Infrared spectroscopy: an introduction, principle, instrumentation, with its application.

What is Infrared spectroscopy?


Infrared spectroscopy deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, which is mostly based on absorption spectroscopy. It can also be used to identify and study chemicals.  Its concept can generally be analyzed in three ways- by measuring reflection, emission, absorption.

It is one of the most widely used spectroscopic techniques which is employed mainly by organic and inorganic compounds, because of its usefulness in determining structures of compounds and identifying them.


Infrared spectroscopy or IR spectroscopy is concerned with the study of absorption of infrared radiation. It is also known as vibrational spectroscopy, which results in vibrational transitions. It is mainly used in structure elucidation to determine the functional groups. It is already known that-

Energy of a molecule = Electronic energy + Vibrational energy + Rotational energy

The study is described that focused on the changes in the vibration of the molecule or absorption of energy due to vibrations.

Infrared spectroscopy is conducted with an instrument called an infrared spectrometer or infrared spectrophotometer that used to produce an infrared spectrum. The most common laboratory instrument used technique is a Fourier transform infrared (FTIR) spectrometer.

Principle of infrared spectroscopy-

In any molecules, the atoms or groups of atoms are connected by bonds, these bonds are analogous to springs and not rigid in nature. Because of the continuous motion of the molecule, they maintain some vibrations with some frequency characteristics to every portion of the molecule. This is called the Natural frequency of vibration.

When the energy in the form of infrared radiation is applied.

In infrared, when Applied IR frequency = Natural frequency of vibration

Absorption of radiation takes place and a peak is observed.

Infrared vibration of Ethanol

Every portion, bond of a molecule, or functional group requires different frequencies for absorption; hence the characteristic peak is observed for each functional group or portion of the molecule. Infrared spectra are nothing but a fingerprint of a molecule. Example- Infrared spectra of Ibuprofen.

Infrared spectra of Ibuprofen

In Infrared spectra, use wavenumbers instead of wavelengths for mentioning the characteristic peak. Because the wavenumber is having larger values and easy to handle than wavelength which will show only small differences between functional groups.

It measures the vibrations of atoms, which is based on it because it is possible to determine the functional groups. Usually, the light atoms and stronger bonds will vibrate at a high stretching frequency or wavenumber.

Wavenumber is nothing but the number of waves present per cm, which can be calculated from the wavelength.


You may read-Spectroscopy.

Criteria for a compound to absorb Infrared radiation-

-Change in dipole moment.

-The applied Infrared frequency always should be equal to the natural frequency of radiation.

Otherwise compounds do not give Infrared peaks.

Types of energy transitions in every region of the electromagnetic spectrum are-

Region of SpectrumEnergy Transitions  
X-raysBond breaking
RadiofrequenciesNuclear spin (nuclear magnetic resonance) Electronic spin (electron spin resonance)

Regions of the Infrared spectrum-

The regions of the infrared in the electromagnetic spectrum is usually divided into three regions- near, middle, and far-infrared. These regions are named for their relation to the visible spectrum.

The higher energy near-infrared can excite overtone or harmonic vibrations. Middle-infrared used for the fundamental vibrations and associated with the rotational-vibrational structure. Far-infrared lying adjacent to the microwave region has low energy and may be used for rotational spectroscopy.

The region of infrared is between 0.8 to 1000 µm, mostly the vibrational bands, which occur in 2.5 to 25 µm. The range of the infrared region are-

IR-RegionWavelength(µm)Wavenumber (cm-1)Frequency (Hz)
Near (overtone)0.8 to 2.5 µm12500 to 4000 cm-13.8x 10 14 to 1.2×10 14 Hz
Middle (Vibration-rotation)2.5 to 50 µm4000 to 200 cm-11.2×10 14 to 6.0×10 12 Hz
Far (Rotation)50 to 1000 µm200 to 10 cm-16.0×10 12 to 3.0×10 12 Hz
Most used2.5 to 25 µm4000 to 400 cm-11.2×10 14 to 2.0×10 13 Hz

Theory of infrared spectroscopy-

To absorb infrared for a molecule, the vibrations or rotations within a molecule must cause a net change in the dipole moment of the molecule. And the alternating electrical field of the radiation interacts with fluctuations in the dipole moment of the molecule with the action.

