The Institute of Pharmaceutical Innovation

Kinetics of phase transformations in crystals of drug compounds using .... x-ray diffraction

Kinetics of phase transformations in crystals of drug compounds using time-resolved powder x-ray diffraction

J. Anwar* & P. Barnes

The kinetics of the structural phase transformation of form IV to form I of sulphathiazole have been studied using time-resolved powder x-ray diffraction (PXRD). The data present significant challenges to any form of rigorous analysis, and serve to illustrate the complications of analysing time-resolved PXRD data. The complications include poor spatial resolution coupled with gross peak-overlap, and kinetic effects due to the use of a scanning detector which cause variation of the relative integrated intensities and distortion of the diffraction patterns with respect to the 2theta axis.

To analyse such time-resolved data a new pattern decomposition method has been proposed. The method is based on the profile fitting approach utilising lattice parameters. The problems of peak overlap and correlation in the fitted intensities are overcome by the use of restraints; the restraints attempt to maintain the relative integrated intensities within each phase in the mixed sample at the values observed for the pure components. The method is ideally suited to the analysis of time-resolved data and enables reduction of the data to a high precision.