Determination of d- and l-enantiomers of methionine and [2H3]methionine in plasma by gas chromatography–mass spectrometry

A method for the stereoselective determination of d- and l-enantiomers of both methionine and [2H3]methionine in rat plasma was developed using gas chromatography–mass spectrometry with selected-ion monitoring (GC–MS-SIM). dl-[2H7]Methionine was used as analytical internal standard to account for losses associated with the extraction, derivatization and chromatography. The amino acids were purified by cation-exchange chromatography using BondElut SCX cartridge and derivatized with HCl in methanol to form methyl ester followed by subsequent N-acylation with optically active (+)-α-methoxy-α-trifluoromethylphenylacetyl chloride to form diastereomeric amide. Quantification was performed by SIM of the molecular-related ions of the diastereomers on the chemical ionization mode. Endogenous l-methionine concentrations in 50 μl of rat plasma were measured with relative intra- and inter-day precision of 4.0 and 6.3%, respectively. The intra- and inter-day reproducibility in the amounts of d- and l-[2H3]methionine determined were in good agreement with actual amount added.