Developments in the use of soluble ionic polymers as pseudo-stationary phases for electrokinetic chromatography and stationary phases for electrochromatography

Molecular imprinting science and technology: a survey of the literature for the years 2004-2011

Herein, we present a survey of the literature covering the development of molecular imprinting science and technology over the years 2004-2011. In total, 3779 references to the original papers, reviews, edited volumes and monographs from this period are included, along with recently identified uncited materials from prior to 2004, which were omitted in the first instalment of this series covering the years 1930-2003. In the presentation of the assembled references, a section presenting reviews and monographs covering the area is followed by sections describing fundamental aspects of molecular imprinting including the development of novel polymer formats. Thereafter, literature describing efforts to apply these polymeric materials to a range of application areas is presented. Current trends and areas of rapid development are discussed.

Advanced CE for chiral analysis of drugs, metabolites, and biomarkers in biological samples

An analysis of recent trends indicates that CE can show real advantages over chromatographic methods in ultratrace enantioselective determination of biologically active compounds in complex biological matrices. It is due to high separation efficiency and many applicable in-capillary electromigration effects in CE (countercurrent migration, stacking effects) enhancing significantly (enantio)separability and enabling effective sample preparation (preconcentration, purification, analyte derivatization). Other possible on-line combinations of CE, such as column coupled CE-CE techniques and implementation of nonelectrophoretic techniques (extraction, membrane filtration, flow injection) into CE, offer additional approaches for highly effective sample preparation and separation. CE matured to a highly flexible and compatible technique enabling its hyphenation with powerful detection systems allowing extremely sensitive detection (e.g. LIF) and/or structural characterization of analytes (e.g. MS). Within the last decade, more as well as less conventional analytical on-line approaches have been effectively utilized in this field and their practical potentialities are demonstrated on many new application examples in this article. Here, three basic areas of (enantioselective) drug bioanalysis are highlighted and supported by a brief theoretical description of each individual approach in a compact review structure (to create integrated view on the topic), including (i) progressive enantioseparation approaches and new enantioselective agents, (ii) in-capillary sample preparation (preconcentration, purification, derivatization), and (iii) detection possibilities related to enhanced sensitivity and structural characterization.

Enantioselective separation using chiral mesoporous spherical silica prepared by templating of chiral block copolymers

In this work, we synthesized chiral mesoporous silica (CMS) spheres, which can be used as a potential candidate for chiral separation. The CMS spheres with controllable pore sizes (of 2-3 nm) and high surface areas of ca. 614 m(2) g(-1) were synthesized by chiral templating with chiral block copolymers based on poly(ethylene oxide) and dl-glutamic acid [PEO(113)-b-(GluA)(10)]. The ordered structure of the chiral mesopores was characterized by high-resolution transmission electron microscopy, and the average pore diameters of chiral mesopores were estimated from the nitrogen adsorption-desorption measurements. The enantioselectivity properties and chiral resolution kinetics of the mesopores of silica spheres, after extraction of chiral polymers of PEO(113)-b-(l/d-GluA)(10), were scrutinized using a racemic solution of valine and measuring the circular dichroism and optical polarimetery. A chiral selectivity factor of 5.22 was found with a specific enantiomer of valine adsorbed preferably. These results raise the new possibilities of CMS spheres for enantiomeric separation and other enantioselective applications.

Electrokinetic chromatography with polymeric pseudostationary phases

Recent research and development efforts concerning polymeric pseudostationary phases (PSPs) for electrokinetic chromatography are reviewed. The introduction of new materials, characterization of structural effects on performance and selectivity, applications, and the use of polymeric PSPs with mass spectrometric detection are considered. Very interesting results concerning the effects of polymer structure have been reported. Significant developments have also been reported in the development of novel applications of polymeric PSPs, particularly for sample preconcentration using micellar affinity gradient focusing. The use of mass spectrometric detection with electrokinetic chromatography has seen significant development, and recent reports indicate that this is a robust and sensitive approach.

Cationic surfactants for micellar electrokinetic chromatography: 1. Characterization of selectivity using the linear solvation energy relationships model

MEKC and the linear solvation energy relationship (LSER) model have been applied to two series of cationic surfactants. The synthetic flexibility of the quaternary ammonium group is exploited to generate the two series, one consisting of linear substitutions and the other incorporating the ammonium into ring structures of varying size. The effects of the head group structure on the CMC, aggregation number, and electrophoretic properties of the surfactants were determined. These surfactants were also characterized with the LSER model, which allowed the contributions of five chemical factors to the interactions between solutes and the micelles to be evaluated. Trends were observed in the cohesivity and polarity of the linear surfactant series, with both increasing with the size of the head group. No trends in the LSER parameters were observed in the cyclic series, but the LSER results do show that the surfactants with cyclic head groups provide a significantly different solvation environment from the linear series. Additional trends were observed in the aggregation behavior and chromatographic properties of the surfactants. These included changes in the CMCs, aggregation numbers, EOF, and electrophoretic mobility of the micelles that correlate to changes in head group size.

