text: The isotopic ratios of a molecule can also be determined by isotope ratio mass spectrometry (IRMS), sample quantity for IRMS is much lower than for NMR, and there is the possibility of coupling the mass spectrometer to a chromatographic system to enable on-line purification or analyses of several components of a complex mixture. However the sample is burnt after a physical transformation such as combustion or pyrolysis. Therefore, it gives a mean value of the concentration of the isotope studied between all sites of the molecule.  IRMS is the official AOAC technique used for the average ratio C/C (or δC) of sugars or ethanol, and the official CEN and OIV method for the 18O/16O in water.
  The SNIF-NMR method (Site-Specific Natural Isotope Fractionation studied by Nuclear Magnetic Resonance) is able to determine, to a high level of accuracy, the isotopic ratios for each of the sites of the molecule, which enables a better discrimination. For example, for ethanol (CHCHOH), the three ratios ((D/H)CH, (D/H)CH and (D/H)) can be obtained.