in aqueous solution with Cd(II)-
imprinted resin (poly-Cd(II)-DAAB-VP) packed columns. Anal Chim Acta, 519,
173-179.
[716] Lu, Y. K. & Yan. X. P. (2004). An imprinted organic-inorganic hybrid sorbent for
selective separation of cadmium from aqueous solution. Anal. Chem., 76, 453-457.
[717] Olsen, S., Pessenda, L. C. R., Růžička, J. & Hansen, E. H. (1983). Combination of
flow injection analysis with flame atomic-absorption spectrophotometry:
determination of trace amounts of heavy metals in polluted seawater. Analyst, 108,
905-917.
[718] Liljegren, G., Pettersson, J., Markides, K. E. & Nyholm, L. (2002). Electrochemical
solid-phase microextraction of anions and cations using polypyrrole coatings and an
integrated three-electrode device. Analyst, 127, 591-597.
[719] He, Y., Tang, L., Wu, X., Hou, X. D. & Lee. Y. I. (2007). Spectroscopy: The best way
toward green analytical chemistry?. Appl Spectrosc Rev, 42, 119-138.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 242
[720] Anastas, P. T. & Warner, J. C. (2000). Green chemistry: Theory and practice; Oxford
University Press: Oxford.
[721] Koel, M. & Kalijurand, M. (2006). Application of the principles of green chemistry in
analytical chemistry. Pure Appl Chem, 78, 1993-2002.
[722] Armenta, S., Garrigues, S. & de la Guardia, M. (2008). Green analytical chemistry.
Trends Anal Chem, 27, 497-511.
[723] Moreda-Piñeiro, A., Barciela-Alonso, M. C., Domínguez-González, R., Peña-
Vázquez, E., Herbello-Hermelo, P. & Bermejo-Barrera, P. (2009). Alternative solid
sample pre-treatment methods in green analytical atomic spectrometry. Spectrosc Lett,
in press.
[724] Zlotorzynski, A. (1995). The application of microwave radiation to analytical and
environmental chemistry. Critical Rev Anal Chem, 25, 43-76.
[725] Jin, Q., Liang, F., Zhang, H., Zhao, L., Huan, Y. & Song, D. (1999). Application of
microwave techniques in analytical chemistry. Trends Anal Chem, 18, 479-484.
[726] Abu-Samra, A., Morris, J. S. & Koirtyohann, S. R. (1975). Wet ashing of some
biological samples in a microwave oven. Anal Chem, 47, 1475-1477.
[727] Agazzi, A. & Pirola, C. (2000). Fundamentals, methods and future trends of
environmental microwave sample preparation. Microchem J, 67, 337-341.
[728] Srogi, K. (2006). A review: Application of microwave techniques for environmental
analytical chemistry. Anal Lett, 39, 1261-1288.
[729] Kingston, H. M. & Jassie, L. B. (1988). Introduction to microwave sample
preparation. Theory and practice; ACS Profesional Referente Book: Washington.
[730] Arain, M. B., Kazi, T. G., Jamali, M. K., Jalbani, N., Afridi, H. I., Sarfraz, R. A. &
Shah, A. Q. (2007). Determination of toxic elements in muscle tissues of five fish
species using ultrasound-assisted pseudodigestion by electrothermal atomic absorption
spectrophotometry: optimization study. Spectrosc Lett, 40, 861-878.
[731] Arain, M. B., Kazi, T. G., Jamali, M. K., Afridi, H. I., Jalbani, N. & Memon, A. R.
(2007). Ultrasound-assisted pseudodigestion for toxic metals determination in fish
muscles followed by electrothermal atomic absorption spectrophotometry:
multivariate strategy. J AOAC Int, 90, 1118-1127.
[732] Brunori, C., Ipolyi, I., Macaluso, L. & Morabito, R. (2004). Evaluation of an
ultrasonic digestion procedure for total metal determination in sediment reference
materials. Anal Chim Acta, 510, 101-110.
[733] Bellotto, V. R. & Miekeley, N. (2007). Trace metals in mussel shells and
corresponding soft tissue samples: A validation experiment for the use of Perna perna
shells in pollution monitoring. Anal Bioanal Chem, 389, 769-776.
[734] Saavedra, Y., González, A., Fernández, P. & Blanco, J. (2004). A simple optimized
microwave digestion method for multielement monitoring in mussel samples.
Spectrochim Acta B, 59, 533-541.
[735] LaBrecque, J. J., Benzo, Z., Alfonso, J. A., Cordoves, P. R., Quintal, M., Gómez,
C. V. & Marcano, E. (2004). The concentrations of selected trace elements in clams,
Trivela mactroidea along the Venezuelan coast in the state of Miranda. Mar Pollut
Bull, 49, 664-667.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 243
[736] Yang, K. X. & Swami, K. (2007). Determination of metals in marine species by
microwave digestion and inductively coupled plasma mass spectrometry analysis.
Spectrochim Acta B, 62, 1177-1181.
[737] Hamilton, M. A., Rode, P. W., Merchant, M. E. & Sneddon, J. (2008). Determination
and comparison of heavy metals in selected seafood, water, vegetation and sediments
by inductively coupled plasma-optical emission spectrometry from an industrialized
and pristine waterway in Southwest Louisiana. Microchem J, 88, 52-55.
[738] Cubadda, F., Raggi, A. & Coni, E. (2006). Element fingerprinting of marine organisms
by dynamic reaction cell inductively coupled plasma mass spectrometry. Anal Bioanal
Chem, 384, 887-896.
[739] Uluozlu, O. D., Tuzen, M., Mendil, D. & Soylak, M. (2007). Trace metal content in
nine species of fish from the Black and Aegean Seas, Turkey. Food Chem, 104,
835-840.
[740] Meucci, V., Laschi, S., Minunni, M., Pretti, C., Intorre, L., Soldani, G. & Mascini, M.
(2009). An optimized digestion method coupled to electrochemical sensor for the
determination of Cd, Cu, Pb and Hg in fish by square wave anodic stripping
voltammetry. Talanta, 77, 1143-1148.
[741] Bocca, B., Conti, M. E., Pino, A., Mattei, D., Forte, G. & Alimonti, A. (2007). Simple,
fast, and low-contamination microwave-assisted digestion procedures for the
determination of chemical elements in biological and environmental matrices by sector
field ICP-MS. Int J Environ Anal Chem, 87, 1111-1123.
[742] Sucharova, J. & Suchara, I. (2006). Determination of 36 elements in plant reference
materials with different Si contents by inductively coupled plasma massspectrometry:
Comparison of microwave digestions assisted by three types of digestion mixtures.
Anal Chim Acta, 576, 163-176.
[743] Tuncel, S. G., Yenisoy-Karakas, S. & Dogangun, A. (2004). Determination of metal
concentrations in lichen samples by inductively coupled plasma atomic emission
spectroscopy technique after applying different digestion procedures. Talanta, 63,
273-277.
[744] Pino, A., Alimonti, A., Botre, F., Minoia, C., Bocca, B. & Conti, M. E. (2007).
Determination of twenty-five elements in lichens by sector field inductively coupled
plasma mass spectrometry and microwave-assisted acid digestion. Rapid Commun
Mass Spectrom, 21, 1900-1906.
[745] Engstroem, E., Stenberg, A., Baxter, D. C., Malinovsky, D., Maekinen, I., Poenni, S.
& Rodushkin, I. (2004). Effects of sample preparation and calibration strategy on
accuracy and precision in the multi-elemental analysis of soil by sector-field ICP-MS.
J Anal At Spectrom, 19, 858-866.
[746] Riebe, G., Pritzkow, W., Vogl, J. & Jochen. (2006). Determination of heavy metals in
soil. A comparison of various digestion methods. GIT Labor-Fachzeitschrift, 50,
688-690.
[747] Marin, B., Chopin, E. I. B., Jupinet, B. & Gauthier, D. (2008). Comparison of
microwave-assisted digestion procedures for total trace element content determination
in calcareous soils. Talanta, 77, 282-288.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 244
[748] Rodríguez, L., Ruíz, E., Alonso-Azcarate, J. & Rincón, J. (2009). Heavy metal
distribution and chemical speciation in tailings and soils around a Pb-Zn mine in
Spain. J Environ Manag 2009, 90, 1106-1116.
[749] Al-Hefne, J., Al-Dyel, O., Chowdhury, D. A. & Al-Ajayan, T. (2005). Distribution
and ICP-MS determination of heavy elements in the surfacial sand along the red sea
coastline of Saudi Arabia. At Spectros, 26, 51-58.
[750] Melaku, S., Dams, R. & Moens, L. (2005). Determination of trace elements in
agricultural soil samples by inductively coupled plasma-mass spectrometry:
Microwave acid digestion versus aqua regia extraction. Anal Chim Acta, 543, 117-123.
[751] Aneva, Z. & Chepanova, L. (2005). Assessment of heavy metals levels in urban dusts
and soils by microwave-assisted aqua regia digestion. J Environ Protec Ecol, 6,
293-304.
[752] Landajo, A., Arana, G., de Diego, A., Etxebarria, N., Zuloaga, O. & Amouroux, D.
