Effect of preferred orientation

Toraya s WPPD approach is quite similar to the Rietveld method it requires knowledge of the chemical composition of the individual phases (mass absorption coefficients of phases of the sample), and their unit cell parameters from indexing. The benefit of this method is that it does not require the structural model required by the Rietveld method. Furthermore, if the quality of the crystallographic structure is poor and contains disordered pharmaceutical or poorly refined solvent molecules, quantification by the WPPD approach will be unbiased by an inadequate structural model, in contrast to the Rietveld method. If an appropriate internal standard of known quantity is introduced to the sample, the method can be applied to determine the amorphous phase composition as well as the crystalline components.9 The Rietveld method uses structural-based parameters such as atomic coordinates and atomic site occupancies are required for the calculation of the structure factor, in addition to the parameters refined by the WPPD method of Toraya. The additional complexity of the Rietveld method affords a greater amount of information to be extracted from the data set, due to the increased number of refinable parameters. Furthermore, the method is commonly referred to as a standardless method, since the structural model serves the role of a standard crystalline phase. It is generally best to minimize the effect of preferred orientation through sample preparation. In certain instances models of its influence on the powder pattern can be used to improve the refinement.12... 

X-ray intensities were measured photometrically. To reduce the effect of absorption as much as possible, the sulfide was diluted with magnesium oxide. The value of the coefficient julR was experimentally determined. Moreover, to eliminate the effect of preferred orientation, which arises when the sample assumes the form of a cylindrical rod, we adopted a spherical sample shape of 0.3-mm. diameter. 

See Chapter 2, seetion 2.10.6 for a description of mathematical models, which may be used to account for the effects of preferred orientation on the scattered intensity. 

Gunter ME, Dyar MD, Delaney JS, Sutton SR, Lanzirotti A (2002) Effects of preferred orientation on microscale XANES measurements of Fe /yFe in biopyriboles. 33rd Aim Lunar Planet Sci Conf, Abstr 1654... 

The effects of preferred orientation on PXRD patterns are illustrated in Fig. 7.6 which presents diffraction patterns of a set of theoretical aspirin crystals. Although only one crystal form of aspirin is known, the outward appearance of... 

Calibration samples and samples for analysis should be treated in the same way. The ground sample should be introduced into the sample holder of the diffractometer using the back-loading method which reduces the effects of preferred orientation. 

---