Solution to phenylbutazone purity challenge.

In the last 15 years NMR has become a reference technique for the determination of purity for organic standards containing NMR active nuclei [1, 2]. The puzzle proposed for this Analytical Challenge was focused on the use of 1H-NMR for the characterization of a phenylbutazone standard. Such material was purchased from Sigma-Aldrich and analyzed in parallel by HPLC-UV and 1H-NMR. Initially, application of the two methodologies gave results which were not in agreement [3]. Whereas the HPLC-UV method found only two minor impurities that did not exceed the 1 % level, the NMR assay was less generous: integration of the proton signal at position 5 (Fig. 1), provided a phenylbutazone purity estimate of only 0.8119 ± 0.0014k=1 g/g [3]. Such evidence could alert the analyst that an impurity, structurally related to phenylbutazone, is present. This