Quantitative NMR (QNMR) is a NMR technique that can be used to accurately quantify organic molecules. Quantitative NMR has multiple applications such as reaction monitoring, purity analysis, and stability studies to list a few. Recently, its use as a method for measuring purity for the pharmaceutical industry has been gaining traction. Since NMR is considered a primary method of analysis, there is no need to use a traceable reference compound unlike other methods such as HPLC with UV.
Quantitative NMR operates by mixing a known quantity of a standard with a known quantity of your compound. Integration of the standard peak corresponds with a known number of hydrogens which can then be used to compare to a peak integration in the sample. The main limitation is that the standard must not have overlapping peaks with the sample NMR spectrum. Various standards can be used to meet this requirement.
The purity of the sample can then be calculated using the equation: