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Mixed-mode acrylamide-based continuous beds bearing tert-butyl groups for capillary electrochromatography synthesized via complexation of N-tert-butylacrylamide with a water-soluble cyclodextrin. Part I: Retention properties
... In our previous publication [55], we reported the synthesis of a monolithic stationary phase based on the solubilization of N-tert-butylacrylamide in aqueous solution via a 1:1 complex formed with statistically methylated β-CD (Me-β-CD). Monolithic stationary phases are prepared by in situ free radical copolymerization of the formed inclusion complex (N-tert-butylacrylamide/Me-β-CD) with piperazine diacrylamide (PDA), methacrylamide (MA), and vinylsulfonic acid (VSA) in the presence of ammonium sulfate in pre-treated fused-silica capillaries (100 μm ID). ...
... The mixed-mode monolithic stationary phases used in CEC measurements were synthesized as described in [55] by copolymerization of a mixture of N-tert-butylacrylamide/ Me-β-CD complex, methacrylamide (MA) as hydrophilic monomer, piperazine diacrylamide (PDA) as crosslinker, ammonium sulfate (AS) as lyotropic salt, vinylsulfonic acid (VSA) as charge-bearing monomer, ammonium persulfate (APS) as free radical polymerization initiator, and N,N,N′,N′-tetramethylethylenediamine (TEMED) as accelerator in the redox initiator system in aqueous phosphate buffer (100 mM, pH 7.0) inside fused-silica capillaries with both circular cross section with varied sizes (100 or 180 µm ID) and square cross section with lateral height of 72 µm which had been pre-treated with 3-(trimethoxysilyl) propyl methacrylate (bind silane). The polymerization process was allowed to proceed overnight at room temperature. ...
... In the first part of this series, we have investigated the impact of the formation constant of the formed inclusion complex N-tert-butylacrylamide/Me-β-CD on the retention properties of the synthesized monolithic stationary phases. These studies also detected a significant influence of the formation constant of the formed host-guest inclusion complex on the morphology (domain size) of the formed monoliths [55]. In the present work, we deepened this aspect with the aim to study more in detail the influence of the formation constant of the formed inclusion complex on the morphology and on the chromatographic efficiency of the formed monolith. ...
In this series, we investigate the impact of the complex formation constant of the hydrophobic monomer with respect to statistically methylated-β-cyclodextrin (Me-β-CD) on the electrochromatographic properties of highly crosslinked amphiphilic mixed-mode acrylamide-based monolithic stationary phases. In the first part, we investigated the retention properties. In the present study, we optimize the synthesis parameters with respect to obtainable morphology and separation efficiency. For this purpose, a series of mixed-mode acrylamide-based continuous beds bearing tert-butyl groups is synthesized under systematic variation of (i) the concentration of the lyotropic salt ammonium sulfate in the polymerization mixture and (ii) the dimension of the capillary. The impact of these parameters on the chromatographic efficiency is studied under isocratic conditions for alkylphenones in the reversed-phase mode via capillary electrochromatography with varied electric field strength. As expected, there is a strong impact of the concentration of ammonium sulfate in the polymerization mixture on the morphology (examined via scanning electron microscopy) and on the chromatographic efficiency, while there is only a minor influence when varying the size or the shape of the capillary cross-sectional area. Morphology and chromatographic efficiency of this new type of stationary phase are compared to those reported in our previous series. The studies reveal a significant influence of the formation constant of the formed inclusion complex on the morphology and the chromatographic efficiency for those monoliths having a large domain size, while there is an insignificant influence for those monoliths having a small domain size.
... In this context, different types of adsorbents either synthetic or natural have been used for the removal of toxic pollutants from aqueous solutions including: activated carbon [7,25,27,29,30], metal oxide nanoparticles [11,20,31], functionalized silica gel [32], hydroxyapatite [33], chitosan [34], bentonite [35], zeolite [27,36,37], humic acid [38], silica-based and metal-oxide monoliths [39,40], organic-inorganic nanocomposite ion exchangers [23], and vegetable and fruit peels [8]. During the last years, macroporous organic-based monoliths have gained great interest as a unique material used in analytical and bioanalytical chemistry applications such as chromatography, extraction, catalyst supports, or purification [41][42][43][44][45][46][47][48]. This is due to their ease of production in various shapes and dimensions, high porosity, reproducibility of the synthesis procedures, high capacity, flexible framework, chemical stability, and availability of a wide variety of suitable monomers [29,43,49,50]. ...
... The macroporous polyacrylamide-based monolith used in this work was synthesized as described in [45] with some minor modifications. The modified detailed synthesis procedures were described in our very recently published work [51]. ...
In the present study, a macroporous polyacrylamide-based monolith bearing negatively charged sulfonic acid groups was synthesized as a new adsorbent for the removal of heavy metals ions (Pb 2+ , Cd 2+ , and Cr 3+) from aqueous solutions. Vinylsulfonic acid was selected as an anionic monomer to introduce a negative charge on the surface of the resulting monolith that forms a complex with investigated metal ions. The influences of solution pH, contact time, monolith dosage, initial concentration , and temperature on Pb 2+ , Cd 2+ , and Cr 3+ removal were determined using the batch equilibrium technique. Adsorption data were modeled with Langmuir, Freundlich, and Dubinin-Raduskevich isotherm models. The experimental equilibrium data for Pb 2+ , Cd 2+ , and Cr 3+ using the synthesized monolith showed a good correlation with the Langmuir isotherm model. Based on the Langmuir model, the maximum monolayer adsorption capacities of the monolith were 22.8 mg g-1 for Cd 2+ , 33.3 mg g-1 for Pb 2+ , and 66.7 mg g-1 for Cr 3+ at 25°C. Kinetic studies revealed that the adsorption of the metal ions onto the monolith followed pseudo-second-order kinetics. The negative and positive values of free energy (ΔG°) and enthalpy (ΔH°) revealed that the adsorption of the metal ions onto the monolith was spontaneous and endothermic, respectively.
... high surface area), reproducibility of the synthesis procedure, flexible framework, and chemical stability [10,[42][43][44][45][46]. For these reasons, these materials have been successfully used in different separation techniques such as chromatography, solid-phase extraction, recovery of metal ions, or purification [10,42,[45][46][47][48][49]. For example, Hu et al. [10] synthesized macroporous silica-based monoliths functionalized with chelating ligand diglycolamide (DGA) using a sol-gel process. ...
... The macroporous polyacrylamide-based monoliths used in adsorption studies were synthesized as described in [48] with some modifications. For polymerization reaction, the hydrophobic monomer N-tert-Butylacrylamide was solubilized in aqueous phosphate buffer (0.1 M, pH 7.0) with the required amount of Me-β-CD inside a glass tube. ...
