A stable isotope dilution assay for the antiparkinsonian drug benztropine in biological fluids.

A quantitative gas chromatographic-mass spectrometric (GC-MS) assay was developed for the determination of benztropine in human urine and plasma. The assay utilizes selected ion focusing to monitor in a GC effluent a specific fragment ion of benztropine generated by electron-impact ionization. Benztropine-d3 was the internal standard. The assay can measure 5 ng/ml of benztropine/ml with about 6% precision. The curve relating the amounts of benztropine added versus the amounts found over a large range of benztropine concentrations was a straight line with a nearly zero intercept and a slope of 0.98 +/- 0.02. The method was used for the analysis of benztropine in urine and plasma of patients on therapeutic dose of the drug.