Comparison of Carbon Isotope Ratio Measurement of the Vanillin Methoxy Group by GC-IRMS and ¹³C-qNMR

Site-specific carbon isotope ratio measurements by quantitative C-13 NMR (C-13-qNMR), Orbitrap-MS, and GC-IRMS offer a new dimension to conventional bulk carbon isotope ratio measurements used in food provenance, forensics, and a number of other applications. While the site-specific measurements of carbon isotope ratios in vanillin by C-13-qNMR or Orbitrap-MS are powerful new tools in food analysis, there are a limited number of studies regarding the validity of these measurement results. Here we present carbon site-specific measurements of vanillin by GC-IRMS and C-13-qNMR for methoxy carbon. Carbon isotope delta (delta C-13) values obtained by these different measurement approaches demonstrate remarkable agreement; in five vanillin samples whose bulk delta C-13 values ranged from -31 parts per thousand to -26 parts per thousand, their delta C-13 values of the methoxy carbon ranged from -62.4 parts per thousand to -30.6 parts per thousand, yet the difference between the results of the two analytical approaches was within +/- 0.6 parts per thousand. While the GC-IRMS approach afforded up to 9-fold lower uncertainties and required 100-fold less sample compared to the C-13-qNMR, the C-13-qNMR is able to assign delta C-13 values to all carbon atoms in the molecule, not just the cleavable methoxy group.