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

This article reviews the development, characterization and application of soluble ionic polymeric materials as pseudo-stationary phases for electrokinetic chromatography and as stationary phases for electrochromatography since 1997. Polymeric pseudo-stationary phases for electrokinetic chromatography, including cationic polymers, anionic siloxane and acrylamide polymers, polymerized surfactants (micelle polymers), and chiral polymers are reviewed. Also reviewed are suspended molecularly imprinted polymer micro-particles. Application of polymeric pseudo-stationary phases with electrospray ionization mass spectrometric detection is presented. Recent progress in the development and characterization of physically adsorbed stationary phases for electrochromatography using polymers of the same or similar chemistry is also reviewed.     

Application of particles as pseudo-stationary phases in electrokinetic chromatography

Since the introduction of micelles by Terabe et al. (1984), many different species have been employed as carriers for electrokinetic chromatography. As it is not possible to separate uncharged solutes or ions with equal mobility in capillary electrophoresis, these phases are added to introduce or improve selectivity.

This review surveys the application of particles as pseudo-stationary phases. Up to now only very few applications exist based either on polymer particles for the separation of catechols and primary amines or silica-based material. With these particles separations are shown for phenols, polycyclic aromatic hydrocarbons and naphthalene derivatives. The characteristic properties of particles as pseudo-stationary phases and their benefits as well as their limitations are discussed.     

Micelles as pseudo-stationary phases in micellar electrokinetic chromatography

This review article describes some general comments on micellar electrokinetic chromatography (MEKC) from the viewpoint of pseudo-stationary phases and presents a compiled list of surfactants used for MEKC, prepared from published papers. We tried to give comments on some typical surfactants from the practical point of view.     

Macromolecular surfactant as a pseudo-stationary phase in micellar electrokinetic capillary chromatography

We examined polymers of sodium 11-acrylamidoundecanoate [poly(Na 11-AAU)] with a very high molecular mass (>10(6)) for their potential use as a pseudo-stationary phase in micellar electrokinetic capillary chromatography (MEKC). Size-exclusion chromatography and capillary electrophoresis studies reveal that the polymers are highly charged, and have a densely packed chain structure. For aromatic compounds, the polymeric surfactant showed significantly different selectivity than sodium dodecyl sulfate (SDS). It was suggested that one molecule of poly(Na 11-AAU) forms one micelle. The structural stability of this pseudo-stationary phase permitted its use with relatively high percentages of organic modifiers in the buffer medium, allowing the separation of highly hydrophobic compounds which are difficult to analyze by conventional MEKC with SDS.     

Effect of pendant chain lengths and backbone functionalities on the chemical selectivity of sulfonated amphiphilic copolymers as pseudo-stationary phases in electrokinetic chromatography

Amphiphilic copolymers of AMPS (2-acrylamido-2-methyl-1-propanesulfonic acid) and hydrophobic monomers with various chemical structures were synthesized, characterized and used as novel electrokinetic chromatography polymeric pseudo-stationary phases, showing significant chemical selectivity differences from that of the conventional monomeric pseudo-stationary phase, sodium lauryl sulphate. Copolymers of AMPS and methacrylates with different pendant chain lengths (C8, C12 and C18) were investigated and no significant difference in chemical selectivity was observed among them. However, the spacer bonding chemistry was shown to contribute to significant chemical selectivity difference, e.g. poly(AMPS-lauryl methacrylate) showed different chemical selectivity from poly(AMPS-lauryl methacrylamide). Linear solvation energy relationship analysis of 20 solutes by eight different polymeric pseudo-stationary phases was employed to investigate the solute molecule structural contributions to the retention. Hydrogen-bonding properties (described by system constants b and a) of poly(AMPS-alkyl methacrylamide) were found stronger than those of poly(AMPS-alkyl methacrylate).     

Chiral separations using polymeric dipeptide surfactants: effect of number of chiral centers and steric factors.

Two polymeric dipeptide chiral surfactants (PDCSs), poly sodium N-undecanoyl isoleucyl-valinate (SUILV) with three chiral centers and poly sodium N-undecanoyl leucyl-valinate (SULV) with two chiral centers, have been evaluated and compared as chiral pseudo-stationary phases in electrokinetic capillary chromatography. The performance of these surfactants, in terms of enantioselectivity was examined using anionic, cationic and neutral analytes. Analyses of the data suggest that the enantiomeric resolutions of the analytes with these two PDCSs are dependent upon steric factors rather than number of stereogenic centers.     

