Infrared spectroscopy, Principle, Instrumentation and Applications

Infrared spectroscopy, Principle, Instrumentation and Applications

Infrared spectroscopy is one of the most powerful analytical techniques which provides the possibility of chemical identification. This techniques when coupled with intensity measurement can be used for quantitative analysis..

This technique is based upon the simple fact that a chemical substance displays marked selective absorption in the infrared region. After absorption of IR radiation, the molecules of a chemical substance vibrate at many rates of vibration, giving rise to close – packed absorption bands. Known as an IR absorption spectrum which can extend over a wide wavelength range..

The Range of Infrared Radiation

The infrared radiation refer widely to that region of electromagnetic spectrum which lies between the visible and microwave region. Althrough, this region can divided into four section.

1. The photographic region : This ranges from visible to 1.2 mu

2. The very Near Infrared region : This is also known as Overton and ranges from 1.2 to 2.5 mu

3. The Near infrared Region : This is also known as vibration region and ranges from 2.5 to 25 mu

4. The far Infrared region : This is know as the rotation region. This ranges from 25 to 300 – 400mu


Underlying principal of infrared spectroscopy is based upon the molecular vibration which is further composed of the stretching and the bending vibration of a molecule.

Molecular Vibration :

It can be regarded as a sort of flexible system comprising of balls of verying masses representing the atom of a molecule and springs of varying strengths representing the chemical bonds of molecular. The vibration for molecules are of two types namely.

1. Stretching
2. Bending ( or deformation )

#1. Stretching :

In which distance the two atoms increase or decreases, but atom remain in the same bond axis, eg A -B where A and B represent the two atom and the double arrow as the streched bond.

The stretching is of two types

a. Symmetrical stretching
b. Asymmetrical stretching

a. Symmetrical Stretching

Here, the two hydrogen atoms either move towards or away from the central carbon atom in unison, by which either altering the interatomic distance or causing no change in valence angle…

Molecular vibration GIF

b. Asymmetrical stretching

In this example, one hydrogen atom approaches the carbon atom while the other moves away from the carbon atom.

molecular vibration in IR

#2. Bending and deformation

In which the position of the atom changes relatives to original bond axis.

A. In plane bending vibration –

i. Scissoring
Here, the two atom connected to a central atom either move toward or away from each other with certain deformation of the valence angle.

animation of molecular vibration in IR

ii. Rocking
In this instance, the structural unit swings back and forth in the plane of The molecule.

Infrared spectroscopy, Principle, Instrumentation and Applications 3

B. Out of Plane Bending Vibrations

i. Wagging
In this case the structural unit swings and forth out of the plane of the molecule.

molecular vibration animation

ii. Twisting
Here, the structural unit rotates about the bond that joint it to the rest of the molecule.

molecular vibration image gif


The main parts of an IR spectrometer are :

1. IR radiation source.
2. Monochromators
3. Sample cell and sampling of substance.
4. Detector.

1. The IR Radiation Sources

Infrared instruments need a source of radiant energy which provides a means for isolating narrow frequency band. The radiation source must emit IR radiation which should be :

a. Intense enough for detection
b. Steady.
c. Extend over the desired wavelengths.

The various popular source of IR radiation are as follows ;

a. Incandescent lamp
b. Nernest glower
c. Globar Source
d. Mercury Arc.

2. Monochromators

The radiation source emit radiation of various frequency as the sample in IR spectroscopy absorb only at certain frequency, it thus becomes essential to select desired frequencies from the radiation source and reject the radiation of other frequencies. Thus selection has been achieved by means of Monochromators which are mainly of two types :

a. Prism Monochromators
b. Grating Monochromators.

3. Sample cells and sampling of substance

As infrared spectroscopy has been used for the characterization of solid, liquid or gas sample. But these sample have to be treated differently. Although, the only common point to the sampling of different phase is that the material containing the sample should be transparent to IR radiation.

4. Detector

The various type of detector are ;

  • a. Balodmeters
  • b.Thermocouples
  • c. Thermistors
  • d. Golay cell
  • e. Photoconuctivity cell

    Single Beam spectrophotometer :


    1. In the single – beam system, the radiation is emitted by the source through the sample and then through a fixed prism and rotating Littrow mirror.

    2. Both prism and Littrow mirror select the desired wave length and then permit it to pass on to the detector..

    3. The detector measures the intensity of radiation after passes through the sample.

    4. Knowing the original intensity of radiation, one can measure how much radiation has been absorbed.

    5. By measuring the degree of absorption at wavelength, the absorption spectrum of the sample can be obtained.

    Process of IR spectroscopy

    Application of Infrared Spectroscopy to organic compounds.

    1. Identification of substance
    Infrared spectroscopy is used to establish whether a given sample or an organic substance is identical with another or not like : Alkanes, Alkenes, Alkynes, Aromatic ring etc.

    2. Determination of Molecular structure
    Infrared spectroscopy is helpful in determining molecular structure of unknown substance.

    From an examination of the position of absorption band in the is possible to establish the nature of the gas groups present in the molecule.

    3. Detection of impurities.

    4. Isomerism in organic chemistry

    5. Identification of functional groups

    6. Miscellaneous Examples

    The following are some important application ;

    a. Determination of purity
    b. Shape of symmetry of a molecule.
    c. Presence of water in a sample.
    d. Measurement of paints And varnishes

    VIdeo Tutorial

    Watch this video tutorials for better understanding.



    So that was all about Infrared spectroscopy, Principle, Instrumentation and Applications. hope it will help you.

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