Isotopic analysis by nuclear magnetic resonance

The SNIF-NMR analytical method was developed to detect the (over) sugaring of wine and enrichment of grape musts, and is mainly used to check the authenticity of foodstuffs (such as wines, spirits, fruit juice, honey, sugar and vinegar) and to control the naturality of some aromatic molecules (such as vanillin, benzaldehyde, raspberry ketone and anethole).

It is also an official method adopted by the Association Of Analytical Chemists (AOAC) for analysis of fruit juices, maple syrup, vanillin, and by the European Committee for Standardization (CEN) for vinegar.

Control measures such as determining the alcoholic strength of the intermediate products of the analysis (fermented juice or distillate) are performed on each sample.

In the AOAC official method, the ratios of (D/H)CH3 and (D/H)CH2 are calculated by comparison with an Internal standard, tetramethylurea (TMU), with a certified (D/H) value.

In the case of wine and fruits, the isotopic parameters of ethanol have been shown to respond even to subtle environmental variations and they efficiently characterize the region of production,.

Private companies involved in food and beverage controls have also collected authentic samples and built up specific data banks.

It allows to control the quality of vinegar and to determine if it comes from sugar cane, wine, malt, cider, and alcohol or from a chemical synthesis.

[12] The naturality of different aroma can also be checked using SNIF-NMR: for example for anethole, abundance of only six monodeuterated isotopomers can be measured by 2H-SNIF-NMR that allows differentiating the botanical origins fennel, star anise or pine.

It is demonstrated that the site specific deuterium contents of benzaldehyde allow the determination of the origin of the molecule: synthetic (ex-toluene and ex-benzal chloride), natural (ex-kernels from apricots, peaches, cherries and ex-bitter almond) and semisynthetic (ex-cinnamaldehyde extracted from cinnamon).

[14] Other applications have also been published: raspberry ketone),[15] heliotropine, ... As described in the work of E. Tenailleau and S. Akoka, an optimization of the technique parameters have enabled to reach a better accuracy for the 13C NMR measurements).

SNIF-NMR in the world
Figure 2
Figure 3 - Isotopic Fractionation Sources
Figure 5 -Steps of SNIF-NMR of ethanol - Official method
Figure 6 - Principle of the IRMS
Figure 7 - Official Isotope method recognition for food application – 2013
Figure 8 - 2H-NMR spectrum of ethanol
Figure 9 - Adulteration triangle: repartition of isotopic ratios on ethanol molecules
Figure 10 - Applications of SNIF-NMR and IRMS to wine authenticity
Figure 11 - Application for vinegar
Figure 12 - With this method C3, C4 and CAM plant metabolism are well separated