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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