FT-IR Spectroscopy

There are two techniques to measure how much light a sample absorbs or emits at each wavelength in the IR region- Dispersive spectroscopy and second is Fourier transform spectroscopy.

Dispersive spectroscopy is a straightforward technique, where a monochromatic beam (single wavelength) of light shines on the sample and absorption is measured. This is repeated for each wavelength.

In FT-IR spectroscopy, a light beam consisting of many frequencies shines on the sample and how much of that beam is absorbed is measured. Next the light beam is modified using ‘Michaelson Interferometer’ – it is a set of mirrors one of which is moving, and the other mirror is fixed. The moving mirror blocks each wavelength of light periodically (blocked, transmitted, blocked……). This is how the beam is modified to contain different combination of wavelengths or frequencies giving a second data set. This is repeated many times over a short span of time giving many data sets corresponding to different frequencies. A computer takes this raw data and converts it into spectrum using a mathematical process known ‘Fourier Transform’. Here, raw data is light absorption at each mirror position into desired result which is light absorption at each wavelength.

FT-IR has many advantages over dispersive IR such as high signal-to-noise spectrum ratio, short scan time, high resolution, reduced interference from stray light.

 

 

IR-SPECTROSCOPY

 

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.

 

 

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