Determination of the enantiomers of ketamine and norketamine in human plasma by enantioselective liquid chromatography–mass spectrometry

A sensitive enantioselective liquid chromatographic assay with mass spectrometric detection has been developed and validated for the simultaneous determination of plasma concentrations of (R)- and (S)-ketamine, and (R)- and (S)-norketamine. The compounds were extracted from human plasma using solid-phase extraction and then directly injected into the LC–MS system for detection and quantification. Enantioselective separations were achieved on a liquid chromatographic chiral stationary phase based upon immobilized α1-acid glycoprotein (the Chiral AGP column). The separations were achieved using a mobile phase composed of 2-propanol–ammonium acetate buffer (10 mM, pH 7.6) (6:94, v/v), a flow-rate of 0.5 ml/min and a temperature of 25 °C. Under these conditions, the analysis time was 20 min. Detection of the ketamine, norketamine and bromoketamine (internal standard) enantiomers was achieved using selected ion monitoring at m/z 238.1, 224.1 and 284.0, respectively. Extracted calibration curves were linear from 1 to 125 ng/ml per enantiomer for each analyte with correlation coefficients better than 0.9993 and intra- and inter-day RSDs of less than 8.0%. The method was applied to samples from a clinical study of ketamine in pain management.