37.4.3—Of tetramethylsilane, TMS, as the standard

Tetramethylsilane (TMS) is used as the reference compound assigned δ = 0 ppm in NMR. Sample peaks are reported relative to this standard, making chemical shifts comparable.

TMS gives one sharp signal because its twelve protons (and four carbons in 13C NMR) are equivalent. It is chemically inert and easy to remove from the spectrum.

A proton signal at 2.1 ppm lies 2.1 ppm downfield from TMS under the same instrument conditions.

The TMS peak is not a sample impurity to interpret as an unknown structure; it defines the scale.