Mixed micelles of short chain alkyl surfactants and bile salts in electrokinetic chromatography: Enhanced separation of corticosteroids

The separation of a complex mixture of 17 corticosteroids was investigated by mixed micellar electrokinetic chromatography (MMEKC) employing various bile salts and/or alkylsulfonates. In this study, influence of individual surfactants and mixed micelles of hydrocarbon-bile salt surfactants on retention behavior, selectivity and the size of the elution window is investigated. Retention behavior of corticosteroids in SDS and bile salt micelles is examined using linear solvation energy relationships (LSER). In addition, the effects of type of bile salt surfactant on elution patterns were investigated. It was found that separation patterns are mostly influenced by the number of hydroxyl functional groups on the steroidal backbone of the bile salts, while the type of ionic head group has little, if any, effect on the steroids separation. Comparisons between mixed micellar techniques acid the inclusion of conventional modifiers to various single and binary surfactant systems were made. The addition of modifiers such as acetonitrile, urea and beta-cyclodextrin to SDS surfactant systems, as well as mixed bile salt systems of sodium taurocholate and sodium glycodeoxycholate, did not improve the separation of the steroids. On the other hand, the addition of the short-chain alkylsulfonate sodium butanesulfonate to the mixture of taurocholate and glycodeoxycholate greatly improved the separation of the 17 corticosteroids and provided a baseline separation of all solutes. The effects of carbon chain length and concentration of alkylsulfonate on capacity factor, selectivity, efficiency and the size of the elution window were investigated.