Determination of site-specific carbon isotope ratios at natural abundance by carbon-13 nuclear magnetic resonance spectroscopy

Abstract

Site-specific natural isotope fractionation of hydrogen studied by deuterium NMR (SNIF-NMR) spectroscopy is a powerful source of information on hydrogen pathways occurring in biosyntheses in natural conditions. The potential of the carbon counterpart of this method has been investigated and compared. Three typical molecular species, ethanol, acetic acid, and vanillin, have been considered. Taking into account the requirements of quantitative 13C NMR, appropriate experimental procedures have been defined and the repeatability and reproducibility of the isotope ratio determinations have been checked in different conditions. It is shown that the carbon version of the SNIF-NMR method is capable of detecting small differences in the carbon-13 content of the ethyl fragment of ethanols from different botanical or synthetic origins. These results are in agreement with mass spectrometry determinations of the overall carbon isotope ratios. Deviations with respect to a statistical distribution of 13C have been detected in the case of acetic acid and vanillin. However, since the method is very sensitive to several kinds of systematic error, only a relative significance can be attached at present to the internal parameters directly accessible. Isotope dilution experiments have also been carried out in order to check the consistency of the results. In the present state of experimental accuracy, the 13C NMR method is of more limited potential than 2H SNIF-NMR spectroscopy. However it may provide complementary information. Moreover it is particularly efficient for detecting and quantifying adulterations that aim to mimic the overall carbon-13 content of a natural compound by adding a selectivity enriched species to a less expensive substrate from a different origin.

Fractionation in position-specific isotope composition during vaporization of environmental pollutants measured with isotope ratio monitoring by 13C nuclear magnetic resonance spectrometry

“…As a result, the development of the technique took longer, starting from the first work (Caer et al, 1991) to the recent attainment of suitable acquisition conditions (Caytan et al, 2007a). A relative method of 13 C NMR was previously proposed as a way to calculate isotope effects in which the NMR signal intensity of the site-active carbon is related to the intensity of the signal of a 'neutral' carbon (far away from the active site), making the assumption that the latter is not modified during the chemical reaction (Singleton and Thomas, 1995).…”

Classification of Wines Based on Combination of 1H NMR Spectroscopy and Principal Component Analysis

“…As a non-invasive method, NMR has played an important role in wine analysis, [1][2][3][4][13][14][15][16] especially in classification and characterization of the geographic origin and age of wine. [12][13][14][15][16][17][18][19][20][21][22][23][24][25] The method is widely used for analysis of metabolites in complex biological samples.…”

“…[12][13][14][15][16][17][18][19][20][21][22][23][24][25] The method is widely used for analysis of metabolites in complex biological samples. [26][27][28] Since NMR spectroscopy needs nearly no sample pretreatment, the inherent chemical or biological properties of the sample are well kept and the error resulting from pretreatment may be reduced.…”

Wine analysis by 1D and 2D NMR spectroscopy

“…Currently 1 H and 13 C NMR spectroscopies are used for the detection of enrichment of musts by addition of sugars and for the identification of geographical origin [1,2]. It has been shown that 13 C NMR can successfully be used for detection of sugars, sugar alcohols and acids [3] and that 13 C resonances of amino acids can be used as fingerprints for the monitoring of European wines [4].…”