IR is an analytical technique
used for the identification of chemical compounds, functional groups, structure
elucidation and molecules in solid, liquid, or gaseous forms. IR spectroscopy
studies the interaction of IR radiation with the sample (solid, liquid or gas)
by absorption, emission, or reflection. This interaction of IR radiation with
sample is conducted on an instrument called IR spectrophotometer and it
produces an IR spectrum.
An IR spectrum is a graph of
transmittance (Y-axis) vs wavenumber cm-1 (X-axis). IR spectrum is
usually taken in mid-infrared region from 4,000–400 cm−1. IR
spectrum is obtained by passing a beam of IR radiation through the sample.
Absorption occurs when the IR frequency of the bon/bonds in the sample is the
same as that of radiation. Absorption at each wavelength is examined by
measuring transmitted light at each wavelength.
Measurement of transmitted IR
light at each wavelength is done by 2 methods, Dispersive spectroscopy, and FT-IR.
IR spectroscopy can be used for
analysis of solid, liquid, or gaseous sample. Liquid samples are analyzed by
sandwiching between two salt plates of NaCl (or CaF2 and KBr). These
salts are transparent to IR light and do not contribute to the spectrum.
For solid samples, one method is
to grind the sample with a mulling agent (Nujol) and apply a thin layer of this
mull on the salt plates for measurement. Second method involves finely grinding
the sample with KBr salt. Fine grinding removes scattering effect of large
crystals. This fine powder is made into a translucent pellet by mechanical
pressing. This pellet is used for measurement.
For polymeric materials, cast
film method is used where the sample is dissolved in a non-hygroscopic solvent
and a drop of it is placed on KBr or NaCl cell. Solution is dried via
evaporation leaving a thin layer of sample on the cell which is then for
measurement.
Applications
IR spectroscopy is widely used in
quality control like measurement of CO2 emissions in air to keep a
check on greenhouse gases. It is also used to detect hydrocarbon gas release
during natural gas and crude oil transportation.
It is used in polymer industry
for measuring the degree of polymerization.
It is also used in food and
pharmaceutical industry to measure the concentration of various components in
food items and to ensure purity of drug and drug products.
