Analysis of 11-nor-9-carbonyl-∆⁹-tetrahydrocannabinol in human blood and bile by liquid chromatography–tandem mass spectrometry after derivatization with ethyl chloroformate and amines

∆⁹-Tetrahydrocannabinol (THC) is a major psychoactive ingredient in cannabis and is metabolized mainly to 11-nor-9-carbonyl-∆⁹-tetrahydrocannabinol (THC-COOH). We have derivatized THC-COOH by the stepwise addition of ethyl chloroformate and 2-methoxyethylamine at room temperature, which converts the phenolic hydroxyl group and the carboxylic acid to ethyl carbonate and 2-methoxyethylamide, respectively. Based on the peak area, this derivatization increased the sensitivity of THC-COOH detection by approximately 100-fold in liquid chromatography–tandem mass spectrometry with electrospray ionization (ESI). In contrast, the sensitivity of THC detection decreased. This shows that the increase in the sensitivity of the THC-COOH detection can be attributed to the presence of the amide group, which ionizes readily during ESI. Quantitative analysis of THC-COOH in blood and bile was performed with this derivatization. Calibration curves were created in the range of 1–50 ng/ml for blood and 10–400 ng/ml for bile. For blood, the limit of detection was 0.25 ng/ml and the limit of quantification was 1 ng/ml. For bile, these limits were 4 and 10 ng/ml, respectively. This method was subsequently used in a medico-legal autopsy case.