Chemical Shift: Internal References and Solvent Effects

Precise measurement of the absolute absorption frequencies of nuclei in a sample is difficult. To overcome this, a standard internal reference compound is added, and the difference between their absorption frequencies is measured.

Internal reference compounds, like tetramethylsilane, or TMS, are chemically inert, soluble in NMR solvents, and easily removable.

With highly shielded methyl protons that yield an intense signal at a lower frequency than most organic molecules, TMS is a primary reference in proton, carbon, and silicon NMR spectroscopy.

If the reference compound is not inert, it is kept in a capillary tube within the NMR tube and termed an external reference.

Further, deuterated NMR solvents contain residual protons whose signal can also be used as a secondary reference.

The signal from deuterium itself can be used to monitor the instrument's magnetic field by a technique called locking.

The deuterium signal is constantly compared to a reference frequency and adjusted if there is any variation.