Mass fragmentographic determination of methadyl acetate in urine using stable isotope labeled analog as internal standard.

A quantitative GLC-mass spectrometric assay was developed for the determination of methadyl acetate in urine. The assay utilized selective ion focusing to monitor, in a GLC effluent, the M--15 ion generated by electron-impact ionization of methadyl acetate. Methadyl acetate-d4 was used as an internal standard. The assay can measure 10 ng of drug/ml with about 6% precision. The curve relating the amounts of drug added to control urine versus the amounts experimentally found over a large concentration range is a straight line with a slope of 0.98 +/- 0.02 and a nearly zero intercept. Assay specificity was confirmed by complete identity of the mass spectrum of methadyl acetate in the biological extract with that of the authentic material. The method was used for the urinary analysis of methadyl acetate in a rabbit given a single intravenous dose. The animal excreted less than 1% of the intact drug with a half-life of approximately 15 hr. Consequently, the long-acting characteristic of methadyl acetate must be attributed to its metabolism into active metabolites.