(2004). Analysis of heavy metal distribution in superficial estuarine sediments (estuary
of Bilbao, Basque Country) by open focused microwave-assisted extraction and ICP-
OES. Chemosphere, 56, 1033-1041.
[753] Tuzen, M., Sari, H. & Soylak, M. (2004). Microwave and wet digestion procedures for
atomic absorption spectrometric determination of trace metals contents of sediment
samples. Anal Lett, 37, 1925-1936.
[754] Gao, B., Liu, Y., Sun, K., Liang, X., Peng, P., Sheng, G. & Fu, J. (2008). Precise
determination of cadmium and lead isotopic compositions in river sediments. Anal
Chim Acta, 612, 114-120.
[755] Bettiol, C., Stievano, L., Bertelle, M., Delfino, F. & Argese, E. (2008). Evaluation of
microwave-assisted acid extraction procedures for the determination of metal content
and potential bioavailability in sediments. Appl Geochem, 23, 1140-1151.
[756] Marrero, J., Polla, G., Jiménez Rebagliati, R., Plá, R., Gómez, D. & Smichowski,
P.(2007). Characterization and determination of 28 elements in fly ashes collected in a
thermal power plant in Argentina using different instrumental techniques. Spectrochim
Acta B, 62, 101-108.
[757] Karanasiou, A. A., Thomaidis, N. S., Eleftheriadis, K. & Siskos, P. A. (2005).
Comparative study of pretreatment methods for the determination of metals in
atmospheric aerosol by electrothermal atomic absorption spectrometry. Talanta, 65,
1196-1202.
[758] Pekney N. J. & Davidson, C. I. (2005). Determination of trace elements in ambient
aerosol samples. Anal Chim Acta, 540, 269-277.
[759] Fernández Álvarez, F., Ternero Rodríguez, M., Fernández Espinosa, A. J. & Gutierrez
Daban, A. (2004). Physical speciation of arsenic, mercury, lead, cadmium and nickel
in inhalable atmospheric particles. Anal Chim Acta, 524, 33-40.
[760] Kulkarni, P., Chellam, S., Flanagan, J. B. & Jayanty, R. K. M. (2007). Microwave
digestion-ICP-MS for elemental analysis in ambient airborne fine particulate matter:
Rare earth elements and validation using a filter borne fine particle certified reference
material. Anal Chim Acta, 599, 170-176.
[761] Karthikeyan, S., Joshi, U. M. & Balasubramanian, R. (2006). Microwave assisted
sample preparation for determining water-soluble fraction of trace elements in urban
airborne particulate matter: Evaluation of bioavailability. Anal Chim Acta, 576, 23-30.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 245
[762] Turkoglu, O., Saracoglu, S., Soylak, M. & Elci, L. (2004). Monitoring copper, nickel,
cobalt, lead, cadmium, manganese and chromium levels in house dust samples from
Kayseri, Turkey. Trace Elem Electrol, 21, 4-9.
[763] Nakatsuka, S., Okamura, K., Norisuye, K. & Sohrin, Y. (2007). Simultaneous
determination of suspended particulate trace metals (Co, Ni, Cu, Zn, Cd and Pb) in
seawater with small volume filtration assisted by microwave digestion and flow
injection inductively coupled plasma mass spectrometer. Anal Chim Acta, 594, 52-60.
[764] Bermejo-Barrera, P., Moreda-Piñeiro, A. & Bermejo-Barrera, A. (2002). Sample pre-
treatment methods for the trace elements determination in seafood products by atomic
absorption spectrometry. Talanta, 57, 969-984.
[765] Lorentzen, E. M. L. & Kingston, H. M. (1996). Comparison of microwave-assisted
and conventional leaching using EPA method 3050B. Anal Chem, 68, 4316-4320.
[766] Puchyr, R. F. & Shapiro, R. (1986). Determination of Trace elements in foods by HCl-
HNO3 leaching and flame atomic absorption spectroscopy. J Assoc Anal Chem, 69,
868-870.
[767] Santos Júnior, D., Krug, F. J., de Godoy Pereira, M. & Korn., M. (2006). Currents on
ultrasound-assisted extraction for sample preparation and spectroscopic analytes
determination. Appl Spectrosc Rev, 41, 305-321.
[768] Chmilenko, F. A. & Baklanov, A. N. (1992). Atomic absorption determination of toxic
and biologically active trace elements in dairy products using ultrasound for
acceleration of mineralization. Zh Anal Khim, 47, 1322-1327.
[769] Nakamura, Y., Etoh, T., Noto, Y. & Murai, Y. (1985). Determination of wear metals
in lubricating oils by inductively coupled plasma emission spectrometry. Bunseki
Kagaku, 34, T85-T88.
[770] Sánchez, J., García, R. & Millán, E. (1994). Ultrasonic bath digestion procedures for
analysis for heavy metals in several reference materials. Analusis, 22, 222-225.
[771] Sánchez, J. & Millán, M. E. (1992). Use of ultrasound in digestion of biological
reference materials for determination of heavy metals. Quím Anal, 11, 3-10.
[772] Mason, T. J. (1999). Sonochemistry; Oxford University Press: Oxford.
[773] Luque-García, J. L. & Luque de Castro, M. D. (2003). Ultrasound: a powerful tool for
leaching. Trends Anal Chem, 22, 41-46.
[774] Mason, T. J. & Lorimer, J. P. (2002). Applied sonochemistry: Uses of power
ultrasounds in chemistry and processing; Wiley-VCH: Weinheim.
[775] Santos, H. M. & Capelo, J. L. (2007). Trends in ultrasonic-based equipment for
analytical sample treatment. Talanta, 73, 795-802.
[776] Capelo, J. L., Maduro, C. & Viena, C. (2005). Discussion of parameters associated
with the ultrasonic solid-liquid extraction for elemental analysis (total content) by
electrothermal atomic absorption spectrometry. An overview. Ultrasounds Sonochem,
12, 225-232.
[777] Kumina, D. M., Gribovskaya, I. F. & Karyakin, A. V. (1985). Method for extraction of
elements from plants into solution using ultrasound. Zh Anal Khim, 40, 1184-1187.
[778] Ladra-Ramos, N., Domínguez-González, R., Moreda-Piñeiro, A., Bermejo-Barrera, A.
& Bermejo-Barrera, P. (2005). Determination of major and trace elements in edible
seaweed by aas after ultrasound-assisted acid leaching. At Spectrosc, 26, 59-67.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 246
[779] Domínguez-González, R., Moreda-Piñeiro, A., Bermejo-Barrera, A. & Bermejo-
Barrera, P. (2005). Application of ultrasound-assisted acid leaching procedure for
major and trace elements determination in edible seaweed by inductively coupled
plasma-optical emission spectrometry. Talanta, 66, 937-942.
[780] Brisbin, J. A. & Caruso, J. A. (2002). Comparison of extraction procedures for the
determination of arsenic and other elements in lobster tissue by inductively coupled
plasma mass spectrometry. Analyst, 127, 921-929.
[781] Borkowska-Burnecka, J., Wisz, J. & Zyrnicki, W. (2003). Applicability of ultrasonic
leaching by diluted acids for determination of total metal contents in plant materials.
Chem Anal, 48, 115-126.
[782] Minami, H., Honjyo, T. & Atsuya, I. (1996). A new solid-liquid extraction sampling
technique for direct determination of trace elements in biological materials by graphite
furnace atomic absorption spectrometry. Spectrochim Acta B, 51, 211-220.
[783] Borkowska-Burnecka, J., Jankowiak, U., Zyrnicki, W. & Wilk, K. A. (2004). Effect of
surfactant addition on ultrasonic leaching of trace elements from plant samples in
inductively coupled plasma-atomic emission spectrometry. Spectrochim Acta B, 59,
585-590.
[784] Filgueiras, A. V., Lavilla, I. & Bendicho, C. (2001). Ultrasound-assisted solubilization
of trace and minor metals from plant tissue using ethylenediaminetetraacetic acid in
alkaline medium. Fresenius' J Anal Chem, 369, 451-456.
[785] García-Rey, R. M., Quiles-Zafra, R. & Luque de Castro, M. D. (2003). New methods
for acceleration of metal sample preparation prior to determination of the metal
content by atomic absorption spectrometry. Anal Bioanal Chem, 377, 316-321.
[786] Maduro, C., Vale, G., Alves, S., Galesio, M., da Silva, M. D. R. G., Fernández, C.,
Catarino, S., Rivas, M. G., Mota, A. M. & Capelo, J. L. (2006). Determination of Cd
and Pb in biological reference materials by electrothermal atomic absorption
spectrometry: a comparison of three ultrasonic-based sample treatment procedures.
Talanta, 68, 1156-1161.
[787] Lavilla, I., Capelo, J. L. & Bendicho, C. (1999). Determination of cadmium and lead
in mussels by electrothermal atomic absorption spectrometry using an ultrasound-
assisted extraction method optimized by factorial design. Fresenius' J Anal Chem,
363, 283-288.
[788] El Azouzi, H., Cervera, M. L. & de la Guardia, M. (1998). Multi-elemental analysis of
mussel samples by atomic absorption spectrometry after room temperature sonication.
