Infra-red (IR) spectroscopy or Infra-red Absorption Spectroscopy is an analytical technique that applies infrared radiations for the chemical identification of a compound. It is a qualitative technique that helps to study the structural properties of a chemical compound, and if coupled with an intensity measuring instrument then it can be used for quantitative analysis.

It is a more quick technique as compared to other structure analysis methods such as X-ray diffraction, electron spin resonance, etc. 

The infrared region of the electromagnetic spectrum extends from the red end of the visible spectrum to the microwave region of the electromagnetic spectrum. The infrared region ranges from 0.8 to 200 microns. The infrared region is divided into the following regions:

  • Near IR – 0.7-2.5 microns(wavelength) or 13000-4000 cm⁻¹(wave number)
  • Middle IR – 2.5-50 microns(wavelength) or 4000-200 cm⁻¹(wave number)
  • Far IR – 16-200 microns(wavelength) or 400-10 cm⁻¹(wave number)
  • IR region – 2.5-16 microns(wavelength) or 4000-400 cm⁻¹(wave number)

The most useful region for the identification of any functional group in a chemical compound lies between 13000-4000 cm⁻¹. While the region between 4000-400 cm⁻¹ is known as the fingerprint region. 

Why IR Spectrum is Called the Fingerprint of a Compound?

IR radiation causes vibrations in the molecules of a chemical compound. So when a compound is exposed to IR radiations, the molecules of the compound vibrate at many rates of vibrations giving rise to closely packed absorption bands, which are known as the IR absorption spectrum.

These various bands are the results of the characteristic functional groups and bonds present in a substance. That’s why an IR spectrum of a chemical compound is known as its fingerprint which is used for the identification of the compound. 

The band positions in an IR spectrum can be expressed in the terms of wavenumber(v), which is the reciprocal of the wavelength while the band intensities can be expressed in the form of transmission or absorbance.

Transmittance(T) is the ratio of radiant power transmitted by a sample to the radiant power incident on the sample whereas the logarithm to the base 10 of the reciprocal of transmittance is called the absorbance(A).

What Causes the IR Spectrum?

IR radiations possess low energy as compared to Ultraviolet-visible radiations, due to which they cannot bring out the electronic changes, but can affect the vibrational and rotational energy levels of a molecule. The effective region IR which causes both vibrational and rotational transition lies between 2.5-1.5 microns.

Therefore the IR spectrum is also known as the vibrational-rotational spectrum. However, the higher IR (i.e. Above 25 microns) can cause only rotational transitions.  

The working principle of IR spectroscopy is Hooke’s law of simple harmonic motion which states that, “The strain of a substance is proportional to the applied stress within the elastic limit of that substance”.

It gives the frequency of motion as

 𝜈 = 1/2𝜋c * √𝜅/𝜇

𝜇 = m₁m₂ / (m₁ + m₂)

𝜈 = frequency
c = speed of light
k = force constant 
𝜇 = reduced mass of individual atoms
m₁ and m₂ = mass of two atoms in the molecule

One essential factor in the vibrational transition of a molecule is the change in the dipole moment during the vibrations. That concludes that if there is no dipole moment then no vibration will occur in the IR region. The quantity produced by this is the vibrational energy which is given by:

E(vib) = [ v + ½] h𝜈

𝜟E(vib) = h𝜈

𝜟E(vib) = energy difference between two vibrational levels
v = number of vibrational levels
h = Planck’s constant
𝜈 = vibrational frequency of molecular bond

The transition of molecular vibrations from the lowest level to the higher level is a result of IR radiation absorption. When the vibrations move from the lowest level to the first level, then the frequency of the radiation is called fundamental vibration frequency. When the transition of vibration occurs from the lowest level to the second level then the frequency of the radiation is overtone frequency.  

Polyatomic Vibrations

Above we have discussed the vibrations in the diatomic molecules. In this section, we’ll be learning about the vibrations in polyatomic molecules.

A polyatomic molecule is one which has multiple numbers of atoms. In such molecules, the atoms and bonds are not so rigidly linked. Therefore the atoms and bonds vibrate from the rest position to higher vibrational levels and give both fundamental and overtone vibrations bands.

The fundamental bands are the important bands that depend on the degree of freedom in a molecule (the sum of coordinates necessary to locate all the atoms of a molecule in space). A non-linear molecule containing N atoms has 3 degrees of freedom that possess 3N-6 vibrational degrees of freedom, while for a linear molecule, it is 3N-5.

