Determination of metoprolol enantiomers in human urine by GC-MS using (-)-α-methoxy-α-(trifluoromethyl)phenylacetyl chloride as a chiral derivatizing agent

A 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-methyl-N-(trimethylsilyl)trifluoroacetamide to form an O-silyl ether, followed by subsequent chiral derivatization with (-)-alpha-methoxy-alpha-(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 . 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 healthy volunteers dosed with racemic metoproiol tartrate.