Characterisation of mixed lithium dodecyl sulphate/lithium perfluorooctanesulphonate pseudo‐stationary phases in MEKC

Lipophilicity and methylene selectivity of mixed pseudo‐stationary phases (PSPs) (containing lithium dodecyl sulphate (LDS) and lithium perfluorooctanesulphonate (LiPFOS) in different molar ratios) applied in MEKC have been investigated. Micellar proportion (t_prop,mic, a quantity expressing that how much time is spent by the analyte in the micellar phase related to its whole migration time), CLOGP₅₀ value (showing the value of hydrophobicity of a molecule spending exactly 50% of its migration time in the PSP) and methylene selectivity have been determined for different LDS/LiPFOS mixed phases. Values of the above‐mentioned parameters have been determined for analytes with different chemical structures (alkylbenzene and alkylphenone homologous series, alcohols). Good linear correlation was obtained between either the micellar proportion, CLOGP₅₀, or methylene selectivity and the phase composition for the mixed phases. Lipophilicity and methylene selectivity of the mixed LDS/LiPFOS PSPs can be calculated and can continuously be changed by mixing the two single phases (LDS and LiPFOS) in the appropriate (and calculable) portion.     

Recent innovation in capillary electrokinetic chromatography with replaceable charged pseudostationary phases or additives

Recent developments of separation of neutral analytes in capillary systems with the mobile phase driven by the electroosmotic flow (EOF) and charged additives acting as a pseudostationary phase are reviewed. As pseudostationary phases a number of additives are used. Soluble polymers, either anionic or cationic, were applied as alternatives to micelles. Monomeric charged additives are also intended to form associates with the analytes, leading to selective retention and separation in a similar way as the polymeric pseudostationary phases. Dendrimers, spherical macromolecules with highly branched chains and charged terminal groups, are successfully applied for the separation of lipophilic analytes. Polymers with covalently stabilized structures are introduced in the form of permanent micelles and are therefore insensitive to the mobile phase composition, enlarging the applicability of micellar electrokinetic capillary chromatography (MEKC).     

Copolymerized polymeric surfactants: characterization and application in micellar electrokinetic chromatography

An achiral monomeric surfactant (sodium 10-undecenyl sulfate, SUS) and a chiral surfactant (sodium 10-undecenoyl L-leucinate, SUL) were synthesized and polymerized individually to form poly-SUS and poly-SUL. These surfactants were then copolymerized at various molar ratios to produce a variety of copolymerized surfactants (CoPSs), possessing both achiral (sulfate) and chiral (leucinate) head groups. The CoPSs, poly-SUS, poly-SUL, and sodium dodecyl sulfate were characterized using several analytical techniques. The aggregation numbers of the polymeric surfactants and the partial specific volumes were determined by the use of fluorescence quenching and density measurements, respectively. These polymeric surfactants were investigated as novel pseudostationary phases in micellar electrokinetic chromatography (MEKC) for the separation of chiral and achiral solutes. Solute hydrophobicity was found to have major influence on the MEKC retention of alkyl phenyl ketones. In contrast, hydrogen-bonding ability of benzodiazepines is the major factor that governs their retention, but hydrophobicity has an insignificant effect on MEKC retention of benzodiazepines.     

Monomeric and polymeric anionic gemini surfactants and mixed surfactant systems in micellar electrokinetic chromatography. Part I: characterization and application as novel pseudostationary phases