J Anal At Spectrom, 13, 533-538.
[789] Moreda-Piñeiro, A., Bermejo-Barrera, P. & Bermejo-Barrera, A. (2006). Chemometric
investigation of systematic error in the analysis of biological materials by flame and
electrothermal atomic absorption spectrometry. Anal Chim Acta, 560, 143-152.
[790] Manutsewee, N., Aeungmaitrepirom, W., Varanusupakul, P. & Imyim, A. (2007).
Determination of Cd, Cu and Zn in fish and mussel by aas after ultrasound-assisted
acid leaching extraction. Food Chem, 101, 817-824.
[791] Väisänen, A. & Suontamo, R. J. (2002). Comparison of ultrasound-assisted extraction,
microwave-assisted acid leaching and reflux for the determination of arsenic,
cadmium and copper in contaminated soil samples by electrothermal atomic
absorption spectrometry. J Anal At Spectrom, 17, 739-742.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 247
[792] Shiowatana, J., Tantidanai, N., Nookabkaew, S. & Nacapricha, D. (2001). A novel
continuous-flow sequential extraction procedure for metal speciation in solids.
J Environ Quality, 30, 1195-1205.
[793] Moreno-Cid, A. & Yebra, M. C. (2002). Flow Injection determination of copper in
mussels by flame atomic absorption spectrometry after on-line continuous ultrasound-
assisted extraction. Spectrochim Acta B, 57, 967-974.
[794] Priego-Capote, F. & Luque de Castro, M. D. (2004). Dynamic ultrasound-assisted
leaching of essential macro and micronutrient metal elements from animal feeds prior
to flame atomic absorption spectrometry. Anal Bioanal Chem, 378, 1376-1381.
[795] Yebra-Biurrun, M. C., Cancela-Pérez, S. & Moreno-Cid-Barinaga, A. (2005).
Coupling continuous ultrasound-assisted extraction, preconcentration and flame
atomic absorption spectrometric detection for the determination of cadmium and lead
in mussel samples. Anal Chim Acta, 533, 51-56.
[796] Cespón-Romero, R. M. & Yebra-Biurrun, M. C. (2008). Application of factorial
designs for optimisation of on-line determination of cadmium, lead and nickel in
welding fumes by atomic absorption spectrometry. Int J Environ Anal Chem, 88,
539-547.
[797] Seco-Gesto, E. V., Moreda-Piñeiro, A., Bermejo-Barrera, A. & Bermejo-Barrera, P.
(2007). Multi-element determination in raft mussels by fast microwave-assisted acid
leaching and inductively coupled plasma-optical emission spectrometry. Talanta, 72,
1178-1185.
[798] Borkowska-Burnecka, J. (2000). Microwave assisted extraction for trace element
analysis of plant materials by ICP-AES. Fresenius’J Anal Chem, 358, 633-637.
[799] Herrera, M. C. & Luque de Castro, M. D. (2002). Dynamic approach based on
iiterative change of the flow direction for microwave-assisted leaching of cadmium
and lead from plant prior to GF-AAS. J Anal At Spectrom, 17, 1530-1533.
[800] Fisher, J. A., Scarlett, M. J. & Stott, A. D. (1997). Accelerated solvent extraction: an
evaluation for screening of soils for selected U.S. EPA Semivolatile organic priority
pollutants. Environ Sci Technol, 31, 1120-1127.
[801] Giergielewicz-Mozajska, H., Dabrowski, L. & Namiesnik, J. (2001). Accelerated
solvent extraction (ASE) in the analysis of environmental solid samples - some aspects
of theory and practice. Critical Rev Anal Chem, 31, 149-165.
[802] Alonso-Rodríguez, E., Moreda-Piñeiro, J., López-Mahía, P., Prada-Rodríguez, D.,
Fernández-Fernández, E., Muniategui-Lorenzo, S., Moreda-Piñeiro, A., Bermejo-
Barrera, A. & Bermejo-Barrera, P. (2006). Pressurized liquid extraction of
organometals and its feasibility for total metal extraction. Trends Anal Chem, 25,
511-519.
[803] Kronholm, J., Hartonen, K. & Riekkola, M. L. (2007). Analytical extractions with
water at elevated temperatures and pressures. Trends Anal Chem, 26, 396-412.
[804] Rico Varadé, C. M. & Luque de Castro, M. D. (1998). Determination of Selenium in
solid samples by continuous subcritical water extraction, flow injection derivatisation
and atomic fluorescence detection. J Anal At Spectrom, 13, 787- 791.
[805] Fernández-Pérez, V., Jiménez-Carmona, M. M. & Luque de Castro, M. D. (1999).
Continuous extraction with acidified subcritical water of arsenic, selenium and
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 248
mercury from coal prior to on-line derivatisation-atomic fluorescence detection. J Anal
At Spectrom, 14, 1761-1765.
[806] Jiménez-Carmona, M. M., Fernández-Pérez, V., Gualda-Bueno, M. J., Cabanás-
Espejo, J. M. & Luque de Castro, M. D. (1999). Acidified subcritical water extraction
of major ash-forming elements from coal. Anal Chim Acta, 395, 113-118.
[807] Salvador, F. & Merchán, M. D. (1995). USA Patent 5 400 642.
[808] Morales-Muñoz, S., Luque-García, J. L. & Luque de Castro, M. D. (2003). Acidified
pressurized hot water for the continuous extraction of cadmium and lead from plant
materials prior to ETAAS. Spectrochim Acta B, 58, 159-165.
[809] Fernández-Pérez, V., Jiménez-Carmona, M. M. & Luque de Castro, M. D. (2001).
Continuous liquid–liquid extraction using modified subcritical water for the
demetalisation of used industrial oils. Anal Chim Acta, 433, 47-52.
[810] Morales-Riffo, J. J. & Richter, P. (2004). Rapid determination of inorganic elements in
airborne particulate matter by using acidified subcritical-water extraction and
inductively-coupled plasma–optical-emission spectrometry. Anal Bioanal Chem, 380,
129-134.
[811] Tavakoli, O. & Yoshida, H. (2005). Effective recovery of harmful metal ions from
squid wastes using subcritical and supercritical water treatments. Environ Sci Technol,
39, 2357-2363.
[812] Heltai, G., Percsich, K., Halász, G., Jung, K. & Fekete, I. (2005). Estimation of
ecotoxicological potential of contaminated sediments based on a sequential extraction
procedure with supercritical CO2 and subcritical H2O solvents. Microchem J, 79,
231-237.
[813] Wanekaya, A. K., Myung, S. & Sadik, O. A. (2002). Pressure assisted chelating
extraction: a novel technique for digesting metals in solid matrices. Analyst, 127,
1272-1276.
[814] Alonso-Rodríguez, E., Moreda-Piñeiro, J., López-Mahía, P., Muniategui-Lorenzo, S.,
Prada-Rodríguez, D., Moreda-Piñeiro, A. & Bermejo-Barrera, P. (2007). Use of
chelating solvent-based pressurized liquid extraction combined with inductively
coupled plasma-optical emission spectrometry for trace element determination in
atmospheric particulate matter. J Anal At Spectrom, 22, 1089-1096.
[815] Maurí-Aucejo, A.R., Arnandis-Chover, T., Marín-Sáez, R. & Llobat-Estellés, M.
(2007). Application of pressurized fluid extraction to determine cadmium and zinc in
plants. Anal Chim Acta, 581, 78-82.
[816] Moreda-Piñeiro, J., Alonso-Rodríguez, E., López-Mahía, P., Muniategui-Lorenzo, S.,
Fernández-Fernández, E., Prada-Rodríguez, D., Moreda-Piñeiro, A., Bermejo-Barrera,
A. & Bermejo-Barrera, P. (2006). Pressurized liquid extraction as a novel sample pre-
treatment for trace element leaching from biological material. Anal Chim Acta, 572,
172-179.
[817] Moreda-Piñeiro, J., Alonso-Rodríguez, E., López-Mahía, P., Muniategui-Lorenzo, S.,
Prada-Rodríguez, D., Moreda-Piñeiro, A., Bermejo-Barrera, A. & Bermejo-Barrera, P.
(2006). As, Cd, Cr, Ni and Pb pressurized liquid extraction with acetic acid from
marine sediment and soil samples. Spectrochim Acta B, 61, 1304-1309.
[818] Moreda-Piñeiro, J., Alonso-Rodríguez, E., López-Mahía, P., Muniategui-Lorenzo, S.,
Prada-Rodríguez, D., Moreda-Piñeiro, A. & Bermejo-Barrera, P. (2007). Development
Analytical Chemistry of Cadmium: Sample Pre-treatment… 249
of a new sample pre-treatment procedure based on pressurized liquid extraction for the
determination of metals in edible seaweed. Anal Chim Acta, 598, 95-102.
[819] Peña-Farfal, C., Moreda-Piñeiro, A., Bermejo-Barrera, A., Bermejo-Barrera, P.,
Pinochet-Cancino, H. & de Gregori-Henríquez, I. (2004). Use of enzymatic hydrolysis
for the multi-element determination in mussel soft tissue by inductively coupled
plasma-atomic absorption spectrometry. Talanta, 64, 671-681.
