Using the energy absorbed when atomic nuclei interact with a magnetic field to identify the chemical environment around each nucleus in a molecule, and hence the molecular structure.
In proton:nuclear magnetic resonance (1H NMR), an applied magnetic field produces a difference in the energy levels of the nuclear spin states. Then, a broadband pulse of radio frequencies causes any hydrogen nuclei within the molecule to flip their spins (‘resonate’). When each nucleus ‘relaxes’ back to the lower energy state, radio waves are emitted at a frequency highly characteristic of the local electronic environment, and different peaks are characteristic of hydrogens in different molecular positions, as illustrated in the spectrum of ethyl benzene
Image copyright, with thanks: Kate Kemsley, University of East Anglia
The technique can be used to analyse foods, yielding a spectral profile containing information on all the compounds present in the sample. In the authenticity context, such profiles can be used in targeted or non-targeted approaches, often involving comparison with a database of spectra from authentic samples.
