Capillary zone electrophoresis of basic drugs in non-aqueous acetonitrile with buffers based on a conventional pH scale

The pK(a)(*) valves of 10 nitrogen-containing basic drugs in non-aqueous acetonitrile were determined from the pH* dependence of their electrophoretic mobilities. The pH* scale in the organic solvent was established using background electrolytes with known conventional pK(a)(*) values, making further calibration with reference pH electrodes unnecessary. In acetonitrile the pK(a)(*) valves of analytes (or their conjugated cation acids, BH+, respectively) were 5.2 - 8.9 pK units > those in water. The observed change in pK(a)(*) values of cationic analytes was, however, much less than the known respective change for neutral acids type HA. From the pK(a)(*) values and the actual mobilities, it is possible to predict pH* conditions to enable separation of analytes, and this was demonstrated for two pairs of common drugs.