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The S-methyl metabolite of captopril was identified and determined in human plasma by positive chemical ionization selected-ion monitoring gas chromatography-mass spectrometry. After oral administration of 100 mg of captopril to healthy subjects, the maximum plasma level was 60-114 ng/mL. These data for the S-methyl metabolite of captopril were correlated to total and unchanged captopril levels. Captopril--identification and determination of the S-methyl metabolite in human plasma, gas chromatography-selected-ion monitoring mass spectrometry Gas chromatography-selected-ion monitoring mass spectrometry--determination of the S-methyl metabolite of captopril in human plasma after oral administration.

Captopril is liberated from covalently protein-bound disulfides and other disulfide metabolites in human plasma by reduction with tri-n-butyl-phosphine. The captopril is then treated with N-ethylmaleimide, purified on XAD-2 resin, eluted with ethyl acetate, and methylated prior to its determination by gas chromatography-mass spectrometry with selected-ion monitoring. The limit of detection is 20 ng/mL of plasma.