While NMR is primarily used for structural determination, it can also be used for purity determination, provided that the structure and molecular weight of the compound is known. This technique requires the use of an internal standard of known purity. Typically this standard will have a high molecular weight to facilitate accurate weighing, but relatively few protons so as to give a clear peak for later integration e.g. 1,2,4,5-tetrachloro-3-nitrobenzene. Accurately weighed portions of the standard and sample are combined and analysed by NMR. Suitable peaks from both compounds are selected and the purity of the sample is determined via the following equation.
Where:
* w: weight of internal standard
* w: weight of sample
* n[H]: the integrated area of the peak selected for comparison in the standard, corrected for the number of protons in that functional group
* n[H]: the integrated area of the peak selected for comparison in the sample, corrected for the number of protons in that functional group
* MW: molecular weight of standard
* MW: molecular weight of sample
* P: purity of internal standard