Quantitative analyses of complex pharmaceutical mixtures by the Rietveld method

The object was to perform simultaneous quantitative analyses of complex pharmaceutical solid mixtures by the Rietveld method. Mixtures consisting of anhydrous β-carbamazepine, anhydrous α-carbamazepine, and carbamazepine dihydrate were chosen as the model system. Lithium fluoride was used as the internal standard. Mixtures of various compositions were prepared and subjected to X-ray powder diffractometry (XRD). The XRD pattern of each mixture was analyzed by the Rietveld method and at the end of the refinement, the goodness of fit was evaluated. When the analyte concentration was high (≥20%), the relative error in the determination was <±5%. The detection of analyte was possible even when its concentration was low (<1% w/w). A unique advantage of the method is that it enables simultaneous quantitative analyses of multiple phases without the requirement of "standard curves."