Micellar electrokinetic chromatography: a review of methodological and instrumental innovations focusing on practical aspects.

This review article addresses recent methodological and instrumental innovations in MEKC with emphasis on practical aspects. Like its predecessors, this review is intended to provide an updated overview covering work on the most salient methodological contributions to enhancing sensitivity and resolution in MEKC-based determinations published over the past two years. The most widespread approaches to enhancing sensitivity, which include improving "classical" online sample concentration techniques, combinations of on- and off-line sample concentration protocols and recent developments are discussed, and so are modifications of existing MEKC systems with various micellar phases, the use of BGE additives (organic modifiers, chiral selectors, gold nanoparticles) and coated capillaries, and the implementation of 2D separations and chemometric methods to enhance resolution. Instrumental approaches such as MS and LIF are also discussed, and proposals for overcoming the problems typically encountered in directly coupling MEKC with MS, and the recent inception of quantum dots with a great potential for LIF detection in MEKC, are also dealt with. Finally, foreseeable developments on potential future directions are also expressed.

A review of developments in the methodology and application of microemulsion electrokinetic chromatography.

MEEKC is a mode of CE, which utilizes microemulsion (ME) as the BGE to achieve separation of a diverse range of analytes. MEs are composed of nanometer-sized oil droplets suspended in aqueous buffer which are stabilized by the presence of a surfactant and co-surfactant. These MEs are commonly referred to as oil-in-water MEs and their application in MEEKC has been extensively examined. This review details advances in the theory, methodology, and application of MEEKC during the period 2010-2012. Areas covered include online sample concentration, advances in chiral separations, use of coated capillaries, chemometric approaches, and the use of novel additives to the ME system. This review also provides the reader with an introduction to MEEKC and a presentation of recent applications.

Capillary electrophoresis of natural products: highlights of the last five years (2006-2010).

An extensive search of the published research and review articles indicated a growing number of works that employed electroseparation techniques. Capillary zone electrophoresis (CZE) and electrokinetic chromatography (EKC), in particular, were found to be popular for the analyses of natural products. This review outlines the important developments in this field over the years, especially during the 2006-2010 period. An overview of the natural product applications such as pharmaceuticals/herbal products, fingerprinting and quality control, food and food contaminants, and toxicological compounds of interest to forensics were presented. Important areas of detection strategies, microchips, sample preconcentration, and chiral separations were also discussed. Hence, quick information was provided on the researches already undertaken and the possibilities of unexplored areas. In addition, fundamental concepts for the understanding of CZE and EKC and their suitability for natural products analyses were briefly discussed.

Recent developments and applications of molecularly imprinted monolithic column for HPLC and CEC.

As a new generation of stationary phases, monolithic supports have attracted significant interest in high-performance liquid chromatography (HPLC) and capillary electrochromatography (CEC) because of their ease of preparation, high reproducibility, versatile surface chemistry and rapid mass transport. Molecularly imprinted polymers (MIPs) are synthetic materials with high specific recognition ability to template molecule. The combination of monolithic column and MIPs integrates the high efficiency of modern chromatography and the high selectivity provided by MIPs. This review focuses on the recent developments and applications of all kinds of monolithic matrix, i.e. organic polymer-based and silica-based MIP monolith in HPLC and CEC mode.

Capillary separation: micellar electrokinetic chromatography.

Micellar electrokinetic chromatography (MEKC), a separation mode of capillary electrophoresis (CE), has enabled the separation of electrically neutral analytes. MEKC can be performed by adding an ionic micelle to the running solution of CE without modifying the instrument. Its separation principle is based on the differential migration of the ionic micelles and the bulk running buffer under electrophoresis conditions and on the interaction between the analyte and the micelle. Hence, MEKC's separation principle is similar to that of chromatography. MEKC is a useful technique particularly for the separation of small molecules, both neutral and charged, and yields high-efficiency separation in a short time with minimum amounts of sample and reagents. To improve the concentration sensitivity of detection, several on-line sample preconcentration techniques such as sweeping have been developed.

Recent trends in the determination of polyphenols by electromigration methods.

An overview mapping recent trends in the determination of polyphenols of natural origin (mostly flavonoids) and their synthetic derivatives by electromigration methods is presented. The overview (covering the period of the recent 5 years and comprising 61 references) is focused on capillary zone electrophoresis (CZE) and micellar electrokinetic chromatography (MEKC) with various detection methods. Techniques comprising on-line pre-separation such as isotachophoresis (ITP)-CZE and flow-injection-CZE, chiral separations and CZE evaluation of antioxidation activity are also discussed.

Recent developments in the field of monolithic stationary phases for capillary electrochromatography.

This review summarizes the contributions to the rapidly growing area of monolithic columns based on both silica and synthetic polymers for capillary electrochromatography and chip electrochromatography, with a focus on those published during the year 2004. A wide variety of both modified approaches to the "old" monoliths and new monoliths have been reported despite the very short period of time covered. This demonstrates that monolithic stationary phases have become a well-established format in the field of electrochromatography. The simplicity of their preparation as well as the good control over their porous properties and surface chemistries make the monolithic separation media an attractive alternative to capillary columns packed with particulate materials.

Recent advances in microemulsion electrokinetic chromatography.

Microemulsion electrokinetic chromatography (MEEKC) is an electrodriven separation technique. Separations are typically achieved using oil-in-water microemulsions, which are composed of nanometre-sized droplets of oil suspended in aqueous buffer. The oil droplets are coated in surfactant molecules and the system is stabilised by the addition of a short-chain alcohol cosurfactant. The novel use of water-in-oil microemulsions for MEEKC separations has also been investigated recently. This report summarises the different microemulsion types and compositions used to-date and their applications with a focus on recent papers (2002-2004). The effects of key operating variables (pH, surfactant, cosurfactant, oil phase, buffer, additives, temperature, organic modifier) and methodology techniques are described.

Further method development for measurement of linezolid in human serum by MEKC.

A method for determining linezolid concentration in human serum using micellar electrokinetic capillary chromatography by direct injection of serum is described. A borate buffer (pH 8.0) containing sodium dodecyl sulfate was used as a run buffer and detection of linezolid was performed at 250 nm (its absorption maximum). The migration time of linezolid was 5.5 min and the detection limit was 0.5 mg/l (S/N = 3). The precision and accuracy of this method was good with no interference with the detection from bilirubin, hemoglobin and chyle of high concentrations. This provides a simple and easy method where samples of micro-quantity are used.

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.