Fourier-transform infrared spectroscopy
Technique using interferometry and Fourier transforms for infrared spectroscopy.
Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid, or gaseous material. It collects high-resolution spectral data over a wide range, from the near- to far-IR region, offering a significant advantage over dispersive spectrometers that measure intensity over a narrow range of wavelengths at a time. The term originates from the Fourier transform, a mathematical process required to convert raw data into the actual spectrum.
- field
- Analytical chemistry, spectroscopy
- known_for
- Using a Michelson interferometer and Fourier transform to measure infrared absorption across a broad spectral range
- first_commercial_instrument
- Digilab Model FTS-14 (1969)
- key_component
- Michelson interferometer with a moving mirror
- data_type
- Interferogram converted to spectrum via Fourier transform
Lore & Background
Later instruments used potassium bromide prisms to extend to 25 μm and caesium iodide to 50 μm. The far-infrared region beyond 50 μm required accurately ruled diffraction gratings and more sensitive detectors like the Golay detector, as well as exclusion of atmospheric water vapor. Far-infrared spectrophotometers were cumbersome, slow, and expensive.
Reader's Guide
The advantages of the Michelson interferometer were well-known, but considerable technical difficulties had to be overcome before a commercial instrument could be built. Digilab pioneered the world's first commercial FTIR spectrometer (Model FTS-14) in 1969. In a Michelson interferometer adapted for FTIR, light from a polychromatic infrared source is collimated and directed to a beam splitter, with light reflected from fixed and moving mirrors back to the beam splitter and into the sample compartment. The interferogram is obtained by varying the optical path difference and recording the detector signal. Commercial spectrometers use various scanning mechanisms, including linear moving mirrors, cube corner reflectors, rotary movements, and moving wedges of IR-transparent material. The interferogram is converted to a spectrum by Fourier transformation, requiring digital storage at equal intervals of path difference, often measured using a helium-neon laser.
Did You Know?
- The Fourier transform converts the displacement of the mirror (in cm) into wavenumbers (in cm⁻¹).
- The raw data from an FTIR spectrometer is called an interferogram.
- The first commercial FTIR spectrometer was the Digilab Model FTS-14, introduced in 1969.
- A Michelson interferometer for FTIR uses a beam splitter that ideally sends 50% of light to a fixed mirror and 50% to a moving mirror.
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