In this study, a series of macroporous polyacrylamide-based monoliths bearing negatively charged interaction sites were synthesized as adsorbents for effective removal of Th4+ from aqueous solutions. The effects of solution pH, contact time, monolith dosage, initial concentration, and temperature on adsorption process was studied. The Langmuir, Freundlich, and Dubinin–Raduskevich adsorption isotherms were applied to describe the adsorption data. The equilibrium data were best fitted with both Freundlich and Langmuir models. The experimental kinetic data were well described by the pseudo-second order kinetic. Thermodynamic studies revealed that the adsorption of Th4+ onto the synthesized monolith was an endothermic and spontaneous process.
... A new highly cross-linked amphiphilic mixed-mode acrylamide-based monolithic stationary phase containing tert-butyl groups has been developed utilizing solubilization of the hydrophobic monomer, N-tertbutylacrylamide, with the water-soluble statistically methylated β-cyclodextrin [113]. The intention of the authors was to systematically investigate the influence of the complex formation constant of the hydrophobic monomer with respect to methylated cyclodextrin. ...
... First, the capillary tubing was packed with 5 μm bare silica particles. Next, polymerization mixture consisting of the monomer glycidyl [113].) methacrylate, the cross-linker pentaerythritol triacrylate, the initiator, and a mixture of porogens was introduced into the particle packed capillary and then polymerization occurred at enhanced temperature. ...
The mixed‐mode phases have become very popular in the last decade and the number of new mixed/multi‐mode sorbents is growing fast. Unlike single‐mode stationary phases perfectly suited for the separation of the analytes possessing similar physicochemical properties, for instance reversed‐phase chromatography for hydrophobic solutes, mixed‐mode sorbents providing multimodal interactions can render a better separation selectivity for complex mixtures of solutes differing significantly in their physicochemical characteristics. The most frequent modern mixed‐mode stationary phases are di/tri‐mode sorbents embracing the following interactions, hydrophobic, electrostatic (coulombic) and hydrophilic. According to their structures, it is possible to distinguish silica‐based, polymer‐based, hybrid, and monolithic mixed‐mode stationary phases. Herewith, newly synthesized mixed‐mode sorbents developed within the last two and half years are categorized, discussed, and summarized. The main attention is devoted to the description of the synthetic routes and characterization methods applied for the new stationary phases.
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... The development of highly crosslinked porous polymer-based and inorganic silica-based monoliths has gained increasing attention due to their ease of preparation, high porosity, high surface area, high adsorption capacity, reusability, and chemical/thermal stability [33,34,[40][41][42][43][44][45][46][47][48]. These advantages make monoliths promising materials that have been successfully used in different applications such as separations, heterogeneous catalysis, adsorption, extraction, pre-concentration, and purification [32][33][34][35]40,44,47,[49][50][51]. ...
... In order to make the stationary phase have the characteristics of being usable in mixed mode, we tried to directly use one amphiphilic monomer instead of using two hydrophilic and hydrophobic monomers to prepare hydrogels. Amphiphilic monomers have both hydrophobic and hydrophilic groups [30], so we do not have to consider how to make the final hydrogel have a certain ratio of hydrophobic monomer and hydrophilic monomer content [31]. There are also more types of amphiphilic monomers, but SBMA is easier to obtain, and related reports have proven its good chemical properties [32]. ...
By introducing functional groups such as quaternary amine groups, sulfonic acid groups, triazine groups, and other mespore nanomaterials into the hydrogel, better separation effect of some organic framework materials has been obtained. Due to a reasonable design and preparation strategy, the hydrogel composite-modified silica can be used in the selective separation of various analytes such as pesticides, alkylbenzenes, polycyclic aromatic hydrocarbons, nucleosides/bases, benzoic acids, antibiotics, and carbohydrates. Through the exploration of chromatographic retention behavior, it is proved that the column can be used in mixed-mode liquid chromatography. The intra-day relative standard deviation for retention time of this new stationary phase is 0.12–0.16% (n = 10), and the inter-day relative standard deviation is less than 0.39% (n = 5). This new stationary phase can also be used for separation in complex samples. The limit of detection (LOD) for chlorotoluron in farm irrigation water is 0.21 µg/L and the linear range is 2–250 µg/L. After optimizing the chromatographic conditions, the highest efficiency of the hydrogel column in RPLC and HILIC modes has reached 32,400 plates/m (chlorobenzuron) and 41,300 plates/m (galactose). This new type of hydrogel composite is a porous network material with flexible functional design and simple preparation method and its application has been expanded in liquid chromatography separation successfully.
Graphical abstract
The hydrogel composed of triallyl cyanate cross-linking agent and 3-(2-(methacryloyloxy) ethyl) dimethylamine) propane-1-sulfonate (SBMA) monomer which were co-modified on the surface of mesoporous silica with MOF-919 for separation in mixed-mode liquid chromatography.
A series of amphiphilic macroporous mixed-mode acrylamide-based continuous beds bearing positively charged quaternary ammonium groups is synthesized for capillary electrochromatography (CEC) under variation of the concentration of the cationic monomer in the polymerization mixture. Positively charged mixed-mode monolithic stationary phases are synthesized in pre-treated fused silica capillaries of 100 µm I.D via single step free radical copolymerization of cyclodextrin-solubilized N-tert-butylacrylamide, a hydrophilic crosslinker (piperazine diacrylamide), a hydrophilic neutral monomer (methacrylamide), and a positively charged monomer ([2-(methacryloyloxy)ethyl]trimethyl ammonium methyl sulfate) in aqueous solution containing the lyotropic salt ammonium sulfate as a pore-forming agent. The synthesized monolithic stationary phases contain hydrophobic, hydrophilic, and charged functionalities. They can be employed for the CEC separations of different classes of neutral and charged solutes (with varied polarity) in the reversed-phase mode, in the normal-phase mode, in the ion-exchange mode, in a mixed-mode, or in the hydrophilic interaction liquid chromatography (HILIC) mode. The influence of the concentration of the cationic monomer in the polymerization mixture on retention factor, electroosmotic mobility, and methylene selectivity (αmeth) is studied under isocratic conditions for alkylphenones in the reversed-phase mode by capillary electrochromatography (CEC). Scanning electron microscopy (SEM) micrographs demonstrate that the morphology of the synthesized monoliths (i.e., the domain size) is strongly influenced by the variation of the concentration of the cationic monomer in the polymerization mixture.
A triphenylmethylamine-functionalized monolithic capillary column was newly designed for reversed-phase capillary electrochromatographic applications. Incorporation of the three phenyl rings-containing selector (also referred to as trityl selector) was achieved through post-polymerization functionalization of a generic monolithic matrix bearing nucleophilic-sensitive hydroxysuccinimide moieties. Such a 3D polymer matrix was obtained through UV-induced in situ free radical copolymerization of N-acryloxysuccinimide and ethylene dimethacrylate. The separation properties of the trityl monolithic capillary column were initially evaluated vis-à-vis polycyclic aromatic hydrocarbons, as model hydrophobic compounds, and compared to the separation ability of a benzylamine-functionalized monolithic capillary column prepared using the same generic monolithic matrix. Electrochromatographic separation of phenols and anilines was also considered, and our preliminary results suggest the occurrence of hydrophobic interactions due to the aromatic and non-polar nature of the surface-grafted trityl selector. The triphenyl monolithic capillary column exhibited relative standard deviation values (% RSD) below 4.1 % for the here-studied chromatographic parameters, namely, retention factor, selectivity, resolution, and efficiency.
