Why use NaCl plates in IR

The IR plates that we use in the IR spectroscopy are made of polished sodium chloride (NaCl). Since it is transparent to infrared radiation it is used to take the IR spectra of liquid samples, these plates work similarly to potassium bromide (KBr) for solid samples. The major difficulty of using a liquid sample is to decide a solvent with which to dilute the sample. No solvent is ideal, however, if some information about the molecule is known, a solvent can be selected accordingly. We can use for CHCl3 or DCM liquid for dissolving the sample since it is suitable for plates. Water is not a suitable solvent as NaCl is water-soluble. If there is water in the sample the plates will be ruined.

NaCl plates are delicate and very sensitive to moisture; hence nowadays the attenuated total reflectance system is used to take IR spectra. In ATR we can use a wide range of sample types such as solid, liquid, gel and powder and does not need to prepare samples. The diamond ATR is most popular because it is very rugged, simple to use and gives high-quality spectra in the region 4000–400 cm-1.

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