Monoclonal behavior of molecularly imprinted polymer nanoparticles in capillary electrochromatography

A new approach based on miniemulsion polymerization is demonstrated for synthesis of molecularly imprinted nanoparticles (MIP-NP; 30-150 nm) with "monoclonal" binding behavior. The performance of the MIP nanoparticles is characterized with partial filling capillary electrochromatography, for the analysis of rac-propranolol, where (S)-propranolol is used as a template. In contrast to previous HPLC and CEC methods based on the use of MIPs, there is no apparent tailing for the enantiomer peaks, and baseline separation with 25,000-60,000 plate number is achieved. These effects are attributed to reduction of the MIP site heterogeneity by means of peripheral location of the core cross-linked NP and to MIP-binding sites with the same ordered radial orientation. This new MIP approach is based on the substitution of the functional monomers with a surfactant monomer, sodium N-undecenoyl glycinate (SUG) for improved inclusion in the MIP-NP structure and to the use of a miniemulsion in the MIP-NP synthesis. The feasibility of working primarily with aqueous electrolytes (10 mM phosphate with a 20% acetonitrile at pH 7) is attributable to the micellar character of the MIP-NPs, provided by the inclusion of the SUG monomers in the structure. To our knowledge this is the first example of "monoclonal" MIP-NPs incorporated in CEC separations of drug enantiomers.

Templating Mesoporous Silica with Chiral Block Copolymers and Its Application for Enantioselective Separation

In this paper we describe the synthesis of chiral mesoporous silica based on chiral block copolymers of poly(ethylene oxide) and of d-phenylalanine (PEO-b-D-Phe) as a surfactant template. The resulting porous structures are characterized by nitrogen sorption experiments, transmission electron microscopy, and small-angle XRD. It is shown that chiral block copolymers of PEO-b-D-Phe are effective as a surfactant template for the preparation of silica materials with highly ordered periodic mesoporous structures of hexagonal symmetry with a pore size of ca. 5 nm and high surface areas of ca. 700 m2/g. The enantioselectivity feature of this porous silica, after the extraction of the chiral copolymers, was examined by selective adsorption of enantiomers and racemic solutions of valine. The selective adsorption was measured by circular dichroism (CD) spectroscopy. A chiral selectivity factor of 2.34 was found with the D enantiomer of valine adsorbed preferably.

Advances in chiral separation using capillary electromigration techniques

This review gives an overview of recent developments in CZE, EKC, and CEC covering the literature since the year 2004. Since there appeared a special issue on applications, this review focuses on the progress in electromigration techniques and new methodological developments. New techniques, new chiral selectors as well as new chiral stationary phases for CEC are discussed.

Recent progress in the use of ionic polymers as pseudostationary phases for EKC

This review concerns the introduction, characterization, and application of polymeric pseudostationary phases (PSPs) for EKC since 2004. Achiral and chiral polymers and separations are reviewed, as is the application of polymeric PSPs for the combination of EKC with mass spectrometric detection.

MEKC: An update focusing on practical aspects

This paper reviews recent methodological and instrumental advances in MEKC. Improvements in sensitivity arising from the use of on-line sample concentration (sweeping, stacking, and combination of both protocols) and derivatization (in-capillary reactions and coupling with flow-injection systems) and improvements in resolution obtained by changing the composition of the BGE (e.g., with organic modifiers, ionic liquids, nonionic and zwitterionic surfactants, mixed micelles, and vesicles) or using coated capillaries are discussed in detail. In addition, MS and LIF spectroscopy are examined in relation to their advantages and restrictions as applied to MEKC analysis. Some thoughts on potential future directions are also expressed.

Porous polyacrylamide monoliths in hydrophilic interaction capillary electrochromatography of oligosaccharides

Capillary electrochromatography (CEC) of oligosaccharides in porous polyacrylamide monoliths has been explored. While it is possible to alter separation capacity for various compounds by copolymerization of suitable separation ligands in the polymerization backbone, "blank" acrylamide matrix is also capable of sufficient resolution of oligosaccharides in the hydrophilic interaction mode. The "blank" acrylamide network, formed with a more rigid crosslinker, provides maximum efficiency for separations (routinely up to 350,000 theoretical plates/m for fluorescently-labeled oligosaccharides). These columns yield a high spatial resolution of the branched glycan isomers and large column permeabilities. From the structural point of view, some voids are observable in the monoliths at the mesoporous range (mean pore radius ca. 35 nm, surface area of 74 m2/g), as measured by intrusion porosimetry in the dry state.

