Stable isotope ratio analysis of amino acids: the use of N(O,S)-ethoxycarbonyl trifluoroethyl ester derivatives and gas chromatography/mass spectrometry.

N(O,S)-Ethoxycarbonyl trifluoroethyl amino acid esters are formed by the reaction of amino acids with ethylchloroformate plus trifluoroethanol plus pyridine. The use of these derivatives for a rapid and sensitive determination of specific enrichment of stable isotopically labeled tracer amino acids in blood plasma and protein hydrolysates, by using gas chromatography/electron impact mass spectrometry, is discussed.