Validation of a method for quantification of ketobemidone in human plasma with liquid chromatography–tandem mass spectrometry

A liquid chromatography tandem mass spectrometry (LC–MS–MS) method for determination of the analgesic aminophenol ketobemidone in human plasma is presented. Two preparation methods for plasma samples containing ketobemidone were compared, liquid–liquid extraction (LLE) and solid-phase extraction (SPE). Both methods showed good precision (n=10), 1.7% and 2.9%, respectively (0.04 μM) and 1.1% and 2.5%, respectively (0.14 μM). The accuracy was 98% and 103%, respectively (0.04 μM) and 105% and 99%, respectively (0.14 μM). Ketobemidone could be quantified at 0.43 nM, with a relative standard deviation of 17.5% (n=19) using LLE and 18.6% (n=10) using SPE. This level was an order of magnitude lower than earlier reported quantification limits. Quantitative data from plasma samples analyzed with LC–MS–MS were in good agreement with those obtained by gas chromatography with chemical ionization mass spectrometry (GC-CI/MS). This indicates that LC–MS–MS is a good alternative method to GC–MS as it is more sensitive and time-consuming derivatization can be avoided.