Sodium di(undecenyl) tartarate monomer (SDUT), a vesicle-forming amphiphilic compound possessing two hydrophilic carboxylate head groups and two hydrophobic undecenyl chains gemini surfactant, was prepared and polymerized to form a polymeric gemini surfactant (i.e., poly-SDUT). These anionic surfactant systems with carboxylate (SDUT and poly-SDUT) and sulfate (sodium dodecyl sulfate, SDS) head groups as well as mixed surfactant systems (SDS/SDUT, SDS/poly-SDUT, and SDUT/poly-SDUT) were then applied as novel pseudostationary phases in micellar electrokinetic chromatography (MEKC). The SDUT and poly-SDUT were characterized using various analytical techniques. Retention factors of 36 benzene derivatives were calculated in 20 mM phosphate buffer of each surfactant system. The retention factor values of the test solutes show that there are distinctive selectivity differences between the surfactant systems. Solute-pseudostationary phase interactions in MEKC were also examined by determining the free energy of transfer of the substituted functional groups from the aqueous buffer phase into the pseudostationary phase.     

Thermodynamic studies of the interaction of molecular micelles and copolymerized molecular micelles with benzodiazepines and alkyl phenyl ketones using MEKC

In this study, polymers of sodium 10-undecenoyl L-leucinate (SUL) and sodium undecenyl sulfate (SUS) as well as their copolymerized molecular micelles (CoPMMs) were applied in MEKC as pseudostationary phases to separate benzodiazepines and alkyl phenyl ketones. SDS, a common pseudostationary phase used in MEKC, was also used for comparison. The van't Hoff relationship was applied to compute the temperature dependence of the MEKC retention factors of the test solutes to estimate the enthalpy, entropy, and the Gibbs free energy. Nonlinear van't Hoff plots were obtained with the majority of benzodiazepines indicating that the thermodynamic parameters were temperature-dependent in all surfactant systems for these solutes. In contrast, all alkyl phenyl ketones resulted in linear van't Hoff plots.     

Solute-solvent interactions in micellar electrokinetic chromatography: V. Factors that produce peak splitting

The experimental conditions that produce analyte peak splitting in micellar electrokinetic capillary chromatography (MEKC) have been systematically investigated. The system studied was a neutral phosphate buffer and sodium dodecyl sulfate (SDS) micelles as pseudostationary phase. A number of analytes showing a wide variety of hydrophobicity values and several organic solvents as sample diluents have been tested. Peak splitting phenomena are mainly due to the presence of organic solvent in the sample solution. They increase with the hydrophobicity of the analyte and decrease with the increase of the surfactant concentration. When hydrophobic compounds are analyzed the suggested ways to avoid split peaks are: (i) the use of 1-propanol or 1-butanol as sample diluent instead of methanol or acetonitrile or (ii) the use of high concentration of surfactant in the separating solution when the analyte must be dissolved in pure methanol or acetonitrile.     

Fundamental studies on electrokinetic chromatography with PEGylated phospholipid micelles.

In this study, micelles prepared from distearoylphosphatidylethanolamine with covalently attached poly(ethylene) glycol) (PEG) of molecular weight 2000 (DSPE-PEG-2000) were employed in micellar electrokinetic chromatography (MEKC) as pseudostationary phases. Since DSPE-PEG-2000 contains long hydrophobic alkyl chains, an anionic phosphate group, and hydrophilic PEG chains, the prepared micelles are expected to provide a characteristic retention behavior for both neutral and ionic compounds. As a typical example, a baseline separation of phenol and 2-naphthol was successfully achieved by using the DSPE-PEG-2000 micelles as a background electrolyte for MEKC; such success clearly shows that the micelles can retain electrically neutral compounds. The MEKC separations of anionic and cationic compounds with a DSPE-PEG-2000 micellar solution and the enantioseparation of binaphthyl compounds with mixed micelles containing bile salt are also discussed.     

