Simultaneous determination of selenomethionine enantiomers in biological fluids by stable isotope dilution gas chromatography–mass spectrometry

A method for the stereoselective determination of d- and l-enantiomers of selenomethionine in mouse plasma was developed using gas chromatography–mass spectrometry with selected-ion monitoring (GC–MS-SIM). dl-[2H3,82Se]selenomethionine was used as analytical internal standard to account for losses associated with the extraction, derivatization and chromatography. Selenomethionine enantiomers in mouse plasma 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. The intra- and inter-day precision for d- and l-selenomethionine spiked to mouse plasma gave good reproducibility with relative standard deviation of 3% and 3% for d-selenomethionine and 6% and 3% for l-selenomethionine, respectively. The estimated amounts were in good agreement with the actual amounts spiked, the intra- and inter-day relative error being 5% and 2% for d-selenomethionine and 2% and 1% for l-selenomethionine, respectively. The present method is sensitive enough to determine pharmacokinetics of selenomethionine enantiomers.