Why use NaCl plates in IR?
The IR plates that we use in IR spectroscopy are made of polished sodium chloride (NaCl). Being transparent to infrared radiation, they are used to take the IR spectrum of liquid samples, these plates work similarly to potassium bromide (KBr) for solid samples. The main difficulty in using a liquid sample is choosing a solvent with which to dilute the sample. No solvent is ideal, but if certain information about the molecule is known, a solvent can be selected accordingly. We can use it for liquid CHCl3 or DCM to dissolve sample as it is suitable for plates. Water is not a suitable solvent because NaCl is soluble in water. If there is water in the sample, the plates will be destroyed.
NaCl plates are delicate and very sensitive to humidity; therefore, today the attenuated total reflectance system is used to take IR spectra. At ATR we can use a wide range of sample types such as solids, liquids, ice and powder and there is no need to prepare samples. Diamond ATR is the most popular because it is very durable, easy to use, and offers high quality spectra in the 4000–400 cm-1 region.
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