Standard for Chemical Shift:

As we cannot measure the NMR of bare nuclei, we do not have a reference to measure the chemical shifts. Thus we required to use some reference standard with respect to which the extent of shielding or deshielding of the external field in various chemical environments could be measured. This is very same to the choice of the standard hydrogen electrode as the reference for defining electrode potentials of half cells in Electrochemistry.  In the case of organic compounds generally tetramethylsilane, (CH₃)₄Si (TMS) is used as a standard with respect to which chemical shift data is reported.

TMS has the subsequent characteristics that make it a molecule of choice to act as a reference.

• It gives a strong and sharp signal at a very high field. Therefore it does not interfere with the signals of all other types of protons in different organic molecules as they absorb at lower field relative to TMS.
• All 12 protons are chemically and magnetically equivalent therefore, these give a reasonably intense signal even at very low concentrations.
• It has low boiling point (27^oC) and is soluble in most organic solvents. Hence it can be easily removed or separated from other organic compounds or solvents after the spectrum is recorded.
• It is highly inert and does not interact with most organic compounds. Hence it does not interfere in NMR measurements.