Variations in crystals of flufenamic acid of its methyl and tert-butyl analogues as impurities as determined by partial dissolutions†

Abstract

Achieving specified levels of impurities is one the key goals of crystallisation processes in manufacturing. The mode of impurity incorporation and the variation of impurity levels throughout crystal batches are key factors affecting the performance of crystallisations in terms of achieving purity specifications. Evaluation of the distribution of impurities in crystals of flufenamic acid (FFA) using a controlled partial dissolution approach is described. 2-(3-Tolylamino)benzoic acid (MeFA) and 2-((3-(tert-butyl)phenyl)amino)benzoic acid (tBuFA), analogues of FFA in which the trifluoromethyl group has been replaced by a methyl group or by a tert-butyl group respectively, were selected as the impurities. Thermal analysis suggests formation of a solid solution between FFA and MeFA isostructural to FFA form III. The stepwise dissolution approach was initially demonstrated on samples of pure FFA crystals and was then extended to evaluate the distribution of levels of MeFA and tBuFA impurities. The impurity levels are shown as varying with dissolution midpoint. Stepwise dissolution was usefully applied to FFA crystal of various morphologies, while for crystals with extremely needle-like morphology, a segmentation analysis approach was more practical. The work presented outlines a method for evaluating the distribution of impurities in crystalline materials using commonly available analytical and particle sizing methods.