Spectroscopic and X-ray Diffraction Study of Structural Disorder in Cryomilled and Amorphous Griseofulvin ANNA ˙ ZARO ´ W, BO ZHOU, XIANQIN WANG, RODOLFO PINAL, and ZAFAR IQBAL* Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102 (A.Z., Z.I.); Otto York Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of Technology, New Jersey 07102 (X.W.); and Department of Industrial and Physical Pharmacy, Purdue University, West Lafayette, Indiana 47907 (B.Z., R.P.) Structural disorder induced by cryogenic milling and by heating to the amorphous phase in the active pharmaceutical ingredient Griseofulvin has been studied using Raman spectroscopy, X-ray powder diffraction (XRPD), and fluorescence spectroscopy. A broad, exciting-frequency- independent scattering background in the Raman spectra and changes in intensities and splitting of some of the Raman lines due to lattice and molecular modes have been observed. In the cryomilled samples this strong background is deconvoluted into two components: one due to lattice disorder induced by cryomilling and the other due to Mie scattering from nanosized crystallites. A single-component background scattering attributed to lattice disorder is seen in the Raman spectrum of the amorphous sample. Fluorescence measurements showed an intrinsic fluorescence signal in as-received Griseofulvin that does not correspond to the inelastic background in the Raman spectra and, moreover, decreases in intensity upon cryomilling, thus excluding an assignment of the Raman background intensity to impurity- or molecular-defect-induced fluores- cence. Wide-angle XRPD measurements on cryomilled Griseofulvin shows a broad two-component background consistent with the background- scattering component in the Raman data associated with lattice disorder, but at longer correlation lengths. Persistence of this disorder to even longer lengths is evident in small-angle synchrotron XRPD data on micronized Griseofulvin taken as a function of temperature from the crystalline to the amorphous phase. Index Headings: Griseofulvin; Active pharmaceutical ingredient; API; Milling; Cryomilling; Raman spectroscopy; Disorder; Lattice disorder; Crystalline defects; Amorphous phase; Fluorescence spectroscopy; X-ray powder diffraction; XRPD. INTRODUCTION Milling is widely used by the pharmaceutical industry to reduce the particle size of active pharmaceutical ingredients (APIs). Size reduction leads to an increase in the surface-to- bulk ratio of the particle, which enhances its bioavailability and dissolution rate. During milling, however, mechanical stress can induce changes in the crystal lattice of the API, which will modify its physical and chemical properties. Both ball and jet milling, commonly used in the industry, can affect crystal morphology and induce structural disorder. 1 These changes can cause crystal defects that are believed to mostly exist at the surface of the crystallites 2 and may originate from molecular conformational changes or unit cell distortions. Many types of defects exist and the most common ones include those due to varying grain sizes, edge dislocations, and grain boundary effects. 3 Crystalline defects act as higher energy sites that are thermodynamically unstable compared to those in the stable crystalline form. Defects can progressively transform during processing into longer-range disorder in atomic positions, resulting eventually in the formation of an amorphous phase or a polymorph. 4 Raman spectroscopy is particularly sensitive to structural changes. The first-order lattice or inter-molecular modes observed in Raman spectra typically occur at frequencies below 200 cm 1 . 5,6 Lattice modes include translational and librational vibrations that are very sensitive to structural disorder and polymorphism. Intra-molecular deformation, rocking, wagging, breathing, and stretching modes, which typically lie in the frequency range above 200 cm 1 , 7 are also sensitive to changes in the local crystal field caused by disorder or polymorph formation. 8 Therefore, changes in peak positions and intensities or the appearance of new lines in the Raman spectra would indicate the formation of defects or disorder, an amorphous phase, or a new structural polymorph. 9–11 In addition, structural defects or disorder in the crystal may generate background scattering in the Raman spectra and diffuse scattering superimposed on X-ray diffraction patterns that resemble X-ray scattering from amorphous solids. 12–15 Background intensity in Raman spectra is often caused by fluorescence when the excitation wavelength is close to an intrinsic electronic transition in the sample or it is caused by impurities introduced during processing or sample preparation. In both these cases the background intensity would vary with the excitation wavelength. If the scattering background is independent of excitation frequency, it may be attributed to other possibilities, especially if the material has undergone mechanical processing that can produce structural defects or disorder. In the present study, cryomilled Griseofulvin was investi- gated for defect and disorder formation, and structural stability using Raman spectroscopy and X-ray powder diffraction (XRPD) techniques. Prior work by Bates et al. 12 and Yamamura et al. 13 focused primarily on X-ray diffraction studies of disorder in organic crystal systems, whereas Veprek et al. 14 investigated the transition to the amorphous phase in silicon, a covalently bonded inorganic crystal, using both X-ray diffraction and Raman spectroscopy. In this paper, Raman spectroscopic and diffuse inelastic light scattering data have been obtained and correlated with X-ray diffraction and scattering data obtained from polycrystalline samples of as- received and processed Griseofulvin, an organic solid, and from amorphous Griseofulvin. Quantitative estimates of crystallinity and particle sizes were obtained by differential scanning calorimetry (DSC) and from scanning electron microscope (SEM) images, respectively. The Raman spectra were examined using two different Raman instruments with Received 9 June 2010; accepted 2 November 2010. * Author to whom correspondence should be sent. E-mail: iqbal@adm. njit.edu. DOI: 10.1366/10-06024 Volume 65, Number 2, 2011 APPLIED SPECTROSCOPY 135 0003-7028/11/6502-0135$2.00/0 Ó 2011 Society for Applied Spectroscopy