Precise δ13C-determination in the Range of Natural Abundance on Amino Acids from Protein Hydrolysates by Gas Chromatography - Isotope Ratio Mass Spectrometry

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Abstract
Routine on-line 13C-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% ° (1 SD) for derivatization, separation and measurement could be attained. From δ13C-values of derivatives δ13C-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 δ13C-value of casein calculated from the individual amino acids differed by less than 1.5%° from the directly determined value. The δ13C-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 δ-value determination of up to nine amino acids from 200 μg of a mixture in less than one hour. By this means the detection of 13C-labeled amino acids in nmole amounts diluted by the 5000-fold amount of carrier becomes possible.

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