[820] Bermejo, P., Capelo, J.L., Mota, A., Madrid, Y. & Cámara, C. (2004). Enzymatic
digestion and ultrasonication: A powerful combination in analytical chemistry. Trends
Anal Chem, 23, 654-663.
[821] Carpenter, R. C. (1981). The Determination of Cadmium, copper, lead and thallium in
human liver and kidney tissue by flame atomic absorption spectrometry after
enzymatic digestion. Anal Chim Acta, 125, 209-213.
[822] Vale, G., Rial-Otero, R., Mota, A., Fonseca, L. & Capelo, J. L. (2008). Ultrasonic-
assisted enzymatic digestion (USAED) for total elemental determination and
elemental speciation: A tutorial. Talanta, 75, 872-884.
[823] Peña-Farfal, C., Moreda-Piñeiro, A., Bermejo-Barrera, A., Bermejo-Barrera, P.,
Pinochet-Cancino, H. & de Gregori-Henríquez, I. (2005). Speeding up Enzymatic
hydrolysis procedures for the multi-element determination in edible seaweed. Anal
Chim Acta, 548, 183-191.
[824] Moreda-Piñeiro, A., Bermejo-Barrera, A., Bermejo-Barrera, P., Moreda-Piñeiro, J.,
Alonso-Rodríguez, E., Muniategui-Lorenzo, S., López-Mahía, P. & Prada-Rodríguez,
D. (2007). Feasibility of pressurization to speed up enzymatic hydrolysis of biological
materials for multielement determinations. Anal Chem, 79, 1797-1805.
[825] Peachey, E., McCarthy, N. & Goenaga-Infante, H. (2008). Acceleration of enzymatic
hydrolysis of protein-bound selenium by focused microwave energy. J Anal At
Spectrom, 23, 487-492.
[826] Peña-Farfal, C., Moreda-Piñeiro, A., Bermejo-Barrera, A., Bermejo-Barrera, P.,
Pinochet-Cancino, H. & de Gregori-Henríquez, I. (2004). Ultrasound bath-assisted
enzymatic hydrolysis procedures as sample pretreatment for the multielement
determination in mussels by inductively coupled plasma atomic absorption
spectrometry. Anal Chem, 76, 3541-3547.
[827] Kurfüst, V. (1998). Solid sample analysis; Springer: Berlin.
[828] Bendicho, C. & de Loos-Vollebregt, M. T. C. (1991). Solid Sampling in
electrothermal atomic absorption spectrometry using commercial atomizers. A review.
J Anal At Spectrom, 8, 353-374.
[829] Cal-Prieto, M. J., Felipe-Sotelo, M., Carlosena, A., Andrade, J. M., López-Mahía, P.,
Muniategui, S. & Prada, D. (2002). Slurry sampling for direct analysis of solid
materials by electrothermal atomic absorption spectrometry (ETAAS). A literature
review from 1990 to 2000. Talanta, 56, 1-51.
[830] Vale, M. G. R., Oleszczuk, N. & dos Santos, W. N. L. (2006). Current status of direct
solid sampling for electrothermal atomic absorption spectrometry. A critical review of
the development between 1995 and 2005. Appl Spectrosc Rev, 41, 377-400.
[831] Carnrick, G. R., Daley, G. & Fotinopoulos, A. A. (1989). design and use of a new
automated ultrasonic slurry sampler for graphite-furnace atomic absorption.
At Spectrosc, 10, 170-174.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 250
[832] Majidi, V. & Holcombe, J. A. (1990). Error analysis for sampling of slurries:
Sedimentation errors. Spectrochim Acta B, 45, 753-761.
[833] Miller-Ihli, N. J. (1997). Ultrasonic slurry sampling graphite furnace atomic
absorption spectrometry: analytical considerations for analysis of high density glass
material. Spectrochim Acta B, 52, 431-436.
[834] Miller-Ihli, N. J. (1994). influence of slurry preparation on the accuracy of ultrasonic
slurry electrothermal atomic absorption spectrometry. J Anal At Spectrom, 9,
1129-1134.
[835] Almeida-Pereira, L., Amorim, I. & Borba da Silva, J. B. (2006). Determination of
Cadmium, Chromium and Lead in Marine Sediment Slurry Samples by Electrothermal
Atomic Absorption Spectrometry Using Permanent Modifiers. Talanta, 68, 771-775.
[836] Ebdon, L., Foulkes, M. & Sutton, K. (1997). Slurry Nebulizations in Plasmas. J Anal
At Spectrom, 12, 213- 229.
[837] Santos, M. C. & Nóbrega, J. A. (2006). Slurry Nebulization in Plasmas for Analysis of
Inorganic Materials. Appl Spectros Rev, 41, 427-448.
[838] Matusiewicz, H. (2003). Chemical Vapour Generation with Slurry Sampling: A
Review of Atomic Absorption Applications. Appl Spectros Rev, 3, 263-294.
[839] Moreda-Piñeiro, J., Moscoso-Pérez, C., López-Mahía, P., Muniategui-Lorenzo, S.,
Fernández-Fernández, E. & Prada-Rodríguez, D. (2006). Use of Aqueous Slurries of
Coal Fly Ash Samples for the Direct Determination of As, Sb and Se by Hydride
Generation-Atomic Fluorescence Spectrometry. At Spectros, 27, 19-25.
[840] Moreda-Piñeiro, J., Moscoso-Pérez, C., Piñeiro-Iglesias, M., López-Mahía, P.,
Muniategui-Lorenzo, S., Fernández-Fernández, E. & Prada-Rodríguez, D. (2007).
Aqueous and Acidified Slurry Sampling Approaches in the As, Sb and Sn
Determination of Urban Dust Samples by GH-ETAAS. At Spectros, 28, 137-143.
[841] Tepleton, D. M., Ariese, F., Cornelis, R., Danielsson, L.G., Muntau, H., VanLeeuwen,
H. P. & Łobinski, R. (2000). Guidelines for terms related to chemical speciation and
fractionation of elements. Definitions, structural aspects, and methodological
approaches (IUPAC Recommendations 2000). Pure Appl Chem, 72, 1453-1470.
[842] Kozelka, P. B. & Bruland, K. W. (1998). Chemical speciation of dissolved Cu, Zn, Cd,
Pb in Narragansett Bay, Rhode Island. Mar Chem, 60, 267-282.
[843] Xue, H. & Sigg L. (1998). Cadmium speciation and complexation by natural organic
ligands in fresh water. Anal Chim Acta, 363, 249-259.
[844] Pongratz, R. & Heumann, K. G. (1996). Determination of monomethylcadmium in the
environment by differential pulse anodic stripping voltammetry. Anal Chem, 68,
1262-1266.
[845] Pongratz, R. & Heumann, K. G. (1999). Production of methylated mercury, lead, and
cadmium by marine bacteria as a significant natural source for atmospheric heavy
metals in polar regions. Chemosphere, 39, 89-102.
[846] Quevauviller, P. (1998). Operationally defined extraction procedures for soil and
sediment analysis - I. Standardization. Trends Anal Chem, 17, 289-198.
[847] López-Sánchez, J. F., Sahuquillo, A., Fiedler, H. D., Rubio, R., Rauret, G., Muntau, H.
& Quevauviller, P. (1998). CRM 601, a stable material for its extractable content of
heavy metals. Analyst, 123, 1675-1677.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 251
[848] Ure, A. M., Quevauviller, P., Muntau, H. & Griepink, B. (1993). Speciation of heavy-
metals in soils and sediments. An account of the improvement and harmonization of
extraction techniques undertaken under the auspices of the BCR of the Commision of
the European Communities. Int J Environ Anal Chem, 51, 135-151.
[849] Tessier, A., Campbell, P. G. C. & Visón, M. (1979). Sequential extraction procedure
for the speciation of particulate trace-metals. Anal Chem, 51, 844-851.
[850] Filgueiras, A. V., Lavilla, I. & Bendicho, C. (2002). Chemical sequential extraction
for metal partitioning in environmental solid samples. J Environ Monit, 4, 823-857.
[851] Rao, C. R. M., Sahuquillo, A. & López-Sánchez, J. F. (2008). A review of the
different methods applied in environmental geochemistry for single and sequential
extraction of trace elements in soils and related materials. Water Air Soil Pollut, 189,
291-333.
[852] Davidson, C. M. & Delevoye, G. (2001). Effect of ultrasonic agitation on the release
of copper, iron, manganese and zinc from soil and sediment using the BCR three-stage
sequential extraction. J Environ Monit, 3, 398-403.
[853] Rauret, G., López Sánchez, J. F., Sahuquillo, A., Barahona, E., Lachica, M., Ure,
A. M., Davidson, C. M., Gómez, A., Luck, D., Bacon, J., Yli-Halla, M., Muntau, H. &
Quevauviller, P. (2000). Application of a modified BCR sequential extraction (three-
step) procedure for the determination of extractable trace metal contents in a sewage
sludge amended soil reference material (CRM 483), complemented by a three-year
stability study of acetic acid and EDTA extractable metal content. J Environ Monitor,
2, 228-233.
