Main article: Michelson interferometer Schematic diagram of a Michelson interferometer, configured for FTIR Digilab pioneered the world's first commercial FTIR spectrometer (Model FTS-14) in 1969 (Digilab FTIRs are now a part of Agilent technologies's molecular product line after it acquired spectroscopy business from Varian). Also an electronic computer was needed to perform the required Fourier transform, and this only became practicable with the advent of minicomputers, such as the PDP-8, which became available in 1965. The advantages of the Michelson interferometer were well-known, but considerable technical difficulties had to be overcome before a commercial instrument could be built. Far-infrared spectrophotometers were cumbersome, slow and expensive. An additional issue is the need to exclude atmospheric water vapour because water vapour has an intense pure rotational spectrum in this region. More sensitive detectors than the bolometer were required because of the low energy of the radiation. Measurements in the far infrared needed the development of accurately ruled diffraction gratings to replace the prisms as dispersing elements, since salt crystals are opaque in this region. The region beyond 50 μm (200 cm −1) became known as the far-infrared region at very long wavelengths it merges into the microwave region. Later instruments used potassium bromide prisms to extend the range to 25 μm (400 cm −1) and caesium iodide 50 μm (200 cm −1). The upper limit was imposed by the fact that the dispersing element was a prism made from a single crystal of rock-salt ( sodium chloride), which becomes opaque at wavelengths longer than about 15 μm this spectral region became known as the rock-salt region. The lower wavelength limit was chosen to encompass the highest known vibration frequency due to a fundamental molecular vibration. This instrument covered the wavelength range from 2.5 μm to 15 μm ( wavenumber range 4,000 cm −1 to 660 cm −1). The first low-cost spectrophotometer capable of recording an infrared spectrum was the Perkin-Elmer Infracord produced in 1957. The raw data is called an "interferogram". The Fourier transform converts one domain (in this case displacement of the mirror in cm) into its inverse domain (wavenumbers in cm −1). The processing required turns out to be a common algorithm called the Fourier transform. Different wavelengths are modulated at different rates, so that at each moment or mirror position the beam coming out of the interferometer has a different spectrum.Īs mentioned, computer processing is required to turn the raw data (light absorption for each mirror position) into the desired result (light absorption for each wavelength). As this mirror moves, each wavelength of light in the beam is periodically blocked, transmitted, blocked, transmitted, by the interferometer, due to wave interference. The light shines into a Michelson interferometer-a certain configuration of mirrors, one of which is moved by a motor. The beam described above is generated by starting with a broadband light source-one containing the full spectrum of wavelengths to be measured. Afterwards, a computer takes all this data and works backward to infer what the absorption is at each wavelength. This process is rapidly repeated many times over a short time span. Next, the beam is modified to contain a different combination of frequencies, giving a second data point. Rather than shining a monochromatic beam of light (a beam composed of only a single wavelength) at the sample, this technique shines a beam containing many frequencies of light at once and measures how much of that beam is absorbed by the sample. (This is how some UV–vis spectrometers work, for example.)įourier-transform spectroscopy is a less intuitive way to obtain the same information. The most straightforward way to do this, the "dispersive spectroscopy" technique, is to shine a monochromatic light beam at a sample, measure how much of the light is absorbed, and repeat for each different wavelength. The goal of absorption spectroscopy techniques (FTIR, ultraviolet-visible ("UV-vis") spectroscopy, etc.) is to measure how much light a sample absorbs at each wavelength. The central peak is at the ZPD position ("zero path difference" or zero retardation), where the maximal amount of light passes through the interferometer to the detector. ( June 2022) ( Learn how and when to remove this template message)Īn FTIR interferogram. Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources in this section. This section needs additional citations for verification.
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