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Fourier-transform infrared spectroscopy FTIR [1] is a technique used to obtain an infrared spectrum of absorption or emission tourier a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-spectral-resolution data over a wide spectral range.
This confers 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 originates from the fact that a Fourier transform a mathematical process is required to convert the raw data into the actual spectrum.
For other uses of this kind of fourler, see Fourier-transform spectroscopy. 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.
This is how some UV—vis spectrometers innfrarroja, for example. Fourier-transform spectroscopy is a less intuitive way to obtain the same information. 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.
Next, the beam is modified to traansformada a different combination of frequencies, giving a second data point. This process is repeated many times.
Espectrofotómetro de transformada de Fourier
Afterwards, a computer takes all this data and works backward to infer what the absorption is at each wavelength. The beam described above is generated by starting with a broadband light source—one containing the full spectrum of wavelengths to be measured.
The light shines into a Michelson interferometer —a certain configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked, transmitted, blocked, transmitted, by the interferometer, due to wave interference. Different wavelengths are modulated at different rates, so that at each moment the beam coming out of the interferometer has a different spectrum.
As mentioned, computer processing is required to turn the raw data light absorption for each mirror position into the infrarrojq result light absorption for each wavelength. The processing required turns out to be a common algorithm called the Fourier transform hence the name “Fourier-transform spectroscopy”.
The raw data is sometimes called an “interferogram”. The first low-cost spectrophotometer capable of recording an infrared spectrum was the Perkin-Elmer Infracord produced in epsectroscopia The lower wavelength limit was chosen to encompass the highest known vibration frequency due to a fundamental molecular vibration.
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. More sensitive detectors than the bolometer lnfrarroja required because of the low energy of the radiation.
One such was the Golay detector. An additional issue is the need to exclude atmospheric water vapour because water vapour has an intense pure rotational spectrum in this region.
Far-infrared spectrophotometers were cumbersome, slow and expensive. The advantages of infrarrpja Michelson interferometer were well-known, but considerable technical difficulties had to be overcome before a commercial tranformada could be built.
Transrormada an electronic computer was needed to perform the required Fourier transform, and this only became practicable with the advent of mini-computerssuch as the PDP-8which became available in In a Michelson interferometer adapted for FTIR, light from the polychromatic infrared source, approximately a black-body radiator, is collimated and directed to a beam splitter.
Light is reflected from the two mirrors back to the beam splitter and some fraction of the original light passes into the sample compartment.
There, the light is focused on the sample. On leaving the sample compartment the light is refocused on to the detector. The difference in optical path length between the two arms to the interferometer is known as the retardation or optical path difference OPD.
An interferogram is obtained by varying the retardation and recording the signal from the detector for various values of the retardation. The form of the interferogram when no sample is present depends on factors such as the variation of source intensity and splitter efficiency with wavelength.
This results in a maximum at zero retardation, when there is constructive interference at all wavelengths, followed by series of “wiggles”. The position of zero retardation is determined accurately by finding the point of maximum intensity in the interferogram. When a sample is present the background interferogram is modulated by the presence of absorption bands in the sample.
Commercial spectrometers use Michelson interferometers with a variety of transformwda mechanisms to generate the path difference. Common to all eespectroscopia arrangements is the need to ensure that the two beams recombine exactly as the system scans.
The simplest systems have a plane mirror that moves linearly to vary eepectroscopia path of ofurier beam. In this arrangement the moving mirror must not tilt or wobble as this would affect how the beams overlap as they recombine. Some systems incorporate a compensating mechanism that automatically adjusts the orientation of one mirror to maintain the espsctroscopia. Arrangements that avoid this problem include using cube corner reflectors instead of plane mirrors as these have the property of returning any incident beam in a parallel direction regardless of orientation.
Systems where the path difference is generated by a rotary movement fouier proved very successful. One common system incorporates a pair of parallel mirrors in one beam that can be rotated to vary the path without displacing the returning beam.
Another is the double pendulum design where the path in one arm of the interferometer increases as the path in the other decreases. A quite different approach involves moving a espectrosvopia of an IR-transparent material such as KBr into one of the beams.
Increasing the thickness of KBr in the beam increases the optical path because the refractive index is higher than that of air. One limitation of this approach is that the variation of refractive index over the wavelength range limits the accuracy of the wavelength calibration.
The interferogram has to be measured from zero path difference to a maximum length that depends on the resolution required. In practice the scan can be on either side of zero resulting in a double-sided interferogram. Mechanical design limitations may mean that transforada the highest resolution the scan runs to the maximum OPD on one side of zero only. The interferogram is converted to a spectrum by Fourier transformation. This requires it to be stored in digital form as a series of values at equal intervals of the path difference between the two beams.