*Highlights:
• Core-shell particles are one of the hottest topics in analytical chemistry
• We review the analytical advantages of columns packed with core-shell particles
• We review fundamentals and applications of columns packed with core-shell particles
• We overview the theory of columns packed with core-shell particles from a practical viewpoint
• We review applications for developing and optimizing methods in liquid chromatography
*Abstract
In this review, we present the latest highlights and trends in the use of core-shell particles as stationary phases in columns for liquid chromatography (LC). These highly homogeneous particles have a diameter of 1.3–5 µm with a solid silica core and a porous surface that make their performance excellent compared to fully-porous particles. Use of these columns has been growing exponentially since their first large-scale manufacture in 2006, and they are an emerging trend in the analysis of biological, toxicological and pharmaceutically interesting compounds. We review the main theoretical aspects responsible for their surprisingly good chromatographic behavior and analytical features. We also summarize state-of-the-art analytical applications taking advantage of this column technology that is leading the current revolution in LC.
In our previous work we have described the synthesis, characterization, and optimization of thechromatographic efficiency of a highly crosslinked macroporous mixed-mode acrylamide-based mono-lithic stationary phase synthesized by in situ free radical copolymerization of cyclodextrin-solubilizedN-adamantyl acrylamide, piperazinediacrylamide, methacrylamide and vinylsulfonic acid in aqueousmedium in pre-treated fused silica capillaries of 100 �m I.D. In the present work, we study with differ-ent classes of neutral analytes (with varied hydrophobicity) the impact of the type of retention mode(influenced by the type of analyte and the mobile phase composition) and the impact of the solutefunctionality on the chromatographic efficiency and peak symmetry with a monolith synthesized underoptimized synthesis parameters. With this monolithic capillary high separation efficiencies (up to ca.220,000 m−1) are obtained for the separation of different analyte classes (alkylphenones, nitrotoluenes,and phenolic compounds with k = 0.2–0.55) in the reversed-phase mode, in the normal-phase mode, andin the mixed mode. For neutral alkylanilines (k < 0.25) plate numbers of about 300,000 m−1are routinelyreached in the reversed-phase elution mode. For phenolic solutes separated in a mixed mode there is asolute-specific influence on peak symmetry and chromatographic efficiency. With increasing efficiency of the monolith, axial diffusion becomes an important mechanism of band broadening. For those peaks,which do not show a significant asymmetry (asymmetry factor ≤1.05), it is confirmed that plate heights gained via the tangent method are equivalent to those gained via moment analysis.
A novel method for the analysis of benzimidazole residues in water samples by capillary electrochromatography-UV detection (290 nm), using laboratory-made packed columns is presented. Capillaries (25 cm packed length×75 µm inner diameter, 34 cm total length, 25.5 cm effective capillary length) were packed with C18 particles (5 µm, non-encapped) following a high pressure packing procedure and using a compact steel unit designed for packing capillary columns. Acetone was employed as solvent to carry the particles through the capillary and pack it under a pressure of 42 MPa. Outlet and inlet frits were made by sintering the particles of the stationary phase by heating the packed material with a nichrome ribbon connected to a 7 V AC power supply. With the aim of achieving a good analytical performance, the variables that affected the separation were studied, using a mobile phase composition of 60:40 (v/v) acetonitrile/water containing ammonium acetate (1 mM, pH 6.5), a separation voltage of 25 kV and a temperature of 25 °C. In addition, a combined hydrodynamic-electrokinetic injection mode was considered and samples were injected for 75 s under a voltage of 12.5 kV and a pressure of 11.5 bar. Finally, the determination of benzimidazoles in water samples was accomplished by capillary electrochromatography using dispersive liquid-liquid microextraction as sample treatment. Variables affecting the extraction efficiency were optimized, using chloroform and ethanol as extraction and disperser solvents, respectively. This method was applied to different water samples, obtaining satisfactory results in terms of linearity (R²≥0.990), repeatability (RSD≤1.2%), reproducibility (RSD≤2.2%) and trueness (R≥87.7%). Detection and quantification limits were lower than 2.8 µg L⁻¹ and 9.3 µg L⁻¹, respectively.
• A brief explanation on how CEC works is provided.
• A deep review of the literature about CEC in food analysis is done.
• Relevant applications in the different CEC modes are discussed.
• Future trends in this field are debated.
Capillary electrochromatography (CEC) is a miniaturized technique which gathers the high separation efficiency of capillary electrophoresis and the selectivity of liquid chromatography, being a very interesting hybrid separation procedure. It requires a small amount of solvents and samples, having a high impact in the total cost of the analysis. For these reasons, CEC has aimed a relatively big potential, especially in the fields of pharmaceutics and biomedicine analysis. Nonetheless, food analysis has also been of interest for a considerable number of researchers who have focused their efforts in this direction. This review article shows a deep examination of the use of CEC in food safety and food quality, from the first application in 1997 to the present, commenting those with a higher impact or novelty.
A monolithic material with dual functionality was successfully synthesized through the combination of UV-initiated free radical copolymerization, nucleophilic substitution reaction and thiol-ene radical photoaddition. Firstly, poly(N-acryloxysuccinimide-co-ethylene dimethacrylate) was prepared by non-solvent induced phase separation occurring within the course of the polymerization of the respective monomers. Toluene was selected as a porogen providing percolating pores in the micrometer range. Secondly, N-hydroxysuccinimide units on the monolith surface were converted into ene functionality through covalent immobilization of allylamine. Finally, thioglycerol served as a hydrophilizing agent of the pore surface via surface confined thiol-ene click grafting. All the preparation steps were performed, in situ, i.e. within the confines of UV-transparent capillary channels, affording a micro-column with electro-osmotic flow generation and hydrogen bond forming abilities. Raman microspectrocopy confirmed that all the synthetic steps were accomplished successfully. Investigation of the electrochromatographic retention behavior of hydrophobic and hydrophilic solutes revealed the possibility of tuning the interfacial character of the diol-functionalized monolithic stationary phases with respect to the water content in the surrounding liquid phase. For the acetonitrile-rich mobile phase, the diol-monolith interface exhibited highly hydrophilic character allowing fast and efficient separation of phenol and aniline derivatives, as well as methylxanthines and pyrimidic bases. Moreover, the relative standard deviation of the retention factor values was less than 3.3%. © 2016 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
A novel strategy for rapidly fabricating ionic liquid (IL)-bonded multifunctional monolithic stationary phase has been developed by an in-situ polycondensation of urea-formaldehyde (UF) and a lab-made acylamino-functionalized IL (1-acetylamino-propyl-3-methylimidazolium bromide, [AAPMIm]Br). Two polycondensation processes of UF with 1-amino-propyl-3-methylimidazolium bromide or [AAPMIm]Br were evaluated. Several parameters including mass ratio of urea-formaldehyde, amount of [AAPMIm]Br, polycondensation time and reaction temperature were optimized, and the [AAPMIm]Br-bonded monolithic stationary phase could be rapidly synthesized in 10 min with a satisfactory permeability and mechanical stability. Used for pressurized capillary electrochromatography (pCEC), a typical hydrophilic interaction (HI) retention could be obtained in the resultant [AAPMIm]Br-bonded monolith when the content of acetonitrile (ACN) in mobile phase exceeded 20%. Multiple retention mechanisms such as hydrophilic interaction (HI), hydrogen bond (HH), anion-exchange and cation-exclude interactions, were acheived in the [AAPMIm]Br-bonded monolith. Various polar compounds including phenols, benzoic acid and its homologues, and enkephalins have been well separated and thus demonstrated a satisfactory separation performance of the obtained monolith. A facile access is lighted for rapid preparation of ionic liquid-bonded monoliths with multiple retention mechanisms for pCEC.
