Gas chromatography-mass spectrometry technique (GC-MS) is mainly recognized as a tool of first choice when volatile compounds are determined. Here, we provide the credible evidence that its application in analysis can be extended to non-volatile sulfur-containing compounds, to which methionine (Met), homocysteine (Hcy), homocysteine thiolactone (HTL), and cysteine (Cys) belong. To prove this point, the first method, based on GC-MS, for the identification and quantification of Met-related compounds in human saliva, has been elaborated. The assay involves simultaneous disulfides reduction with tris(2-carboxyethyl)phosphine (TCEP) and acetonitrile (MeCN) deproteinization, followed by preconcentration by drying under vacuum and treatment of the residue with a derivatizing mixture containing anhydrous pyridine, N-trimethylsilyl-N-methyl trifluoroacetamide (MSTFA), and trimethylchlorosilane (TMCS). The validity of the method was demonstrated based upon US FDA recommendations. The assay linearity was observed over the range of 0.5-20 µmol L-1 for Met, Hcy, Cys, and 1-20 µmol L-1 for HTL in saliva. The limit of quantification (LOQ) equals 0.1 µmol L-1 for Met, Hcy, Cys, while its value for HTL was 0.05 µmol L-1. The method was successfully applied to saliva samples donated by apparently healthy volunteers (n = 10).
Keywords: N-trimethylsilyl-N-methyl trifluoroacetamide; amino acid; aminothiol; cysteine; gas chromatography–mass spectrometry; homocysteine; homocysteine thiolactone; human saliva; methionine; sulfur amino acid.