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Determination of
90
Sr in milk and milk powder in Tehran
and estimation of annual effective dose
Neda Saraygord-Afshari •Fereshteh Abbasisiar •
Parviz Abdolmaleki •Mahdi Ghiassi-Nejad •
Ali Attarilar
Published online: 7 July 2011
ÓSpringer Science+Business Media, LLC 2011
Abstract Thirty-eight different milk and milk powder
samples from Tehran-Iran were collected and analyzed for
90
Sr activity using a method in which the daughter product
of
90
Sr decay (
90
Y) was extracted by tributyl phosphate
from ashed milk.
90
Y was then back extracted with water,
and oxalate was precipitated . Following the sample ana-
lyzing, beta counting was performed with an ultralow-level
liquid scintillation spectrometer. The quality control and
assurance of the method were obtained by standard sam-
ples prepared with an IAEA-certified reference material.
The mean determined
90
Sr activity concentration in the
analyzed milk and milk powder (0.225 ±0.042 and
0.216 ±0.024 Bq kg
-1
, respectively) showed that the
radioactivity concentration in our samples was too low to
induce biological hazards. These data can provide useful
information of the background level of contamination,
which in turn can be used in the following environmental
monitoring programs.
Keywords Strontium-90 Milk Effective dose
Activity determination
1 Introduction
The biologically hazardous radionuclide
90
Sr, which is
present in our environment, is an artificial radionuclide,
produced essentially by the
235
U and
239
Pu fission reaction,
which has occurred during the previous atmospheric
nuclear tests and nuclear reactor accidents (Brun et al.
2003; Stamoulis et al. 2007).
Strontium is a bone seeker element. Due to its chemical
and biochemical similarities with calcium, more than 99%
of strontium is efficiently incorporated into bone tissue and
teeth. Characterized by a long physical and biological half-
life (28.15 and &7 years, respectively),
90
Sr may cause
damage to bone marrow and induce bone sarcoma and
leukemia, because of its high-energy b-particles; E
bmax
:
546 keV; (Brun et al. 2002).
90
Sr decays to
90
Y (half-life:
64.1 h), which emits hard b-particles with maximum
energy of 2,280 keV
.90
Y also contributes to the internal
dose of
90
Sr (Brun et al. 2003).
90
Sr transfers into humans mainly via foodstuffs. Since
milk is the principle source of calcium in human diet, it is a
substantial contributor especially for infants (Bem et al.
1991). Moreover, because strontium transfer from soil and
plant to cow milk is efficient and rapid, milk contamination
level can give an indication of
90
Sr deposition over a wide
area (Brun et al. 2002; Galle 1988). Therefore,
90
Sr mea-
surement especially in milk has acquired considerable
attention in environmental and personal monitoring pro-
grams (Alvarez et al. 1995; Froidevaux et al. 2006,2004;
Landstetter and Wallner 2006; Mietelski et al. 2004;
Al-Masri et al. 2004). Also, there are a lot of reports,
focusing on the methodology of its measurement (Chang
et al. 2004; Jassin 2005; Lee et al. 2002; Baron et al. 2004;
Horwitz et al. 1992; Mikulaj and Svcc 1993; Tait et al.
1997). Accordingly, this paper represents the results of
N. Saraygord-Afshari P. Abdolmaleki (&)M. Ghiassi-Nejad
Department of Biophysics, Faculty of Biological Sciences,
Tarbiat Modares University, P.O. Box 14115/175, Tehran, Iran
e-mail: parviz@modares.ac.ir
F. Abbasisiar A. Attarilar
Environmental Radiation Protection Division, National
Radiation Protection Department (NRPD), Tehran, Iran
F. Abbasisiar A. Attarilar
Atomic Energy Organization of Iran (AEOI), Tehran, Iran
123
Environmentalist (2011) 31:308–314
DOI 10.1007/s10669-011-9337-6
radioactivity analysis carried out for
90
Sr in the milk
samples consumed in Tehran-Iran followed by the esti-
mation of its annual effective dose, in order to assess of the
toxic effects of this radio isotope in the consumers.
