Accurate and comparable site-specific stable carbon isotope ratio measurement of vanillin methoxy group by IRMS and 13C-qNMR

Site specific isotope ratio measurements are an emerging field of study which could provide a new dimensionality to conventional bulk stable isotope ratio measurements used in food provenance, forensics and a number of other applications. Most often, this site specific stable isotope ratio analysis is done by NMR, which is also possible for each individual carbon atom in a molecule (13C-qNMR). However, for site-specific carbon isotopes, there is little data to confirm the accuracy of NMR measurements using other independent analytical methods.

Here we present a study of d¹³C values of the methoxy group of vanillin standards using ¹³C-qNMR and GC-IRMS. For the IRMS analysis site-specific carbon isotope analysis is accessible by converting the methoxy group into gaseous iodomethane using hydroiodic acid.

Values obtained by these different measurement principles demonstrated remarkable agreement. In the five studied vanillin material bulk carbon isotope delta values ranged from -31.30 to -25.85 ‰ whilst the methoxy (C₈) carbon delta values ranged from -62.47 to -30.59 ‰. The difference observed between the two analytical approaches were between 0.14 to 0.71 ‰ well within the analytical uncertainty. Overall, the GC-IRMS approach offers 3-5-fold better uncertainties and requires 100-fold less sample material compared to qNMR. Conversely the ¹³C-qNMR approach was able to assign values to all carbons in the molecule not just the cleavable methoxy group.