Infrared Spectroscopy

Infrared Spectroscopy is a powerful analytical technique that provides information about the interaction of IR light with molecules. It is used to identify substances and can be used to determine the structure by analysing the frequencies at which it absorbs IR light. It works by passing infrared light through a sample and measuring the wavelengths absorbed, providing information about the sample’s composition and structure. This topic is all about IR Spectroscopy and explains the Principle, key components, working of IR, Types of IR, Types of Vibrations, Applications of IR and concludes with the importance of IR in the current research and development field.

Principle of IR Spectroscopy:

Infrared spectroscopy works on the fact that molecules vibrate at specific frequencies when exposed to infrared radiation. When a molecule absorbs IR radiation, its bond starts to vibrate either by stretching or by bending. These vibrations are determined by the chemical bonds and the masses of the atoms involved. When a molecule absorbs infrared radiation, it excites these vibration modes, creating a unique absorption pattern. The IR spectrum is plotted as absorbance v/s Wave numbers. Each functional group has its unique absorption band, and it helps to determine molecular structure.

Components of IR:

An IR spectrometer generally consists of a radiation source, a Monochromator, a sample compartment, a detector, and a recorder.

Radiation Source:

The source emits infrared radiation, which is typically generated by heating an inert solid to high temperatures (1000-1800 °C). These sources produce a broad spectrum of IR radiation that can be focused onto the sample.

Monochromator:

The monochromator is a device that separates the IR radiation into its wavelengths. It uses a combination of prisms, gratings, or filters, and slit mechanisms to disperse the light. Narrow slits improve resolution by allowing for the differentiation of closely spaced frequencies, while wider slits enhance sensitivity by allowing more light to reach the detector.

Sample Compartment:

The sample is placed into to sample compartment to interact with the IR radiation. Samples can be solids, liquids, or gases, and are typically held in specialised cells or cuvettes. For solid samples pallet technique is generally used. Mull technique is used for the liquid samples, these samples can be held by using liquid cells. These cells are made up of alkali halides sampling technique for liquid is also used for the sampling of gas samples.

Detector: 

The detector measures the amount of IR radiation that passes through the sample. Common detectors include bolometers and pyroelectric detectors, which convert the infrared absorption into electrical signals.

Recorder:

The recorder displays the resulting spectrum, which shows the absorption of IR radiation at different wavelengths by the sample. The spectrum provides information about the functional groups and structure of the sample. The source emits infrared radiation, which is then passed through the sample. A Monochromator separates the light into its component wavelengths. The detector measures the amount of light transmitted through the sample, and the recorder displays the resulting spectrum. Some IR spectrometers, like FTIR (Fourier Transform Infrared) spectrometers, use an interferometer instead of a monochromator to separate wavelengths, offering advantages like higher resolution and sensitivity. 

Types of Vibrations and IR regions:

There are two types of vibrations, called stretching and bending vibrations. The IR portion of the electromagnetic spectrum is divided into three regions as near, mid and far IR regions. The near region is approximately 14000 to 4000 cm-1. The mid-IR region is at 4000-400cm-1.and the far-IR region appears at 400-10cm-1. The near IR region has high energy, and the far IR region has the lowest energy. The spectrograph is generally interpreted as having two regions: a functional group and finger fingerprint region.

Factors Affecting Vibrational Frequency:

Different factors such as symmetry, Fermi resonance, Hydrogen bonding, electronic effect and bond angle influence the vibration frequency. Symmetrical compounds do not have dipole moments and hence symmetric molecules are IR Inactive. Fermi resonance results in an unexpected shift in the energy and intensity of bands. Due to hydrogen bonding downward shift in the frequency is observed. Electronic effects, such as inductive and field effects, result in a shift of absorption bands. A difference in the bond angle also results in a change in the absorption bands.

Advantages and Disadvantages of IR Spectroscopy:

IR Spectroscopy offers a number of advantages. It is non non-destructive technique that offers a very high speed of scanning. It scans the entire frequency within a few seconds, and thus it makes it easy to analyse unstable samples also.IR spectrometers offer high resolution and can be used for analysis of almost all organic and a few inorganic molecules.IR spectra provide information regarding structure, type of compound, its functional groups and also stereochemistry. It provides cost-effective analysis as it is non-destructive and requires a small amount of sample. It consumes very little electricity, it requires very little space and can be operated in both daylight and at night. At the same time, it has its limitations also, as IR cannot provide complete structural information as other techniques like NMR or mass spectrometry. Interference may be observed in some samples due to fluorescence. IR applications may not be suitable for all applications..It may be affected by environmental factors such as temperature and humidity.

Application of IR Spectroscopy:

IR Spectroscopy is a powerful, sensitive and specific analytical tool used in different industries in different applications. It is used in chemistry, biology and in various industries for qualitative and quantitative applications. A few important applications are listed.IR Spectroscopy helps to determine the presence and type of functional groups in the molecule. Based on the functional group information structure elucidation of the unknown compound can be done.IR can be used to determine the concentration of a specific compound in the mixture. IR Technique can be useful for the identification test of various raw materials, finished products excipients. IR spectroscopy is used to characterise the properties of materials such as polymers, ceramics. IR spectroscopy is a valuable tool in forensic investigations for identifying materials like paints, fibres, and drugs, providing valuable evidence in criminal cases. It’s used in detecting contaminants like pesticides in food quality analysis and monitoring reactions during food processing. IR spectroscopy can be useful to monitor the progress of chemical reactions, which is useful for process optimisation and troubleshooting. Hence, it is concluded that IR spectroscopy has a wide range of applications in different types of industries.

Conclusion:

Infrared (IR) spectroscopy is a versatile analytical technique that provides valuable information about the chemical structure and composition of materials. The technique analyses how molecules absorb infrared light, which corresponds to specific vibrational modes like stretching and bending of chemical bonds. This technique is widely accepted in different industries with its variety of Applications, although it has certain limitations.

FAQ

Reference:

• https://www.geeksforge.org.
•  “Guide to FT-IR Spectroscopy.” Bruker.com, 2025, www.bruker.com/en/products-and-solutions/infrared-and-raman/ft-ir-routine-spectrometer/what-is-ft-ir-spectroscopy.html.
• Ferrer, Núria. “Forensic Science, Applications of IR Spectroscopy*.” Encyclopedia of Spectroscopy and Spectrometry, Elsevier, 2010, pp. 681–92, https://doi.org/10.1016/b978-0-12-374413-5.00151-2.