The chiral separation of pharmaceuticals is one of the major research topics in the pharmaceutical industry. Chromatographic techniques are most frequently used in this context. Separations in capillary electrochromatography (CEC) are an alternative and achieved by chromatographic retention and electrophoretic mobility principles. As a result, CEC is characterized by a high selectivity and efficiency. The limited number of stationary phases specifically developed for CEC, the low number of commercially available CEC columns, the frits to maintain the stationary phase, which forms fragile spots in the columns, and the limited column robustness and reproducibility, make CEC not very attractive for industrial application. However, CEC is still applied and studied in the academic field. This review discusses the enantioseparation of drugs in CEC published during the last four years, with a critical view on the reproducibility and the practical utility of these applications.
Here we have reviewed separation studies utilizing monolithic capillary columns for separation of compounds preceding MS analysis. The review is divided in two parts according to the used separation method, namely CEC and capillary LC (cLC). Based on our overview, monolithic CEC-MS techniques have been more focused on the syntheses of highly specialized and selective separation phase materials for fast and efficient separation of specific types of analytes. In contrast, monolithic cLC-MS is more widely used and is often employed, for instance, in the analysis of oligonucleotides, metabolites, and peptides and proteins in proteomic studies. While poly(styrene-divinylbenzene)-based and silica-based monolithic capillaries found their place in proteomic analyses, the other laboratory-synthesized monoliths still wait for their wider utilization in routine analyses. The development of new monolithic materials will most likely continue due to the demand of more efficient and rapid separation of increasingly complex samples. This article is protected by copyright. All rights reserved.
Cyclodextrins are cyclic oligosaccharides which have recently been recognized as useful pharmaceutical excipients. The molecular structure of these glucose derivatives, which approximates a truncated cone or torus, generates a hydrophilic exterior surface and a non polar cavity interior. As such, cyclodextrins can interact with appropriately sized molecules to result in the formation of inclusion complexes. These non-covalent complexes offer a variety of physicochemical advantages over the un manipulated drugs including the possibility for increased water solubility and solution stability. Further, chemical modification to the parent cyclodextrin can result in an increase in the extent of drug complexation and interaction. In this short article, the effects of substitution of paracetamol in α, β & γ cyclodextrin and the forces involved in the drug-cyclodextrin complex formation are discussed. Some general observations are made which predict that γ complexes are most stable than β complexes which are more stable then α complex.
In HPLC, analytes injected into a separation column broaden naturally during the separation procedure. In this paper, analyte zone sharpening of peptides was achieved in pressurized capillary electrochromatography, which is a separation method that combines capillary HPLC and capillary electrophoresis (CE), by employing dynamic pH junction for CE. When the pH of the mobile phase was altered from basic to acidic in a step gradient, the analyte peptides were focused at the basic/acidic interface with the application of voltage. The effect of both pH and pressurized flow velocity on the zone sharpening was investigated. With the proposed method, the peak height of angiotensin II, [Asn1, Val5]-angiotensin II, and angiotensin III were enhanced 12, 10, and 12 times, respectively. Selective peak zone sharpening for angiotensin II was also demonstrated.
The primary need for advanced porous materials with well-understood and enhanced properties is identified for implementations in the fields of life science and engineering. The current contribution aims at developing a review perspective of the current generation of porous organic monolithic materials as hierarchically structured materials. This structure is derived from fundamental events occurring during preparation. Understanding the evolution of their porous structure from a free-radical cross-linking (co)polymerization process provides insight for an advanced understanding of structure-to-function relationships. A number of existing and emerging characterization techniques and cross-correlation to synthetic design of experiments are also of importance. Some exciting new approaches for creation of porous polymer monoliths are discussed, for example, living polymerizations and routes based on "click chemistry." It has been further suggested that the new synthetic concepts must be accompanied with an advanced structural understanding of the materials to identify a desired step-change for their performance to be used in separations, catalysis, and extraction.
Considerable developments have taken place in the area of supports for high performance liquid chromatography (HPLC) over the last ten years or so since the commercial introduction of smaller sub-2 m particle columns1. These particles offer particular advantages in terms of increasing the speed of analysis, which is of paramount importance when large numbers of samples must be analysed. Shorter analysis times, particularly in conjunction with the use of columns of smaller internal diameter, offer considerable savings in the cost of mobile phase solvents and their disposal. Small superficially porous "core shell" or simply "shell" materials have gained considerable popularity in recent years and advances in methods of preparation, performance and comparison with totally porous particle columns will be a feature of this review. The review will cover the use of very small particles in capillary column formats to determine their potential compared with the use of columns of more conventional diameters (1- 4.6 mm ID), particularly with the objective of increasing the detection sensitivity when combined with mass spectrometry.
Open tubular capillary electrochromatography (OT-CEC) separates analyte mixtures by a combination of electrophoretic, electro-osmotic and/or chromatographic effects. OT-CEC research is an active and growing field, with studies encompassing a wide range of investigations related to new strategies for chemical modification of the inner surface of the capillary, leading to the introduction of novel stationary phase coatings. This review has examined the literature on OT-CEC from 2013 to August 2015 and highlights the developments in the fabrication of highly selective stationary phases, based on materials that include cyclodextrin chiral selectors, graphene and graphene oxide, metal-organic frameworks, molecularly imprinted polymers, nanoparticles, nanolatex particles, nanocomposites, in situ generated polymers, block polymers, tentacle-type polymers, polyelectrolyte multilayers, polysaccharides, phospholipids and proteins. This review, while considering the development of novel OT-CEC coating materials, specifically examines different immobilization or coating methodologies and approaches and also discusses the separation mechanisms that occur with these new materials. These OT-CEC coatings are intended mainly to separate low molecular weight molecules relevant to the pharmaceutical, agricultural and food industries as well as for use in environmental monitoring. This article is protected by copyright. All rights reserved.
