Modeling of transmitted X-ray intensity variation with sample thickness and solid fraction in glycine compacts

The previous paper in this series introduced an X-ray diffraction quantitation method for the polymorphic content in tablets made of pure components. Before the method could be transferred, further studies were required to explain the commonly observed X-ray intensity variation in analyzing compacts. The literature typically attributes the variation to partial amorphization under compression and/or to preferred orientation, without much viable explanation or compelling evidence. In this study, changes in intensity in compacts analyzed in transmission geometry were found to be primarily a function of sample thickness and solid fraction. A theoretical model was developed to describe the X-ray powder diffraction (XRPD) intensity as a function of solid fraction, mass absorption coefficient, and thickness. The model was tested on two sets of glycine compacts: one with varying thickness at constant solid fraction, and the other with various solid fractions at a given thickness. The results show that the model predicts the XRPD intensity at any given sample thickness and solid fraction. With this model, the intensity variation of compacts made under different compression conditions can be normalized, making the method transferable to various tablet geometries and facilitating the analysis over expected ranges of formulation and process variation.