Deuterium site-specific distribution

Deuterium site-specific distribution stores infonnation about the history of a benzaldehyde product. Products from the same source should have similar isotope distribution. Therefore, comparing a test sample s deuterium site-specific distribution with that of standard samples can identify the origin of the test sample. The precondition for a successful identification is that tlie... 

The first paper dealing with the NMR determination of non-random distribution of deuterium appeared in 1981 and the site-specific natural isotope fractionation studied by nuclear magnetic resonance (SNIF-NMR) terminology was proposed.36 Since this time several review articles on application of deuterium and 13C NMR natural abundance spectroscopy were published.37 1... 

Table 6.11 Site-specific deuterium distribution in benmldehyde by source. Adapted from [280] with kind permission, Copyright [1992] American Chemical Society. Values f[i] are the mole fractions of site-deuterated molecular species i. ADR (aromatic distribution ratio) = f ]ortho]/f ]meta + para]. The (D/H) are expressed in total parts per million [ppm] and are determined by standard combustion and IRMS...

Benzaldehyde obtained from bitter almond oil and from the retro-aldol reaction of ciimamic aldehyde from cinnamon originate from different biosynthetic pathways. Therefore, benzaldehyde from bitter almond oil can be easily differentiated from benzaldehyde ex-cassia by comparing tlieir site-specific deuterium distributions. 

While the deuterium spectra of benzaldehydes from different sources do not appear (upon visual inspection) to have significant differences from one another, other than some variation in intensity, the deuterium distribution of benzaldehyde does form clusters on a principal component analysis plot. Benzaldehyde products from the same source have similar deuterium distributions and are therefore close to each otlier on the plot (Figure 1). Thus, the origin of benzaldehyde can be differentiated based on site-specific deuterium distribution. Products outside of the clusters of knoivn samples are normally considered as originating from an unknown source or as a mixture of benzaldehyde from different known sources. 

One simple test is to measure the level of radioactivity from the sample. Synthetic vanillin is not radioactive. However, natural vanilla, like all natural products, is. This is, of course, because atmospheric carbon dioxide contains some radioactive 14C formed by exposure to cosmic radiation in the upper atmosphere. Plants then incorporate this into their photosynthetic pathway and produce metabolites, which exhibit a low level of radioactivity. Synthetic vanillin is prepared from coal tar, which is not radioactive since the 14C has long since decayed. However, unscrupulous dealers know this and can synthesise radiolabelled or hot vanillin and dose it into synthetic material so that the level of radioactivity matches that of a natural sample. Another method of checking for naturalness must therefore be found. When plant enzymes synthesise molecules, they, like all catalysts, are susceptible to isotope effects. The vanilla plant is no exception and examination of the distribution of hydrogen and carbon isotopes in the vanillin molecule reveals that the heavier deuterium and 13C isotopes accumulate at certain specific sites. A suitable NMR spectrometer can determine the isotopic distribution in a sample and the cost of using 2H, 13C and 14C labelled synthetic materials to replicate the NMR spectra and radioactivity of natural vanillin in a synthetic sample would not be financially attractive. Furthermore, the 2H and 13C labelling patterns in the vanilla bean are different from those of other natural shikimate sources and so the NMR technique can also distinguish between vanillin from vanilla and vanillin produced by... 

SNIF-NMR is a powerful method for the authentication analysis of benzaldehyde products. However, manufacturing processes may cause the deuterium to shift on the carbonyl site. In some cases, the current SNIF NMR method cannot differentiate abnormal distribution of deuterium caused by adulteration or by production processes that lead to false negative or false positive conclusions of a product. Deuterium on the aromatic sites of benzaldehyde is more stable and is not affected by the known manufacturing processes. The aromatic deuterium distribution is very specific and can be used to classify benzaldehyde products from fossil or different botanical origins. The proposed improvement needs to be further developed and validated. 

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