Remember that electromagnetic radiation consists of an oscillating electric field and an oscillating magnetic field, perpendicular to each other. when the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, it causes a change in the amplitude of molecular vibration. The frequency of the vibration is by a formula is-



Where v is the frequency,

F is the force content,

µ is the reduced mass,

It is given by the formula-


Where w1 andw2 are masses of the individual atoms.

Vibration– Where the change in the shape of the molecule-stretching of bonds, bending of bonds, or internal rotation around single bonds, is known as vibration in a molecule. It is studied because whenever the interaction between EMR (Electromagnetic radiation) waves and matter so change appears in these vibrations.

Types of Vibrations-

-Stretching vibrations– These are vibrations in which the bond length is altered, in increased or decreased form. There are 2 sub-types-

Symmetrical stretching– in which two bonds increase or decrease in length, symmetrically.

Asymmetrical stretching– in which when one bond length increases, the other one decreases.

-Bending vibrations

+In plane bending- In these vibrations, there is a change in bond angle. The bending of bonds takes place within the same plane.

Scissoring– In which the bond angle decreases is called scissoring.

Rocking– In which the bond angle is maintained, but both bonds move within the plane is called rocking.

ofplane bending– There is a change in bond angle and the bending of bonds takes place within outside the plane of the molecule.

Wagging– In which both atoms move to one side of the plane is called wagging.

Twisting– In which one atom is above the plane and the other is below the plane is called twisting.

Types-of-bending-in vibration

Different types of vibration in a molecule-

If a molecule contains ‘n’ atoms, then the total number of fundamental vibrations can be expressed in-

(3n-6) in a Non-linear molecule and (3n-5) in a Linear molecule

But sometimes, it will not be possible to obtain the predicted number of peaks, because of spectral overlapping, low resolution, or due to weak peaks, etc.

Factors Affecting the Vibrational frequency-

The calculated value of the frequency of absorption for a particular bond is never exactly equal to its experimental value. There are many factors that are responsible for vibrational shifts-

-Vibrational coupling– It is observed in the compounds containing the –CH2 and –CH3 groups such as carboxylic acids, amides, and aldehydes.

-Bonding of hydrogen shows the absorption shift towards the lower wavelength. This is observed in the alcohols, phenols, and enols. Hydrogen bonding brings about remarkable downward frequency shifts. Stronger the hydrogen bonding, greater is the absorption shift towards lower wavelength than the normal value.

There are 2 types of hydrogen bonding

Intermolecular →broadbands

Intramolecular → sharp bands

Bonding of hydrogen in O-H and N-H compounds deserve special attention. Example- alcohols & phenols, enols & chelates.

Electronic effects such as conjugation, mesomeric effects, and inductive effect lower the absorption frequencies.

1. conjugation– It lowers the absorption frequency of C=O stretching whether the conjugation is due to α, β- unsaturation, or due to an aromatic ring.

2. mesomeric effect– A molecule can be represented by two or more structures that differ only in the arrangement of electrons.

3. inductive effect– It depends upon the intrinsic tendency of a substituent to either release or withdraws electrons.

Instrumentation of infrared spectroscopy-

In the infrared, the middle infrared region is energetic adequate to excite molecular vibrations to higher energy levels. In infrared absorption bands the wavelength is characteristic of specific types of chemical bonds, and finds its greatest utility for the identification of organic and inorganic molecules.

infrared spectrophotometer

It is an instrument that passes infrared light through an organic molecule and produces a spectrum. In the spectrum of infrared, that contains a plot of the amount of light transmitted on the vertical axis against the wavelength of infrared radiation on the horizontal axis.


In infrared spectra, the absorption of peaks points downward because the vertical axis is the percentage transmittance of the radiation through the sample in it. Absorption of radiation lowers the percentage transmittance value because all bonds in an organic molecule interact with infrared radiation, so the infrared spectra provide a considerable amount of structural data.

Source- Sample – Monochromator – Detector – Recorder

-Light Source-

The source of continuous infrared radiation is required for measuring the infrared absorption and for a sensitive infrared transducer, or detector. These are required for measuring infrared absorption. The sources are consisting of an inactive solid that is electrically heated to a temperature between 1,500 and 2,200 K.

