Preferred orientation effects

Determination of whether preferred orientation effects can be eliminated through control of the sample particle size... 

X-Ray Powder Diflraction Pattern of Brinzolamide (Ground to Minimize Preferred Orientation Effects)... 

Preferred orientation effects are addressed by introducing the preferred orientation factor in Eq. 2.65 and/or by proper care in the preparation of the powdered specimen. The former may be quite difficult and even impossible when preferred orientation effects are severe. Therefore, every attempt should be made to physically increase randomness of particle distributions in the sample to be examined during a powder diffraction experiment. The sample preparation will be discussed in Chapter 3, and in this seetion we will discuss the modelling of the preferred orientation by various functions approximating the radial distribution of the crystallite orientations. 

In both cases the most affected is the intensity of Bragg peaks that correspond to reciprocal lattice points that have their corresponding reciprocal lattice vectors parallel or perpendicular to df hki, while the effect on intensity of other Bragg peaks is intermediate. Hence the preferred orientation effect on the intensity of any reflection hkl can be described as a radial function of angle ha between the corresponding vector d / and a specific d w, which is the preferred orientation direction. The angle hki can be calculated from ... 

Cylindrical samples, which are common in the Debye-Scherrer cameras Figure 3.2), are also used in powder diffractometry. Similar to flat transmission samples, small amounts of powder are required in the cylindrical specimen geometry. This form of the sample is least susceptible to the non-random distribution of particle orientations, i.e. to preferred orientation effects. 

The reference intensity ratio method is based on the experimentally established intensity ratio between the strongest Bragg peaks in the examined phase and in a standard reference material. The most typical reference material is corundum, and the corresponding peak is (113). The reference intensity ratio k) is quoted for a 50 50 (wt. %) mixture of the material with corundum, and it is known as the corundum number . The latter is commonly accepted and listed for many compounds in the ICDD s Powder Diffraction File. Even though this method is simple and relatively quick, careful account and/or experimental minimization of preferred orientation effects are necessary to obtain reliable quantitative results. 

After all three independent atoms (peaks 1 through 3 in Table 6.6) have been included in computations assuming identical displacement parameters in an isotropic approximation (5 = 0.5 A ), the resulting Rp = 6.9% without refinement. This value is excellent because i) the powder diffraction pattern is relatively simple with minimum overlap, and ii) the powder particles used in the diffraction experiment were nearly ideal (spherical), thus preferred orientation effects were also minimized. The following electron density distribution Figure 6.13 and Table 6.7) was obtained using the newly determined set of phase angles. 

When preferred orientation effects are strong, the intensities of Bragg reflections become biased by systematic errors. A correction is nearly impossible before the crystal structure is solved and the preferred orientation refined using an acceptable model. These errors are in addition to the errors introduced by deconvolution of the overlapped Bragg peaks. 

Positions (coordinates) of atoms in the unit cell are the strongest contributors into the computed integrated intensities of Bragg reflections assuming that preferred orientation effects are weak. For this powder, preferred orientation was expected (and later found) to be minor due to small particle sizes and predominantly isotropic particle shapes. 

The present Collection serves as a source of reference patterns for pure zeolite phases. The data will be helpful in establishing the structural purity of experimental phases and in indexing their diffraction patterns. The data will also aid in the determination of changes in the lattice parameters with changing composition, assessing preferred orientation effects, background evaluation, and line broadening. We have also included diffraction patterns of several common dense silicate phases to facilitate their detection in mixed phase syntheses. 

A recent analytical study stresses the growing need, prompted partly by l islatory requirements, to differentiate polymorphs and to quantify polymorphic mixtures in pharmaceutical production [126]. The compounds benzil and benzoic add were chosen as a model system for the development of an XRD protocol which could be extended to the quantification of mixtures of drug polymorphs. The study involved the evaluation of sample thickness, the determination of preferred orientation effects, optimum milling conditions and the construction of diffraction intensity-composition calibration curves for mixtures of benzil and benzoic acid. Since the composition of such mixtures can be accurately determined by an independent method, namely HPLC, vaUdation of the quantification of mixtures by the XRD protocol was possible. It was concluded that the protocol is accurate for the model system to within a few percent. It is desirable that the general validity of the approach suggested be tested on a range of real polymorphic systems. 

The powder samples were back loaded in the sample holders to mitigate preferred orientation effects for XRD data collection. A PANalydeal X Pert Pro diffractometer in conventional Bragg-Brentano 0-20 geometry with a... 

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