Simultaneous determination of methamphetamine and amphetamine in human urine using pipette tip solid-phase extraction and gas chromatography-mass spectrometry

Methamphetamine and amphetamine were extracted from human urine samples using pipette tip solid-phase extraction (SPE) with MonoTip C18 tips (pipette tip volume, 200 microl), in which C18-bonded monolithic silica gel was fixed. A sample of human urine (0.5 ml) containing methamphetamine, amphetamine, and N-methylbenzylamine as internal standard (IS), was mixed with 25 microl of 1M sodium hydroxide solution. The mixture was extracted into the C18 phase of the SPE tip by 25 repeated aspirating/dispensing cycles using a manual micropipettor. Analytes retained in the C18 phase were then eluted with methanol by five repeated aspirating/dispensing cycles. After derivatization with trifluoroacetic anhydride, analytes were measured by gas chromatography/mass spectrometry with selected ion monitoring in the positive-ion electron impact mode. Recoveries of methamphetamine, amphetamine, and IS spiked into urine were more than 82.9, 82.2, and 78.2%, respectively. Regression equations for methamphetamine and amphetamine showed excellent linearity in the range of 0.25-200 ng/0.5 ml. Limit of detection was 0.04 ng/0.5 ml for methamphetamine and 0.05 ng/0.5 ml for amphetamine. Intra- and inter-day coefficients of variations for both stimulants were not greater than 10.8%. The data obtained from actual determination of methamphetamine and amphetamine in autopsy urine samples are also presented for validation of the method.