Determination of indomethacin polymorphic contents by chemometric near-infrared spectroscopy and conventional powder X-ray diffractometry

A chemoinfometric method for the quantitative determination of the crystal content of indomethacin (IMC) polymorphs, based on Fourier-transform near-infrared (FT-NIR) spectroscopy, was established. A direct comparison of the data with those collected using the conventional powder X-ray diffraction method was performed. Pure alpha and gamma forms of IMC were prepared using published methods. Powder X-ray diffraction profiles and NIR spectra were recorded for six kinds of standard material with various contents of the gamma form of IMC. Principal component regression (PCR) analyses were performed on the basis of the normalized NIR spectral sets of standard samples with known contents of the gamma form of IMC. A calibration equation was determined to minimize the root mean square error of the prediction. The predicted gamma form contents were reproducible and had a relatively small standard deviation. The values of the gamma form contents predicted by the two methods were in close agreement. The results indicated that NIR spectroscopy provides an accurate quantitative analysis of crystallinity in polymorphs compared with the results obtained by conventional powder X-ray diffractometry.