Eve J. Tenailleau

The use of ¹³C isotopic distribution as an efficient means to determine the origin of vanillin has been substantiated. Using quantitative ¹³C NMR, the ¹³C/¹²C ratios at all eight carbon positions can be exploited. On a set of 21 samples of vanillin from five different origins, complete discrimination can be achieved. It is shown that, for many purposes, a rapid analysis in which only five sites are used is sufficient. However, improved discrimination using all eight sites is preferable to differentiate between different methods of production from natural ferulic acid or between natural and lignin-derived vanillin on the basis of the ¹³C/¹²C ratios characteristic of different origins. The C1 and C8 positions are demonstrated to be the most significant sites for discrimination using principle component analysis. However, aromatic carbon positions make an essential contribution, notably in differentiating between natural and lignin-derived vanillin. Keywords: Nuclear magnetic resonance spectroscopy; carbon-13; quantitative; vanillin; ferulic acid; authentication

Quantitative ¹³C NMR conditions have been established that permit the precise determination of site-specific ¹³C/¹²C ratios at low or natural abundance. Spectral acquisition parameters have been optimized in order to obtain minimum intensity distortions over the spectral width and in relation to the major sources of inaccuracy: the relaxation times, the decoupling pulse and power, and nuclear Overhauser effects. A major reduction in experimental time resulting from a study of the relaxation times and variance analysis has been achieved. The influence of ¹H decoupling conditions on peak areas was shown to be critical in that different relative peak areas are obtained according to the decoupling power. The efficiency with which the quantitative ¹³C NMR method can determine site-specific ¹³C/¹²C ratios in natural products has been tested for 12 independent samples of vanillin from different sources. Discriminatory analysis performed in the space defined by the site-specific carbon isotope ratios allows natural vanillin and that from different synthetic origins to be unambiguously distinguished.