A novel capillary electrochromatography (CEC) method was developed by using a polymethacrylate ester-based monolithic column for the simultaneous separation and determination of five nucleoside and purine compounds in urine. The porous monolithic column was firstly designed by mean of in situ co-polymerizing stearyl methacrylate (SMA), trihydroxymethylpropyl trimethylacrylate (TMPTMA) and 2-acrylamido-2- methyl-1-propanesulfonic acid (AMPS) in a ternary porogenic solvent including cyclohexanol, 1, 4-butanediol and water. The performance and influence factors of the monolithic columns were investigated and evaluated by a CEC method. The five compounds including guanine, adenine, adenosine, cytidine and N(6) -methyladenosine were selected as analytes. Under the optimized condition of 6.0 mM phosphate buffer at pH 5.0 without ACN, five analytes were rapidly separated in 4 min with the high separation efficiency. The recoveries of spiked samples were ranged form 89.0% to 109.0% with RSDs less than 4.83%. Therefore, the proposed method could possibly provide for rapid separation and detection of the nucleoside and purine compounds in biomedicine sample. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
A pressurized capillary electrochromatography (pCEC) method combined with an online concentration was developed for the separation and determination of five β2 -blockers (terbutaline, salbutamol, formoterol, procaterol and salmeterol) in human urine. A stearyl methacrylate-based monolithic column was prepared as the separation column. The various separation parameters including acetontrile concentration, applied voltage, pH and concentration of the running buffer were investigated. On-line concentration methods in combination of the chromatographic zone-sharpening effect and field-enhanced sample-stacking effect were utilized to improve detection sensitivity. The on-line concentration parameters including injection voltage, injection time, as well as sample matrix were systematically studied. Compared with the conventional sample injection, the on-line concentration technique appeared to improve the corresponding sensitivities 20ཞ50-fold. Furthermore, good precision was obtained with relative standard deviations (RSDs) for migration times within 1.09% and for peak areas less than 3.55% (n = 5). The established method was successfully applied to the determination of above-mentioned β2 -agonists in urine samples. The recoveries of spiked urine samples were between 85.0% and 113.3% with RSDs less than 5.39%. This article is protected by copyright. All rights reserved.
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A novel mixed-mode monolithic stationary phase based on bonding the 4,5-imidazoledicarboxy acid to the surface of an organic-silica hybrid monolith was prepared and characterized. Characteristics in terms of mechanical stability, permeability and reproducibility of the resultant monolithic column preformed satisfactory. A typical per aqueous chromatographic behavior was observed in water-rich mobile phases. The resultant monolithic column was applied for the efficient resolution of various polar and hydrophilic compounds under per aqueous or reversed-phase chromatographic conditions, which successfully demonstrated its suitability for the analysis of these polar compounds in highly aqueous mobile phases. The highest column efficiency obtained for the amines was 14 9000 N m-1. The experimental results showed that the mixed-mode mechanism of hydrophobic and ion-exchange interactions was involved in the separation under the given conditions. The successful applications suggested that the mixed-mode organic-silica hybrid monolithic column could offer a wide range of retention behaviors and flexible selectivities toward polar compounds, achieving various target separations.
The overloaded band profiles of the protonated species of propranolol and amitriptyline were recorded under acidic conditions on four classes of stationary phases including a conventional silica/organic hybrid material in reversed-phase liquid chromatography mode (BEH-C18 ), an electrostatic repulsion reversed-phase liquid chromatography C18 column (BEH-C18 +), a poly(styrene-divinylbenzene) monolithic column and a hydrophilic interaction chromatography stationary phase (underivatized BEH). The same amounts of protonated bases per unit volume of stationary phase were injected in each column (16, 47 and 141 μg/cm(3) ). The performance of the propranolol/amitriptyline purification was assessed on the basis of the asymmetry of the recorded band profiles and on the selectivity factor achieved. The results show that the separation performed under reversed-phase liquid chromatography-like conditions (with BEH-C18 , BEH-C18 +, and polymer monolith materials) provide the largest selectivity factors due to the difference in the hydrophobic character of the two compounds. However, they also provide the most distorted overloaded band profiles due to a too small loading capacity. Remarkably, symmetric band profiles were observed with the hydrophilic interaction chromatography column. The larger loading capacity of the hydrophilic interaction chromatography column is due to the accumulation of the protonated bases into the diffuse water layer formed at the surface of the polar adsorbent. This work encourages purifying ionizable compounds on hydrophilic interaction chromatography columns rather than on reversed-phase liquid chromatography columns. This article is protected by copyright. All rights reserved.
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This review summarizes developments in the field of monolithic columns for chromatographic applications that mostly appeared during the last two or so years and that we found most fascinating. The truly useful and widely applicable monoliths are now 25 years old. Despite their mature age and commercial availability, monoliths remain a hot topic in the separation science as demonstrated with a large number of papers published each year that originate from numerous laboratories all around the world, and numerous presentations concerned with these materials at major chromatographic conferences. The recent developments in the field of monoliths focus on three major areas: (i) improvements in column efficiency, (ii) modulation of selectivity, and (iii) applications. This review mostly concerns the first two items since we see the greatest advances occurring right there. Noticeably, our search of the literature revealed that the majority of the recently published papers are related to monoliths prepared from organic polymers. It is likely due to the simplicity enabling the preparation and handling of the polymer-based monoliths as opposed to the more sensitive and complex techniques required for the inorganic counterparts. This is also why this review is biased more towards the organic polymers.
The crucial point affecting the separation time in comprehensive two-dimensional liquid chromatography is the performance of the column used in the second dimension, which should allow highly efficient fast chromatographic separations in the short time available for the analysis of fractions transferred from the first to the second dimension (often 1 min or less). This can be accomplished on short columns packed with sub-2-μm particles, at the cost of very high operation pressure. Core-shell or silica monolithic columns have better permeability, and their use in the second dimension of comprehensive two-dimensional liquid chromatography with conventional liquid chromatography instrumentation is continuously increasing. Monolithic columns based on organic polymer matrices offer a wide selection of stationary phase chemistries, including new hydrophilic interaction liquid chromatography materials, which can be used in the design of novel two-dimensional separations. Some organic polymer monolithic materials offer a dual retention mechanism (reversed-phase hydrophilic interaction liquid chromatography), so a single column can be used in alternating runs for highly orthogonal off-line two-dimensional and even three-dimensional separations. In the present work, the properties of core-shell and silica gel monolithic columns are briefly summarized and their applications in two-dimensional separations of peptides, proteins, oligomer surfactants, fats and oils, carotenoids, phenolic and flavone compounds in plant extracts, food, and beverages are reviewed.
