An improved combined gas chromatographic mass spectrometric assay for the estimation of norphenazone in urine.

An improved, combined gas chromatographic mass spectrometric assay for the estimation of norphenazone in urine is described. A structural analogue of norphenazone is used as internal standard and derivatization of both norphenazone and the analogue gives these compounds better gas chromatographic properties. These modifications increase the precision and accuracy of the assay.