PRECISE DELTA-C-13-DETERMINATION IN THE RANGE OF NATURAL-ABUNDANCE ON AMINO-ACIDS FROM PROTEIN HYDROLYSATES BY GAS-CHROMATOGRAPHY - ISOTOPE RATIO MASS-SPECTROMETRY

Routine on-line C-13-analysis by coupled gas chromatography - combustion - isotope ratio - mass spectrometry of individual amino acids out of mixtures in the natural abundance range demands their strictly standardized derivatization. For the N-acetyl-propylesters of amino acids from protein hydrolysates and blood serum a mean precision of 0.5 parts per thousand (1 SD) for derivatization, separation and measurement could be attained. From deltaC-13-values of derivatives deltaC-13-values of amino acids could be calculated by a carbon balance equation, implying an isotope effect on the C-1 of acetate of about 1.04 for nine amino acids. The global deltaC-13-value of casein calculated from the individual amino acids differed by less than 1.5 parts per thousand from the directly determined value. The deltaC-13-values of amino acids from plant protein hydrolysates analyzed by the on-line method were in agreement with results obtained by classical procedures. The method permits the delta-value determination of up to nine amino acids from 200 mug of a mixture in less than one hour. By this means the detection of C-13-labeled amino acids in nmole amounts diluted by the 5000-fold amount of carrier becomes possible.