STABLE NITROGEN ISOTOPE ANALYSIS OF AMINO-ACID ENANTIOMERS BY GAS-CHROMATOGRAPHY COMBUSTION ISOTOPE RATIO MASS-SPECTROMETRY/

The analysis of the stable nitrogen isotope compositions of individual amino acid stereoisomers through the use of gas chromatography/combus tion/isotope ratio mass spectrometry (GC/C/IRMS) is presented. Nitroge n 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 iso tope analysis. Following hydrolysis and derivatization, single-compone nt isotope analysis is accomplished on nanomole quantities of each of the stereoisomers of an amino acid, utilizing the effluent stream of g as chromatographic separation. Nitrogen isotope fractionation is minim al during acylation of the amino acid, with no additional nitrogen bei ng added stoichiometrically to the derivative. Thus, the isotopic comp osition of the nitrogen in the derivative is that of the original comp ound. Replicate stable nitrogen isotope analyses of 11 amino acids, an d their trifluoroacetyl (TFA)/isopropyl (PP) ester derivatives, determ ined by both conventional isotope ratio mass spectrometry (IRMS) and G C/C/IRMS, indicate that the GC procedure is highly reproducible (stand ard deviations typically 0.3-0.4 parts per thousand) and that isotopic differences between the amino acid and its TFA/IP derivative are, in general, less than 0.5 parts per thousand.