The most widely used method for isotope analysis at natural abundance is isotope ratio monitoring by Mass Spectrometry (irm-MS) which provides bulk isotopic composition in H, C, N, O or S. However, in the 1980s, the direct access to Site-specific Natural Isotope Fractionation by Nuclear Magnetic Resonance (SNIF-NMR ) was immediately recognized as a powerful technique to authenticate the origin of natural or synthetic products. The initial - and still most popular - application consisted in detecting the chaptalization of wines by irm- H NMR. The approach has been extended to a wide range of methodologies over the last decade, paving the way to a wide range of applications, not only in the field of authentication but also to study metabolism. In particular, the emerging irm- C NMR approach delivers direct access to position-specific C isotope content at natural abundance. After highlighting the application scope of irm-NMR ( H and C), this article describes the major improvements which made possible to reach the required accuracy of 1‰ (0.1%) in irm- C NMR. The last part of the manuscript summarizes the different steps to perform isotope analysis as a function of the sample properties (concentration, peak overlap) and the kind of targeted isotopic information (authentication, affiliation). Copyright © 2016 John Wiley & Sons, Ltd.

For the last ten years, quantitative isotope ratio monitoring C NMR (irm-C NMR) has been successfully tested and proven as an efficient tool for the determination of position-specific C/C ratios. Several applications in different domains have shown the interest in this technique. In the context of origin assignment, the possibility to track the distribution network of illicit drugs or cutting agents is of prime importance. However irm-C NMR still suffers from a relative lack of sensitivity limiting its dissemination among control laboratories. Improvements were proposed to reduce experiment time by using the INEPT sequence ("Insensitive Nuclei Enhanced by Polarization Transfer") based on polarization transfer from highly sensitive H to less sensitiveC. Several applications based on the use of the one bond scalar coupling between H andC ( J) have shown the potential of this methodology in terms of short experimental duration. However, the isotopic information given by quaternary carbons was lost. The aim of this study is to extend this approach by using short- and long-range coupling ( J, J, and J) in order to have access to all C/C position-specific ratios, i.e., acquisition of the full spectrum (FS-INEPT). It is shown that this innovative tool provides both sensitivity gain-thanks to the long-range polarization transfer-and appropriate repeatability. The relative isotopic profiles allowed the classification of two cutting agents, caffeine and paracetamol (acetaminophen), according to their origin, as it was previously observed with "classical" irm-C NMR but consuming much less sample and/or reducing the experimental time.

The position-specific 15 N isotope content in organic molecules, at natural abundance, is for the first time determined by using a quantitative methodology based on 15 N Nuclear Magnetic Resonance (NMR) spectrometry. 15 N NMR spectra are obtained by using an adiabatic "Full-Spectrum" INEPT sequence in order to make possible 15 N NMR experiments with a high signal-to-noise ratio (>500), to reach a precision with a standard deviation below 1‰ (0.1%). This level of precision is required for observing small changes in 15 N content associated to 15 N isotope effects. As an illustration, the measurement of an isotopic enrichment factor ε for each 15 N isotopomer is presented for 1-methylimidazole induced during a separation process on a silica column. The precision expressed as the longterm repeatability of the methodology is good enough to evaluate small changes in the 15 N isotope contents for a given isotopomer. As observed for 13 C, inverse and normal 15 N isotope effects occur concomitantly, giving access to new information on the origin of the 15 N isotope effects, not detectable by other techniques such as isotope ratio measured by Mass Spectrometry for which bulk (average) values are obtained. KEYWORDS 15 N enrichment factor, 15 N INEPT, 15 N isotopic profiling, 15 N NMR, position-specific 15 N isotope analysis