Infra Red (IR) spectroscopy

Infra Red (IR) spectroscopy

Infra Red (IR) spectroscopy

Infra Red (IR) spectroscopy

Infra Red (IR) spectroscopy is a powerful analytical technique used in the field of chemistry to determine the identity and structure of chemical compounds. It involves the absorption of infrared radiation by a molecule, leading to the vibration of its chemical bonds. This absorption produces a unique spectrum that can be used to identify the functional groups present in a compound and provides valuable information about its structure and properties.

Principles of IR Spectroscopy

IR spectroscopy is based on the principle that molecules absorb specific frequencies of infrared light, which are characteristic of their chemical bonds. Each chemical bond has a specific vibrational frequency, and the absorbance of infrared light causes these bonds to vibrate. The amount of light absorbed at a particular frequency is related to the strength of a specific chemical bond. Therefore, when a molecule is exposed to infrared radiation, it absorbs specific frequencies of light, resulting in an IR spectrum, which can be used to identify the different functional groups present in the molecule.


The basic components required for an IR spectroscopy experiment are a source of infrared radiation, a sample holder, and a detector. The most commonly used sources are a Globar (a silicon carbide rod) or a Nernst Glower (a mixture of oxides of yttrium, zirconium, and other rare earth elements). These sources emit a broad spectrum of IR radiation, covering the entire mid-infrared range (4000–400 cm-1).

The sample holder used in IR spectroscopy varies depending on the nature of the sample. For liquids, a salt plate or a liquid cell is used, while for solids, a KBr pellet is used. For gases, a gas cell is used, where the sample is placed at reduced pressure. The detector used in IR spectrometers are mainly of two types: Thermal detectors, such as a thermocouple or a thermopile, and photon detectors, such as a photomultiplier tube or a charge-coupled device (CCD) camera.


IR spectroscopy has a wide range of applications in various fields, such as pharmaceuticals, forensics, environmental science, and materials science. It is used in the identification and quantification of drug compounds and their metabolites in pharmaceutical analysis. IR spectroscopy is also widely used in forensic analysis to determine the composition and origin of trace evidence found at crime scenes.

In environmental science, IR spectroscopy is used to analyze air and water samples for contaminants. It helps in the determination of the types and quantities of pollutants present in the sample, aiding in the monitoring and regulation of environmental pollution. In materials science, IR spectroscopy is used to determine the chemical composition and structure of materials, such as polymers, ceramics, and minerals.

Advantages and Disadvantages

One of the main advantages of IR spectroscopy is its non-destructive nature, as the sample does not undergo any chemical or physical changes during the analysis. It is also relatively fast and requires minimal sample preparation, making it a popular choice for routine analysis. However, one limitation of IR spectroscopy is that it cannot provide information about the spatial distribution of the functional groups in a sample. Hence, it is often coupled with other techniques such as microscopy or chromatography to overcome this limitation.

P Q R Bands

Fermi Resonance

IR Spectra of Fe2(CO)9