The simultaneous determination of pethidine and norpethidine in biofluids by nitrogen selective gas chromatography.

A rapid sensitive and selective gas chromatographic method has been developed for the simultaneous determination of pethidine and its major basic metabolite, norpethidine, using a nitrogen selective detector. The procedure involved a preliminary ethereal extraction of the drug, its metabolite and internal marker (lignocaine) from the alkalinised biological fluids (plasma or urine). The extract, after concentration, was analysed by a GC system (3 per cent OV 17 on Gas Chrom Q, 80-100 mesh) linked to a nitrogen selective detector. The calibration graphs (relating peak height ratios of the drug to internal marker and concentration) of pethidine and norepethidine were linear and reproducible over the ranges of 5 ng/ml to 100 ng/ml for plasma samples and 50 ng/ml to 1000 ng/ml for urine samples. The recovery of the drug and metabolite from plasma samples at 5 ng/ml and 100 ng/ml is 100 per cent and 84.9 per cent respectively for pethidine, and 100 per cent and 90.9 per cent respectively for norpethidine. Similar recovery from urine samples of pethidine and norpethidine is also achieved at 50 ng/ml and 1000 ng/ml levels. The reproducibility of the assay procedure for pethidine and norpethidine is 100 +/- 0.02 per cent and 100 +/- 0.04 per cent at 100 ng/ml level, and 100 +/- 0.15 per cent and 100 +/- 0.12 per cent at 5 ng/ml level.