Capillary zone electrophoresis of cationic and anionic drugs in methanol

Abstract

The actual mobilities and dissociation constants of acidic and basic pharmaceuticals were determined in methanol. Actual mobilities were derived from the dependence of the effective mobilities of the analytes on the pH of the methanolic background electrolyte solution (pH MeOH). The p K a values of the pharmaceuticals in methanol (p K a,MeOH) were calculated by non-linear curve fitting to the measured mobility values. It was found that the shift in p K a value (when compounds were transferred from water to methanol) increased with the acidity of the analyte. The average p K a shift for compounds exhibiting acidic properties in water was ca. 5.5 units, and the shift for basic compounds about 2 units. As was shown for a mixture of β-blockers, the calculated actual mobilities and p K a values can be utilised in the optimisation of pH conditions for separation. The practical value of the method was illustrated by the analysis of urine samples.