Why are salt plates used in IR instead of glass?

Sodium chloride (NaCl) cells are infrared transparent and do not absorb infrared radiation in the region between 4000 and 450 cm-1; therefore, it is used to analyze the sample in IR. When looking at the IR spectrum, glass or quartz cells cannot be used because they absorb IR radiation. Liquid samples are often cleaned between plates of NaCl salt. Since solvents absorb infrared radiation, infrared cells usually have narrow paths to keep solvent absorption at adequate levels. Solid samples are often prepared with potassium bromide (KBr) in pellet form in the hydraulic pellet press. Currently, attenuated total reflectance (ATR) spectroscopy is another typical application for determining the functional group of molecules. No need to prepare samples. This facilitates the direct analysis of samples in solid, liquid, ice or powder state without any preparation.