Pipette tip solid-phase extraction and gas chromatography - mass spectrometry for the determination of methamphetamine and amphetamine in human whole blood

Methamphetamine and amphetamine were extracted from human whole blood samples using pipette tip solid-phase extraction (SPE) with MonoTip C₁₈ tips, on which C₁₈-bonded monolithic silica gel was fixed. Human whole blood (0.1 mL) containing methamphetamine and amphetamine, with N-methylbenzylamine as an internal standard, was mixed with 0.4 mL of distilled water and 50 μL of 5 M sodium hydroxide solution. After centrifugation, the supernatant was extracted to the C₁₈ phase of the tip (pipette tip volume, 200 μL) by 25 repeated aspirating/dispensing cycles using a manual micropipettor. Analytes retained in the C₁₈ phase were 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 and amphetamine spiked into whole blood were more than 87.6 and 81.7%, respectively. Regression equations for methamphetamine and amphetamine showed excellent linearity in the range of 0.5-100 ng/0.1 mL. The limits of detection for methamphetamine and amphetamine were 0.15 and 0.11 ng/0.1 mL, respectively. Intra- and interday coefficients of variation for both stimulants were not greater than 9.6 and 13.8%, respectively. The determination of methamphetamine and amphetamine in autopsy whole blood samples is presented, and was shown to validate the present methodology.