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What Is an Infrared Spectrophotometer?

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  • Written By: C.H. Seman
  • Edited By: Angela B.
  • Last Modified Date: 20 November 2016
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An infrared spectrophotometer is a device used in organic chemistry to gather information about the structural properties of organic molecules and compounds. In this particular kind of spectrophotometer, infrared light is absorbed by the chemical compounds, and the movement of the chemical bonds is analyzed. Infrared spectrophotometers can be used to identify unknown chemicals and to determine sample purity. They frequently are used in research applications for universities and chemical process industries.

The infrared spectrophotometer, often known as an IR spectrophotometer, uses infrared light to cause movement in the bonds of organic molecules. Infrared light falls between visible light and microwave radiation on the electromagnetic radiation spectrum. This type of light can be further divided into the near, mid, and far IR ranges, with the mid IR range being the most helpful in infrared spectroscopy. The light in this region may have a wavelength, or λ, of 3x10-4 to 3x10-3 centimeters. This range also can be expressed in terms of the wavenumber, or ν, which is the inverse of the wavelength.

Organic molecules can absorb infrared light and, as a result, they can vibrate in different ways. Absorption of infrared light occurs whenever the radiant energy of the light itself matches the energy of a given molecular vibration. The movement can be described by symmetrical and asymmetrical stretching of the molecular bonds and bending of the molecular bonds.

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An infrared spectrophotometer that uses a prism or a grating to split the infrared radiation source into separate frequencies is known as a dispersive infrared spectrophotometer. A more modern design, the Fourier transform infrared spectrophotometer, is the preferred device in research and industrial settings. The precision of reported wavenumbers is constant throughout the device's scanning region due to constant resolving power.

A Fourier transform infrared spectrophotometer consists of five basic parts — the infrared radiation source, the interferometer, the sample, the detector and the computer. The infrared radiation source usually is a glowing black-body source, and the amount of energy emitted is controlled by an aperture. The interferometer is an optical device that performs spectral coding on the infrared radiation beam. The beam passes through the sample and then through the detector, which decodes the interferogram signals from the interferometer. The final step is the computer, which performs a Fourier transform on the data and presents it in a usable interface.

The infrared spectrophotometer is unique in that it can be used to identify functional groups in an unknown sample. Certain functional groups have a "fingerprint", or a unique absorption peak that can be identified from an infrared spectrophotometer output graph. Libraries and databases of organic chemical readings can be used to identify unknown organic samples.

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