Infrared spectroscopy analyzes molecules by exposing them to radiation in a unique wavelength range. Chemical bonds in a molecule vibrate at a specific wavelength, allowing for identification. Conventional IR spectroscopy uses a radiation source, sample container, and IR sensors, while Fourier transform IR spectrometers offer more precise results. The presence of chemical groups in a molecule is determined through an elimination process.
Infrared (IR) spectroscopy is used to analyze molecules. There are many types of spectroscopy which are used to determine different properties and characteristics of a molecule. IR spectroscopy instrumentation is used to clarify which groups are present in a sample.
The IR radiation band includes wavelengths of 800-1,000,000 nanometers. This light is invisible to the human eye, although the effects of IR radiation are perceived as heat. The range of radiation 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 stretch, bend or twist when exposed to IR radiation. This occurs at a wavelength that is unique to each bond and each type of vibration. Thus, 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 radiation source, sample container, and IR sensors to detect which wavelengths have passed through the sample. The traditional IR spectrometer is called a dispersive grating spectrometer. It works by splitting the radiation from the IR source into two streams, one of which passes through the sample and the other is used as a control. The spectrometer compares the relative absorption from the control and sample to calculate the relative absorption for each wavelength.
The IR source is typically 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 solid, liquid or gaseous. It can also 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. The analysis of IR spectra, originally conducted manually, has become computerised. Fourier transform IR (FTIR) spectrometers offered far more precise, accurate and sensitive results than dispersive grating IR technology.
In practice, the presence of chemical groups in a molecule is determined through an elimination process. For example, absorption at a particular set of wavelengths implies the presence of a carbon-oxygen double bond, meaning that the compound could contain a range of organic groups. Further absorption at another wavelength suggests that a carbon-oxygen single bond also exists, meaning that the sample contains a carboxyl group (-CO2-). The presence of at least one carboxylic acid group (-CO2-H) would be confirmed if absorption were observed at a wavelength corresponding to a hydroxyl group (-OH).
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