Intramolecular Carbon Isotope Distribution of Acetic Acid in Vinegar

Compound-specific carbon isotope analysis of acetic acid is useful for origin discrimination and quality control of vinegar. Intramolecular carbon isotope distributions, which are each carbon isotope ratios of the methyl and carboxyl carbons in the acetic acid molecule, may be required to obtain more detailed information to discriminate such origin. In this study, improved gas chromatography pyrolysis gas chromatography combustion isotope ratio mass spectrometry (GC-Py-GC-C-IRMS) combined with headspace solid-phase microextraction (HS-SPME) was used to measure the intramolecular carbon isotope distributions of acetic acid in 14 Japanese vinegars. The results demonstrated that the methyl carbons of acetic acid molecules in vinegars produced from plants were mostly isotopically depleted in C-13 relative to the carboxyl carbon. Moreover, isotopic differences (delta C-13(carboxyl) - delta C-13(methyl)) had a wide range from -0.3 to 18.2 parts per thousand, and these values differed among botanical origins, C3, C4, and CAM plants.