The intensity of the magnetic fields B₀ used for NMR experiments can be very diverse (from 5.9 to 14.1 T, corresponding to a respective frequency of resonance of the ¹H from 250 to 600 MHz in routine). It is therefore not possible to express the chemical shifts of the protons in frequency if the spectra have not been acquired with the same range of B₀ field. In order not to depend on B₀ and make this comparison, a reference is used. In proton NMR, tetramethylsilane (TMS or Si(CH₃)₄) is used as a reference. The chemical shift is given by the following equation: (expressed in ppm).