text: Chemical shift  is usually expressed in parts per million (ppm) by frequency, because it is calculated from:
  where  is the absolute resonance frequency of the sample and  is the absolute resonance frequency of a standard reference compound, measured in the same applied magnetic field .  Since the numerator is usually expressed in hertz, and the denominator in megahertz,  is expressed in ppm.
  The detected frequencies (in Hz) for H, C, and Si nuclei are usually referenced against TMS (tetramethylsilane), TSP (Trimethylsilylpropanoic acid), or DSS, which by the definition above have a chemical shift of zero if chosen as the reference.  Other standard materials are used for setting the chemical shift for other nuclei.
  Thus, an NMR signal observed at a frequency 300 Hz higher than the signal from TMS, where the TMS resonance frequency is 300 MHz, has a chemical shift of:
  Although the absolute resonance frequency depends on the applied magnetic field, the chemical shift is independent of external magnetic field strength. On the other hand, the resolution of NMR will increase with applied magnetic field.