Method development for identification of ketobemidone metabolites in microdialysate samples by coupled-column capillary liquid chromatography–tandem mass spectrometry

Methodologies for identification of ketobemidone metabolites in microdialysate samples utilizing coupled-column capillary liquid chromatography–electrospray quadrupole time-of-flight tandem mass spectrometry are presented. Two different methods were developed to efficiently analyze the metabolites norketobemidone, ketobemidone N-oxide and hydroxyketobemidone, respectively. Both methods include on-line desalting and trapping of the analytes on micro-solid-phase extraction columns with different retention mechanisms. Norketobemidone and ketobemidone N-oxide were trapped on a C18 column and then eluted by back-flush followed by isocratic separation on a fluorinated reversed-phase type silica gel column (Fluofix). Retention equations are proposed for the chromatographic observations made on the Fluofix column. Hydroxyketobemidone was trapped on a phenylboronic acid column by complex formation at basic pH and then eluted at acidic pH directly into to the mass spectrometer. Oxidation of hydroxyketobemidone to its corresponding quinone was also observed. The methods were successfully used to analyze synthetic ketobemidone metabolites in dilute low-volume microdialysis samples.