Norbert Naulet

Precision, accuracy and referencing of isotope ratios determined by nuclear magnetic resonance

SummaryThe precision and accuracy of site-specific and average hydrogen isotope ratios determined by NMR are checked according to the ISO norms. The consistency of the results is investigated by comparison with the values of the mean isotope ratios measured by mass spectrometry. Several kinds of molecular species from different natural origins have been studied: water, ethanol, anethol, vanillin. A very good agreement between MS and NMR results is found for water. The limitations and advantages of NMR for the determination of average deuterium contents of organic molecules are discussed. A satisfactory precision of the NMR measurements can usually be reached, however systematic deviations of a few ppm between MS and NMR results may occur. By using either MS or NMR mean isotope ratios, site-specific parameters can be obtained by SNIF-NMR with an accuracy which is strongly dependent on the structure of the molecular species. The role and the importance of the reference materials in quantitative isotope determinations are stressed.

Natural isotope tracers and water filiation in the vine ecosystem

Abstract A systematic investigation of the water cycle in the vine ecosystem is presented. The H chain from water to products via photosynthesis and fermentation is analysed by following the behavior of the natural H isotope tracer. The analytical techniques used are mass spectrometry of isotope ratios, the Site-specific Natural Isotope Fractionation-NMR method (SNIF-NMR) and multivariate analysis. The contents in deuterium in soil water extracted under controlled conditions are shown to vary with the depth of the soil layer and with the pedological properties. However, the isotope content of leaf water, of berry water, and of the fermentation products, is mainly governed by climatic effects and the nature of the soil plays only a secondary role. On average, a continuous increase in the isotope ratio is observed from soil water to leaf water, berry water and wine water but strong deuterium depletions intervene in the photosynthesis of glucose and fermentation into ethanol. The multivariate analysis of the data shows that the climatic significance of the isotope parameters of the end wine products is not a simple translation of the relation between the isotope content of precipitation and meteorological parameters. The behavior of soil water, leaf water and berry water is in fact somewhat decoupled from that of meteoric water and the final climatic dependency combines several responses for successive effects. From a practical point of view, these results which concern vineyards with very different pedological properties in a small area of the Saumur-Champigny region of France, show that the isotope parameters of wine are better criteria of the climatic features which prevail during grape elaboration than of the nature of the soil.