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Chemical
shifts, or peak positions, are always referenced to an internationally
agreed-upon compound, whose location is defined as 0 ppm. For both 1H
and 13C NMR, this compound is tetramethylsilane, or TMS [(CH3)4Si].
In the past, it was necessary to include a few drops of TMS in each sample
in order to insure that the chemical-shift scale was accurate. Most modern
instruments can calculate shifts without any TMS in the sample. However,
if very exact chemical shifts are needed, it is still good practice to
include a reference compound. The concentration of TMS used is usually
around 1%; some solvents are sold with the reference included (list
of vendors). |