Starburst dendrimer-supported pseudostationary phases for electrokinetic chromatography

Pseudo-stationary phases for electrokinetic chromatography were prepared by the alkylation of starburst dendrimers (SBDs). The SBD-supported pseudo-stationary phase with dodecyl groups showed higher efficiency than short-akyl derivatives, and maintained the hydrophobic property inthe presence of methanol. The dodecyl-modified SBD provided wide migration time windows ar high methanol content to effect the separation of sixteen aromatic hydrocarbons, the priority pollutants designated by EPA, in 65% methanol. The selectivity of polymer-supported pseudo-stationary phase can be varied simply by changing the length of the alkyl groups. The dodecyl SBD showed relatively similar selectivity as sodium dodecyl culfate micelle, whereas short alkyl derivatives showed preference towards rigid and planar compounds based on the rigid and planar compounds based on the rigid polymer backbones. The selectivity of SBD-supported pseudo-stationary phases was dominated by the chain length of the alkyl groups, with the minor effect of the structure of the core and the generation of SBD where alkyl groups were attached.     

Polymeric and polymer-supported pseudostationary phases in micellar electrokinetic chromatography: performance and selectivity

Several types of synthetic ionic polymers have been employed as pseudostationary phases in electrokinetic chromatography. The polymers have been shown to have some significant advantages and different chemical selectivity relative to conventional surfactant micelles. Polymeric phases are effective for the separation and analysis of hydrophobic and chiral compounds, and may be useful for the application of mass spectrometric detection. Additionally, the polymeric phases often demonstrate unique selectivity relative to micellar phases, and can be designed and synthesized to provide desired selectivity. This review covers efforts to develop and characterize the performance, characteristics, and selectivity of synthetic polymeric pseudostationary phases since their introduction in 1992. Some ideas for the future development of polymeric pseudostationary phases and the role they may play in electrokinetic separations are presented.     

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.     

聚合物在胶束电动色谱中的应用

本文回顾了自1992 年聚合物第一次被用作胶束电动色谱准固定相以来聚合胶束、聚合物表面活性剂和枝状高分子在这方面的研究进展, 并与常用表面活性剂的性能进行了对比。     

Retention indices in micellar electrokinetic chromatography

The use of retention indices in micellar electrokinetic chromatography (MEKC) is evaluated both from a theoretical and a practical point of view. Fundamental equations for the determination of retention indices in MEKC are described, showing that retention indices are independent of the surfactant concentration. Possibilities as well as limitations of different homologous series as reference standards are described. In addition, the practical application of retention indices for identification, investigation of solute-micelle interactions, characterization and classification of pseudo-stationary phases and determination of solute lipophilicity are discussed.     

Application of retention factors in affinity electrokinetic chromatography and capillary electrophoresis.

Affinity capillary electrophoresis has become an established approach for performing interaction studies. In affinity electrokinetic chromatography the retention factor, as in liquid chromatography, is useful for describing the migration behavior of the analytes, and is instrumental for assessing the affinity of an analyte for the pseudo-stationary phase. Erroneous use of the retention factor concept in affinity capillary electrophoretic studies has appeared in a number of recent papers. The errors and their origin are pointed out, and the correct use of retention factors in affinity electrokinetic chromatography and capillary electrophoresis is summarized.     

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.     

Analysis of sulfobutyl ether-beta-cyclodextrin mixtures by ion-spray mass spectrometry and liquid chromatography-ion-spray mass spectrometry

A comparison is made of the retention properties of additives applied as positively charged pseudo-stationary phases for electrokinetic chromatography of neutral analytes. All additives have a quaternary ammonium as functional group. The polymeric additive [poly(N,N,N',N'-tetramethyl-N-trimethylenehexamethylenediammonium), Polybrene] has a concentration of 2% (w/w) in the background electrolyte (acetate, pH 5.2). Monomeric octyltrimethylammonium (OTMA) was used at a concentration below or above its critical micelle concentration (CMC) (140 mmol/l). At a concentration (259 mmol/l) above the CMC the system is that normally used for micellar electrokinetic chromatography with cationic micelles. However, even below the CMC, where OTMA is present as monomer, retention of the neutral analytes is observed as well. In all systems coating of the capillary wall with Polybrene establishes an electroosmotic flow directed towards the anode, counter-migrating to the electrophoretic movement of the additive. Based on the measurement of the mobility of the analytes (15 small, monofunctional aromatic compounds with different functional groups), their capacity factors, k(i), were determined in all systems. Low correlation of the k(i) values is observed between the particular systems, indicating their different selectivity at least for individual pairs of analytes. Based on the log k(i) values, a linear free energy relationship was applied to elucidate the main types of chemical interaction responsible for retention. As a result, cavity formation and n or pi electron interactions were found being significant for the micellar OTMA system, which agrees with findings described in the literature for other (cationic and anionic) micellar systems. For the polymeric system and for the monomeric OTMA system, the significant retention parameter is indicating n and pi electron interactions.     