The heated material will then emit infra-red radiation. The source of IR radiation is Nernst glower, globar source, or carbon dioxide laser is used.

Nernst glower– It is constructed of rare earth oxides in the form of a hollow cylinder and at the ends of the cylinder the platinum leads are consisting which allow the passage of electricity. Nernst glowers are fragile. These are having a large negative temperature coefficient of electrical resistance and they must be preheated to be conductive.

Globar source– It is a rod of silicon carbide in 5 mm diameter, 50 mm long which is electrically heated to about 1,500 K. The process of water cooling of the electrical contacts is needed to prevent arcing in a globar source. The spectral output is comparable with the Nernst glower, except at short wavelengths less than 5 mm, where its output becomes larger.

Carbon dioxide laser– It is a tuneable carbon dioxide laser that is used as an infrared source. It is used for monitoring certain atmospheric pollutants and for determining absorbing species in aqueous solutions as an infrared source.

-Sample handling-

 Infrared spectroscopy is used for the characterization of solid, liquid, or gas samples.

Solid– For preparing solid samples various techniques are used such as pressed pellet technique, solid run in solution, solid films, mull technique, etc.Mainly for solid sample handling the process of the pressed pellet technique is used.

In this process the solid samples are mixed with Potassium bromide and compressed into a thin transparent pellet by using a hydraulic press and are used for analysis. Alternatively, the sample can be mixed with Nujol (mineral oil) and a film of liquid (Nujol Mull) is applied on a liquid sample cell.

Liquid– For liquid samples, the process can be held using a liquid sample cell which is made up of alkali halides. Aqueous solvents are not be used because they will dissolve alkali halides. Only organic solvents like chloroform are used.

Normally the liquid samples are analyzed in the rectangular cells made up of NaCl or KBr. In the case of organic liquids, it must be dried before taking into the sample cells.

Gas– For a sampling of gas, the process is similar to the sampling of liquids. The spectrum of a gas can be obtained by the process of permitting the sample to expand into an evacuated cell, and it is also called a cuvette. The gaseous samples are measured in the special sample cells which are made up of NaCl.


The radiation source emits the polychromatic light which contains a wide range of frequencies. It is used to convert the polychromatic light into monochromatic light. The most commonly used monochromators are prism and grating monochromators.

Filters made up of Lithium fluoride or prisms made of Potassium bromide, Caesium iodide, Sodium Chloride or Thallium Bromo iodide are used.  Diffraction gratings made up of alkali halides are also used.


These are used to measure the intensity of unabsorbed infrared radiation. Detectors like thermal detectors, thermocouples, bolometers, thermisters, Golay cells, photo conducting, and pyro-electric detectors are used.

Thermal detectors– These can be used over a wide range of wavelengths and they operate at room temperature. The main disadvantages of the thermal detectors are slow response time and lower sensitivity relative to other types of detectors.

Thermocouple– It consists of a pair of junctions of different metals; for example, two pieces of bismuth fused to either end of a piece of antimony. The potential difference or voltage between the junction’s changes according to the difference in temperature between the junctions. In a series, the several thermocouples connected are called a thermopile.

Bolometer- The functions by changing resistance when heated and constructed of strips of metals such as platinum or nickel or from a semiconductor.

Pyroelectric detector- Consists of a pyroelectric material which is an insulator that having the special thermal and electric properties. For pyroelectric infrared detectors, triglycine sulphate is the most common material is used.

Unlike other then thermal detectors, it depends on the rate of change of the detector temperature rather than on the temperature itself. This allows it to operate with much faster response time and makes these detectors the choice for Fourier transform infrared (FTIR) spectrometers where rapid response is essential.

Photo conducting detectors– It is the most sensitive detectors. They rely on interactions between photons and a semiconductor. It consists of a thin film of semiconductor material like- lead sulphide, mercury cadmium telluride, or indium antimonide. These semiconductors are deposited on a nonconducting glass surface and then sealed into a deviate wrapper to protect them from the atmosphere.

