Sodium chloride IR spectroscopy plates. The spectroscopy plates are the round, clear objects. The chemical to be analysed is dissolved in a suitable solution and compressed between the two plates in the holder seen here. Infrared (IR) radiation is then shone through the plates and the sample to obtain an IR spectrum for the sample (example seen in background). Pure sodium chloride is used for the plates because it has a low absorbance for infrared radiation, and so does not interfere with the process. The IR spectrum can then be studied to deduce the molecular structure and chemical identity of the sample.