Novel anionic copolymerized surfactants of mixed achiral and chiral surfactants as pseudostationary phases for micellar electrokinetic chromatography

One disadvantage of amino acid-based chiral selectors for micellar electrokinetic chromatography (MEKC) is that either they have very low solubility or are insoluble at acidic pHs. In order to increase solubilities at lower pHs, we have synthesized a highly water-soluble achiral surfactant and copolymerized it with an amino acid-based chiral surfactant. These two surfactants were polymerized either separately or at various molar rations of binary solutions, yielding pure molecular or copolymerized surfactant (CoPS), respectively. All surfactants were characterized by use of several analytical techniques prior to using them as novel pseudostationary phases in MEKC. The chromatographic performance of the CoPS in MEKC was tested with chiral and achiral analytes. The highly soluble sulfate head group significantly increased the solubility of amino acid-based CoPS over a wide range of pH. Three chiral binaphthyl derivatives were tested and each surfactant system was found to have different selectivity.     

New Pseudostationary Phases for Electrokinetic Chromatography: A High-Molecular Surfactant and Proteins

To extend the applicability of electrokinetic chromatography (EKC), two new types of pseudostationary phases have been introduced. A high-molecular surfactant, butyl acrylate/butyl methacrylate/methacrylic acid copolymer (BBMA) is employed as a micellar forming surfactant for miccllar electrokinetic chromatography (MEKC). The critical micelle concentration of BBMA is essentially zero, which means the micellar concentration is constant irrespective of temperature and buffer. Some characteristic features of BBMA as the pseudostationary phase for MEKC is investigated in comparison with conventional ionic surfactants. Ovomucoid and avidin, which are proteins isolated from egg white, have been found to be useful chiral selectors in affinity EKC. A few examples of the separation of enantiomers with these proteins are shown.     

Characterization and application of sodium di(2-ethylhexyl) sulfosuccinate and sodium di(2-ethylhexyl) phosphate surfactants as pseudostationary phases in micellar electrokinetic chromatography

Sodium di(2-ethylhexyl) sulfosuccinate (DOSS) and sodium di(2-ethylhexyl) phosphate (NaDEHP) surfactants, with double alkyl chains and negatively charged headgroups, were characterized using fluorescence quenching, densitometry, and tensiometry techniques to determine their aggregation number, partial specific volume, and critical aggregation concentration. These two surfactants were then applied as pseudostationary phases in micellar electrokinetic chromatography (MEKC) for separations of alkyl phenyl ketones. The aggregation number of NaDEHP was found to be more than two-fold higher than that of DOSS. The partial specific volumes of NaDEHP and DOSS were found to be 0.9003 and 0.8371 mL/g, respectively. The critical aggregation concentrations are 5.12 and 1.80 mM for NaDEHP and DOSS, respectively. The DOSS surfactant provided a wider separation window and had a greater hydrophobic environment than the NaDEHP surfactant under the MEKC experimental conditions studied.     

Monomeric and polymeric anionic gemini surfactants and mixed surfactant systems in micellar electrokinetic chromatography. Part II: characterization of chemical selectivity using two linear solvation energy relationship models

Sodium di(undecenyl) tartarate monomer (SDUT), a vesicle-forming amphiphilic compound possessing two hydrophilic carboxylate headgroups and two hydrophobic undecenyl chains, was prepared and polymerized to form a polymeric vesicle (i.e., poly-SDUT). The anionic surfactants of SDUT and poly-SDUT (carboxylate head group) and sodium dodecyl sulfate, SDS (sulfate head groups) as well as mixed surfactant systems (SDS/SDUT, SDS/poly-SDUT, and SDUT/poly-SDUT) were applied as pseudostationary phases in micellar electrokinetic chromatography (MEKC). Two linear solvation energy relationship (LSER) models, i.e., solvatochromic and solvation parameter models, were successfully applied to investigate the effect of the type and composition of pseudostationary phases on the retention mechanism and selectivity in MEKC. The solvatochromic and solvation parameter models were used to help understand the fundamental nature of the solute-pseudostationary phase interactions and to characterize the properties of the pseudostationary phases (e.g., solute size and hydrogen bond-accepting ability for all pseudostationary phases). The solute types were found to have a significant effect on the LSER system coefficients and on the predicted retention factors. Although both LSER models provide the same information, the solvation parameter model is found to provide much better results both statistically and chemically than the solvatochromic model.     

