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Enantioselective separation of triadimenol, a component of systemic agricultural fungicide, by electrokinetic chromatography (EKC) using cyclodextrins (CDs) as chiral selectors was investigated. Both a neutral CD derivative, hydroxypropyl-gamma-CD (HP-gamma-CD), and an ionic one, heptakis-6-sulfato-beta-CD (HS-beta-CD), were employed as an additive in cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) and as a chiral pseudostationary phase in CDEKC, respectively. In each system, four stereoisomeric peaks were completely or partially separated from each other. To enhance the detectability or the concentration sensitivity, on-line preconcentration techniques were applied to both EKC systems. Sweeping was used in the CD-MEKC system under an acidic condition, whereas stacking with a reverse migrating pseudostationary phase (SRMP) in the CDEKC system. Around 10-fold increase in the detection sensitivity for each peak was attained with both sweeping and SRMP systems. Good repeatabilities in the migration time, corrected peak area, and peak height were recognized in terms of the relative standard deviation. The limit of detection for each peak in the SRMP-CDEKC system, calculated from the calibration curve, was found to be 0.8-3.8 ppm.

This review provides an overview as well as a practical understanding of on-line sample concentration techniques in micellar electrokinetic chromatography (MEKC). MEKC as well as other capillary electrophoretic modes suffer from low concentration sensitivity due to minute sample volume and limited optical pathlength for on-capillary photometric detection. Two on-line sample preconcentration techniques, sample stacking and sweeping are known to be effective techniques for enhancement of the concentration sensitivity in MEKC. Sample stacking occurs as ions cross a boundary that separates regions of the high electric field sample zone and the low electric field background solution zone. The difference in migration velocity of pseudostationary phases within the two zones is the key to achieving the focusing effect. Sweeping is defined as the picking and accumulating of analytes by the pseudostationary phase that penetrates the sample zone devoid of pseudostationary phase. In this review, several examples of the sample stacking and sweeping under different experimental conditions are given, besides many references to applications.

A simple and inexpensive approach to modify a commercial atmospheric pressure chemical ionization (APCI) interface was proposed for capillary electrophoresis–mass spectrometry (CE–MS). In order to accommodate lower flow rates in the range 1–10 μl/min, both sheath liquid and nebulizing gas were coaxially supplied to the nebulizer as an arrangement of pneumatically assisted electrospray interface. Since this paper focused on the primary study of the modified APCI interface, the performance of the interface was first evaluated by the direct infusion of a reserpine solution. Optimization of several APCI parameters, such as temperature of APCI vaporizer, nebulizing gas flow and APCI corona discharge current, were accomplished. The orifice dimension for the nebulizing gas flow was largely independent of the MS sensitivity when the nebulizing gas flow rate was ca. 0.4 l/min. A successful CE–APCI–MS separation is obtained using the modified APCI interface.

Electrokinetic chromatography (EKC) is one branch of capillary electrophoresis that permits the separation of electrically neutral solutes by the electrophoretic technique. The separation principle of EKC is based on that of chromatography, and various modes of EKC have been developed along with the partition mechanism. Micellar electrokinetic chromatography (MEKC), in which an ionic surfactant solution is employed instead of a buffer solution in capillary zone electrophoresis (CZE) at a higher concentration than the critical micelle concentration (CMC), has become the most popular technique among various EKC modes. Besides the separation of the electrically neutral or non-ionic solutes, MEKC is also effective for the separation of ionic solutes as well as CZE, hence it suits the analysis of pharmaceuticals, including cationic, anionic and neutral. Separation selectivity in MEKC can be manipulated easily through the modification of the buffer as well as changing the surfactants. Direct enantiomeric separation is successful by using chiral surfactants or chiral additives. Determination of drugs in plasma is also successfully achieved by a direct sample injection method, similar to micellar HPLC. In this paper, we summarize the principle separation characteristics of MEKC and some applications to pharmaceutical analysis, including direct enantiomeric separation and direct assay of drugs in plasma.

Micellar electrokinetic chromatography (MEKC) permits the separation of electrically neutral analytes by chromatographic principles in a capillary electrophoresis system. The most effective way to obtain high resolution in MEKC is to increase the separation factor, as in conventional chromatography. The separation factor in MEKC depends on the molecular structure of the micelle and hence on the surfactant used, the pH of solution, and the nature of any additives to the micellar solution. The hydrophilic moieties of surfactant molecules generally affect selectivity more than do the hydrophobic moieties. Chiral surfactants enable the enantiomeric separation of mixtures of chiral solutes to be achieved. Mixed micelles consisting of ionic and nonionic surfactants display different selectivity from that of single ionic micelles. Additives such as cyclodextrins, ion-pair reagents, urea, organic solvents and metals can also serve as useful modifiers of the micellar solution for improving separation. In particular, cyclodextrins are useful for the separation of aromatic isomers and enantiomers. A general introductory guide to the design of successful separations by MEKC is proposed, based primarily on the author's work.

In enantiomeric separation by capillary zone electrophoresis using dimethyl-beta-cyclodextrin (DM-beta-CD) as a chiral selector, enantioselectivities were sometimes significantly different among DM-beta-CDs from five different suppliers. The reason was due to the difference in the compositions among these commercial products, which was shown by the liquid chromatographic (LC) analysis. As for commercial DM-beta-CD from one supplier, two major components were obtained by preparative LC. NMR spectroscopic and mass spectrometric analyses of these two components were performed to estimate the structure of each component. The results implied that the commercial products consist of heptakis(2,6-di-O-dimethyl)-beta-CD and hexakis(2,6-di-O-methyl)-mono(2,3,6-tri-O-methyl)-beta-CD. Sometimes different enantioselectivities were observed between these two components including the original DM-beta-CD.