International hydrogen, analysis positioning

To ensure reproducibility and to set up a scale, the positions of NMR absorption peaks are measured relative to the signal of an internal standard, known as TMS. Tetramethylsilane (TMS), (CH3)4Si (Figure 21.1), has 12 hydrogens in an identical chemical environment and it therefore produces a sharp single peak which is located away from the majority of the peaks found in many organic molecules. A small amount of this substance is added to a sample before it is introduced into the NMR machine. It is non-toxic, unreactive and can be readily removed after analysis by evaporation. 

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