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Distinguishing Impurities … Part 2

A previous blog described how integrals and coupling information can assist in discerning an unknown structure signal from an impurity(ies).  Before proceeding to acquire 2D NMR data to confirm the information, there is the option to purify/rewash the sample (assuming there is enough sample to do this) followed by a spectral comparison of the before and after the wash; any differences in the integrals may assist in identifying signals from an impurity.

Illustrated below are 2 1H NMR spectra differing in sample preparation. The top spectrum, washed once before acquisition, shows 2 impurity signals at 1.68 and 2.14 ppm. The bottom spectrum is for the same sample but washed a second time. When comparing the 2 spectra, only the 2 singlets have changed significantly in area, thus, indicating a separate entity in relation to the remaining signals.  


One advantage of knowing which signals belong to an unknown structure  is that these signals, and only these signals, are used in searching across a library or database. This will help in reducing false positives and allow more flexibility to narrow down the number of hits.