2 Materials and methods
2.1 Reagents
All reagents used were of analytical grade and from
MERCK or FLUKA companies. The radioisotopes
90
Sr/
90
Y solution was obtained from Amersham. The
method was tested and certified with reference milk
(Milk-152) received by AQCS (Analytical Quality Control
Services) laboratory of the International Atomic Energy
Agency (IAEA), Vienna, Austria.
2.2 Equipment
Beta counting was performed with a Wallac (model
Quantulus 1220) ultralow-level liquid scintillation. Other
required instruments included an oven, a muffle furnace, a
freeze/dryer set, an analytical balance, hot plates, and
magnetic stirrers, all of which are normally available in
chemical laboratories.
2.3 Sampling and sample preparation
Milk samples were obtained from local markets in Tehran
(the capital city of Iran). Around 12 million inhabitants live
in this area, and the large population of this city was one of
the main considerations for the selection of the sampling
site. Twenty-eight milk and ten milk powder samples were
collected during the year of the experiment. All the sam-
ples were prepared before analysis. First, about 1.5–2 l of
each milk sample was dried, and each time, 100–120 g of
the dried milk or milk powder was weighted in a porcelain
crucible and dried in an oven at 80°C for 6 h to a constant
weight. After well drying the samples, complete ashing in a
muffle furnace at 700°C was carried out for about 2 h (note
that the temperature should be increased gradually). As a
result of this step, a large amount of organic materials such
as fats and proteins will be decomposed.
2.4 Chemical procedure
The applied method; which is the combination of two
common methods with some modifications, was based on
the direct determination of
90
Y that is in secular equilib-
rium with
90
Sr, by measuring Cerenkov irradiation using
liquid scintillation counter (Bem et al. 1991; IAEA 1993a,
b). The method used TBP (tributyl phosphate) extraction of
90
Y as follows (Fig. 1):
1. Ten–twenty grams of the ash obtained was weighed
into a 250-ml beaker, and 1 ml of each Cs
?
,Ba
2?
,
La
3?
, and Sr
2?
carrier solutions, with 10 ml of Y
3?
carrier solution, was added (each containing 1 mg
element.ml
-1
).
2. Ten milliliters of conc. HNO
3
(65%) per 1 g of ash
(about 100 ml) was added, and the sample was gently
boiled at 200°C for about 2 h on a hot plate while the
beaker was covered with a glass watch (leaching
process).
3. After the leaching step, the sample was cooled enough
to be filtered through a medium–fast filter paper.
4. The filtrate was transferred into a 250-ml separatory
funnel and extracted for 3–5 min with 30 ml TBP
(previously equilibrated with 14 M HNO
3
). The time
of the first extraction was recorded for the decay
correction.
5. After the separation of the two phases, the organic
phase was transferred into another separatory funnel
and the acid phase was treated again with 30 ml of
TBP. This step was repeated once more.
6. The organic phases obtained from all the three
mentioned steps were combined and washed with
50 ml of 14 M HNO
3
to remove possible contami-
nation from other radio nuclides with lower distribu-
tion coefficients between TBP and 14 M HNO
3
. The
aqueous phase was discarded.
7. The organic phase was back extracted 2 times with
50 ml of water and then with 50 ml of 2 M HNO
3
to
strip yttrium from TBP.
8. The aqueous phases were combined and then evap-
orated to less than 50 ml on a hot plate. Following
that, the pH was adjusted to 9–10 with ammonia
solution.
9. Two milliliters of Fe
3?
carrier solution was then
added to the sample, and after heating in a water
bath, it was centrifuged for 6–8 min at 6,000 rpm.
10. The precipitate was dissolved using a minimum
amount of 6 M HNO
3
by heating. Then, 30 ml of 2%
ammonium oxalate solution was added and pH was
adjusted up to 2–2.5 by adding ammonia again.
11. After heating in a water bath, the sample was
centrifuged for 6–8 min at 6,000 rpm. The steps 10
and 11 were repeated 3 times in order to eliminate the
possible presence of Fe
3?
ions.
12. Finally, the solution containing yttrium oxalate
precipitate was filtered through an accurately
weighed filter paper (blue band) and washed twice
with a minimum amount of distilled water and
ethanol.
Environmentalist (2011) 31:308–314 309
123
2.5 Sample counting
As already mentioned, in this research,
90
Sr concentration
is determined by its daughter (
90
Y) in radioactive equilib-
rium with its parent. For the Cerenkov counting, yttrium
oxalate together with the filter paper was dissolved with
5 ml 6 M HCl in a 20-ml Copper–Teflon scintillation vial
by heating in an oven at 80°C for 3–5 min. Then, 10 ml of
distilled water and 5 ml of 2 M HCl were added and mixed
well by shaking.
After preparing the vials, they were counted in a
Quantulus 1,220 liquid scintillation spectrometer that has
an active liquid scintillation guard counter and a 4pold
lead-passive shielding that protects the spectrometer
against the external and cosmic radiations. Each sample
was counted once for 10,600 s in Cerenkov mode. In the
beta spectrum,
90
Y window was selected in channel region
5–400. The detection efficiency for
90
Y was calibrated
using some sources prepared from standard
90
Sr/
90
Y solu-
tion after separation of
90
Y and equaled to 74%. Blanks
were prepared in the same way as the sample using stable
yttrium carrier, and the amount of the average background
count rate was determined as 1.323 cpm (count per
minute).
Carrier
addition
&
Sample
preparation
Yittrium
extraction
Activity
Determinatio
36-39 h. 3-4 h. 3 h.
MILK
Drying;
Ashing;
Carrier addition
Leaching with
3
HNo con.
Extraction with
clean TBP;
(3 times)
Back extraction of total
organic phases with water
(2 times)
&
with 3
HNo 2 M
(1 time)
Fe & Y
p
reci
p
itation in the
presence of Fe/carrier at
pH ≅9-10
Solving of the precipitates
and ammonium oxalate
addition at pH ≅2-2.5
(
3 times
)
Y oxalate precipitation
Ultra low level Liquid
scintillation
(Cerenkov counting)
Sr, Y, Cs, Ba, alkaline, alkaline
earth, transition elements,
proteins fats
Sr, Y, Cs, Ba, alkaline, alkaline
earth, transition elements
Y, Fe, Ca
Y
Proteins & fats
Fe, Ca
Sr, Ba,
Alkaline.
Alkaline
earth &
transition
elements
Fig. 1 Schematic flowchart for
analytical procedure
310 Environmentalist (2011) 31:308–314
123
2.6 Calculations
2.6.1 Activity determination
After sample counting, the activity concentration A
(Bq kg
-1
) in the samples was calculated using the fol-
lowing expression:
A¼GbB
RWae60 ekDtfa:d:ð1Þ
where G
b
is the gross beta count rate (cpm) for the sample,
Bis the count rate of the blank sample (cps), Ris the
chemical recovery of yttrium determined by gravimetry,
and the calculation is based on the standardized yttrium
carrier solution, W
a
is the analyzed ash weight (kg), eis the
Cerenkov beta counting efficiency for
90
Y, kis the decay
constant of
90
Y (0.0108 h
-1
), Dtis the decay time from the
first extraction to the middle of the counting time (hours),
and f
a.d.
is the ash to dry weight ratio.
2.6.2 Uncertainties
For the 2rstandard deviation, uncertainties, U(Bq kg
-1
)
for the Cerenkov method, were calculated according to the
following relation (Scarpitta et al. 1999):
U¼2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
GbCT
p
CT WsReð2Þ
where CT is the counting time (second) and W
s
is the
original weight of the sample (kg).
