Capillary electrophoresis of diuretics and probenecid in methanol

Actual mobilities and dissociation constants of six diuretics (benzthiazide, bumetanide, ethacrynic acid, furosemide, hydrochlorothiazide and chlorothiazide) and probenecid were investigated in methanol by capillary zone electrophoresis (CZE). The actual mobilities were derived from the dependence of the effective mobilities of the analytes on the pH* of the methanolic background electrolyte (BGE) solution. The measurement of effective mobilities was carried out mainly by a pressure-mediated capillary electrophoresis (CE) method. For comparison, parallel measurements were carried for two of the analytes by the conventional capillary electrophoresis approach, without pressure. The pKa* values in methanol were calculated by non-linear curve fitting to the measured mobility values. The difference between the pKa values in water and methanol was about 5 units for the loop diuretics (furosemide, bumetanide, ethacrynic acid) and probenecid and around 3–4 units for the thiazide diuretics. Knowledge of the ionisation behaviour of compounds in methanol paves the way for the wider use of methanol as background electrolyte solvent in capillary electrophoresis. Moreover, as is demonstrated in the current study, the calculation of actual mobilities and pKa values facilitates the optimisation of pH conditions for the separation, thereby applications can be expanded, and the combination of capillary electrophoresis with electrospray ionisation mass spectrometry becomes easier.