Position-specific 15 N isotope analysis in organic molecules: A high-precision 15 N NMR method to determine the intramolecular 15 N isotope composition and fractionation at natural abundance.

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 long-term 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.