The vibrations in a molecule can be divided into two principal groups namely:

Types of Bond Vibrations.
Types of Bond Vibrations

1. Stretching Vibrations

The vibrations in which the two bonded atoms oscillate continuously without altering the bond axis or bond angles. They are further classified as:

  • Symmetric Vibrations– In these vibrations the atoms move away from the central atom.
  • Asymmetric Vibrations– The vibrations in which one atom moves away from the central atom whereas the other atom moves towards the central atom.

2. Bending Vibrations

Unlike the stretching vibrations, in these vibrations, there is a continuous change in the band axis and angles with the common atom. They are also further categorized as:

  • Scissoring– The type of vibrations produced due to the back-and-forth motion of the atoms.
  • Rocking– These vibrations result from the oscillation of atoms back and forth out of the equilibrium plane.
  • Wagging– When the atoms oscillate in an equilibrium plane formed by the atoms, wagging vibrations are produced.
  • Twisting– The vibrations that occur when a structural unit rotates around the bond which joins to the molecule.

Infra-Red Spectroscopy Instrumentation

An IR instrument is composed of similar components as in Ultraviolet-Visible spectroscopy but has few differences and modifications. A typical IR spectroscopy instrument is composed of the following components:

a) Radiation source

The IR radiation sources are usually hot bodies similar to black bodies that continuously emit radiation. IR spectroscopy uses the following sources:

  1. Incandescent Lamps– In this lamp, a nichrome coil is used which can be raised to incandescence by resistive heating. A black oxide film formed on the coil gives acceptable emissivity. The maximum temperature can reach up to 1100 degrees Celsius. It produces less intense radiation but is a very reliable source. 
  2. Nernst Glower– It is the most commonly used radiation source in IR spectroscopy. The oxides of zirconium, yttrium, and thorium are fused together and molded in the form of hollow tubes or rods about 1-3 mm in diameter and 2-5 cm in length. The rod ends are cemented to short ceramic tubes for mounting and short platinum leads are provided for power connections.  It produces intense radiation and almost thrice that from nichrome wire and globar sources.
  3. Globar Source– A globar source is a rod of diameter 6-8 mm and length 50 mm, made up of sintered silicon carbide, enclosed in a water-cooled brass tube with a lot for emission of radiations. It is usually operated at 1300 degrees Celsius. One feature is that it is self-starting and is electrically heated.
  4. Mercury Arc Lamp– A mercury arc is enclosed in a quartz jacket to reduce the loss of radiation. It produces radiation similar to the black body. Usually used for producing very far IR radiations.
  5. Tungsten Filament– It produces radiation when a tungsten filament is heated. It is useful only for the near-infrared region.

b) Monochromators

The radiations emitted from the IR source are of varied frequencies, therefore a monochromator is installed so that only desired frequencies can pass through the sample. The two types of monochromators used in IR spectroscopy are:

  1. Prism Monochromators– Prisms are very simple and have a greater range, therefore used as a monochromator. Prisms made up of halogen salts are used in IR spectroscopy. Crystalline sodium chloride prisms are used for analytical work from 5-15 microns and are also adequate for 2.5 microns. Potassium bromide and cesium bromide are used for far IR regions. For near IR regions lithium fluoride is used.
  2. Grating Monochromators– Grating is a series of parallel straight lines cut out into a plane surface (usually plastic or glass coated with aluminum). The gratings minimize the number of scattered radiations and concentrate them into a single order.

c) Sample Holder

Demountable sample cells made from rock salt are used. Teflon spacers are used to adjust the path length. Since they are made up of alkali metals, they become foggy easily due to moisture. Therefore, regular polishing is required to render them useful again. 

d) Detectors and Recorder

As the name suggests, detectors detect the radiation transmitted from the sample. The following detectors are used by IR spectroscopy:

  1. Thermal Photodetectors– These detectors produce potential differences depending upon the amount of radiation. There are various kinds of thermal detectors which are as follows:

    i) Thermocouple DetectorIt is constructed by joining dissimilar metal stripes having different thermoelectric properties. These strips are welded with blackened gold foil. One welded joint is kept at a constant temperature, called the cold joint, and the other welded joint is exposed to radiation called the hot joint. The hot joint due to exposure causes a rise in temperature while there is no rise at the cold junction. Due to this difference in the temperatures at the two ends, there is a generation of potential difference that is used to produce signals.

    ii) Bolometers– It is basically a Wheatstone bridge, which is based on the fact that the electrical resistance of a metal increases approximately 0.4% for every celsius degree increase in temperature. When IR radiation falls on the metal, there is a change in the temperature which results in a change of resistance. This change in resistance is regarded as the measure of the amount of radiation that has fallen on the bolometer.