As a typical miniaturized analytical technique, capillary electrochromatography (CEC) has attracted much attention because of its low sample and solvent consumption, high efficiency, high selectivity, high resolution and fast speed. In this review, we mainly cover the development of capillary columns and detection techniques in the CEC since 2009. Herein, three types of capillary columns, namely, open-tubular capillary columns, monolithic columns and packed columns, and several types of detectors are reviewed in detail. Moreover, a two-dimensional separation system based on CEC is also reported.This article is protected by copyright. All rights reserved
The mass transport properties of a non-retained (thiourea) and three retained low molecular weight compounds (acetophenone, valerophenone, and octanophenone) along a 4.6mm×45mm PROSWIFT™ RP-1S monolithic column made of rigid cross-linked poly(styrene-divinylbenzene) copolymer was investigated in depth. Accurate protocols (peak parking experiments, measurement of the first and second central moments of peak profiles by numerical integration) combined with the use of validated models of effective diffusion along monolithic structures were applied for the determination of the longitudinal diffusion, the eddy dispersion, and the skeleton-eluent mass transfer resistances due to the finite analyte diffusivity across the polymer skeleton and to the slow absorption kinetics into the polymer volume. Experimental results show by increasing order of importance evidence that the resolution performance of this short and wide polymer-based monolithic HPLC column is limited by the slow analyte diffusivity across the polymer skeleton (smaller than one tenth of the bulk diffusion coefficient for k'>1), its large eddy dispersion HETP (Heddy≃100μm), and the slow rate of absorption (≃10Hz only) in the polymer volume for retained analytes. The column performance could be improved by preparing a more homogeneous material with a rigid internal mesoporous structure. This would provide a column bed having a larger specific surface area, allowing faster analyte diffusion across the mesoporous skeleton, a smaller eddy dispersion HETP, and a faster absorption kinetics in the polymeric monolith than those observed for the currently available materials.
Chromatographic analytical columns containing porous monolithic beds based on cross-linked polymers and derivatized silica have now been commercially available for several years and, despite some apparent conceptual similarities, are marketed and utilized for quite different chromatographic applications. While this distinction is well-accepted by users, the fundamental differences in chromatographic behavior of these materials that lead to this clear distinction in their primary application areas have not yet been systematically studied. To this end, the present experimental study investigates differences in the apparent chromatographic characteristics when using small molecules with commercially available monolithic reversed-phase analytical columns based on poly(styrene-co-divinyl benzene) and C18-derivatized silica. Relevant practical information is obtained from measurements made by "arrested elution" of non-retained and retained solutes and chromatographic elution performance across a wide range of retention factors with a set of structurally similar small molecules. Observations of apparent diffusion probed with "arrested elution" experiments and mass transport inferred from the observed efficiency at increased flow velocity in the monolithic structures (both under retained and non-retained conditions) lead to the conclusion that fundamentally different solute transport behavior is operative. The silica-based monolithic materials are used to establish a "reference" for comparison to observations with cross-linked porous polymeric monolithic materials. Despite the differences in morphology, chromatographic properties have their origin in the underlying physical structure of pore space. The derivatized surfaces in silica-based materials have their counterpart in pore-fluid gel interfaces in polymeric monoliths. The pore-fluid gel interfaces have their origin in varying solvation of polymer by eluent components. Consequently, they allow varying permeation of small molecules into the solvated polymer via partition. The traversing of small molecules through the polymer monolith's complex nanoscale physical structure plays a key factor when rationalizing any chromatographic performance as seen in the slopes of plate height curves which vary dramatically with mobile phase composition and solute identity.
Porous monolithic poly(styrene-co-divinylbenzene) stationary phases in 4.6mm ID analytical format have been investigated with respect to their transport properties probed by solutes of biological origin varying vastly in size. Elucidation of several properties of these benchmark and robust materials gave complementary insight. These are: (i) the porous polymers' apparent dry-state microscopic appearance, (ii) the columns porosity probed by the biomolecules and modulated by mobile phase solvent composition, (iii) the impact of probe solute size on apparent retention at varying mobile phase solvent compositions, and (iv) the elution performance under both nonretained and retained elution conditions. By varying the volume percentage of acetonitrile in the mobile phase, it is demonstrated that the monolithic scaffold shows a variable porosity experienced in particular by the larger sized solutes, while the smaller solutes are gradually less affected. The nanoscale swelling and solvation of porous monolithic adsorbents resulting in gel porosity varied with mobile phase solvent composition was, therefore, indicated. The plate height curves for the solutes under nonretained conditions show a moderate increase at increased flow velocity while approaching plateau values. These plateau values were in conjunction with a trend of a decreased performance at an increased molecular weight of the solute. The systematic shape of the plate height curves at increased flow velocity indicates pre-asymptotic dispersion. This is because the column bed aspect ratio of length-to-diameter is equal or smaller than 10. Imposing retention on the solutes at a constant flow velocity deteriorates isocratic elution performance, more pronouncedly for the larger sized solutes at even weak retention. This is explained with slow pore fluid-gel interface diffusion. Additionally, the apparent retention factor for elution of the probe solutes becomes a function of flow rate, consequently a function of imposed pressure experienced by the scaffold.
A review of stationary phases in 2013
In our previous article we have described the synthesis of a new amphiphilic monolithic stationary phase by in-situ free radical copolymerization of cyclodextrin-solubilized N-adamantyl acrylamide, piperazinediacrylamide, methacrylamide and vinylsulfonic acid in aqueous medium in pre-treated fused silica capillaries of 100 μm I. D. In the present work, we study the morphology of different monolithic stationary phases synthesized under variation of the concentration of ammonium sulfate in the polymerization mixture. The pore size distribution is determined with inverse size exclusion chromatography (ISEC) using the retention data of a series of polystyrene standards with narrow molecular size distribution and known average molar mass ranging from 1560 to 2 010 000 g mol-1.The impact of the concentration of the lyotropic salt ammonium sulfate in the polymerization mixture on the formed morphology, the pore size distribution, and the fractional volume of mesopores and macropores is determined. The homogeneity and uniformity of the formed monolith over the length of the capillary and the covalent attachment to the confining walls are confirmed. Repetition of the synthesis procedure shows that these morphology parameters are well controlled as there is an excellent capillary-to-capillary, day-to-day, and run-to-run reproducibility reached for the electroosmotic mobility and the retention factor determined with alkylphenones in the reversed-phase mode.
The review brings a comprehensive survey of recent developments and applications of high performance capillary and microchip electroseparation methods (zone electrophoresis, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography and electrochromatography) to analysis, micropreparation, purification and physicochemical and biochemical characterization of peptides in the years 2013, 2014 and ca. up to the middle of 2015. Advances in the investigation of electromigration properties of peptides, in the methodology of their analysis, including sample preseparation, preconcentration and derivatization, adsorption suppression and EOF control, as well as in detection of peptides, are described. New developments in particular CE and CEC modes are presented and several types of their applications to peptide analysis are reported: conventional qualitative and quantitative analysis, determination in complex (bio)matrices, monitoring of chemical and enzymatical reactions and physical changes, amino acid, sequence and chiral analysis, and peptide mapping of proteins. Some micropreparative peptide separations are shown and capabilities of CE and CEC techniques to provide important physicochemical characteristics of peptides are demonstrated. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
In this paper, polystyrene-based monoliths with highly ordered macroporous structure were synthesized by using SiO 2 colloidal crystal as template. SEM observation shows that the macropores are highly ordered and are interconnected by small windows. The BET surface area of PS monolith is about 36.17 m 2/g. The polymer monoliths can resist 5 MPa pressure, showing high mechanical and compressive strength. © 2012 Quan Zhou Wu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
The binding constants for β- cyclodextrin inclusion in aqueous solution of a few simple adamantane salts have been determined by a conductimetric technique at 25°C over the pH range 4-10. Binding was largely dictated by the closeness of fit of the adamantyl moiety within the β- cyclodextrin cavity with the charge on the substituent group playing a minor role. The strongest binding occurred when the charge was neutralized by protonation or deprotonation.
