Why the Sample Holder and Preparation Matter in XRD: Preferred Orientation of Pharmaceutical Samples
Aspirin and paracetamol are two of the most popular painkillers. X-ray diffraction, a common characterization technique in research, development and quality control of such pharmaceuticals, regularly encounters problems with preferred orientation. It is shown that these issues can be avoided with optimized measurement setups, sample holder selection and sample preparation.
Proper sample preparation is one of the most crucial steps to guarantee high-quality data in powder X-ray diffraction (XRD). The choice of the right sample holder, preparation technique, measurement geometry and measurement parameters all have a significant influence on the data quality.
One of the most regularly observed problems stemming from the sample preparation is preferred orientation or texture effects. These can occur in most samples and can be caused by a number of different sample properties. The easiest way to visualize the phenomenon is by imagining a powder sample that does not consist of spherical grains, but of elongated needles, or plates. The probability that one of these needles or plates orients on a thin edge is small compared to the probability that it orients aligned with neighboring particles. Texture effects can also have other origins, such as crystal properties or crystallization behavior, and can also be observed in samples that only contain spherical particles1.
Texture leads to the overexposure of some lattice planes while others are rarely in the correct position to diffract the X-ray beam which in turn results in large errors in the relative intensities of different peaks. These differences become clearly visible when comparing the measured diffraction pattern to the theoretical (ideal) pattern calculated from the single crystal structure solution. When working with new substances or phase mixtures, texture effects can seriously complicate any quantitative phase analysis or the determination of the crystal structure.
For this application report, preferred orientation effects were investigated for two different pharmaceutical samples using the Automated Multipurpose Powder X-Ray Diffractometer XRDynamic 500 by Anton Paar. Two types of commercially available painkiller tablets, one containing aspirin (acetylsalicylic acid) and the other paracetamol (acetaminophen), were analyzed and compared to their pure active pharmaceutical ingredients (API). The large variety of sample holder types available for XRDynamic 500 was employed to identify the best way to avoid texture effects, which were primarily observed for the pure APIs.
1. E. Prince, International Tables for Crystallography. Volume C, Mathematical, Physical and Chemical Tables. Dordrecht ; Boston ; London :Published for the International Union of Crystallography by Kluwer Academic Publishers, 2004.
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