Determination of thiol drugs in pharmaceutical formulations as their S-pyridinium derivatives by high-performance liquid chromatography with ultraviolet detection

Acetylcysteine and captopril can be determined in pharmaceutical formulations after precolumn derivatization of the thiol with 1-benzyl-2-chloropyridinium bromide (BCPB) by reversed-phase ion-pair HPLC separation and UV detection. The thiol group of the drugs reacts with BCPB in 0.1 mol/L phosphate buffer (pH 8) to form S-pyridinium derivatives showing an absorption maximum at 314 nm. The S-pyridinium derivative was separated isocratically on an Asahipak ODP-50 column at 45°C with 0.2 mol/L citrate buffer containing 10 mmol/L of sodium octanesulfonate (pH 2.5) and acetone (4:1, v/v). Calibration curves for both analytes were linear over the range of 2–20 μg/mL with variation coefficients of 3.12–1.28% for acetylcysteine and 9.22–1.51% for captopril.