Infrared (IR) Spectroscopy- Definition, Principle, Parts, Uses

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What is Infrared (IR) Spectroscopy?

Infrared (IR) spectroscopy or vibrational spectroscopy is an analytical technique that takes advantage of the vibrational transitions of a molecule.

It is one of the most common and widely used spectroscopic techniques employed mainly by inorganic and organic chemists due to its usefulness in determining the structures of compounds and identifying them.

The method or technique of infrared spectroscopy is conducted with an instrument called an infrared spectrometer (or spectrophotometer) to produce an infrared spectrum.

Infrared (IR) Spectroscopy

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Principle of Infrared (IR) Spectroscopy

  1. Infrared Spectroscopy is the analysis of infrared light interacting with a molecule.
  2. The portion of the infrared region most useful for analysis of organic compounds have a wavelength range from 2,500 to 16,000 nm, with a corresponding frequency range from 1.9*1013 to 1.2*1014 Hz.
  3. Photon energies associated with this part of the infrared (from 1 to 15 kcal/mole) are not large enough to excite electrons, but may induce vibrational excitation of covalently bonded atoms and groups.
  4. It is known that in addition to the facile rotation of groups about single bonds, molecules experience a wide variety of vibrational motions, characteristic of their component atoms.
  5. Consequently, virtually all organic compounds will absorb infrared radiation that corresponds in energy to these vibrations.
  6. Infrared spectrometers, similar in principle to other spectrometer, permit chemists to obtain absorption spectra of compounds that are a unique reflection of their molecular structure. 
  7. The fundamental measurement obtained in infrared spectroscopy is an infrared spectrum, which is a plot of measured infrared intensity versus wavelength (or frequency) of light.
  • IR Spectroscopy measures the vibrations of atoms, and based on this it is possible to determine the functional groups.
  • Generally, stronger bonds and light atoms will vibrate at a high stretching frequency (wavenumber).

Instrumentation of Infrared (IR) Spectroscopy

The main parts of the IR spectrometer are as follows:

  1. Radiation source
  2. Sample cells and sampling of substances
  3. Monochromators
  4. Detectors
  5. Recorder

A. IR radiation sources

IR instruments require a source of radiant energy which emits IR radiation which must be steady, intense enough for detection, and extend over the desired wavelength.

Various sources of IR radiations are as follows.

  1. Nernst glower
  2. Incandescent lamp
  3. Mercury arc
  4. Tungsten lamp
  5. Glober source
  6. Nichrome wire

B. 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.

C. 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.

D. Detectors 

  • Detectors are used to measure the intensity of unabsorbed infrared radiation.
  • Detectors like thermocouples, Bolometers, thermisters, Golay cell, and pyro-electric detectors are used.

E. Recorders

  • Recorders are used to record the IR spectrum.

Applications of Infrared (IR) Spectroscopy

It has been of great significance to scientific researchers in many fields such as:

  • Protein characterization
  • Nanoscale semiconductor analysis and
  • Space exploration.
  • Analysis of gaseous, liquid or solid samples
  • Identification of compounds
  • Quantitative analysis
  • Information regarding functional groups of molecules and constitution of molecules can be deduced from IR spectrum
  • To know about interaction among molecules

Reference

  1. https://chem.libretexts.org
  2. https://en.wikipedia.org/wiki/Infrared_spectroscopy
  3. https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/spectrpy/infrared/infrared.htm
  4. http://www.wag.caltech.edu/home/jang/genchem/infrared.htm
  5. https://web.vscht.cz/~poustkaj/EN%20ASFA%20AU%20Koplik_Infrared_spectroscopy.pdf
  6. https://www.pharmatutor.org/pharma-analysis/analytical-aspects-of-infra-red spectroscopy-ir/instrumentation-of-ir-spectrophotometry

About Author

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Sagar Aryal, PhD

Dr. Sagar Aryal is a microbiologist, researcher, and science communicator from Nepal. He is also co-founder and Research Head of the Department of Natural Products at the Kathmandu Research Institute for Biological Sciences (KRIBS), where he leads research on natural product discovery and microbial biodiversity. Dr. Aryal earned his Ph.D. in Microbiology from Tribhuvan University, Kathmandu, Nepal, in 2025, supported by a DAAD Research Grant and a University Grants Commission (UGC) Nepal Ph.D. Fellowship. His research interests include Actinomycetes, Myxobacteria, microbial taxonomy, and natural products, particularly the discovery of bioactive compounds with potential pharmaceutical applications. He isolated and identified Nocardia nepalensis, a novel bacterial species from Nepal, contributing to understanding microbial diversity in the region. Dr. Aryal has published more than 16 research articles in international journals and contributed three book chapters to established academic publications. He has presented scientific research findings and developed international research collaborations through visits to more than 18 countries across Asia, Europe, North America, and Oceania. From 2023 to 2025, he served as the American Society for Microbiology (ASM) Young Ambassador to Nepal. Since December 2025, he has served as an Editorial Board Member of Scientific Reports, a Nature Portfolio journal. In recognition of his scientific contributions, Dr. Aryal received the NAST Science and Technology Youth Award 2025 from the Nepal Academy of Science and Technology. He previously received the Frs. Locke and Stiller Research Award (LSRA) for the 2017-2018 cycle from St. Xavier's College, Kathmandu, and a Nepal Academy of Science and Technology (NAST) Research Grant for 2017-2018. Alongside his scientific career, Dr. Aryal has more than 15 years of experience in blogging, content writing, and search engine optimization (SEO).