How Does IR Spectroscopy Work?

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How Does IR Spectroscopy Work?
Molecular bonds show different infrared absorption frequencies. (cocaine molecule 1 image by Yurok Aleksandrovich from Fotolia.com)

Infrared spectroscopy, also known as IR spectroscopy, can reveal the structures of covalently bonded chemical compounds such as organic compounds. As such, for students and researchers who synthesise these compounds in the laboratory, it becomes a useful tool for verifying the results of an experiment. Different chemical bonds absorb different frequencies of infrared, and infrared spectroscopy shows vibrations at those frequencies (displayed as 'wavenumbers') depending on the type of bond.

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Function

Infrared spectroscopy serves as one useful tool in the chemist's toolbox for identifying compounds. It does not give the exact structure of a compound, but rather shows the identity of the functional groups, or moieties, in a molecule - the different segments of the molecule's composition. As such an inexact tool, IR spectroscopy works best when used in conjunction with other forms of analysis such as melting point determination.

In professional chemistry, IR has largely gone out of fashion, replaced by more informative methods like NMR (nuclear magnetic resonance) spectroscopy. It still enjoys frequent use in student laboratories, as IR spectroscopy remains useful in identifying important characteristics of molecules synthesised in student lab experiments, according to Colorado University Boulder.

Method

Generally, the chemist grinds a solid sample with a substance like potassium bromide (which, as an ionic compound, does not show up in IR spectroscopy) and places it in a special device to allow the sensor to shine through it. Sometimes she or he mixes solid samples with solvents like mineral oil (which gives a limited, known reading in the IR printout) to use the liquid method, which involves placing a sample between two plates of pressed salt (NaCl, sodium chloride) to allow the infrared light to shine through, according to Michigan State University.

Significance

When infrared 'light' or radiation hits a molecule, the bonds in the molecule absorb the energy of the infrared and respond by vibrating. Commonly, scientists call the different types of vibrations bending, stretching, rocking or scissoring.

According to Michele Sherban-Kline at Yale University, an IR spectrometer has a source, an optical system, a detector and an amplifier. The source gives off infrared rays; the optical system moves these rays in the correct direction; the detector observes changes in the infrared radiation, and the amplifier improves the detector signal.

Types

Sometimes spectrometers use single beams of infrared and then split them into component wavelengths; other designs use two separate beams and use the difference between those beams after one has passed through the sample to give information about the sample. Old-fashioned spectrometers amplified the signal optically, and modern spectrometers use electronic amplification for the same purpose, according to Michele Sherban-Kline at Yale University.

Identification

IR spectroscopy identifies molecules based on their functional groups. The chemist using IR spectroscopy can use a table or chart to identify these groups. Each functional group has a different 'wavenumber', listed in inverse centimetres, and a typical appearance---for example, the stretch of an O-H group, such as that of water or alcohol, occupies a very wide peak with a wavenumber near 3500, according to Michigan State University. If the compound synthesised does not contain any alcohol groups (also known as hydroxyl groups) this peak can indicate the inadvertent presence of water in the sample, a common student error in the laboratory.

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