Research paper Influence of sample characteristics on quantification of carbamazepine hydrate formation by X-ray powder diffraction and Raman spectroscopy F. Tian a , F. Zhang a , N. Sandler a , K.C. Gordon b , C.M. McGoverin b , C.J. Strachan c , D.J. Saville a , T. Rades a, * a School of Pharmacy, University of Otago, Dunedin, New Zealand b Department of Chemistry, University of Otago, Dunedin, New Zealand c Drug Discovery and Development Technology Centre (DDTC), University of Helsinki, Finland Received 15 October 2006; accepted in revised form 7 December 2006 Available online 13 December 2006 Abstract This study aimed to assess the suitability of two widely utilized solid state characterization techniques namely powder X-ray diffrac- tion (XRPD) and Raman spectroscopy, in polymorph detection and quantification for carbamazepine anhydrate and dihydrate mix- tures. The influences of particle size, particle morphology, mixing, and in particular, surface bias on quantitation were investigated. Binary mixtures of carbamazepine anhydrate (form III) and dihydrate were prepared and analyzed using both XRPD and Raman spec- troscopy in combination with partial least squares analysis. It was found that in principle both XRPD and Raman spectroscopy could be used to build calibration models for quantitative analysis, and a satisfactory correlation between the two techniques could be achieved. However, Raman spectroscopy appeared to be a more reliable quantification method because problems such as different particle size, morphology, and special distribution of the two solid state forms of the drug seemed to have no significant influence on Raman scattering in this study. The robust nature of Raman analysis greatly facilitates the whole quantification process from the preparation of calibration models to the quantification of in situ CBZ–DH conversion. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Raman spectroscopy; X-ray powder diffraction; Carbamazepine; Carbamazepine dihydrate; Quantification 1. Introduction The identification and quantitative characterization of crystalline forms of pharmaceutical materials are recog- nized as critical issues in the development of new drug products. There is a growing awareness of the significant influence of polymorphic and pseudopolymorphic transi- tions on the physico-chemical properties and pharmaceuti- cal performance of drug products [1,2]. Carbamazepine (CBZ) is a typical example demonstrating the importance of polymorphism and pseudopolymorphism. CBZ has been routinely used for the treatment of epilepsy for over 20 years, however, CBZ has a low therapeutic index [3]. Six different CBZ polymorphs and pseudopolymorphs have been identified each with different physico-chemical prop- erties [4–7]. Consequently, CBZ is an ideal candidate for attempting to establish relationships between polymorphic structures and properties, such as solubility/bioavailability characteristics or physical/chemical stability. The CBZ sys- tem also serves as an excellent model to study how analyt- ical techniques perform in solid-state analyses. There are a number of solid-state characterization tech- niques, classified by Brittain (1995) [8], based on the level at 0939-6411/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ejpb.2006.12.002 * Corresponding author. School of Pharmacy, University of Otago, P.O. Box 913, Dunedin, New Zealand. Tel.: +64 3 479 5410; fax: +64 3 479 7034. E-mail address: thomas.rades@stonebow.otago.ac.nz (T. Rades). www.elsevier.com/locate/ejpb European Journal of Pharmaceutics and Biopharmaceutics 66 (2007) 466–474