Migration behaviour of monovalent weak acids in micellar electrokinetic chromatography; mobility model versus retention model

Migration of charged compounds in micellar electrokinetic chromatography (MEKC) is based on micellar solubilization as well as electrophoretic migration. Consequently, the migration behaviour of these compounds can be described with a retention model or a mobility model. Both models are discussed and evaluated for the migration behaviour of monovalent weak acids in a sodium dodecyl sulphate micellar system. We have shown that the calculation of retention factors may be troublesome for hydrophobic compounds, due to interaction with surfactant molecules in the aqueous phase. Therefore, the mobility model is preferable. The simultaneous determination of dissociation constants by spectroscopic and electrophoretic methods is discussed and apparent dissociation constants in micellar media are determined, showing that pKa shifts may occur in MEKC analyses, due to micellar solubilization. Furthermore, a sample stacking procedure is described, based on differences in overall effective mobilities between aqueous and micellar electrolyte systems