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Infrared (IR) spectroscopy is used to analyze molecules. There are many types of spectroscopy that are used to determine different properties and characteristics of a molecule. IR spectroscopy instrumentation is used to elucidate what groups are present in a sample.
The IR radiation band comprises wavelengths of 800-1,000,000 nanometers. This light is invisible to the human eye, although the effects of IR radiation are felt as heat. The radiation range used in IR spectroscopy instrumentation is 2,500-16,000 nanometers. This range is called the group frequency region.
Chemical bonds in a molecule can be made to stretch, bend or twist when exposed to IR radiation. This occurs at a wavelength that is unique for each bond and each type of vibration. Therefore, the presence of a specific bond is characterized on an IR spectrum by the absorption of radiation at a discrete set of wavelengths.
Conventional IR spectroscopy instrumentation requires a source of radiation, a container for the sample and IR sensors to detect which wavelengths have passed through the sample. The traditional IR spectrometer is called a dispersive grating spectrometer. This works by dividing the radiation from the IR source into two streams, with one stream passing though the sample and the other being used as a control. The spectrometer compares relative absorption from the control and the sample to calculate relative absorption for each wavelength.
The IR source typically is a solid that has been heated to more than 2,700 degrees Fahrenheit (about 1,500 degrees Celsius). Sources include wound electrical wires or filaments, silicon carbide and rare earth metal oxide. The sample can be a solid, liquid or gas. It also can be in liquid solution, but in this state, care must be taken to distinguish between absorptions by the solvent and absorptions by the dissolved sample.
The late 20th century and early 21st century saw many advances in IR spectroscopy instrumentation. Analysis of IR spectra, originally conducted manually, became computerized. Fourier transform IR (FTIR) spectrometers offered far more precise, accurate and sensitive results than dispersive grating IR technology.
In practice, the presences of chemical groups in a molecule are determined via a process of elimination. For example, absorption at a particular set of wavelengths implies the presence of a carbon-to-oxygen double bond, meaning that the compound could contain a range of organic groups. Further absorption at another wavelength suggests that there also is a carbon-to-oxygen single bond, meaning that the sample contains a carboxylic group (-CO2-). Presence of at least one carboxylic acid group (-CO2-H) would be confirmed if absorption at a wavelength corresponding to a hydroxyl (-OH) group is observed.
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