Discontinuous double mechanism for the retention of some cation-type oximes on a hydrophilic stationary phase in liquid chromatography

Abstract

Retention data (retention factor versusacetonitrile content in the mobile phase) on a zwitterionic hydrophilic stationary phase (ZIC-HILIC) of four cationic-type aldoximes (containing one or two pyridinium rings) showed atypical V-shape profiles. Such an unusual behavior is obviously different from reported U-shaped retention plots obtained for non-ionic compounds, where the ionic strength is constant only in an aqueous component. A double retention mechanism may explain such curves: a reversed phase in highly aqueous mobile phases (more than 60%), and a normal phase for mobile phases with a high concentration of acetonitrile (% acetonitrile > 40%). Polynomial and linear equations were used to describe the dependence of the retention factor on the acetonitrile content in the mobile phase. The experimental inflexion point for each analyte is confirmed through calculation of the content of the organic solvent in the mobile phase for which the two retention functions become equal. When ionic strength becomes constant in the mobile phase the reversed phase is dominant and the retention factor versusacetonitrile content in the mobile phase becomes linear over the entire domain.