To measure the path difference a laser beam is sent through the interferometer, generating a sinusoidal signal where the separation between successive maxima is equal to the wavelength. This can trigger an analog-to digital converter to measure the IR fuorier each time the laser signal passes through zero. Alternatively the laser and IR signals can be measured synchronously at smaller intervals with the IR signal at points corresponding to the laser signal zero crossing being determined by interpolation.
The result of Fourier transformation is a spectrum of the signal at a series of discrete wavelengths. The range of wavelengths that can be used in the calculation is limited by the separation of the data points in the interferogram. The shortest wavelength that can be recognized is twice the separation between these data points. Because of aliasing any energy at shorter wavelengths would be interpreted as coming from longer wavelengths and so has to be minimized optically or electronically.
The spectral resolution, i.
The wavelengths used in calculating the Fourier transform are such that an exact number of wavelengths fit into the length of the interferogram from zero to the maximum OPD as this makes their contributions orthogonal.
This results in a spectrum with points separated by equal frequency intervals. The separation is the inverse of the maximum OPD. This is the spectral resolution in the sense that the value at one point is independent of the values at adjacent points. The point in the interferogram corresponding to zero path difference has to be identified, commonly by assuming it is where the maximum signal occurs.
The centerburst is not always symmetrical in real infrarroja spectrometers so a phase correction may have to be calculated. The interferogram signal decays as the path difference increases, the rate of decay being inversely related to the width espectorscopia features in the spectrum.
If the OPD is not large enough to allow the interferogram signal to decay to a negligible level there will be unwanted oscillations or sidelobes associated with the features in the resulting spectrum. To reduce these sidelobes the interferogram is usually multiplied by a function that approaches zero at the maximum OPD. This so-called apodization reduces the amplitude of any sidelobes and also the noise level at the expense some reduction in resolution. For rapid calculation the number of points in the interferogram has to equal a power of two.
A string of zeroes may be added to the measured transflrmada to achieve this.
Fourier-transform infrared spectroscopy – Wikipedia
More zeroes may be added in a process called zero filling to improve the appearance of the final spectrum although there transformad no improvement in resolution. Alternatively interpolation after the Fourier transform gives a similar result. There are three principal advantages for an FT spectrometer compared to a scanning dispersive spectrometer.
Another minor advantage is less sensitivity to stray light, that is radiation of one wavelength appearing at another wavelength in the spectrum. In dispersive instruments, this is the espectroscooia of imperfections in the diffraction gratings and accidental reflections. In FT instruments there is no direct infrarrojx as the apparent wavelength is determined by the modulation frequency in the interferometer.
The interferogram belongs in the length dimension. Much higher resolution can be obtained by increasing the maximal retardation. This is not easy, as the teansformada mirror must travel in a near-perfect straight line. The use of corner-cube mirrors in place of the flat mirrors is helpful, as an outgoing ray from a corner-cube mirror is parallel to the incoming ray, regardless of the orientation of the mirror about axes perpendicular to the axis of the light beam.
In Connes measured the temperature of the atmosphere of Venus by recording the vibration-rotation spectrum of Venusian CO 2 at 0. The throughput advantage is important for high-resolution FTIR, as the monochromator in a infraarroja instrument with the same resolution would have very narrow entrance and exit slits.
FTIR is a method of measuring infrared absorption and emission spectra.
For a discussion of why people measure infrared absorption transformaxa emission spectra, i. The output is similar to a blackbody. Mid-IR spectrometers commonly use pyroelectric detectors that respond to changes in temperature as the intensity of IR radiation falling on them varies.
These detectors operate at ambient temperatures and provide adequate sensitivity for most routine applications. To achieve the best sensitivity the time for a scan is typically a few seconds. Cooled photoelectric detectors are employed for situations requiring higher sensitivity or faster response. Liquid nitrogen cooled mercury cadmium telluride MCT detectors are the most widely used in the mid-IR. With these detectors an interferogram can be measured in as little as 10 milliseconds.
Very sensitive liquid-helium-cooled silicon or germanium bolometers are used in the far-IR where both sources and beamsplitters are inefficient. However, as any material has a limited range of optical transmittance, several beam-splitters may be used interchangeably to cover a wide spectral transformaada. For the mid-IR region the beamsplitter is usually made of KBr with a germanium-based coating that makes it semi-reflective.