[854] Ho, M. D. & Evans, G. J. (2000). Sequential extraction of metal contaminated soils
with radiochemical assessment of readsorption effects. Environ Sci Technol, 34,
1030-1035.
[855] Szakova, J., Tlustos, P., Balik, J., Pavlikova, D. & Vanek, V. (1999). The sequential
analytical procedure as a tool for evaluation of As, Cd, and Zn mobility in soil.
Fresenius’ J Anal Chem, 363, 594-595.
[856] Davidson, C. M., Duncan, A. L., Littlejohn, D., Ure, A. M. & Garden, L. M. (1998).
A critical evaluation of the three-stage BCR sequential extraction procedure to assess
the potential mobility and toxicity of heavy metals in industrially-contaminated land.
Anal Chim Acta, 363, 45-55.
[857] Ho, M. D. & Evans, G. J. (1997). Operational speciation of cadmium, copper, lead and
zinc in the NIST standard reference materials 2710 and 2711 (Montana soil) by the
BCR sequential extraction procedure and flame atomic absorption spectrometry. Anal
Commun, 34, 363-364.
[858] Tokalioglu, S., Kartal, S. & Gueltekin, A. (2006). Investigation of heavy-metal uptake
by vegetables growing in contaminated soils using the modified BCR sequential
extraction method. Int J Environ Anal Chem, 86, 417-430.
[859] Bacon, J. R., Hewitt, I. J. & Cooper, P. (2005). Reproducibility of the BCR sequential
extraction procedure in a long-term study of the association of heavy metals with soil
components in an upland catchment in Scotland. Sci Total Environ, 337, 191-205.
[860] Fernández, E., Jiménez, R., Lallena, A. M. & Aguilar, J. (2004). Evaluation of the
BCR sequential extraction procedure applied for two unpolluted Spanish soils.
Environ Pollut, 131, 355-364.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 252
[861] Pueyo, M., Sastre, J., Hernández, E., Vidal, M., López-Sánchez, J. F. & Rauret, G.
(200). Prediction of trace element mobility in contaminated soils by sequential
extraction. J Environ Quality, 32, 2054-2066.
[862] Larner, B. L., Seen, A. J. & Townsend, A. T. (2006). Comparative study of optimised
BCR sequential extraction scheme and acid leaching of elements in the certified
reference material NIST 2711. Anal Chim Acta, 556, 444-449.
[863] Sahuquillo, A., López-Sánchez, J. F., Rubio, R., Rauret, G., Thomas, R. P., Davidson,
C. M. & Ure, A. M. (1999). Use of a certified reference material for extractable trace
metals to assess sources of uncertainty in the BCR three-stage sequential extraction
procedure. Anal Chim Acta, 382, 317-327.
[864] Rauret, G., López-Sánchez, J. F., Sahuquillo, A., Rubio, R., Davidson, C., Ure, A. &
Quevauviller, P. (1999). Improvement of the BCR three step sequential extraction
procedure prior to the certification of new sediment and soil reference materials. J
Environ Monit, 1, 57-61.
[865] Davidson, C. M., Wilson, L. E. & Ure, A. M. (1999). Effect of sample preparation on
the operational speciation of cadmium and lead in a freshwater sediment. Fresenius’
J Anal Chem, 363, 134-136.
[866] Quevauviller, P., Rauret, G., Muntau, H., Ure, A. M., Rubio, R., López-Sánchez, J. F.,
Fiedler, H. D. & Griepink, B. (1994). Evaluation of a sequential extraction procedure
for the determination of extractable trace-metal contents in sediments. Fresenius’
J Anal Chem, 349, 808-814.
[867] Fiedler, H. D., López-Sánchez, J. F., Rauret, G., Rubio, R., Quevauviller, P., Ure, A.
M. & Muntau, H. (1994). Study of the stability of extractable trace-metal contents in a
river sediment using sequential extraction. Analyst, 119, 1109-1114.
[868] Heltai, G., Percsich, K., Fekete, I., Barabas, B. & Jozsa, T. (2000). Speciation of waste
water sediments. Microchem J, 67, 43-51.
[869] da Silva, C. L. & Masini, J. C. (2000). Determination of Cu, Pb, Cd, and Zn in river
sediment extracts by sequential injection anodic stripping voltammetry with thin
mercury film electrode. Fresenius’ J Anal Chem, 367, 284-290.
[870] Marin, B., Valladon, M., Polve, M. & Monaco, A. (1997). Reproducibility testing of a
sequential extraction scheme for the determination of trace metal speciation in a
marine reference sediment by inductively coupled plasma-mass spectrometry. Anal
Chim Acta, 342, 91-112.
[871] Thomas, R. P., Ure, A. M., Davidson, C. M., Littlejohn, D., Rauret, G., Rubio, R. &
López-Sánchez, J. F. (1994). 3-Stage sequential extraction procedure for the
determination of metals in river sediments. Anal Chim Acta, 286, 423-429.
[872] Svete, P., Milačič, R. & Pihlar, B. (2001). Partitioning of Zn, Pb and Cd in river
sediments from a lead and zinc mining area using the BCR three-step sequential
extraction procedure. J Environ Monit, 3, 586-590.
[873] Cuong, D. T. & Obbard, J. P. (2006). Metal speciation in coastal marine sediments
from Singapore using a modified BCR-sequential extraction procedure. Appl
Geochem, 21, 1335-1346.
[874] Filgueiras, A. V., Lavilla, I. & Bendicho, C. (2004). Evaluation of distribution,
mobility and binding behavior of heavy metals in surficial sediments of Louro River
Analytical Chemistry of Cadmium: Sample Pre-treatment… 253
(Galicia, Spain) using chemometric analysis: a case study. Sci Total Environ, 330,
115-129.
[875] Scancar, J., Milačič, R., Strazar, M. & Burica, O. (2000). Total metal concentrations
and partitioning of Cd, Cr, Cu, Fe, Ni and Zn in sewage sludge. Sci Total Environ,
250, 9-19.
[876] Greenway, G. M. & Song, Q. J. (2002). Heavy metal speciation in the composting
process. J Environ Monit, 4, 300-305.
[877] Walter, I., Martínez, F. & Cala, V. (2006). Heavy metal speciation and phytotoxic
effects of three representative sewage sludges for agricultural uses. Environ Pollut,
139, 507-514.
[878] Kim, B. & McBride, M. B. (2006). A test of sequential extractions for determining
metal speciation in sewage sludge-amended soils. Environ Pollut, 144, 475-482.
[879] Kazi, T. G., Jamali, M. K., Kazi, G. H., Arain, M. B., Afridi, H. I. & Siddiqui, A.
(2005). Evaluating the mobility of toxic metals in untreated industrial wastewater
sludge using a BCR sequential extraction procedure and a leaching testimate. Anal
Bioanal Chem, 383, 297-304.
[880] Fuentes, A., Llorens, M., Saez, J., Soler, A., Aguilar, M. I., Ortuno. J. F. & Meseguer,
V. F. (2004). Simple and sequential extractions of heavy metals from different sewage
sludges. Chemosphere, 54, 1039-1047.
[881] Margui, E., Queralt, I., Carvalho, M. L. & Hidalgo, M. (2006). Assessment of metal
availability to vegetation (Betula pendula) in Pb–Zn ore concentrate residues with
different features. Environ Pollut, 145, 179-184.
[882] Margui, E., Salvado, V., Queralt, I. & Hidalgo, M. (2004). Comparison of three-stage
sequential extraction and toxicity characteristic leaching tests to evaluate metal
mobility in mining wastes. Anal Chim Acta, 524, 151-159.
[883] Hlavay, J., Polyák, K. & Weisz, M. (2001). Monitoring of the natural environment by
chemical speciation of elements in aerosol and sediment samples. J Environ Monit, 3,
74-80.
[884] Sysalova, J. & Szakova, J. (2006). Mobility assessment and validation of toxic
elements in tunnel dust samples- Subway and road using sequential chemical
extraction and ICP-OES/GF AAS measurements. Environ Res, 101, 287-293.
[885] Soylak, M., Uzek, U., Narin, I., Tuezen, M., Turkoglu, O. & Elci, I. (2004).
Application of the sequential extraction procedure for dust samples from Kayseri-
Turkey. Fresenius Environ Bull, 13, 454-457.
[886] Petit, M. D. & Rucandio, M. I. (1999). Sequential extractions for determination of
cadmium distribution in coal fly ash, soil and sediment samples. Anal Chim Acta, 401,
283-291.
[887] Arain, M. B., Kazi, T. G., Jamali, M. K., Jalbani, N., Afridi, H. I. & Baig, J. A. (2008).
Speciation of heavy metals in sediment by conventional, ultrasound and microwave
assisted single extraction methods: A comparison with modified sequential extraction
procedure. J Hazard Mat, 154, 998-1006.
[888] Jamali, M. K., Kazi, T. G., Arain, M. B., Afridi, H. I., Jalbani, N., Kandhro, G. A.,
Shah, Q. A. & Baig, J. A. (2009). Speciation of heavy metals in untreated sewage
sludge by using microwave assisted sequential extraction procedure. J Hazard Mat,
163, 1157-1164.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 254
[889] Krasnodębska-Ostręga, B., Kaczorowska, M. & Golimowski, J. (2006). Ultrasound-
assisted extraction for the evaluation of element mobility in bottom sediment collected
at mining and smelting Pb-Zn ores area in Poland. Microchim Acta, 154, 39-43.
[890] Domouhtsidou, G. P., Dailianis, S., Kaloyianni, M. & Dimitriadis, V. K. (2004).
Lysosomal membrane stability and metallothionein content in Mytilus
galloprovincialis (L.), as biomarkers - Combination with trace metal concentrations.
Mar Pollut Bull, 48, 572-586.
[891] Romeo, M., Mourgaud, Y., Geffard, A., Gnassia-Barelli, M., Amiard, J. C. &
Budzinski, H. (2003). Multimarker approach in transplanted mussels for evaluating
water quality in Charentes, France, coast areas exposed to different anthropogenic
conditions. Environ Toxicol, 18, 295-305.
[892] Giguere, A., Couillard, Y., Campbell, P. G. C., Perceval, O., Hare, L., Pinel–Alloul, B.
& Pellerin, J. (2003). Steady-state distribution of metals among metallothionein and
other cytosolic ligands and links to cytotoxicity in bivalves living along a polymetallic
gradient. Aquatic Toxicol, 64, 185-200.
[893] Dragun, Z., Erk, M., Raspor, B., Ivankovic, D. & Pavicic, J. (2004). Metal and
metallothionein level in the heat-treated cytosol of gills of transplanted mussels
Mytilus galloprovincialis Lmk. Environ Int, 30, 1019-1025.
[894] Mourgaud, Y., Martínez, E., Geffard, A., Andral, B., Stanisiere, J. I. & Amiard, J. C.
(2002). Metallothionein concentration in the mussel Mytilus galloprovincialis as a
biomarker of response to metal contamination: validation in the field. Biomarkers, 7,
479-490.
[895] Nordberg, M. (1998). Metallothioneins: historical review and state of knowledge.
Talanta, 46, 243-254.
[896] Hunziker, P. E. & Kägi, J. H.R. (1985). Metallothionein; P. Harrison, Ed.,
Metalloproteins Part 2. Metal proteins with non–redox roles; Verlag Chemie:
Weinheim.
[897] Vasak, M. (2005). Advances in metallothionein structure and functions. J Trace
Element Med Biol, 19, 13-17.
[898] Suzuki, K. T. (1992). Preparation of metallothioneins; M. J. Stillman, C. F. Shaw III,
& K. T. Suzuki, (Eds.), Metallothioneins. Synthesis, structure and properties of
metallothioneins, phytochelatins and metal–thiolate complexes; VCH Publishers Inc.:
Weinheim.
[899] Chassaigne, H. & Łobiński, R. (1998). Polymorphism and identification of
metallothionein isoforms by reversed-phase HPLC with on-line ion spray mass
spectrometric detection. Anal Chem, 70, 2536-2543.
[900] Chassaigne, H. & Łobiński, R. (1998). Characterization of metallothionein isoforms
by reversed-phase high-performance liquid chromatography with on-line post-column
acidification and electrospray mass spectrometric detection. J Chromatogr A, 829,
127-136.
[901] Mounicou, S., Połeć, K., Chassaigne, H., Potin–Gautier, M. & Łobiński, R. (2000).
Characterization of metal complexes with metallothioneins by capillary zone
electrophoresis (CZE) with ICP-MS and electrospray (ES)-MS detection. J Anal At
Spectrom, 15, 635-642.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 255
[902] Ferrarello, C. N., Fernández de la Campa, M. R., Goenaga–Infante, H., Fernández–
Sánchez, M. L. & Sanz–Medel, A. (2000). Improved separation of rabbit liver
metallothioneins by FPLC-ICP-MS: a comparison with the conventional anion-
exchange chromatography. Analusis, 28, 351-357.
[903] Polec, K., Szpunar, J., Palacios, O., González-Duarte, P., Atrian, S. & Łobinski, R.
(2001). Investigation of metal binding by recombinant and native metallothioneins by
capillary zone electrophoresis (CZE) coupled with inductively coupled plasma mass
spectrometry (ICP-MS) via a self-aspirating total consumption micronebulizer. J Anal
At Spectrom, 16, 567-574.
[904] Polec-Pawlak, K., Schaumlöffel, D., Szpunar, J., Orange, A. & Łobinski, R. (2002).
Analysis or metal complexes with metallothionein in rat liver by capillary zone
electrophoresis using ICP double-focussing sector-field isotope dilution MS and
electrospray MS detection. J Anal At Spectrom, 17, 908-912.
[905] Chassaigne, H. & Łobinski, R. (1998). Polymorphism and identification of
metallothioneins isoforms by reversed-phase HPLC with on-line ion-spray and
inductively coupled plasma mass spectrometric detection. Fresenius J Anal Chem,
361, 267-273.
[906] Polec, K., Mounicou, S., Chassaigne, H. & Łobinski, R. (2000). Probing metal-
complexes with metallothioneins by reversed phase microbore chromatography and
capillary zone electrophoresis coupled with inductively coupled plasma and
electrospray mass spectrometry. Cell Mol Biol, 46, 221-235.
[907] Polec, K., García-Arribas, O., Peréz-Calvo, M., Szpunar, J., Ribas-Ozonas, N. &
Łobinski, R. (2000). Identification of cadmium-bioinduced ligands in rat liver using
parallel HPLC-ICP-MS and HPLC-electrospray MS. J Anal At Spectrom, 15, 1363-
1368.
[908] Dal Corso, G., Farinati, S., Maistri, S. & Furini, A. (2008). How plants cope with
cadmium: Staking all on metabolism and gene expression. J Integrative Plant Biol, 50,
1268-1280.
[909] Domenech, J., Mir, G., Huguet, G., Capdevila, M., Molinas, M. & Atrian, S. (2006).
Plant metallothionein domains: Functional insight into physiological metal binding
and protein folding. Biochim, 88, 583-593.
[910] Gupta, R. K. Dobritsa, S. V. Stiles, C. A. Essington, M. E. Liu, Z. Y. Chen, C. H.
Serpersu, E. H. & Mullin, B. C. (2002). Metallohistins: A new class of plant metal-
binding proteins. J Protein Chem, 21, 529-536.
[911] Roesijadi, G. & Fowler, B. A. (1991). Purification of invertebrate metallothioneins.
Meth Enzymol, 205, 263-273.
[912] Wolf, C., Rösick, U. & Brätter, P. (2002). Sampling and processing of biopsy samples
for speciation studies of cytosolic metalloproteins. Anal Bional Chem, 372, 491-494.
[913] Rudolph, C., Adam, G. & Simm, A. (1999). Determination of copy number of c-Myc
protein per cell by quantitative Western blotting. Anal Biochem, 269, 66-71.
[914] Santiago-Rivas, S., Moreda-Pineiro, A., Bermejo-Barrera, P., Moreda-Pineiro, J.,
Alonso-Rodríguez, E., Muniategui-Lorenzo, S., López-Mahía, P. & Prada-Rodríguez,
D. (2007). Pressurized liquid extraction-assisted mussel cytosol preparation for the
determination of metals bound to metallothnionein-like proteins. Anal Chim Acta, 603,
36-43.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 256
[915] Pedersen, K. L., Pedersen, S. N., Knudsen, J. & Bjerregaard, P. (2008). Quantification
of metallothionein by differential pulse polarography overestimates concentrations in
crustaceans. Environ Sci Technol, 42, 8426-8432.
[916] López, M. J., Arino, C., Díaz-Cruz, S., Díaz-Cruz, J. M., Tauler, R. & Esteban, M.
(2003). Voltammetry assisted by multivariate analysis as a tool for speciation of
metallothioneins: Competitive complexation of α- and β-metallothionein domains with
cadmium and zinc. Environ Sci Technol, 37, 5609-5616.
[917] Gómez-Ariza, J. L., García-Barrera, T., Lorenzo, F., Bernal, V., Villegas, M. J. &
Oliveira, V. (2004). Use of mass spectrometry techniques for the characterization of
metal bound to proteins (metallomics) in biological systems. Anal Chim Acta, 524,
15-22.
[918] Prange, A. & Schaumlöffel, D. (2002). Hyphenated techniques for the characterization
and quantification of metallothionein isoforms. Anal Bioanal Chem, 373, 441-453.
[919] Ferrarello, C. N., Fernández de la Campa, M. R. & Sanz-Medel, A. (2002).
Multielement trace-element speciation in metal-biomolecules by chromatography
coupled with ICP-MS. Anal Bioanal Chem, 373, 412-421.
[920] Minami, T., Ichida, S. & Kubo, K. (2002). Study of metallothionein using capillary
zone electrophoresis. J Chromatogr B, 781, 303-311.
[921] Ferrarello, C. N., Fernández de la Campa, M., Muñiz, C. S. & Sanz–Medel, A. (2000).
Metal distribution patterns in the mussel Mytilus edulis cytosols using size-exclusion
chromatography and double focusing ICP-MS detection. Analyst, 125, 2223-2229.
[922] Ferrarello, C. N., Fernández de la Campa, M., Montes-Bayón, M. & Sanz–Medel, A.
