MIGRATION BEHAVIOR OF MONOVALENT WEAK ACIDS IN MICELLAR ELECTROKINETIC CHROMATOGRAPHY MOBILITY MODEL VERSUS RETENTION MODEL

Migration of charged compounds in micellar electrokinetic chromatograp hy (MEKC) is based on micellar solubilization as well as electrophoret ic migration. Consequently, the migration behaviour of these compounds can be described with a retention model or a mobility model. Both mod els are discussed and evaluated for the migration behaviour of monoval ent weak acids in a sodium dodecyl sulphate micellar system. We have s hown 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 s imultaneous determination of dissociation constants by spectroscopic a nd electrophoretic methods is discussed and apparent dissociation cons tants in micellar media are determined, showing that pK(a) shifts may occur in MEKC analyses, due to micellar solubilization. Furthermore, a sample stacking procedure is described, based on differences in overa ll effective mobilities between aqueous and micellar electrolyte syste ms.