 For the near-infrared region of the spectrum, the lead sulphide detector is used. For middle and far-infrared radiation the mercury cadmium telluride detector is used. It must be cooled with liquid nitrogen to minimize disturbances.


They are used to record the IR spectrum, on white paper or transparent sheets. Recorders are used to record the IR spectrum.

Types of instruments-

Also, FT-IR (Fourier Transform) Infrared Spectrophotometers with advanced features like matching of spectra, identification of functional group/compound using library databases, and software are available.

The advantages of FT-IR over dispersive instruments are that it is rapid, more sensitive, accurate, and has more computational capabilities.

Fourier transform infrared (FTIR)-


Fourier transform infrared, more commonly known as FTIR, is the preferred method for infrared spectroscopy. Developed in order to overcome the slow scanning limitations encountered with dispersive instruments, with FTIR the infrared radiation is passed through a sample. In FTIR, the measured signal is described as an interferogram.

Performing a Fourier transform on this signal data results in a spectrum identical to that from conventional or dispersive infrared spectroscopy, but results are much faster with results in seconds, rather than minutes. FTIR is the method of obtaining an infrared spectrum by measuring the interferogram of a sample.

Instrumentation of FTIR-


Source- Laser- Interferometer- Detector

Source– Electronically Temperature controlled (ETC)- It is an efficient ceramic, with a refractory united that suddenly increases to operating temperature. It is also thermally insulated to maintain a constant operating temperature.

It provides energy for the spectral region from 7400-50 cm-1. The temperature of the source is constantly monitored and controlled at 1140*C by the ETC.

Laser- A Helium-neon lar (He-Ne laser) is used which is a type of gas laser whose gain medium consists of a mixture of helium and neon (10:1) inside of a small-bore capillary tube, usually exited by a DC electrical discharge. The laser creates the drive volt for the moving mirror. It is used as an internal reference.

Interferometer– It is the heart of the spectrometer and a device in which two or more radiation beams interfere with each other after passing through different optical paths. These produce a unique type of signal which has all of the infrared frequencies encoded into it, and output is an interferogram or interference record.

The two domains of distance and frequency are interring convertible by the mathematical method of furrier transformation.

Michelson Interferometer– It is the basic part of the FTIR spectrometer which is used to split one beam of light into two beams so that the paths of the two beams are different. Then it recombines the two beams and conducts them into the detector where the difference of the intensity of these two beams is measured as a function of the difference of the paths.

Schematic-diagram-of-Michelson Interferometer

To produce an optical path, difference an optical device causing two beams of light to travel different distances. This allows constructive and destructive interference to occur. The changing the optical path difference allows the measurement of an interferogram. It is consisting of the parts are-

-Beam splitter

-Fixed mirror

-Moving mirror

Detector- Pyroelectric detectors and photoconductive detectors are used.

You may read- Electron Spin Resonance spectroscopy.

Application of infrared spectroscopy-

-Identification of functional group and Structure elucidation-

The entire IR region is divided into –

Group frequency region – 4000cm-l to 1500cm-l

Fingerprint region- 1500cm-l to 400cm-l

In the group frequency region, the peaks corresponding to different functional groups can be observed, example-Amino group, alcoholic group, etc. Every part of the molecule has different atoms and are connected by bonds, each bond requires a different IR region for absorption and so characteristic peaks are observed. Hence this region of IR spectrum is called the fingerprint region of a molecule.

-in the identification of drug substances-

 Infrared spectrum of sample and standard can be compared to identify a substance. If the spectra are the same, then the identity of the sample can be confirmed. This technique is called spectral matching.

-In the identification of the impurities in a drug sample

 Impurities have different chemical natures as compared to pure drugs, hence these impurities give rise to additional peaks than that of pure drugs. By comparing these, it can identify and also the presence of impurity.

Study of hydrogen bonding

whether it is of intermolecular or intramolecular type.

Study of polymers.

-The ratio of Cis-trans isomers in a mixture of compounds.-Quantitative analysis-

The quantity of a substance can be determined either in pure form or as a mixture of two or more components. In this, the peak which is characteristic for the drug is chosen and a comparison of the extinction (log 10/10 of peaks for standard and sample is done. This method is called a Baseline technique and is thus used to determine the quantity of a substance.

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