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 micro M) and 1.1% and 2.5%, respectively (0.14 micro M). The accuracy was 98% and 103%, respectively (0.04 micro M) and 105% and 99%, respectively (0.14 micro M). 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.