The Quantitative Analysis of Crystallinity Using FT-Raman Spectroscopy

Purpose. To establish if FT-Raman Spectroscopy can be used to quantitate the degree of crystallinity in a model compound. Methods. Mixtures containing different proportions of amorphous and crystalline indomethacin were prepared. Using the peak intensity ratio 1698 cm^–1 (crystalline) to 1680 cm^–1 (amorphous), a correlation curve was prepared. This correlation curve was validated by testing further samples of known composition. Partially crystalline indomethacin was prepared by milling crystalline indomethacin. Results. A linear correlation curve was obtained across the entire range of 0–100% crystallinity. Using this method, it was possible to detect down to either 1% amorphous or crystalline content. The largest errors were found to result from inhomogeneities in the mixing of the calibration and validation samples. The spectra of the mechanically processed samples were similar to the spectra of the calibration samples, and the degree of crystallinity could be estimated in these samples. Conclusions. FT-Raman Spectroscopy is a potentially useful method to complement existing techniques for the quantitative determination of crystallinity.