Stable isotope labelling and quantitative determination of enalaprilate in human plasma by GC-NICI-MS

A method for the quantitative determination of enalaprilate, the active metabolite of the angiotensin‐converting enzyme inhibitor enalapril, is presented. The asay is based on gas chromatography–negative ion chemical ionization–mass spectrometry. The preparation of 18O4‐labelled enalaprilate for use as an internal standard is described. The method involves solid phase extraction and derivation to the pentafluorobenzyl diester–trifluoroacetamide derivatives. The detection limit was found to be 0.5 pg, and a lower limit of quantitation was reached down to 40 pg ml−1 plasma. The method was applied to pharmacokinetics of the metabolite in humans.