Stable Nitrogen Isotope Analysis of Amino Acid Enantiomers by Gas Chromatography/Combustion/Isotope Ratio Mass Spectrometry

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Abstract
The analysis of the stable nitrogen isotope compositions of individual amino acid stereoisomers through the use of gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) is presented. Nitrogen isotopic compositions of single amino acids or of their enantiomers is possible without the labor-intensive and time-consuming preparative-scale chromatographic procedures required for conventional stable isotope analysis. Following hydrolysis and derivatization, single-component isotope analysis is accomplished on nanomole quantities of each of the stereoisomers of an amino acid, utilizing the effluent stream of gas chromatographic separation. Nitrogen isotope fractionation is minimal during acylation of the amino acid, with no additional nitrogen being added stoichiometrically to the derivative. Thus, the isotopic composition of the nitrogen in the derivative is that of the original compound. Replicate stable nitrogen isotope analyses of 11 amino acids, and their trifluoroacetyl (TFA)/isopropyl (IP) ester derivatives, determined by both conventional isotope ratio mass spectrometry (IRMS) and GC/C/IRMS, indicate that the GC procedure is highly reproducible (standard deviations typically 0.3−0.4‰) and that isotopic differences between the amino acid and its TFA/IP derivative are, in general, less than 0.5‰.