    Thermistors – They are similar to bolometers but made up of metal oxides. Another difference is that in thermistors with the increase in temperature, the resistance is decreased. 

    Golay Cells It is basically a small metallic cylinder closed by a blackened metal plate at one end and at the other end a flexible metalized diaphragm is used. Xenon gas is filled in the cylinder and the cylinder is sealed. When IR radiation falls on the blackened metal plate, the heat produced by it expands the gas-filled which results in the movement of the metallic diaphragm. The movement of the diaphragm results in the production of signals in the form of light reflected onto a photocell.

    2. Photodetectors These detectors detect the intensity of the light radiations from the sample. They are of the following types:

    Photoconductivity CellIt is a detector that consists of a thin layer of lead sulfide or lead telluride supported on glass and enclosed into an evacuated glass envelope. When IR radiation falls on the lead telluride, its conductance increases and causes more current flow. 

    Semiconductor DetectorsSemiconductors such as doped germanium are used. When IR radiation falls on the detector, an electron is displaced in the detector, changing its conductivity greatly. This results in the production of signals which are easily recorded.

Sample Preparation

One of the most important processes in IR spectroscopy is the sample preparation of the analyte. There are various methods applied for the sampling of the analyte which depends on the state of the analyte. 

  • Solid Samples The solid analytes are prepared by various methods such as:

    1. Dissolving in a Solvent This is the simplest method in which the solid analyte is dissolved in appropriate solvents such as chloroform, carbon tetrachloride, alcohols, cyclohexane, acetone, and carbon sulfide.

    2. Solid Film In this method, the sample solution is placed on a potassium bromide or sodium chloride surface and allowed to evaporate. As a result, the sample forms a thin film on the surface.

    3. Mull Technique The solid sample is mixed with heavy mineral oil called Nujol to form a paste. This paste is placed in between the two salt plates and then used for the spectral measurements.

    4. Disk Method This is also known as the pressed pellet method, in which a small amount of finely ground solid sample is intimately mixed with 100 times its weight of powdered potassium bromide in a mortar pestle. This mixture is then pressed under a high pressure of 25000 psi/g in an IR tablet press to form a transparent pellet.
  • Liquid Samples Liquid samples are usually handled in their pure forms in a variety of absorption cells such as demountable cells, sandwich cells, or cavity cells. These cells are made up of sodium bromide, thallium bromide, or potassium bromide. 
  • Gas Samples The dried gas sample is introduced into the gas cell made up of a glass or metal cylinder about 10 cm long whose end walls are made up of sodium chloride.

Limitations of Infra-Red Spectroscopy

  • It only determines the molecular structure of a compound and not the molecular mass.
  • It is unable to differentiate between the enantiomers of a compound.
  • The prisms used as monochromators can easily be corroded, therefore they need regular maintenance.
  • The relative positions of the atoms and different functional groups in the molecule cannot be determined.
  • A single spectrum is not enough to conclude the results.
  • It is a slow process.

Applications of Infra-Red Spectroscopy

  • It is helpful to study the progress of a chemical reaction.
  • It is widely used to determine the structure of a chemical compound.
  • It is highly used to identify organic compounds.
  • It can detect the impurities in the sample.
  • It is useful to differentiate the various isomers of a compound. 

Forensic Application of IR Spectroscopy

  • In forensic toxicology, IR spectroscopy can be used in the analysis of various organic and inorganic poisons. Various drugs and their purity can be examined using IR spectroscopy.
  • It is useful in the analysis of petroleum products and other chemical compounds such as alcohol and other solvents found on the scene of occurrence.
  • It is helpful in the analysis of paints and varnishes. Also, It can be utilized to examine old paintings and artifacts.
  • It is helpful in the examination of questioned documents, which can analyze the inks, handwriting, etc.
  • Explosives can also be examined by IR spectroscopy.
  • Hair and fiber analysis through IR spectroscopy is also common.


IR spectroscopy is a qualitative as well as quantitative (sometimes) analytical technique accepted with open arms by various scientific disciplines. It is a technique that is based on the vibrational and rotational motions of the atoms and the bonds present in a molecule. A peculiar feature of this technique is that it can determine the fingerprint of a chemical compound. 

It is a widely used technique in the forensic examination of various kinds of physical evidence. It is a very helpful technique but has a few drawbacks which are hoped to be countered in the future.

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