Micellar electrokinetic chromatography

Abstract

Electrophoresis is a separation technique based on the migration of the analyte in a solution under the influence of electric field. In electrophoretic separation it is essential for the analyte to be charged or ionic. The migration velocity of the analyte is primarily depends on the charge and the size of the analyte under a homogeneous electric field but it can also be modified by several chemical or physical interactions between the analyte and the electrophoretic media including additives or polymer networks. Neutral molecules were not considered to be separated by electrophoresis until micellar electrokinetic chromatography (MEKC) was developed byTerabe et al. in 1984 [1]. The idea using ionic micelles in electrophoresis was suggested by Nakagawa in 1981 as described by Terabe et al. [1]. The neutral analyte will gain an apparent electrophoretic mobility when it is incorporated into the ionic micelle and will migrate at the same velocity as the micelle under electrophoretic conditions. Since the distribution equilibrium of the analyte between the micelle and the surrounding aqueous phase can be quickly established, the apparent electrophoretic mobility or migration velocity is directly related to the distribution coefficient. The more the analyte is incorporated, the higher the mobility. The analyte also migrates by electroosmotic flow (EOF) under capillary electrophoresis (CE) conditions, although EOF does not contribute to the separation at all. Thus, the migration time in MEKC is a function of electrophoretic velocity of the micelle, distribution ratio, and EOF velocity. A schematic principle of MEKC is shown in Figure 3.1, where EOF is stronger than the electrophoretic migration of the anionic micelle and hence the anionic micelle migrates toward cathode at a retarded velocity. The neutral analyte migrates at the velocity between EOF velocity and that of the micelle and the migration velocity depends on how much fraction of the analyte is incorporated into the micelle. It should be noted that there is optimum distribution of the analyte to the micelle for good separation, that is, totally incorporated analyte by the micelle or free from the micelle cannot be separated by MEKC. MEKC introduced the chromatographic principle into CE by adding ionic micelles into the electrophoretic solution. MEKC is a branch of CE and extended significantly the applicability of CE to a wide range of analytes, particularly small molecules. Since MEKC also belongs to a chromatographic technique, it is familiar for most chromatographers to design separation conditions.