تسجيل الدخول

تسجيل

Solids

المؤلف:  Sue Jickells , Adam Negrusz (Editors)

المصدر:  Clarkes Analytical Forensic Toxicology

الجزء والصفحة:  p433-434

2026-11-14

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Solids

Solids are by far the most common sample types analysed by IR. They are generally examined either as thin films or as dispersions in either liquids or solids. The ideal sample for IR trans mission measurements is clear, visually trans parent and homogeneous. This can be difficult to achieve with solids. Heterogeneous samples that are optically poor with large sample particles can introduce light scattering and the Christensen effect. Light scattering becomes significant when the particle size is more than 1/20th the wavelength of the incident light. Light scattering produces a spectrum offset that is curved, with high light scattering at shorter wavelengths/higher wavenumbers and lower light scattering at longer wavelengths/lower wavenumbers. The Christensen effect, which results from severe refractive index changes at the sample surface, leads to distorted band shapes. Peaks take on an S-shape with an apparent reduced (or even negative) absorption at the longer wavenumber edge. As a result, asymmetric bands may be observed that vary in position and intensity from true values. For transmission measurements, the sample must, ideally, have the appearance of a ‘perfect glass’. To reduce the light scattering and the Christensen effect, all components of the sample must have a very small particle size (≤1 μm) and must be dry; samples should be ground until particle sizes are less than 1 μm. The polymorphic form of a solid sample can affect the IR spectrum. This is an important issue in the pharmaceutical industry, as the rate of dissolution of a solid drug can depend upon its crystal morphology. Polymorphic forms and the factors that influence their formation can be investigated simply by means of a diamond anvil in which the sample is crushed between two diamond windows (see Fig. 1). This means of sampling does not change the polymorphism of the sample, unlike some sample presentation techniques such as alkali halide discs.

Figure 1 Diamond anvil.