The resonance radio frequency is commonly expressed as a chemical shift: the difference between the resonance frequency of a particular nucleus and the resonance frequency of a reference nucleus (TMS as per convention). Whereas the chemical shift is initially measured in Hz, it is usually expressed in the unit-less ppm deviation from the B0 magnetic field by calculating the relative difference between the chemical shift and the B0 magnetic field as follows: 10 6 *(ν-B0)/B0.