Key WordsGas chromatography -mass spectrometry Chiral separation Derivatization Metoprolol Urine samples S u m m a r yA method for the assay of R-(+)-and S-(-)-metoprolol in human urine has been developed using gas chromatography-mass spectrometry. The method involved purification by liquid-liquid extraction and derivatization with N-methyI-N-(trimethylsilyl)trifluoroacetamide to form an O-silyl ether, followed by subsequent chiral derivatization with (-)-~-methoxy-~-(trifluoromethyl)phenylacetyl chloride to form diastereomeric amide. The reaction was rapid and the diastereomeric derivatives were well resolved. Quantitation was performed by selected-ion monitoring of fragment ions of the diastereomers in electron impact ionization mode. No racemization was found during the reaction. The detection limit was 0.5 ng 9 mL 1. The intra-day variation ranged between 0.38 and 7.86% in relation to the measured concentration and inter-day variation was 2.26-8.06%. The method has been applied to the determination of R-(+)-and S-(-)-metoprolol in human urine from healthyvolunteers dosed with racemic metoproIol tartrate.Methods involving gas chromatography [5], high performance liquid chromatography [4,6,7,8,9] and capillary electrophoresis [10] have been developed for the quantitation of metoprolol enantiomers in biological samples. Shin [11] has described the stereospecific derivatization of amphetamine, phenol alkylamine and hydroxyamines and quantitation of the enantiomers by capillary GC-MS.This paper describes the O-silylation of the hydroxyl group of metoprolol using N-methyl-N-(trimethylsilyl) trifluoroacetamide(MSTFA) as a silylating reagent and the N-acylation of the amino group using ( )-e~-methoxy-e~-(trifluoromethyl)-phenylacetyl chloride(()-MTPA-C1) as a chiral derivatizing agent. This is followed by enantioselective determination of the metoprolol derivative in human urine by gas chromatography mass spectrometry with selected-ion monitoring.

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