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Chemical Shift

The environmental effect on any type of hydrogen atom is expressed quantitatively in terms of a quantity called chemical shift. It is generally represented by the letter and is defined by the equation.
Where HS is the magnetic field strength at which the maximum (peak) appears for a particular type of protons of the sample taken while Hr is the field strength at which absorption occurs for some reference substance such as water. For example, in case of ethyl alcohol, for protons in CH3 group, 4.1 whereas for the proton in the OH group,-0.1.
      As H <V, equation can also be written as

However, the most common compound used as a reference is tetraethyl silane (TMS) i.e., (CH3)4 Si. It has many advantages. Firstly, from is structure
we see that all the 12 protons present in it are equivalent i.e. they have the same environment. Hence it gives a single well defined absorb piton peak in the NMR spectrum. Secondly, the protons are highly shielded by their electrons form the external magnetic field so that it shows NMR at a very height external magnetic field strength as compared to other proton. Thirdly, it is chemically, finery. Fourthly, its boiling point is low (270 C)so that it evaporates off quickly anted the sample compound is easily recovered.

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