Author(s): Shigeru Terabe, Koji Otsuka
Micellar electrokinetic chromatography (MEKC), which was first introduced in 1984 (1), has become one of major separation modes in capillary electrophoresis (CE), owing to its applicability to the separation of neutral compounds as well as charged ones (2,3). The separation principle of MEKC with an anionic micelle, such as sodium dodecyl sulfate (SDS), is schematically shown in Fig. 1. A separation capillary is filled with an SDS micellar solution. Under neutral or basic conditions, the entire solution migrates toward the cathode by the electroosmotic flow (EOF) when a high voltage is applied, while the micelle is forced toward the anode by electrophoresis. Normally the EOF is stronger than the electrophoretic migration of the SDS micelle, and hence, the micelle migrates toward the cathode at a slower velocity than the aqueous phase. When a neutral analyte is injected into the micellar solution at the anodic end, it will be distributed between the micelle and the aqueous phase. An analyte that is not incorporated into the micelle at all migrates at the same velocity as the EOF toward the cathode, whereas an analyte that is totally incorporated into the micelle migrates at the lowest velocity or the same velocity as the micelle toward the cathode. The more the analyte is incorporated into the micelle, the slower the analyte will migrate. As long as the analyte is electrically neutral, it migrates at a velocity between the two extremes or between the velocity of the EOF and that of the micelle. The analytes are detected in an increasing order of the distribution coefficients at the cathodic end.