Characterisation of retention in micellar high-performance liquid chromatography and in micellar electrokinetic chromatography using lipophilicity and polarity indices

Abstract

The retention of various barbiturates, phenylurea and triazine herbicides was measured in high-performance liquid chromatography (HPLC) on a C 8 column in aqueous micellar and non-micellar mobile phases containing anionic and cationic surfactants. The retention was characterised using the lipophilicity and polarity indices suggested earlier. The migration of the compounds tested was measured in the same mobile phases using micellar electrokinetic chromatography (MEKC) in fused-silica capillaries. The effects of the surfactants on the separation of uncharged compounds were less in the non-micellar than in the micellar region, but were still significant. The lipophilicity and polarity indices can be applied here, too, to characterise and predict the retention behaviour as a function of the concentration of the organic modifier and of the surfactant in the mobile phase or in the working electrolyte. Unlike the situation in micellar HPLC, the effects of the concentration of the surfactant on the lipophilicity selectivity in an homologous series are negligible in MEKC, where the polarity parameters may be used to characterise the contribution of the electrophoretic migration to the retention. The behaviour observed may possibly be explained by the lipophilic part of the surfactant molecules and of the organic solvent in aqueous–organic mobile phases having similar effects on the separation. The present approach offers the possibility of determining the critical micellar concentration from the migration times obtained using MEKC.