Recent progress in the development,characterization and application of polymeric pseudophases for electrokinetic chromatography

This review article details the development, characterization and application of polymeric materials as pseudostationary phases for electrokinetic chromatography over the past two years. Recent developments in cationic polymers and anionic siloxane, acrylamide and polymerized surfactants (micelle polymers) are reviewed. Also reviewed is recent progress in the development and characterization of chiral polymeric phases for chiral separations by electrokinetic chromatography, and application of a polymeric pseudophase with electrospray ionization mass spectrometric detection.     

Enantiomeric separations by use of polymeric surfactant electrokinetic chromatography

This review surveys the enantiomeric separation of drugs by electrokinetic chromatography using polymeric chiral surfactant pseudostationary phases. These phases have recently been shown to provide better mass transfer and increased rigidity and stability than regular micelles in micellar capillary electrophoresis. Characterization of the polymeric chiral surfactants is presented. Solution interactions of the pseudostationary phases via thermodynamics and fluorescence probe studies are evaluated. Also, case studies of enantiomeric separation of drugs using a single amino acid surfactant and the synergistic effect of the addition of gamma-cyclodextrin to the buffer is discussed. The use of dipeptide surfactants for chiral drug separations is described as well.     

Pharmaceutical applications of micelles in chromatography and electrophoresis

This review surveys the use of micelles as separation media in chromatography and electrophoresis. Applications to pharmaceuticals whose molecular masses are relatively small are focused on in this review. In high-performance liquid chromatography (HPLC), chromatography using micelles and reversed-phase stationary phases such as octadecylsilylized silica gel (ODS) columns is known as micellar liquid chromatography (MLC). The main application of MLC to pharmaceutical analysis is the same as in ion-pair chromatography using alkylsulfonate or tetraalkylammonium. In most cases, selectivity is much improved compared with other short alkyl chain ion-pairing agents such as pentanesulfonate or octanesulfonate. Direct plasma/serum injection can be successful in MLC. Separation of small ions is also successful by using gel filtration columns and micellar solutions. In electrophoresis, especially capillary electrophoresis (CE), micelles are used as pseudo-stationary phases in capillary zone electrophoresis (CZE). This mode is called micellar electrokinetic chromatography (MEKC). Most of the drug analysis can be performed by using the MEKC mode because of its wide applicability. Enantiomer separation, separation of amino acids and closely related peptides, separation of very complex mixtures, determination of drugs in biological samples etc. as well as separation of electrically neutral drugs can be successfully achieved by MEKC. Microemulsion electrokinetic chromatography (MEEKC), in which surfactants are also used in forming the microemulsion, is successful for the separation of electrically neutral drugs as in MEKC. This review mainly describes the typical applications of MLC and MEKC for the analysis of pharmaceuticals.     

Micellar electrophoresis using ionic liquids

In this study, ionic liquid based cationic surfactants were evaluated as pseudo-stationary phases in micellar electrokinetic chromatography (MEKC). The aggregation behaviour of long-chain (C(12) and C(14)) alkylimidazolium ionic liquids in water and aqueous phosphate buffer was investigated by spectrophotometry. The critical micelle concentrations of these salts were determined and compared to those of tetradecyl- and dodecyltrimethylammonium chloride, salts commonly used in capillary electrophoresis. The practical utilization of a new type of surfactant in MEKC was evaluated by introducing an ionic liquid into the running aqueous buffer to separate neutral analytes-methylresorcinol isomers and benzene derivatives.     

Recent advances in peptide chiral selectors for electrophoresis and liquid chromatography

The application of peptides in chiral separations using techniques such as capillary electrophoresis (CE), electrokinetic capillary chromatography (EKC) and liquid chromatography is the focus of this review. Methods for finding peptide selectors using combinatorial library approaches are discussed, as well as recent advances in the use of peptides as general chiral selectors for electrophoresis and liquid chromatography. One example shows the effectiveness of polymeric dipeptide surfactants as general chiral selectors for electrophoresis. Another example shows the versatility of oligoproline chiral stationary phases, exhibiting resolution for a number of racemic analytes comparable to other well-established chiral stationary phases.     

准静态扫集胶束电动毛细管色谱法测定扇贝样品中的贝类毒素

采用准静态扫集胶束电动毛细管色谱(MEKC)法测定了扇贝样品中的2种贝类毒素。毛细管内首先充满含十二烷基硫酸钠(SDS)的缓冲溶液,调节缓冲溶液的pH值,使电渗流等于SDS胶束的电泳流速,电动进样时,带正电荷的贝类毒素离子被SDS扫集吸附,由于SDS在毛细管内处于准静止状态,可使进样时间延长至320 s。与常规电动进样MEKC相比,石房蛤毒素和软骨藻酸的检测灵敏度分别提高950和810倍。该方法对石房蛤毒素和软骨藻酸的检出限分别为0.05,0.12 ng/m L。方法可实现对扇贝样品中2种贝类毒素的快速、灵敏检测。