Quantitative determination of oxprenolol and timolol in urine by capillary zone electrophoresis

A simple capillary zone electrophoretic method with UV detection has been developed for the quantitative determination of the β-adrenoreceptor antagonists (β-blockers) oxprenolol and timolol in human urine, preceded by a solid-phase extraction step. The electrophoretic separation was performed on a 78 cm×75 μm I.D. fused-silica capillary (effective capillary length: 70 cm). The electrolyte consisted of a Na2B4O7–H3BO3 (50 mM), pH 9. The introduction of the sample was made hydrostatically for 20 s and the running voltage 25 kV at the injector end of the capillary. Photometric detection was used at a wavelength of 229 nm for oxprenolol and 280 nm for timolol. Under these conditions oxprenolol migrated at 4.76±0.05 min and timolol at 4.97±0.05 min. The solid-phase extraction methods were optimised for each β-blocker and provided recoveries of 72.8% for timolol and 94.52% for oxprenolol. Good resolution from the endogenous compounds present in the urine matrix were achieved for both compounds. The method was applied to the determination of both β-blockers in pharmaceutical formulations and urine samples obtained from hypertensive patients after the ingestion of a therapeutic dose (in a 24-h time interval after the ingestion). The quantitative results were compared with results previously obtained at our laboratories by HPLC and were found to be in good agreement. Good reproducibility, linearity, accuracy and quantitation limits (in urine) of 0.19 μg/ml for timolol and 0.20 μg/ml for oxprenolol were obtained, allowing the method to be applied to pharmacokinetic studies of these compounds.