Gas chromatography-isotope dilution mass spectrometry preceded by liquid–liquid extraction and chemical derivatization for the determination of ketamine and norketamine in urine

An analytical scheme using gas chromatography (GC)-isotope dilution mass spectrometry (MS) assisted by precedent liquid–liquid extraction (LLE) and chemical derivatization (ChD) is described for the simultaneous determination of ketamine (KT) and its major metabolite, norketamine (NK), in urine. The simultaneous ChD of the two analytes, one primary amine and one secondary amine, with pentafluorobenzoyl chloride (PFBC) has not only enhanced their instrumental responses and mass-spectrum uniqueness but also afforded more proper yet easier selection of qualifier and quantifier ions and hence achieved more evidential identification and quantitation. Thus, the regression calibration curves for KT and NK in urine are linear within 100–5000 ng/ml, with correlation coefficients typically exceeding 0.99 and NK curves exclusively showing larger slopes than KT curves. The method limits of detection (LODs) determined by two definitions for KT and NK range from 3 to 75 ng/ml, and limits of quantitation (LOQs) from 9 to 100 ng/ml. The mean recoveries (N=3) calculated for the LLE/ChD of KT and NK from 50 and 100 μl, respectively, of a 100 μg/ml urinary spike vary from 71.0 to 97.8%, with NK consistently giving better recoveries than KT. The precisions (N=3) calculated for the total analyses of four real-case samples are typically below 12.3%. GC–MS operated in the positive ion chemical ionization (PCI) mode can offer both qualitative and quantitative information complementary to those given by the EI mode. The proposed scheme is simple, effective, reliable, and robust. It may serve as a confirmatory protocol for forensic urine drug testing.