聚(苯乙烯-co-甲基丙烯酸)自组装胶束假固定相电动色谱的特性

研究了两亲性无规共聚物聚(苯乙烯-co-甲基丙烯酸)(P(St-co-MAA))(单体摩尔比分别为6:4和7:3)自组装胶束的物理化学性质,及其作为假固定相(PSP)的胶束电动色谱性能。测定了聚合物胶束的临界胶束浓度(CMC),对胶束内核微环境的极性、表面电荷密度和流体力学直径等微结构参数进行了表征,对时间窗口、亚甲基选择性等电动色谱参数进行了测定,并与聚(甲基丙烯酸甲酯-co-甲基丙烯酸)(P(MMA-co-MAA))胶束、十二烷基硫酸钠(SDS)胶束体系进行了比较;利用线性溶剂化能关系(LSER)研究了聚合物PSP的选择性差异。结果表明:P(St-co-MAA)体系具有最小的CMC、最宽的时间窗口和最好的亚甲基选择性;LSER表明,疏水作用是决定聚合物PSP选择性的最主要因素,氢键酸度其次,特别是P(St-co-MAA)(单体摩尔比7:3)体系具有最高的作用参数,显示了该PSP具有较高的分离选择性。     

Separation of aromatic sulfonate compounds by coelectroosmotic micellar electrokinetic chromatography

Summary Coelectroosmotic micellar electrokinetic chromatography (coelectroosmotic MEKC) has been investigated for the separation of twelve aromatic sulfonate compounds. The advantage of this method is that it combines the efficient separation characteristic of MEKC and the short analysis time of the coelectroosmotic mode. MEKC was performed with either cetyltrimethylammonium bromide (CTAB) or polyethylene glycol dodecyl ether (Brij 35) surfactants as pseudostationary phases and 2-propanol as organic modifier. The electroosmotic Flow (EOF) was reversed by adding two types of EOF modifier, an alkylammonium salt (cetyltrimethylammonium bromide, CTAB) or a cationic polyelectrolyte (hexadimetrine bromide, HDB). The surfactant concentration, applied voltage, and temperature were optimized, the influence of 2-propanol on the MEKC resolution of the compounds was studied. The effect of the osmotic modifier on the separation was also investigated.     

On-line sample concentration in micellar electrokinetic chromatography with cationic micelles in a coated capillary

The electroosmotic flow was successfully suppressed even in the presence of cationic surfactants, when a polyacrylamide-coated capillary was employed. Two on-line sample concentration techniques of sample stacking and sweeping were evaluated in micellar electrokinetic chromatography (MEKC) using the polyacrylamide-coated capillary. Cationic surfactants were used as pseudostationary phases in MEKC. At least 60-fold and about 600-fold increases in detection sensitivity were obtained in terms of peak heights by sample stacking and sweeping, respectively.     

Determination of amino acids by micellar EKC: recent advances in method development and novel applications to different matrices

The extensive use of CE for the analysis of amino acids has been well documented in a series of research articles and reviews. Aim of this report is to address the attention of the reader on the recent advances of micellar electrokinetic chromatography for the separation and determination of these analytes. Enhancements in selectivity of this technique through the use of pseudostationary phases containing mixed micelles, polymers, and chiral selectors are presented. Selected applications concerning separation and quantitation of even minute amounts of amino acids in: (i) biological fluids; (ii) microdialysates; (iii) plant cells; (iv) food stuff; and (v) pharmaceutical formulations have also been covered. Advantages of MEKC over other techniques for the amino acid analysis have been underlined.     

