Micellar Electrokinetic Chromatography

Abstract

Micellar electrokinetic chromatography (MEKC) is a highly efficient capillary‐format separation technique that is performed with the same experimental setup, which is used with capillary electrophoresis (CE). While the separation principle of CE is based on the differences in the effective electrophoretic mobility of the solutes to be separated (only charged species can be separated), MEKC separation is also based on the differences in the distribution constants between two phases migrating at a different velocity because of electrokinetic effects. In general, the second phase is introduced into the separation electrolyte/background electrolyte (BGE) by adding an ionic surfactant (anionic or cationic) in a concentration above the critical micellar concentration (CMC). Under these conditions, dynamic association colloids (micelles) are formed. These aggregates have an electrophoretic mobility and are able to incorporate solutes or interact strongly with species dissolved in the surrounding medium. MEKC extends the applicability of CE to neutral solutes. In addition, MEKC has been proven to be superior to CE in the separation of very complex mixtures of solutes with similar electrophoretic mobility because of the various parameters that are available for the manipulation of the separation selectivity. MEKC can be regarded as a separation technique with a similar scope to that of reversed‐phase high‐performance liquid chromatography (RPHPLC) and other modes of high‐performance liquid chromatography HPLC competing with these modes regarding the efficiency of the separation system, separation speed, cost, and tolerance to matrix constituents, as in MEKC, no stationary phase is present, which can be fouled by irreversible sorption of matrix constituents of the sample. Owing to the small injection volume (few nanoliters) and the small detection volume, which is necessary to avoid intolerable instrumental band broadening, there is a strong need for online focusing methods and sensitive miniaturized detection techniques.