2.6.3 Detection limits
According to the Currie criteria, the minimum detectable
levels, MDLs (Bq kg
-1
), is defined so that, if an amount of
a radioisotope equals to the MDL exits in the sample, it
will be detected with 95% probability (Brun et al. 2003;
Alvarez et al. 1995). In the condition of the present
research, we used the following relation to determine the
MDL values:
MDL = 2:71 þ4:65 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
BCT
p
CT WsRe:ð3Þ
2.6.4 Decay test for
90
Y samples
To check the interfering radionuclides such as
40
K in the
counted samples, the decay curve should be investigated
for each of the prepared sample by counting the
90
Y source
obtained from milk for several hours, but by considering
the large number of samples, it was done randomly only for
some of them. The decay curves were then controlled by
the half-life of
90
Y (64.1 h). Figure 2presents the results
for one of the attempts and can elucidate our statements.
2.6.5 Assessment of effective annual dose due to ingestion
For the assessment of the effective annual dose rate D(e.g.,
nSv year
-1
), following equation was applied (ICRP 1994,
1996; Till and Moore 1988):
D¼AUgð4Þ
where Ais the
90
Sr concentration in milk samples
(Bq kg
-1
), Uis annual consumption of
90
Sr, and gis
effective dose coefficient of
90
Sr, which is 28 nSv Bq
-1
for
adults and 230 nSv Bq
-1
for the children younger than
1 year old.
3 Result
Activity concentrations of
90
Sr (Bq kg
-1
, dry weight) in
the milk and milk powder samples and accuracy results are
summarized in Tables 1and 2, respectively. The average
activity concentration of
90
Sr in the investigated milk
samples was 0.225 ±0.042 Bq kg
-1
. Also, the average
activity concentration for the milk powder samples was
determined as 0.216 ±0.024 Bq kg
-1
. Comparison of the
determined activity concentrations and the minimum
detectable levels shows that the results are mostly close to
or below the detection limits. The data below the MDL
values were omitted in our calculations.
To estimate biological hazard from strontium-90, which
can occur due to milk consumption, the effective dose is
calculated using Eq. 4. Effective dose is based on the risks of
radiation-induced health effects and the use of International
Commission on Radiological Protection (ICRP) biokinetic
model that provides relevant conservation factors to calcu-
late effective dose from the total activity concentration of
radioisotope measured in the food samples (ICRP 1994,
Fig. 2
90
Y decay curve. This curve is fitted to the half-life of
90
Y
(R
2
=0.9) and confirms the purity of our samples
Environmentalist (2011) 31:308–314 311
123
1996). Estimation of the radiation-induced health effects
associated with the intake of radionuclide in the body is
proportional to the dose delivered by the radionuclide while
resident in the various organs. By considering that the sam-
ples used in this study do not consumed merely in their
production area, the average activity concentration was used
for the dose estimation. By these explanations, with regard to
the average activity concentration of
90
Sr in the milk and
milk powder samples (0.225 ±0.042 and 0.216 ±0.024
Bq kg
-1
, respectively) and the rate of milk consumption
(75 kg year
-1
for milk consumption in adults reported by the
Milk Industry of Iran, 63 g day
-1
for milk powder con-
sumption in children at the breast younger than 5 months old,
and 158 g day
-1
for children at the breast older than
5 months old), the effective dose of
90
Sr due to milk
consumption was calculated as 472.50 nSv year
-1
for adults
and 1,142.39–2,865.04 nSv year
-1
for infants.