(2000). Multi-elemental speciation studies of trace elements associated with
metallothionein-like proteins in mussels by liquid chromatography with inductively
coupled plasma time of fligh mass spectrometric detection. J Anal At Spectrom, 15,
1558-1563.
[923] Montes-Bayón, M., Proefrock, D., Sanz-Medel, A. & Prange, A. (2006). Direct
comparison of capillary electrophoresis and capillary liquid chromatography
hyphenated to collision-cell inductively coupled plasma mass spectrometry for the
investigation of Cd-, Cu- and Zn-containing metalloproteins. J Chromatogr A, 1114,
138-144.
[924] Digilio, G., Bracco, C., Vergani, L., Botta, M., Osella, D. & Viarengo, A. (2009). The
cadmium binding domains in the metallothionein isoform Cd7-MT10 from Mytilus
galloprovincialis revealed by NMR spectroscopy. J Biol Inorg Chem, 14, 167-178.
[925] Wang, J., Dreessen, D., Wiederin, D. R. & Houk, R. S. (2001). Measurement of trace
elements in proteins extracted from liver by size exclusion chromatography-
inductively coupled plasma-mass spectrometry with a magnetic sector mass
spectrometer. Anal Biochem, 288, 89-96.
[926] Ogra, Y. & Suzuki, K. T. (1999). Biological significance of non-acetylated
metallothionein. J Chromatogr B, 735, 17-24.
[927] Nischwitz, V., Michalke, B. & Kettrup, A. (2003). Extraction and characterization of
trace element species from porcine liver samples using online HPLC-ICP-MS and off
line HPLC-ESI-MS. J Anal At Spectrom, 18, 444-451.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 257
[928] Wolf. C., Rösick, U. & Brätter, P. (2000). Quantification of the metal distribution in
metallothioneins of the human liver by HPLC coupled with ICP-AES. Fresenius J
Anal Chem, 368, 839-843.
[929] Boulyga, S. F., Loreti, V., Bettmer, J. & Heumann, K. G. (2004). Application of SEC-
ICP-MS for comparative analyses of metal-containing species in cancerous and
healthy human thyroid samples. Anal Bioanal Chem, 380, 198-203.
[930] Richarz, A. N., Wolf, C. & Brätter, P. (2003). Determination of protein-bound trace
elements in human cell cytosols of different organs and different pathological states.
Analyst, 128, 640-645.
[931] Richarz, P. & Brätter, A. N. (2002). Speciation analysis of trace elements in the brains
of individuals with Alzheimer's disease with special emphasis on metallothioneins.
Anal Bioanal Chem, 372, 412-417.
[932] Van Campenhout, K., Goenaga Infante, H., Goemans, G., Belpaire, C., Adams, F.,
Blust, R. & Bervoets, L. (2008). A field survey of metal binding to metallothionein
and other cytosolic ligands in liver of eels using an on-line isotope dilution method in
combination with size exclusion (SE) high pressure liquid chromatography (HPLC)
coupled to inductively coupled plasma time-of-flight mass spectrometry (ICP-
TOFMS). Sci Total Environ, 394, 379-389.
[933] Goenaga Infante, H., Van Campenhout, K., Schaumlöffel, D., Blust, R. & Adams,
F. C. (2003). Multi-element speciation of metalloproteins in fish tissue using size-
exclusion chromatography coupled "on-line" with ICP-isotope dilution-time-of-flight-
mass spectrometry. Analyst, 128, 651-657.
[934] Rodríguez-Cea, A., Linde Arias, A. R., Fernández de la Campa, M. R., Costa Moreira,
J. & Sanz-Medel, A. (2006). A Metal speciation of metallothionein in white sea
catfish, Netuma barba, and pearl cichlid, Geophagus brasiliensis, by orthogonal liquid
chromatography coupled to ICP-MS detection. Talanta, 69, 963-969.
[935] Van Campenhout, K., Goenaga Infante, H., Adams, F. & Blust, R. (2004). Induction
and binding of Cd, Cu, and Zn to metallothionein in carp (Cyprinus carpio) using
HPLC-ICP-TOFMS. Toxicol Sci, 80, 276-287.
[936] Goenaga Infante, H., Van Campenhout, K., Blust, R. & Adams, F. C. (2002).
Inductively coupled plasma time-of-flight mass spectrometry coupled to high-
performance liquid chromatography for multi-elemental speciation analysis of
metalloproteins in carp cytosols. J Anal At Spectrom, 17, 79-87.
[937] Goenaga Infante, H., Cuyckens, F., Van Campenhout, K., Blust, R., Claeys, M., Van
Vaeck, L. & Adams, F. C. (2004). Characterization of metal complexes with
metallothioneins in the liver of the carp Cyprinus carpio by reversed-phase HPLC with
ICP-MS and electrospray ionization (ESI)-MS. J Anal At Spectrom, 19, 159-166.
[938] Ferrarello, C. N., Fernández de la Campa, M. R., Carrasco, J. F. & Sanz-Medel, A.
(2002). Speciation of metallothionein-like proteins of the mussel Mytilus edulis by
orthogonal separation mechanisms with inductively coupled plasma-mass
spectrometry detection: effect of selenium administration. Spectrochim Acta B, 57,
439-449.
[939] Vacchina, V., Łobinski, R., Oven, M. & Zenk, M. H. (2000). Signal identification in
size-exclusion HPLC-ICP-MS chromatograms of plant extracts by electrospray
tandem mass spectrometry (ES MS/MS). J Anal At Spectrom, 15, 529-534.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 258
[940] Goenaga Infante, H., Fernández Sánchez, M. L. & Sanz-Medel, A. (1999). Cadmium-
bound species in human urine using high-performance liquid chromatography-
vesicular hydride generation-inductively coupled plasma mass spectrometry. J Anal At
Spectrom, 14, 1343-1348.
[941] Nöstelbacher, K., Kirchgessner, M. & Stangl, G. I. (2000). Separation and quantitation
of metallothionein isoforms from liver of untreated rats by ion-exchange high-
performance liquid chromatography and atomic absorption spectrometry.
J Chromatogr B, 744, 273-282.
[942] Goenaga Infante, H., Van Campenhout, K., Blust, R. & Adams, F. C. (2006). Anion-
exchange high performance liquid chromatography hyphenated to inductively coupled
plasma-isotope dilution-time-of-flight mass spectrometry for speciation analysis of
metal complexes with metallothionein isoforms in gibel carp (Carassius auratus
gibelio) exposed to environmental metal pollution. J Chromatogr A, 1121, 184-190.
[943] Rodríguez-Cea, A., Fernández de la Campa, M. R., Blanco González, E., Andon
Fernández, B. & Sanz-Medel, A. (2003). Metal speciation analysis in eel (Anguilla
anguilla) metallothioneins by anionic exchange-FPLC-isotope dilution-ICP-MS.
J Anal At Spectrom, 18, 1357-1364.
[944] Chassaigne, H. & Łobinski, R. (1998). Speciation of metal complexes with
biomolecules by reversed-phase HPLC with ion-spray and inductively coupled plasma
mass spectrometric detection. Fresenius J Anal Chem, 361, 267-273.
[945] Goenaga Infante, H., Fernández Sánchez, M. L. & Sanz-Medel, A. (2000). Vesicle-
mediated high performance liquid chromatography coupled to hydride generation
inductively coupled plasma mass spectrometry for cadmium speciation in fish
cytosols. J Anal At Spectrom, 15, 519-524.
[946] Ferrarello, C. N., Ruiz Encinar, J., Centineo, G., García Alonso, J. I., Fernández de la
Campa, M. R. & Sanz-Medel, A. (2002). A Comparison of three different ICP-MS
instruments in the study of cadmium speciation in rabbit liver metallothionein-1 using
reversed-phase HPLC and post-column isotope dilution analysis. J Anal At Spectrom,
17, 1024-1029.
[947] Lu, Q., Bird, S. M. & Barnes, R. M. (1995). Interface for capillary electrophoresis and
inductively-coupled plasma-mass spectrometry. Anal Chem, 67, 2949-2956.
[948] Lu, Q. & Barnes, R. M. (1996). Evaluation of an ultrasonic nebulizer interface for
capillary electrophoresis and inductively coupled plasma mass spectrometry.
Microchem J, 54, 129-143.
[949] B’Hymer, C., Day, J. A. & Caruso, J. A. (2000). Evaluation of a microconcentric
nebulizer and its suction effect in a capillary electrophoresis interface with inductively
coupled plasma mass spectrometry. Appl Spectr, 54, 1040-1046.
[950] Taylor, K. A., Sharp, B. L., Lewis, D. J. & Crews, H. M. (1998). Design and
characterisation of a microconcentric nebuliser interface for capillary electrophoresis-
inductively coupled plasma mass spectrometry. J Anal At Spectrom, 13, 1095-1100.
[951] Majidi, V. & Miller-Ihli, N. J. (1998). Two simple interface designs for capillary
electrophoresis inductively coupled plasma mass spectrometry. Analyst, 123, 803-808.