An equation which relates the volume term (V=M/(ρL-ρg)) of unassociated liquids to pressure P and temperature T has been obtained by the combination of (a) 03V(∂I/∂V)P0=Tx-T for the effect of temperature on V at low (atmospheric) pressure and (b) - V(∂P/∂V)T = Px Vx/6/V6p0+9(P-p) for the effect of pressure on volume at constant temperature. In the equations, p is the vapour pressure; pL the density of the liquid and pg the vapour density. Often pg can be neglected compared with pL and p is small compared with the large pressures required to affect the densities of liquids appreciably. There are three constants: Tx, Px, which equals 4.455 × 109 N m2, and Vx which can be calculated by the addition of atomic values for all the atoms in the molecule and subtraction of a value (6.56 × 106 m3 mol1) for each bond. When V approximates to M/ρL, the molar volume, the equation can be integrated to give the work and heat of isothermal compression. The viscosity of a liquid is related to the work of compression and solubilities in a liquid to the work required to bring the solute to the compressibility of the liquid. Many relationships can be derived and can be used to estimate properties of unassociated liquids.
The volumes, heat capacities, and enthalpies of solution of adamantane in cycloxane,n-hexane, and carbon tetrachloride have been measured as a function of concentration at 25°C (15, 25, and 35°C for the volumes). The results extrapolated to infinite dilution have been resolved into cavity formation and interaction terms. The former have been calculated from the equations of the scaled-particle theory. To estimate the contribution from the latter, we have assumed some proportionality between adamantane-solvent and cyclohexane-solvent interactions. This assumption has been verified with the three different solvents for the three studied thermodynamic functions. The diameter of adamantane in solution has been determined to be 6.36 Å.
Electro-osmosis has been used to pump solvents in both thin-layer and high-speed liquid chromatography. The advantages of this technique over conventional methods of driving solvent are discussed.
Concerning polymeric monolithic materials utilized in separation science, the structural and mechanical characteristics from the nanoscopic to the macroscopic scale remain of great interest. Suitable analytical tools are urgently required to understand the polymer monolith's constituent structure, particularly in the case of nanoscale polymer properties. that tend to develop gel porosity in contact with a mobile phase ultimately affecting the chromatographic performance. Herein described are our first findings from a characterization of commercially available analytical polymer monoliths based on styrene/divinylbenzene and methacrylate chemistries utilizing confocal Raman spectroscopy imaging and atomic force microscopy (AFM). Confocal Raman spectroscopy can be used to generate a three-dimensional representation of monoliths in both dry state and in contact with solvent. AFM force-indentation measurements on individual cross-sectioned globular features permit detailed assessment of mechanical properties of the stationary phase. This approach allowed so far unprecedented insight and identification of a heterogeneous cross-link density distribution of polymer material within individual globular features on a sub-micrometer scale.
A new synthesis procedure for highly crosslinked macroporous amphiphilic N-adamantyl-functionalized mixed-mode acrylamide-based monolithic stationary phases for capillary electrochromatography (CEC) is investigated employing solubilization of the hydrophobic monomer by complexation with a cyclodextrin. N-(1-adamantyl)acrylamide is synthesized and characterized as a hydrophobic monomer forming a water soluble-inclusion complex with statistically methylated-β-cyclodextrin. The stoichiometry, the complex formation constant and the spatial arrangement of the formed complex are determined. Mixed-mode monolithic stationary phases are synthesized by in situ free radical copolymerization of cyclodextrin-solubilized N-adamantyl acrylamide, a water soluble crosslinker (piperazinediacrylamide), a hydrophilic monomer (methacrylamide), and a negatively charged monomer (vinylsulfonic acid) in aqueous medium in bind silane-pretreated fused silica capillaries. The synthesized monolithic stationary phases are amphiphilic and can be employed in the reversed- and in the normal-phase mode (depending on the composition of the mobile phase), which is demonstrated with polar and non-polar analytes. Observations made with polar analytes and polar mobile phase can only be explained by a mixed-mode retention mechanism. The influence of the total monomer concentration (%T) on the chromatographic properties, the electroosmotic mobility, and on the specific permeability is investigated. With a homologues series of alkylphenones it is confirmed that the hydrophobicity (methylene selectivity) of the stationary phase increases with increasing mass fraction of N-(1-adamantyl)acrylamide in the synthesis mixture.
Porous monoliths based on organic precursors undergoing free-radical cross-linking polymerization in porogenic solvents emerged approximately two decades ago as an alternative stationary phase material for diverse applications including liquid chromatography. Though having a profound difference in morphology to their earlier generation polymer bead-based counterparts, they are often based on similar chemistries and as such show certain peculiarities with respect to transport and performance in liquid chromatography applications. Polymer monoliths typically consist of a globule-like, three-dimensionally adhered backbone, which is in a contrast to the silica monoliths having a bi-continuous mesoporous skeleton. Both material types possess large flow-through pores making them desirable for high performance liquid chromatography and other flow-through applications. The current review is devoted to a critical appraisal of the major challenges that researchers face in the retrieval of the never-ending demand of efficiency at often forgotten and desired selectivity and retention in separations using porous polymer monoliths. Therefore, an attempt is made to establish profound links of polymer monoliths to their earlier generation polymer-based particulate beds and differences to silica-based materials. These links are associated with an emerging morphological understanding of the polymer monoliths porous flow-through pore structure, the nano-scale backbone chemistry, and related chromatographic performances in both theoretical and experimental studies. Associated with this understanding, existing attempts in improving flow and transport performance of polymer monoliths are described and discussed. Such developments are addressing morphological concerns with respect to homogeneity and detailed design of pore space, but also tailoring backbone nano-structural chemistry to modulate mass transfer.
