BP-701T - UNIT –II

IR spectroscopy


What is IR? IR is abbreviation (short form) of Infrared, also known as infrared light. IR is electromagnetic radiation (EMR) with wavelengths longer than the visible light. The wavelength of IR is more than 700 nanometres where as the wavelength of visible light is anything between 380 to 700 nanometers.


IR spectroscopy - IR spectroscopy is the measurement of the interaction of infrared radiation with matter by absorption, emission, or reflection. It is used to study and identify chemical substances or functional groups in solid, liquid, or gaseous forms. IR Spectroscopy detects frequencies of infrared light that are absorbed by a molecule. Molecules tend to absorb these specific frequencies of light as they correspond to the frequency of the vibration of bonds in the molecule.


Theory of Infrared Spectroscopy

The IR spectroscopy theory is based on the concept that molecules tend to absorb specific frequencies of light that are characteristic of the corresponding structure of the molecules. The energies are reliant on the shape of the molecular surfaces, the associated vibronic coupling, and the mass corresponding to the atoms.

For instance, the molecule can absorb the energy contained in the incident light and the result is a faster rotation or a more pronounced vibration.

IR Spectroscopy Instrumentation

The instrumentation of infrared spectroscopy:

The main parts are – Radiation source, sample cell, reference cell, Monochromators, detectors, recorder and display.

The Instrumentation of spectroscopy is illustrated below.

IR radiation sources - IR instruments require a source of radiant energy which emit IR radiation which must be steady, intense enough for detection and extend over the desired wavelength. Various sources of IR radiations are - Nernst glower, Incandescent lamp, Mercury arc etc.

Sample cells and sampling of substances - IR spectroscopy has been used for the characterization of solid, liquid or gas samples.           

 i.      Solid - Various techniques are used for preparing solid samples such as pressed pellet technique, solid run in solution, solid films, mull technique etc.

ii.      Liquid – samples can be held using a liquid sample cell made of alkali halides. Aqueous solvents cannot be used as they will dissolve alkali halides. Only organic solvents like chloroform can be used.

iii.      Gas – sampling of gas is similar to the sampling of liquids.


  1. Monochromators – Various types of monochromators are prism, gratings and filters. Prisms are made of Potassium bromide, Sodium chloride or Caesium iodide. Filters are made up of Lithium Fluoride and Diffraction gratings are made up of alkali halides.

  2. Detectors – Detectors are used to measure the intensity of unabsorbed infrared radiation. Detectors like thermocouples, Bolometers, thermisters and Golay cell detectors are used.

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