Micellar Electrokinetic Capillary Chromatography

Neutral compounds present a problem in CE since they have no differential mobility because of their lack of charge and are carried without separation by the EOF. One solution is to operate with surfactants, which form micelles above a certain concentration, and then differentiate between the compounds on the basis of both electrophoretic migration and micellar partitioning. This technique has been so successful that it is now one of the commonest procedures used in CE. Initially the emphasis was on achiral compounds, but it was quickly extended to chiral compounds by the introduction of either chiral surfactants or chiral additives, such as CDs. Surfactants are long chain molecules, characterized by a hydrophobic tail pointing inwards and a hydrophilic head pointing outwards into the aqueous buffer. Micelles are am-phiphilic aggregates of surfactant molecules which form above a surfactant concentration, known as the critical micelle concentration (CMC) (Table 1). There are four classes of surfactants - anionic,

Table 1 The critical micelle concentrations of chiral surfactants and achiral surfactants

Surfactant Critical micelle concentration (mmol L"1)

Chiral

Sodium cholate Sodium taurocholate Sodium deoxycholate Sodium taurodeoxycholate Sodium A/-dodecanoyl-L-valinate

Achiral

Sodium dodecyl sulfate (SDS) Sodium decane sulfate Tetradecyltrimethylammonium bromide (TTAB)

Cetyltrimethylammonium bromide (CTAB)

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cationic, zwitterionic and nonionic - which can be both synthetic and naturally occurring. The synthetic group includes the anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium bromide (CTAB).

A slightly different process, microemulsion elec-trokinetic chromatography, has recently been developed. In this process a combination of oil-water-surfactant and co-surfactant such as an alkyl chain alcohol is used as a pseudo-stationary phase.

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