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Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-resolution spectral data over a wide spectral range. This provides a significant advantage over a dispersive spectrometer, which measures intensity over a narrow range of wavelengths at a time.
The term Fourier-transform infrared spectroscopy is derived from the need for a Fourier transform (a mathematical process) to transform the raw data into original spectra.
Absorption spectroscopy techniques (FTIR, ultraviolet-visible (“UV-vis”) spectroscopy, etc.) aim to measure how much light a sample absorbs at each wavelength. The easiest way to do this, the “dispersive spectroscopy” technique, is to shine a beam of monochromatic light on a sample, measure how much light is absorbed, and repeat for each different wavelength. (For example, some UV-vis spectrometers work this way.)
Fourier transform spectroscopy is a less intuitive way to obtain the same information. Instead of shining a monochromatic beam of light (a beam composed of only a single wavelength) at the sample, this technique shines a beam of light of many frequencies at once and measures how much of that beam is absorbed by the sample. Next, the beam is modified to contain a different frequency combination, giving a second data point. This process is repeated many times rapidly in a short period of time. Next, a computer takes all this data and works backwards to estimate what the absorbance is at each wavelength.
The beam described above is created by starting with a broadband light source – one that contains the full spectrum of wavelengths to be measured. The light shines on a Michelson interferometer – a fixed configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is successively blocked, transmitted, blocked, transmitted, by the interferometer due to wave interference. Different wavelengths are modulated at different rates, so that the beam exiting the interferometer has a different spectrum at each moment or mirror position.
As mentioned, computer processing is required to transform the raw data (light absorption for each mirror position) into the desired result (light absorption for each wavelength). The processing required appears to be a simple algorithm called the Fourier transform. The Fourier transform transforms one domain (mirror displacement in cm) into its inverse domain (wavenumber in cm−1). The raw data is called an “interferogram”.
