Solid-phase extraction-high-performance liquid chromatography determination of verapamil and norverapamil enantiomers...

A simple, rapid, sensitive and selective method has been developed for the stereospecific determination of verapamil and its metabolite, norverapamil in urine. For sample preparation we utilized a membrane-based solid-phase extraction (SPE) disk consisting of a thin, particle-loaded membrane inserted in a plastic syringe-like barrel. The particles, which may be C8 or C18 bonded phase (C8 in this work), are embedded within a matrix of PTFE (Teflon) fibrils. Overall analyte recoveries were above 85%, even at low concentration of 3.0 ng/ml with reproducibilities (C.V. values) below 13.1%. This method of extraction has the advantage of speed and considerable reduction in solvent volumes compared to conventional SPE and solvent extraction. The separation of all the enantiomers was achieved using a single chiral stationary phase column, the cellulose-based reversed-phase, Chiralcel OD-R. Analyte concentrations of less than 3.0 ng/ml could be quantitated with C.V. values below 14%. Calibration curves were linear in the range 2.5-300 ng/ml. Intra-day and inter-day reproducibilities were 10.5-14.2% at 3 ng/ml, 4.8-9.3% at 138.5 ng/ml and 7.8-10.1% at 280 ng/ml level, respectively, for all the enantiomers.