4 Discussion and conclusion
The aim of the present study was monitoring the back-
ground level of radioactivity in milk, which is a reliable
indicator of the general population intake of certain
radionuclides, since it is consumed fresh by a large seg-
ment of the population and contains several of the bio-
logically significant radionuclides (ERD 2001). By
considering this purpose, although many rapid methods
have been developed for the determination of strontium in
milk, we used a combination of two common methods
Table 1 Activity concentration of
90
Sr (Bq kg
-1
, dry weight) in the milk samples and accuracy results
Sample ID Gross beta
count rate
(cpm)
Chemical
recovery of
Yttrium (%)
Analyzed ash
weight (kg)
Ash to dry
weight ratio
The decay
time (h)
Activity concentration
of radio Strontium
a
(Bq kg
-1
)
Minimum
detectable Level
(Bq kg
-1
)
M-1 3.867 92.97 0.00373 0.06544 37.17 1.615 ±0.126 0.178
M-2 1.650 52.90 0.00485 0.06831 49.77 0.336 ±0.116 0.250
M-3 1.768 100.00 0.00980 0.06490 42.80 0.105 ±0.030 0.062
M-4 1.638 89.64 0.00860 0.06143 57.37 0.105 ±0.035 0.075
M-5 2.140 89.91 0.01069 0.06640 52.88 0.225 ±0.034 0.065
M-6 1.847 85.14 0.00625 0.06579 16.48 0.174 ±0.057 0.116
M-7 1.903 100.00 0.01280 0.06632 15.15 0.080 ±0.024 0.049
M-8
b
1.362 90.60 0.01115 0.05000 12.08 0.005 ±0.020 0.047
M-9 2.522 95.16 0.01302 0.06817 10.87 0.167 ±0.030 0.052
M-10 2.055 96.63 0.01241 0.05826 18.33 0.098 ±0.024 0.046
M-11 3.041 92.07 0.01252 0.06804 10.65 0.256 ±0.035 0.056
M-12 2.928 95.61 0.01076 0.07126 13.57 0.290 ±0.040 0.065
M-13 2.280 88.32 0.00971 0.06185 11.88 0.177 ±0.037 0.068
M-14 2.055 89.13 0.01249 0.05098 2.80 0.078 ±0.022 0.043
M-15 1.790 84.99 0.01310 0.04629 12.35 0.050 ±0.019 0.039
M-16 1.796 91.68 0.01266 0.08275 0.80 0.077 ±0.032 0.067
M-17 1.853 91.92 0.01334 0.06841 13.38 0.077 ±0.026 0.052
M-18
b
1.368 90.00 0.01082 0.06479 14.37 0.008 ±0.026 0.063
M-19 2.365 90.48 0.00801 0.04740 17.23 0.185 ±0.034 0.062
M-20 2.190 56.73 0.00993 0.08345 12.52 0.331 ±0.074 0.139
M-21 3.473 96.96 0.01043 0.06907 13.68 0.383 ±0.043 0.064
M-22 2.089 89.79 0.01203 0.06365 14.20 0.119 ±0.029 0.055
M-23
b
1.548 92.31 0.00999 0.03568 13.95 0.023 ±0.016 0.036
M-24
b
1.661 94.20 0.01187 0.06182 17.05 0.051 ±0.024 0.052
M-25 1.807 92.16 0.00622 0.06479 14.92 0.145 ±0.051 0.106
M-26 1.909 97.53 0.00660 0.05893 17.82 0.146 ±0.043 0.086
M-27 2.066 92.13 0.01343 0.04553 12.58 0.071 ±0.018 0.035
M-28 2.055 92.43 0.01285 0.06178 27.48 0.115 ±0.025 0.049
a
Values are the activity obtained ±SD
b
Below minimum detectable level (MDL)
312 Environmentalist (2011) 31:308–314
123
routinely used in monitoring process with some modifica-
tions. The methods were well established and suitable for
our experiments. The quality control and assurance of the
combined methods were also obtained by standard samples
prepared with an IAEA-certified reference materials (Milk-
152) to ensure its quality. The quality control and assur-
ance of the two methods have also been investigated in
some researches (Bem et al. 1991; IAEA 1993a), and our
results confirmed their data too (our assay gave the average
chemical recovery above 91%, and the average of the
method efficiency was obtained 85% which is in accor-
dance with the previous investigations). Moreover, rapid
methods are required for the application of the EURATOM
regulation, in emergency situations, in order to screen a
large number of milk samples in a short period of time and
to minimize the contamination of all the diary food chain.
