Micellar electrokinetic chromatography , generally

Capillary electrophoresis is a general term that is used to describe a number of different separation techniques. Capillary zone electrophoresis (CZE) is the classic technique and is therefore usually referred to as just CE. Other techniques include micellar electrokinetic chromatography (MEKC), capillary isoelectric focusing, and capillary isotachophoresis. CZE and MEKC are the predominant techniques and are those used herein, so only they will be discussed in detail here. 

The distribution coefficient, K, of a solute for an equilibrium between the aqueous phase and micelle, or the micellar solubilization, depends on temperature generally, the distribution coefficient decreases with an increase in temperature. This means that the migration time of a solute, will be reduced when the temperature is elevated under typical micellar electrokinetic chromatography (MEKC) conditions, where, for example, sodium dodecyl sulfate (SDS) is employed as a pseudo-stationary phase at a neutral condition (i.e., pH 7). Also, the velocity of the electro-osmotic flow (EOF), Meof> and the electrophoretic velocity of the micelle, (me), will be increased by an increase in temperature because of a reduced viscosity of the micellar solution employed in a MEKC system. 

The most frequently applied CE techniques for chiral separations are capillary zone electrophoresis (CZE), micellar electrokinetic chromatography (MEKC) and chiral capillary electrochromatography (CEC). Two strategies are generally adopted for the enantiomer resolution in these electrophoretic techniques ... 

The general theory of micellar electrokinetic chromatography represents a confluence of chromatographic and electrophoretic principles. The expressions for electrophoretic mobility under different separation conditions are summarized in Table 8.4 [161,162]. These relationships allow the determination of the critical micelle concentration and equilibrium distribution constants for solute-micelle association complexes under typical conditions for micellar electrokinetic chromatography [60-64,161-164]. These properties change significantly with the composition of the electrolyte solution, and are generally different to common reference values for pure water. 

The general resolution equation for two neutral analytes with similar retention factors in micellar electrokinetic chromatography, is similar to the relationship for chromatography (section 1.6) with an additional term that arises from the limited migration time window [11,12,166,177,178]. 

Micellar electrokinetic chromatography (MEKC) has been used by several authors for the separation of phenolic compoimds [103] and in some cases for the determination in water [104], Off-line SPE using pol)nneric sorbents and MEKC with ECD were used for the determination of chlorinated phenols in a river at a low pg/L level [104]. Generally, one problem associated with miniaturized techniques such as CE when combined with UV detection is its limitation to small injection volumes. Therefore, efficient enrichment steps in the sample preparation are necessary. 

The first reports of micellar electrokinetic capillary chromatography (MECC or MEKC) appeared in the literature in 1984 [16]. Through April 2002, there have been over 1100 English-language papers published in the field. The subject is covered in all general textbooks on capillary electrophoresis. While a vast number of surfactants and related reagents can be employed, most separations can be accomplished with a few simple recipes. 

Micelles and cyclodextrins are the most common reagents used for this technique. Micellar electrokinetic capillary chromatography (MECC or MEKC) is generally used for the separation of small molecules [6], Sodium dodecyl sulfate at concentrations from 20 to 150 mM in conjunction with 20 mM borate buffer (pH 9.3) or phosphate buffer (pH 7.0) represent the most common operating conditions. The mechanism of separation is related to reversed-phase liquid chromatography, at least for neutral solutes. Organic solvents such as 5-20% methanol or acetonitrile are useful to modify selectivity when there is too much retention in the system. Alternative surfactants such as bile salts (sodium cholate), cationic surfactants (cetyltrimethy-lammonium bromide), nonionic surfactants (poly-oxyethylene-23-lauryl ether), and alkyl glucosides can be used as well. 

The main separation modes used in CE are capillary zone electrophoresis (CZE), micellar electrokinetic capillary chromatography (MEKC), capillary isotachophoresis, capillary gel electrophoresis, and capillary isoelectric focusing. CZE and MEKC are used most often. CE buffers are generally aqueous-based, though nonaqueous systems are exploited as well, particularly for analytes that are insoluble or sparingly soluble. 

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