Chiral analysis of dextromethorphan and levomethorphan in human hair by liquid chromatography-tandem mass spectrometry.

Abstract

Methorphan exists in two enantiomeric forms including dextromethorphan and levomethorphan. Dextromethorphan is an over-the-counter antitussive drug, whereas levomethorphan is strictly controlled as a narcotic drug. Chiral analysis of methorphan could, therefore, assist clinicians and forensic experts in differentiating between illicit and therapeutic use and in tracing the source of the drug. A method for enantiomeric separation and quantification of levomethorphan and dextromethorphan in human hair was developed and validated using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Hair was extracted in hydrochloric acid/methanol (1:20, v/v). The supernatant were separated using a Supelco Astec Chirobiotic™ V2 column (250 × 2.1mm, i.d., 5μm particle size) and analyzed on a triple quadrupole linear ion trap mass spectrometer in multiple reaction monitoring mode. The limits of detection for dextromethorphan and levomethorphan were 2 and 1pg/mg, respectively; the lower limit of quantification was 2pg/mg for both drugs. Good linearity (r > 0.995) was observed for both analytes over the linear range. Precision values were below 10% for both analytes; accuracy values ranged from 87.5 to 101%. The extraction recoveries were 78.3-98.4%, and matrix effects were 70.5-88.6%. This method was applied to human hair samples from 120 people suspected of methorphan use to further distinguish the drug chirality. Dextromethorphan was detected in all 120 samples at a concentration range of 2.7-19,100pg/mg, whereas levomethorphan was not detected in any sample. A sensitive quantitative method was established for the enantiomeric separation of dextromethorphan and levomethorphan in hair. This is the first study to achieve chiral analysis of methorphan in human hair.