Reststrahlen effect

As with transmission analysis, DRIFTS test samples are generally ground and mixed with an IR transparent salt, such as potassium bromide (KBr), prior to sampling. However, to ensure that there is no catalytic effect, most systems use CaF2 or powdered crystalline carbon (diamond). One drawback is the presence of reststrahlen bands whose contrast can be reduced by... 

The principle problem with diffuse reflectance is that the specular component of the reflected radiation, that which does not penetrate the sample, is measured along with the diffuse reflected light which penetrates the sample. Generally, the change in specular reflection with frequency is small except in regions of strong absorption bands where the anomalous dispersion leads to Reststrahlen bands in the specular reflection spectrum. When the Reststrahlen bands are observed, the absorption bands can appear inverted at their center. This effect makes quantitative measurements on samples with strong absorptivity very difficult. 

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