Most chromatographic methods, including capillary electrochromatography (CEC), require gradient elution for high resolution of proteins. The gradients used in the CEC experiments described herein were generated by an HPLC instrument and pumped past one end of the capillary column. Part of the gradient was at the same time transported into the capillary solely by electroendosmosis. Employing these gradients, positively charged proteins were separated on a column filled with a continuous bed derivatized with C18 groups (for reversed-phase separation) and with ammonium groups (for generation of electroendosmotic flow (EOF)). Both the proteins and the EOF-generating ligands thus had positive charges to eliminate electrostatic interactions. The gradient and the sample were introduced at the same end of the capillary as in conventional (electro)chromatography or in a new approach, at different ends. In the former mode, the electroendosmotic velocity must be higher than the electrophoretic velocity, whereas in the latter mode, it must be lower. Accordingly, gradient elution in electrochromatography can be used for many CEC columns since the magnitude of their EOF is not critical. The EOF is a function of the concentration of the gradient constituents and may, therefore, be different in different segments of the capillary. The possible attendant effects on zone broadening have been treated, as well as the electrophoretic zone broadening and zone sharpening caused by the gradient. Special precaution was taken in order to ensure that the electrophoretic contribution to the recorded separation did not dominate over the chromatographic one. We used a new approach to synthesize continuous beds with ligands of high concentration. It can briefly be described as follows. By a suspension−polymerization off-capillary procedure (in the absence of stabilizers and surfactants), very small gel particles derivatized with C18 ligands are prepared under ultrasonication for 45 min. Then, piperazine diacrylamide (cross-linker) and dimethyl diallylammonium chloride (both EOF-generating ligand and cross-linker) are added. This suspension is propelled into the capillary (with a methacryloyl-activated inner surface). At this stage, the concentration of nonterminated polymer chains on the surface of the gel particles is sufficiently high for further polymerization reactions. The polymer bed becomes attached covalently to the capillary wall concomitantly with the formation of channels in the bed.
A one-step method is described for in situ preparation of macroporous polymeric matrixes to be used as stationary phases for capillary electrochromatography. The monomers (acrylamide, bisacrylamide, and acrylic or vinylsulfonic acid), including hydrophobic ligands (C4, C6, or C12), and poly(ethylene glycol) have been polymerized in formamide (or N-methylformamide) aqueous solutions inside the capillary. The capillary wall had been activated first by a bifunctional reagent, to couple covalently the resulting gel inside the fused-silica tubing. Thus, no frit is necessary to keep the stationary phase in place. High efficiencies were obtained for a mixture of alkyl phenones (up to 398 000 plates/m). Good separations are achieved in less than 5 min. The migration time reproducibility is better than 1% (RSD) from run to run and 2.5% from day to day. The gel is stable up to at least 50% acetonitrile used as a mobile phase. On-column UV and fluorescence detection can readily be employed. Applications to peptides and carbohydrates are also shown.
The effect of the inclusion complex formation of solutes with beta -cyclodextrin added in the mobile phase on their retention and selectivity has been investigated. A simple equation has been derived that reveals the hyperbolic dependence of the capacity factor, k prime , on the total concentration of cyclodextrin, left bracket CD right bracket //T, in the mobile phase. A plot of the reciprocal of the capacity factor against left bracket CD right bracket //T gives a straight line, from the slope of which the dissociation constant k//D, of the inclusion complex can be calculated. The K//D values thus obtained in aqueous methanol were somewhat larger than those observed in 100% water.
Owing to their favorable porous structure with pore size distribution shifted towards large flow-through pores, organic polymer monoliths have been mainly employed for the separation of macromolecules in gradient elution liquid chromatography. The absence of significant amounts of small pores with a stagnant mobile phase and the resulting low surface area were considered as the main reason for their poor behavior in the isocratic separation of small molecules. Several recent efforts have improved the separation power of organic polymer monoliths for small molecules offering column efficiency up to tens of thousands of plates per meter. These attempts include optimization of the composition of polymerization mixture, including the variation of functional monomer, the cross-linking monomer, and the porogen solvents mixture, adjustment of polymerization temperature, and time. Additionally, post-polymerization modifications including hypercross-linking and the use of carbon nanostructures showed significant improvement in the column properties. This review describes recent developments in the preparation of organic polymer monoliths suitable for the separation of small molecules in the isocratic mode as well as the main factors affecting the column efficiency.
The characteristic pressure, Px (=4.455×109 Nm−2), appears to be the same for all unassociated liquids, and the characteristic volume, Vx, may be found by the addition of atomic factors. The van der Waals constant a, latent heat of evaporation Δ1gHE and solubility parameter can all be estimated for such liquids, since a, Δ(Δ)gHE-RT) V and δV equal Fx1/2Vx/3, where T is the temperature, R is the gas constant, and V=M/(∂L0-∂g0): M is the molecular mass; ∂L0 is the density of the liquid and ∂g0 the vapour density. Molecular attraction constants used for the estimation of solubility parameters can be obtained by the multiplication of Vx by the constant Px1/2/3.
The influence of the alkyl chain length on methylene selectivity has been examined, both in reversed-phase liquid chromatography and liquid-liquid extraction. While log values remained constant in liquid-liquid extraction, the reversed-phase results demonstrated an increase in selectivity with increasing chain length of the bonded phase.
In the correlation of reversed-phase liquid chromatography capacity factors through the equation,
logk¢ = log k¢0 + mV/100 + sp2* + bb2 + aa2\log k' = log k'_0 + mV/100 + s\pi _2^* + b\beta _2 + a\alpha _2
the use of McGowans characteristic volume, Vx, which can be trivially calculated, is entirely equivalent to the use of Leahy's computer-calculated intrinsic volumes, V1, for the cavity term mV/100. It is shown that for 209 gaseous, liquid, and solid solutes, the two sets of volumes are related
through the equation,
V1 = 0.597 + 0.6823 VxV_1 = 0.597 + 0.6823 V_x
with a standard deviation of only 1.24cm3 mol−1, and a correlation coefficient of 0.9988.
Utilizing the concurrence of polymerization-induced phase separation and sol-gel transition in the hydrolytic polycondensation of alkoxysilanes, a well-defined macroporous structure is formed in a monolithic wet gel. By exchanging the fluid phase of the wet gel with an appropriate external solution, the nanometer-range structure of the wet gel can be reorganized into structures with larger median pore size essentially without affecting the macroporous framework. The double-pore structure thus prepared is characterized by open pores distributed in discrete size ranges of micrometers and nanometers. A new type of chromatographic column (silica rod) has been developed using monolithic double-pore silica instead of packed spherical gel particles. Typical silica rod columns had significantly reduced pressure drops and improved analytical efficiencies which do not deteriorate even at higher sample flow rates, both arising from a greater macropore volume than particle packed columns.
Binding constants between adamantane carboxylic acids and -cyclodextrin derivatives were determined by capillary electrophoresis (CE) using indirect detection. In this procedure, a mixture of adamantane derivative and non-interacting anionic standards, is injected and analysis of the electrophoretic mobility, relative to the noninteracting anionic standards, as a function of the concentration of cyclodextrin, yields values for their binding constants to the adamantane derivative. The usefulness of the method for quantification of binding constants is demonstrated.
The driving forces leading to the inclusion complexation of cyclodextrins were reviewed, which included the electrostatic interaction, van der Waalsinteraction, hydrophobic interaction, hydrogen bonding, release of conformational strain,exclusion of cavity-bound high-energy water, and charge–transferinteraction. It was shown that except for the release of conformation strain and exclusion of cavity-bound water, the otherinteractions were indeed contributive to the complex formation. However, it was concludedthat the enthalpy and entropy changes of the complexation were not good criteria to be used injudging whether a particular driving force was present or important, mainly because of theoccurrence of enthalpy-entropy compensation. On the other hand, the multivariate quantitativestructure-activity relationship analyses usually could illustrate which driving forces wereimportant in certain inclusion complexation systems.