Attenuated Total Reflectance (ATR) spectroscopy can be used for samples that are difficult to analyze by a transmission technique. Internal reflectance measurements are made by using an ATR crystal in contact with the sample. Choice of an appropriate ATR accessory is wholly dependent upon the amount and physical state of the sample that you wish to analyze.
The physical state of the sample is an important factor in determining an appropriate ATR system. The sample must be in very good contact with the ATR crystal to obtain a good ATR spectrum. Hence liquids usually exhibit better ATR spectra than solid samples. A clamping system is used with solid samples to force the sample into good contact with the ATR crystal. It helps if the solid sample has a high degree of homogeneity to obtain consistent spectra between like samples. Issues such as reproducibility and sensitivity can become important, due to the way a sample makes contact with a specific ATR crystal.
In summary, along with the sample amount and type, it is often a compromise between chemical performance and spectral range of a crystal material that determines the choice of an ATR accessory for a particular experimental application.