Stability of thiopental and pentobarbital in human plasma determined with a new easy and specific gas chromatography-mass spectrometry assay

A gas chromatographic-mass spectrometric (GC-MS) assay for the determinatio n of thiopental and its main metabolite pentobarbital in human plasma is pr esented in this study. The sample preparation consists only in the addition of the internal standard barbital and an acidic extraction with ethyl acet ate, Analytical separation is accomplished on a RTX-1 15 m x 0.25 mm capill ary column with a film thickness of 0.5 mu m. The effluent is observed by a mass selective detector operating in the single ion monitoring mode. The l imits of detection are 5 ng/ml for pentobarbital and 10 ng/ml for thiopenta l, the intra-day variabilities are 2.2% and 4.0% and the inter-day variabil ities are 3.3% and 7.1% at concentrations of 5 mu g/ml, respectively. Apply ing this assay, the stability of thiopental and pentobarbital in human plas ma was tested at concentrations of 5 mu g/ml each. Thiopental is stable in human plasma at least over 41 days stored at -20 degrees C and 5 degrees C, respectively. A decay of about 2%/day is observed under storage at ambient temperature (19-20 degrees C). Pentobarbital is stable under all storage c onditions. Methanolic solutions of thiopental are stable for 83 days under storage at 5 degrees C. Aqueous solutions of thiopental-sodium are stable f or at least 23 days under storage at 5 degrees C or ambient temperature.