The EURATOM regulation No. L2218/89 published in the
Official European Community Journal on the July 22, 1989
(Re
`glement EURATOM 1989) gives the limits of radio
strontium activity in infant food and dairy products as 75
and 125 Bq kg
-1
. This regulation defines the levels for
radionuclides in foods, which are set into power in early
case of nuclear emergencies. For these situations, by con-
sidering the high allowable concentration limits, a low
detection limit is not of first importance in comparison with
the analytical time (Baron et al. 2004; Chang et al. 2004;
Horwitz et al. 1992; Jassin 2005; Tait et al. 1997,1999;
Lee et al. 2002; Mikulaj and Svcc 1993) and fast methods
are required, but in the present study, by the aim of the
detection of very low background concentrations, it is not
necessary to use new methods, which are also very
expensive because of the low regeneration of the exchange
and affinity resins.
A lot of studies have been carried out to estimate the
transmission of strontium from soil forage to cow milk, and
their finding has been used for predicting radionuclide
concentration in food stuffs and dose impact to man.
Although these studies have shown that the values are
strongly influenced by many physical, chemical, and bio-
logical factors such as geographic site, plant, and cow
species, they are in agreement that the strontium trans-
mission to food chain is highly efficient (Comar and
Wasserman 1964; Paasikallio et al. 1994). In this study, by
considering the strontium concentrations in the analyzed
milk samples (0.225 ±0.042 Bq kg
-1
), we can deduce
that the strontium level in the studied area is obviously low.
However, to get more accurate results, some other studies
should be carried out to determine the strontium-90 con-
centration in forages and soil samples or probably esti-
mation of the transfer factor for this radionuclide.
Data obtained here can also provide an opportunity to
verify any impact from the ingestion of strontium-90 in the
people who consume milk. The amount of calculated
effective dose (472.50 nSv year
-1
for adults and 1,142.39–
2,865.04 nSv year
-1
for infants) show that infants under the
age of one are more sensitive but all the doses are still a very
small fraction of the natural background average annual dose
received by human (&2.4 nSv year
-1
), so they are suffi-
ciently too low to pose a risk to human health.
Since
90
Sr is an artificial radioisotope and can occur
only during uncontrolled nuclear activities (such as Cher-
nobyl accident, weapon fallout and), so in this research, a
suitable method for monitoring process was investigated
and useful information of the amount of
90
Sr deposition
over Tehran province was obtained. These data will be
useful in emergency situations.
Table 2 Data indicating the activity concentration of 90Sr (Bq kg
-1
, dry weight) in the milk powder samples and accuracy results
Sample ID Gross beta
count rate
(cpm)
Chemical
recovery of
Yttrium (%)
Analyzed
ash weight
(kg)
Ash to
dry weight
ratios
The
decay
time (h)
Activity concentration
of radio Strontium
a
(Bq kg
-1
)
Minimum
detectable Level
(Bq kg
-1
)
MP-1 2.500 93.81 0.00523 0.02710 61.00 0.283 ±0.030 0.052
MP-2 2.466 86.97 0.00791 0.02737 10.67 0.115 ±0.021 0.037
MP-3 1.723 88.71 0.01132 0.03262 14.50 0.034 ±0.014 0.031
MP-4 2.218 86.04 0.00994 0.02873 13.62 0.078 ±0.017 0.032
MP-5 3.035 95.13 0.01184 0.04199 16.28 0.171 ±0.022 0.035
MP-6 2.939 89.07 0.00611 0.02057 14.63 0.161 ±0.022 0.036
MP-7
b
1.633 85.52 0.01053 0.03761 17.47 0.035 ±0.018 0.039
MP-8 8.253 91.05 0.00576 0.02341 11.60 0.790 ±0.043 0.042
MP-9
b
1.368 89.37 0.00681 0.02702 12.95 0.005 ±0.018 0.042
MP-10 2.145 74.61 0.02610 0.08392 13.12 0.092 ±0.021 0.041
a
Values are the activity obtained ±SD
b
Below minimum detectable level (MDL)
Environmentalist (2011) 31:308–314 313
123
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