These effects are of critical importance for biological NMR because they give rise to different shielding constants for chemically identical nuclei. Therefore, nuclei which occupy chemically equivalent positions on a molecule, such as the protons on a methyl group, must have the same absorption frequency. However, different methyl groups within a compound may have different absorption frequencies. The primary reason for the chemical shift scale is to make the frequency of resonance independent of the magnetic field. The units of chemical shift are in parts per million or ppm.
Organic chemists use tetramethyl silane as a standard.
Most often the water protons are used as the reference and set to 4.8 ppm. This is done to avoid any interaction between the standard and the protein. Chemical shifts of protons, carbons, and nitrogens. Clearly, some nuclei are more sensitive to their environment than others. This is due to the number of electrons and bonding interactions.