[952] Majidi. V. & Miller-Ihli, N. J. (1998). Potential sources of error in capillary
electrophoresis inductively coupled plasma mass spectrometry for chemical
speciation. Analyst, 123, 809-813.
Analytical Chemistry of Cadmium: Sample Pre-treatment… 259
[953] Prange, A., Schaumlöffel, D., Brätter, P., Richarz, A. N. & Wolf, C. (2001). Species
analysis of metallothionein isoforms in human brain cytosols by use of capillary
electrophoresis hyphenated to inductively coupled plasma-sector field mass
spectrometry. Fresenius J Anal Chem, 371, 764-774.
[954] Schaumlöffel, D., Prange, A., Marx, G., Heumann, K. G. & Brätter, P. (2002).
Characterization and quantification of metallothionein isoforms by capillary
electrophoresis-inductively coupled plasma-isotope-dilution mass spectrometry. Anal
Bioanal Chem, 372, 155-163.
[955] Wang, Z. & Prange, A. (2002). Use of surface-modified capillaries in the separation
and characterization of metallothionein isoforms by capillary electrophoresis
inductively coupled plasma mass spectrometry. Anal Chem, 74, 626-631.
[956] Álvarez-Llamas, G., Fernández de la Campa, M. R. & Sanz-Medel, A. (2005). An
alternative interface for CE-ICP-MS cadmium speciation in metallothioneins based on
volatile species generation. Anal Chim Acta, 546, 236-243.
[957] Álvarez-Llamas, G., Fernández de la Campa, M. R., Fernández Sánchez, M. L. &
Sanz-Medel, A. (2002). Comparison of two CE-ICP-MS interfaces based on
microflow nebulizers: application to cadmium speciation in metallothioneins using
quadrupole and double focusing mass analyzers. J Anal At Spectrom, 17, 655-661.
[958] Schaumlöffel, D., Prange, A., Marx, G., Heumann, K. G. & Brätter, P. (2002).
Characterization and quantification of metallothionein isoforms by capillary
electrophoresis-inductively coupled plasma-isotope-dilution mass spectrometry. Anal
Bioanal Chem, 372, 155-163.
[959] Baker, S. A. & Miller-Ihli, N. J. (1999). Comparison of a cross-flow and
microconcentric nebulizer for chemical speciation measurements using CZE-ICP-MS.
Appl Spectrosc, 53, 471-478.
[960] Wolf, C., Schaumlöffel, D., Richarz, A. N., Prange, A. & Brätter, P. (2003). CZE-ICP-
MS separation of metallothioneins in human brain cytosols: comparability of
electropherograms obtained from different sample matrices. Analyst, 128, 576-580.
[961] Proefrock, D., Leonhard, P. & Prange, A. (2003). Determination of sulfur and selected
trace elements in metallothionein-like proteins using capillary electrophoresis
hyphenated to inductively coupled plasma mass spectrometry with an octopole
reaction cell. Anal Bioanal Chem, 377, 132-139.
[962] Profrock, D., Prange, A., Schaumlöffel, D. & Ruck, W. (2003). Optimization of
capillary electrophoresis-inductively coupled plasma mass spectrometry for species
analysis of metallothionein-like proteins extracted from liver tissues of Elbe-bream
and Roe deer. Spectrochim Acta B, 58, 1403-1415.
[963] Álvarez-Llamas, G., Fernández de la Campa, M. R. & Sanz-Medel, A. (2003). Sample
stacking capillary electrophoresis with ICP-(Q)MS detection for Cd, Cu and Zn
speciation in fish liver metallothioneins. J Anal At Spectrom, 18, 460-466.
[964] Álvarez-Llamas, G., Rodríguez-Cea, A., Fernández de la Campa, M. R. & Sanz-
Medel, A. (2003). Metallothionein isoforms separation and cadmium speciation by
capillary electrophoresis with ultraviolet and quadrupole-inductively coupled plasma
mass spectrometric detection. Anal Chim Acta, 448, 105-119.
Antonio Moreda-Piñeiro and Jorge Moreda-Piñeiro 260
[965] International Standard Organization. (1994). Water quality. Determination of
cadmium by atomic absorption spectrometry. ISO 5961. International Organization for
Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[966] International Standard Organization. (1986). Water quality. Determination of cobalt,
nickel, copper, zinc, cadmium and lead. Flame atomic absorption spectrometric
methods. ISO 8288. International Organization for Standardization, Case Postale 56,
CH-1211, Geneva 20 Switzerland.
[967] International Standard Organization. (2003). Water quality. Determination of trace
elements using atomic absorption spectrometry with graphite furnace. ISO 15586.
International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20
Switzerland.
[968] International Standard Organization. (2002). Water quality. Digestion for the
determination of selected elements in water. Part 1. Aqua regia digestion. ISO 15587-
1. International Organization for Standardization, Case Postale 56, CH-1211, Geneva
20 Switzerland.
[969] International Standard Organization. (2002). Water quality. Digestion for the
determination of selected elements in water. Part 2. Nitric acid digestion. ISO 15587-
1. International Organization for Standardization, Case Postale 56, CH-1211, Geneva
20 Switzerland.
[970] International Standard Organization. (2007). Water quality. Determination of selected
elements by inductively coupled plasma optical emission spectroscopy (ICOP-OES).
ISO 11885. International Organization for Standardization, Case Postale 56, CH-1211,
Geneva 20 Switzerland.
[971] International Standard Organization. (2004). Water quality. Application of inductively
coupled plasma mass spectrometry (ICP-MS). Part 1: General guidelines. ISO 17294-
1. International Organization for Standardization, Case Postale 56, CH-1211, Geneva
20 Switzerland.
[972] International Standard Organization. (2003). Water quality. Application of inductively
coupled plasma mass spectrometry (ICP-MS). Part 2: Determination of 62 elements.
ISO 17294-2:2003. International Organization for Standardization, Case Postale 56,
CH-1211, Geneva 20 Switzerland.
[973] Official methods of analysis of The Association of Official Analytical Chemists,
Methods Manual, 18th edition, 2005. Cadmium, chromium, copper, iron, lead,
magnesium, manganese, silver, and zinc in water. Atomic absorption spectrometric
method. AOAC Official method 974.27.
[974] APHA. (1999). Standard Methods for the Examination of Water and Wastewater. 3111
Metals by Flame Atomic Absorption Spectrometry. American Public Health
Association (APHA), American Water Works Association (AWWA), Water
Environment Federation publication (WPCF). APHA, Washington, DC.
[975] APHA. (2004). Standard Methods for the Examination of Water and Wastewater. 3113
Metals by Electrothermal Atomic Absorption Spectrometry. American Public Health
Association (APHA), American Water Works Association (AWWA), Water
Environment Federation publication (WPCF). APHA, Washington, DC.
[976] APHA. (1999). Standard Methods for the Examination of Water and Wastewater. 3120
Metals by Plasma Emission Spectroscopy. American Public Health Association
Analytical Chemistry of Cadmium: Sample Pre-treatment… 261
(APHA), American Water Works Association (AWWA), Water Environment
Federation publication (WPCF). APHA, Washington, DC.
[977] APHA. Standard Methods for the Examination of Water and Wastewater. 3125 Metals
by Inductively Coupled Plasma/Mass Spectrometry. American Public Health
Association (APHA), American Water Works Association (AWWA), Water
Environment Federation publication (WPCF). APHA, Washington, DC.
[978] APHA. (2004). Standard Methods for the Examination of Water and Wastewater. 3130
Metals by Anodic Stripping Voltammetry. American Public Health Association
(APHA), American Water Works Association (AWWA), Water Environment
Federation publication (WPCF). APHA, Washington, DC.
[979] International Standard Organization. (1995). Water quality. Soil quality. Extraction of
trace elements soluble in aqua regia. ISO 11466. International Organization for
Standardization, Case Postale 56, CH-1211, Geneva 20 Switzerland.
[980] International Standard Organization. (2001). Soil quality. Dissolution for the
determination of total element content. Part 1: Dissolution with hydrofluoric and
perchloric acids. ISO 14869-1. International Organization for Standardization, Case
Postale 56, CH-1211, Geneva 20 Switzerland.
[981] International Standard Organization. (1998). Soil quality. Determination of cadmium,
chromium, cobalt, copper, lead, manganese, nickel and zinc in aqua regia extracts of
soils. Flame and electrothermal atomic absorption spectrometric methods. ISO 11047.
International Organization for Standardization, Case Postale 56, CH-1211, Geneva 20
Switzerland.
[982] International Standard Organization. (2008). Soil quality. Determination of trace
elements in extracts of soil by inductively coupled plasma - atomic emission
spectrometry (ICP – AES). ISO 22036. International Organization for Standardization,
Case Postale 56, CH-1211, Geneva 20 Switzerland.
[983] European Standard (2004). Stationary sources emissions. Determination of the total
emission of As, Cd, Cr, Co, Cu, Mn, Ni, Pb, Sb, Tl and V. EN 14385. European
Committee for Standardization, Brussels, Belgium.
[984] European Standard (2005). Ambient air quality. Standard method for measurement of
Pb, Cd, As and Ni in the PM10 fraction of suspended particulate matter. EN 14902.
European Committee for Standardization, Brussels, Belgium.
