What is IR Spectroscopy

Infrared (IR) or vibrational spectroscopy is a method used for analyzing the particle's vibratory transformations. This is one of the very popular spectroscopic approaches employed by inorganic as well as organic laboratories because it is helpful in evaluating and distinguishing the frameworks of the molecules. The infra-red spectroscopy process or procedure is carried out using a tool called an infrared spectrometer to obtain an infrared spectral (or spectrophotometer).

Properties of Infra-red Spectroscopy

  • Infrared spectroscopy is an examination of the association between infra - red light with an atom.
  • For the study of organic molecules the most important component of the infrared field is wavelength from 2,500 to 16,000 nmand the acceptable frequency spectrum ranges are 1.9×1013 to 1.2×1014 Hz.
  • Similarly, infrared spectrometers will allow experimenters to acquire the absorption spectrum of molecules that reflect their molecular framework.
  • Infrared spectroscopy is a basic calculation of an infrared (IR) spectrum that is a diagram of the observed amplitude of infra - red according to the wavelength and otherwise intensity of light.
  • IR Spectroscopy tests atom motions, from which functional groups can be determined.
  • The photon frequencies linked to this infra - red element ( 1-15 kcal/mole) are not highly sufficient to stimulate the electrons but can cause the covalently bound atoms or groups to vibrate.
"An image showing Infrared wavelength."

Principles of Infrared Spectroscopy

Atoms are constantly moving in the atom, with three degrees of freedom for each. The atom contains a measurable energy quantity in both these motions. The sum of the mechanical (Eelec), vibrational (Evib), and rotational energy (Erot) is the overall atomic energy.

tot  = E  elec  + E  vib  + E  rot

The radiations from the middle infrared field have low energies, but are enough to induce vibrational as well as rotational excitement. This energy exchange induces variations in the electric patterns of vibration or rotation of the attached atoms. We deal with the vibrational (and rotating) power of the molecule in the middle infrared (IR) spectroscopy. The molecular vibrations can be of two types, stretching and bending. A stretching pulse is a syncopated motion of interatomic distance variations along the axis of the bond as bending movement shows a bond angle shift. 

Instrumentation of Infrared Spectroscopy

Infrared spectrometer consists of the following parts:

  • Source of radiation.
  • Sample cells.
  • Monochromators.
  • Detectors.
  • Recorders. 
Source of radiation

Inert solids, usually zirconium oxide either rare earth oxides (Nernst glowers, goobers of silicon carbide) and nichrome coils are the typical sources of IR radiation for an IR spectrophotometer. Heated IR origins to 1000-1800°C. It generates constant radiation. FT – IR water cools the source to improve energy as well as stability.

Preparation of samples

The samples to be examined can be in liquid state, gaseous state, solution state or in a solid state.

Gas sample

In IR spectrophotometer, gas specimens can be immediately examined.  The specimen is normally located in a little special cell about 10 cm immediately along the occurring radiation line. Wide cells are also used. The cell's two ends generally consist of sodium chloride that is translucent and not absorbent into the IR field.

Liquid sample

Within sodium chloride plates, a drop of pure liquid sample is mounted and analyzed.

Solid sample

Small powdered solid is blended and pasted with hydrocarbon fluid, such as nujol or hexachlorobutadiene. It is positioned between flat sodium chloride plates inserted in IR spectrophotometer in the course of incident IR radiation. The sample is subsequently combined and compressed with potassium bromide, which is then inserted into the spectrophotometer.

Solution sample

Solid is normally dissolved at 1-5%concentration level in carbon-tetrachloride or chloroform. The solution is in a sodium chloride special cell.


Prism, gratings and filters are numerous kinds of monochromators. Prisms are composed of bromide of potassium, iodide of caesium, or sodium chloride. Filters consist of lithium fluoride and gratings of diffraction consist of alkaline halides.


In IR spectroscopy different detectors are employed.  Pyroelectric detectors and photo diodes and diodes arrays are commonly used in FT-IR instruments. The Pyroelectric detector comprises a split among two electrodes mono-crystals of deuterated Triglycin sulphate and lithium tanslate.


Recorders are utilized to measure the infrared spectrum.

"An image showing parts of Infrared spectroscopy."

Interpretation of Infrared Spectroscopy Results

For the analysis of spectrum there is no one golden law. The continuum must be interpreted by the fundamental criteria or cautionary steps.

An IR continuum is a dynamic plot in general. Multiple organisms have spectrum absorption bands. By evaluating the range with previously existing knowledge criteria, a cautious analytical evaluation is expected. The chemical composition of unidentified chemical compounds is identified by a maximum to high relationship with connoted IR absorption properties for a given functional group and bond.

Applications of Infra-red Spectroscopy

Infrared spectroscopy has a wide range of applications in identifying and quantification of various chemical components.

Food analysis

The detection and measurement of contaminants in foods can be done by spectroscopy. Other examples of contamination have been registered, including juicy, vegetable oils and milk as well the quality of caffeine, protein, lipid or sugar.

Paper industry

Infrared spectroscopy is used in the pulp and paper industry for quality management. The cellulose fibers, inorganic fillers and binders are used in the material. IR can identify the incorporation of polymers with calcium carbonate. Infra - red spectroscopy could also be used to track changes in the processing of cellulose.

Environmental analysis

A wide spectrum of environmental assessments covering climate, water or soil samples has been performed using infrared (IR) spectroscopy. Industrial gas contamination fire emissions even astronomical installations are other popular implementations. In order to consider climate change, the assessment of atmospheric aerosol concentrations is crucial and infrared spectroscopy can be employed to measure the most common and crucial greenhouse gases namely CO 2 , CH 4 and N 2 O.

Forensic investigation

Infrared spectroscopy is used for forensic research to examine the specimens under analysis. ATR technology has been very effective in this regard since it needs little sample and it really is non-destructive. Blood or dye stain examination can be quickly analyzed on clothes, fabrics including other materials. In addition, any contamination can be inspected on hair fiber.

FTIR Spectroscopy

By developing an infrared absorption spectrum, Fourier Transform Infrared Spectroscopy (FTIR) detects chemical bonds in a molecule. It's a fast, non-destructive method for detecting a variety of functional groups that's also responsive to changes in molecular structure.

Common Mistakes

The most common mistake in infrared spectroscopy one can make is while reading the graph. All the peaks should be analyzed properly to find the functional group present in the chemical compound. 

 Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.

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