Site-specific hydrogen isotope measurements of vanillin by 2H-qNMR and GC-IRMS.

Abstract

Stable isotope analyses are powerful techniques to detect counterfeiting of food products, especially in vanillin, one of the world's most popular flavors used in food and pharmaceutical industries. Although a stable carbon isotopic ratio provides an invaluable new source of isotopic information, the analysis of the hydrogen site-specific (²H/¹H) isotopic distribution in vanillin gives valuable complementary isotopic information allowing a complete isotopic fingerprint of the vanillin molecule for authentication. However, there are only a limited number of studies that compare the validity of the results obtained by IRMS and NMR. Here, we present site-specific isotope analysis (also known as position-specific isotope analysis, PSIA) of hydrogen isotopic measurements of vanillin by two independent measurement methods, ²H-qNMR and GC-IRMS, and compare the results obtained by these two methods. ²H-qNMR allows isotopic measurements of all hydrogen atoms (except for OH) of the vanillin molecule while GC-IRMS measures only the hydrogen atoms of the methoxy group (H5). ²H/¹H values of the vanillin H5 from six vanillin demonstrate remarkable agreement between the two techniques with uncertainty well below 1%. Position-specific GC-IRMS on the methoxy group measurements provides three-fold smaller measurement uncertainties while requiring a considerably smaller sample size compared to ²H-qNMR. This quantitative hydrogen isotopic study extends our previous successful work comparing and validating the stable carbon isotopic of the vanillin methoxy group using ¹³C-qNMR and GC-IRMS. Compound-specific isotope analysis (CSIA) using GC-IRMS was also described in this work. Both qNMR and partial PSIA GC-IRMS can be considered as complementary analytical methods, and their combined use provides reliable results.