Cationic and perfluorinated polymeric pseudostationary phases for electrokinetic chromatography

Separation selectivity in electrokinetic chromatography (EKC) is directly affected by the chemistry and solvent characteristics of the pseudostationary phase (PSP). The chemical selectivity of micellar PSPs has been previously demonstrated to vary significantly between anionic and cationic surfactants as well as between hydrocarbon and fluorocarbon surfactants. Polymeric PSPs have also been demonstrated to provide unique selectivity. In the current study, four cationic polymeric pseudo-stationary phases, two of which have perfluorinated pendant groups, are introduced and characterized as PSPs in EKC. Their performance and selectivity is compared to conventional micellar PSPs with similar structure. The solvation characteristics and selectivity of the four polymers most closely resemble those observed for cationic micelles. The polymers are all more cohesive and more polar than their hydrocarbon micellar counterparts. The fluorocarbon PSPs did show preferential interaction with fluorocarbon solutes, were somewhat more cohesive, and were stronger hydrogen bond donors. However, the presence of fluorocarbon moieties did not have as dramatic an effect on selectivity as was observed and published previously for fluorocarbon micelles. This may result from the selectivity being dominated by the presence of the cationic head groups or from the fluorocarbon character of the pendant groups being moderated by the presence of hydrocarbon functionality on the polymer backbones.

Glycine-based polymeric surfactants with varied polar head group: I. synthesis, characterization, and application in micellar electrokinetic chromatography

The monomers and polymers of four anionic amide type sodium undecenoxy carbonyl glycinate (SUCG) surfactants and four anionic carbamate type sodium undecenoyl glycinate (SUG) surfactants with 1-, 2-, 3-, and 4-glycine unit as head group were synthesized and characterized. The CMC and aggregation number (A) for all eight surfactants were determined using fluorescence spectroscopy. In addition, the CMC values of these surfactants were also projected by surface tension and CE. The CMC of the monomers decreases with increases in the size of glycine head groups and correlates well when the fluorescence method was compared to CE. The A number increases and partial specific volume (V) decreases with increase in size of the head group of both monomers and polymers. However, A and V are always lower for the polymers than the corresponding monomers. The electrophoretic and chromatographic parameters of micelle polymers of SUG and SUCG were also examined. The coefficient of EOF increases with the increase in size of the head group but the electrophoretic mobility decreases which results in a decrease in the elution range. The retention data suggest that the selectivity differences among the mono-, di-, and tripeptide derivatives of poly-SUCG surfactants are relatively higher compared to the derivatives of poly-SUG series.

Performance and selectivity of polymeric pseudostationary phases for the electrokinetic separation of amino acid derivatives and peptides

Two polymeric pseudostationary phases, one an acrylamide polymer and the second a siloxane polymer, have been investigated for the separation of naphthalene-2,3-dicarboxaldehyde (NDA)-derivatized amino acids and small peptides. The dervatized amino acids were detected by UV absorbance and laser-induced fluorescence (LIF) detection. The polymers provided very high efficiency and good selectivity for the separation of the amino acids. The separation selectivity using the polymers was significantly different from that of SDS micelles, and there were subtle differences in selectivities between the polymers. Although very good detection limits were obtained with LIF detection, a significant background signal was observed when the polymers were not washed to remove fluorescent impurities. The polymers did not separate the peptides very well. It is postulated that the fixed covalent structure of the polymers prevents them from interacting strongly or efficiently with the peptides, which are large in relation to the analytes typically separated by electrokinetic chromatography using polymers.

Conformational effects on the performance and selectivity of a polymeric pseudostationary phase in electrokinetic chromatography

The effect of the conformation of a polymeric pseudostationary phase on performance and selectivity in electrokinetic chromatography was studied using an amphiphilic pH-responsive polymer that forms compact intramolecular aggregates (unimer micelles) at low pH and a more open conformation at high pH. The change in conformation was found to affect the electrophoretic mobility, retention, selectivity, and separation efficiency. The low-pH conformer has higher electrophoretic mobility and greater affinity for most solutes. The unimer micelle conformation was also found to provide a solvation environment more like that of micelles and other amphiphilic self-associative polymers studied previously. It was not possible to fully characterize the effect of conformation on efficiency, but very hydrophobic solutes with long alkyl chains appeared to migrate with better efficiency when the unimer micelle conformation was employed. The results imply that polymers with a carefully optimized lipophilic-hydrophilic balance that allow self-association will perform better as pseudostationary phases. In addition, the results show that electrokinetic chromatography is a useful method for determining the changes in solvation environment provided by stimuli-responsive polymers with changes in the conditions.

Chiral nanotechnology

A review of chiral, nanoscale science and technology is presented, with the subject divided into two topics. The first discusses nanotechnology in the service of asymmetric synthesis, chiral separations, and analysis. The second topic concerns broader research in the nanotechnology realm, where molecular chirality plays a role in the properties of materials, including molecular devices, chiral supramolecules, chiral nanotubes, chiral fullerenes, and DNA nanotechnology.

Recent progress in the use of soluble ionic polymers as pseudostationary phases for electrokinetic chromatography

This review concerns the development, characterization, and application of soluble ionic polymeric materials as pseudostationary phases for electrokinetic chromatography since 2002. Cationic polymers, anionic siloxanes, polymerized surfactants (micelle polymers), and chiral polymers are considered. The use of stable suspensions of polymer nanoparticles in electrokinetic chromatography is also reviewed.