Gas-chromatographic determination of verapamil and norverapamil, with a nitrogen-selective detector

We present a procedure for the determination of verapamil and its metabolite, norverapamil, in serum. The drugs are extracted under basic conditions into n-heptane/2-butanol (96/4 by vol) and then extracted again into 1 mol/L H2SO4. The acidic solution is made basic with sodium hydroxide, re-extracted with diethyl ether, and the extract evaporated. The residue is redissolved in ethanol and analyzed by gas chromatography with a nitrogen-selective detector. By use of two internal standards, prazepam and D-517 (a verapamil analog), concentration and instrument response are linearly related from 50 micrograms/L to 5 mg/L. Within-run precision (CV) was 3 and 5% for both verapamil and norverapamil at concentrations of 100 and 250 micrograms/L; between-run precision was 11 and 9% at those respective concentrations. Interference studies indicate that most commonly prescribed basic drugs will not interfere with this procedure.