Infrared Spectroscopy

Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum.

Spectroscopy is the study of the interaction of electromagnetic waves with matter. The wavelength of the color corresponds to the energy level.

The wavelength of the near-infrared region is 0.72 to 0.25 micrometer, Mid-region 2.5 to 25 and far region 25 to 300. The near region allows the study of harmonic combination and vibration whereas the mid-region allows the study of rotation and vibration of small molecules and Far for vibration of heavy atoms.

The spectroscopy spread randomly for the characterization of chemical material present on our planet as well as beyond the planet and stars. By 1900, this becomes an important tool for the identification and characterization of chemical compounds and materials. By 1950, spectroscopy is used for the analysis of complicated molecules such as protein. Later, used for study in the membrane, biomolecules, and even DNA.

Types of IR Spectroscopy

The infrared spectroscopy is of two types:

• Classical infrared spectroscopy

Classical infrared spectroscopy consists:

• Diffraction grating
• Detector
• Optical setup of mirrors
• A sample chamber.


Infrared radiation was discovered by William Herschel in the year 1800. He was investigating the energy level associated with the wavelength in the visible spectrum. He noticed that the temperature increase from the blue to red region of the spectrum. He also measured the temperature outside the red region thinking that the increase should stop. But, the temperature there was even higher. He called these rays infrared rays of invisible rays.

The first infrared spectrometer was built in 1835.

Absorption of infrared is restricted to compounds with small energy differences in the possible vibrational and rotational states. For a molecule to absorb infrared the vibrations and rotations within a molecule must cause a change in the dipole moment of the molecule. The bonds between the atoms vibrate.

They possess vibrational motion that is stretching and bending. In stretching the bond length would change.

There are two types of stretching vibration namely;
• Symmetric
• Asymmetric.

The bending vibrations are the bending vibrations which are lead to changes in bond angle concerning the original bond axis. In this vibration one bond may rock back and forth within the plane or bend back or forth outside the plane.

There are four types of bending vibration namely:
• Wagging,
• Scissoring,
• Rocking,
• Twisting.

A photon of light that has a frequency of infrared range will be absorbed if the bonds between atoms in the target material allow these atoms to vibrate in these frequencies.

Working Principle

Infrared radiation is emitted from a glowing black body source and a beam is passed through an aperture that controls the amount of energy present to the sample. The beam enters the interferometer which consists of the components where the spectral encoded takes place and the resulting interferogram signal exit the interferometer.



The beam enters into the sample compartment where it is transmitted through or reflected on the surface of the sample depending upon the type of analysis. Here the interaction takes place and specific frequencies, characteristics to that of the sample are absorbed.

• Source of Radiation

The various popular sources of IR radiations are:
• Incandescent Lamp
• Nernst Glower
• Glower Source
• Mercury Arc


• Various types of monochromator are prisms, grating and filters.
• Prisms are made of Potassium Bromide and Sodium Chloride.
• Filters are made up of Lithium Fluoride and diffraction grating are made up of alkali halides.


The detector is specially designed to measure the special interferogram signal. The microprocessor using the Fourier transform analysis and the final spectra is presented to the chemist for interpretation.

The various types of Detectors used in IR Spectroscopy are:
• Bolometers
• Thermocouple
• Thermistors
• Golay Cell
• Photoconductivity Cell
• Semiconductor Detectors
• Pyroelectric Detectors
• Fourier Transform Systems

Fingerprint IR Region

• In the IR region below 1500cm is rich in many absorption bands and the region is known as FP region.

• Here the number of bending vibrations are more than stretching vibrations.

• In this region small difference in structure and constitute of molecule result significant changes in absorption bands.

• Many compounds show unique absorption band in the region which is very useful in compound identification.

Advantages of FTIR

• This is the non-destructive technology the exact information concerning the molecular structure and characterization at a variety of levels.

As all the frequencies are measured at the same time that is within a few seconds is an advantage.

The detectors are much more sensitive and optical are throughout is much higher resulting in low-level noise and the fast scanning enable random measurement at the desired level.

Mechanical Simplicity:
The moving mirror is the only component that keeps on continuously moving. This means there is little possibility for mechanical breakdown.

Internally Calibrated:
The instrument employed laser as an internal wavelength calibration standard so these increases are self-calibrating. Thus, it is a very reliable technology for identification for virtually any sample. The sensitivity enables the identification of the smallest contaminants also. This makes a tool of quality control or quality assurance applications.


• Identification of Organic Compounds.
• Determination of Molecular Structure.
• Studying the Progress of Reactions.
• Detection of Impurities.
• Isomerism in Organic Chemistry.
• Shape or Symmetry of a Molecule.
• Determination of Purity.
• Presence of Water in a Sample.
• Measurement of Paints and Varnishes
• Examination of Old Paintings and Artifacts

Limitations of Infrared Spectroscopy

• By IR spectroscopy, it is not possible to know the molecular weight of a substance.

• Generally, IR spectroscopy does not provide information on the relative positions of different functional groups on a molecule.

• From the single IR spectrum of an unknown substance, it is not possible to know whether it is a pure compound or a mixture of compounds.

An interesting example is that a mixture of paraffin and alcohols will give the same IR spectra as by higher molecular weight alcohols.

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