Micellar electrokinetic chromatography: current developments and future

This review highlights recent methodological and instrumental advances in micellar electrokinetic chromatography (MEKC). Enhancements in sensitivity and selectivity of the technique through the use of on-line preconcentration approaches (stacking and sweeping) and nonconventional pseudostationary phases, namely nonionic and zwitterionic surfactants, mixed micelles and polymers, are discussed in detail. Laser-induced fluorescence and mass spectrometry, as alternatives to UV-absorption detection, have been covered to evaluate their advantages and limitations when applied to analysis in an MEKC format. Some thoughts on future directions in this area such as in-capillary reactions, coated capillaries and MEKC on microchips are also presented.     

Enantiomer separation of drugs by micellar electrokinetic chromatography using chiral surfactants

A review surveying enantiomer separations by micellar electrokinetic chromatography (MEKC) using chiral surfactants is described. MEKC is one of the most popular techniques in capillary electrophoresis, where neutral compounds can be analyzed as well as charged ones, and the use of chiral micelles enable one to achieve the enantioseparation. The chiral MEKC systems are briefly reviewed according to the types of chiral surfactants along with typical applications. As chiral micelles or pseudostationary phases in MEKC, various natural and synthetic chiral surfactants are used, including several low-molecular-mass surfactants and polymerized surfactants or high-molecular-mass surfactants. Cyclodextrin modified MEKC using chiral micelles is also considered.     

Polymeric sulfated surfactants with varied hydrocarbon tail: I. Synthesis, characterization, and application in micellar electrokinetic chromatography

The influence of surfactant hydrocarbon tail on the solute/pseudostationary phase interactions was examined. Four anionic sulfated surfactants with 8-, 9-, 10-, and 11-carbon chains having a polymerizable double bond at the end of the hydrocarbon chain were synthesized and characterized before and after polymerization. The critical micelle concentration (CMC), polarity, and aggregation number of the four sodium alkenyl sulfate (SAIS) surfactants were determined using fluorescence spectroscopy. The partial specific volume of the polymeric SAIS (poly-SAIS) surfactants was estimated by density measurements and capillary electrophoresis (CE) was employed for determination of methylene selectivity as well as for elution window. The CMC of the monomers of SAIS surfactants decrease with increase in chain length and correlated well when fluorescence method was compared to CE. The physicochemical properties (partial specific volume, methylene selectivity, electrophoretic mobility, and elution window) increased with an increase in chain length. However, no direct relationship was found between the aggregation number and the length of hydrophobic tail of poly-SAIS surfactants. These polymeric surfactants were then used as pseudostationary phases in micellar electrokinetic chromatography (MEKC) to study the retention behavior and selectivity factor of 36 benzene derivatives with different chemical characteristics. Although variation in chain length of the polymeric surfactants significantly affects the retention of nonhydrogen bonding (NHB) benzene derivatives, these effects were less pronounced for hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) benzene derivatives. Therefore, hydrophobicity of poly-SAIS surfactants was found to be a major driving force for retention of NHB derivatives. However, for several benzene derivatives (NHB, HBA, and HBD) significantly higher selectivity factor was observed with longest chain polymeric surfactant (e.g., poly(sodium 10-undecenyl sulfate), poly-SUS) compared to shorter chain polymeric surfactant (e.g., poly(sodium 7-octenyl sulfate), poly-SOcS). In addition, the effect of the surfactant hydrophobic chain was also found to have some impact on migration order of NHB, HBA, and HBD benzene derivatives.     

Marker compounds for the determination of retention factors in EKC

EKC and its sub‐techniques, such as MEKC and microemulsion EKC, have attracted wide interest in recent years. Investigations on this topic have covered several analytical applications, but attention has also been paid more and more to basic studies. This review provides an overview of the different approaches to calculating retention factors, which express the ratio of the amount of sample component in the pseudostationary and mobile phases. Special attention is given to the selection of markers for the determination of the electrophoretic mobility or migration time of a marker describing the behavior of the pseudostationary phase in EKC. Introduction of a hydrophobic marker is by far the most common approach, but the use of a homologous series of compounds is also quite popular. In addition, other possible approaches found in the literature will be described.