Fourier Transform Infrared Spectroscopy (FTIR)
Fourier Transform Infrared Spectroscopy (FTIR) is an extremely useful form of infrared spectroscopy that utilizes an interferogram to collect all of the frequencies to be collected simultaneously. This can be used easily for substance identification by comparing the resultant spectra with a library of known substances. The frequencies are initially passed through an interferogram, which then undergoes a Fourier transformation that combines to produce a spectrum. This spectrum is essentially identical to a normal infrared spectrum for the same compound, except it is produced much faster.
Initially the sample is inserted into the machine and exposed to infrared light. Molecular bonds will then vibrate at a variety of frequencies that depend on the elements and type of bonds. The lower frequencies signify the ground state and the higher frequencies indicate excited energy states. These vibrations reflect a wavelength of light off of the molecules, and these are then split on a half-silvered mirror through an interferometer to produce a spectrum. The sample’s transmittance and reflectance of the infrared rays at different frequencies is key to the translation into an IR absorption plot.
Interferograms function by interfering wavelengths to a pattern of interference by the superposition of waves. The interferogram is based off the Michelson interferometer, which is the most common configuration. The interference pattern is created by splitting a beam of light into two paths, bouncing them back and then recombining them. There is a semi-transparent mirror that is located in the interferometer that allows the light source to pass through to one mirror and then return back to reach the detector. After hitting the mirrors (which is where the samples are held presumably), they are joined together again and a spectrum is derived from the difference caused by constructive and deconstructive interference in wavelengths.
The Fourier transform is essentially a mathematical function at origin that transforms one complex-valued function into another, which when applied to spectroscopy and other applications can create a combined spectra of the wavelength interferences. The Fourier transform can be related through time (which is represented by x in this equation), where the transform variable ξ represents frequency in Hertz. The function below is a representation of how the complex function is transformed into another at a real number:
Some advantages of this method include the fact that FTIR does not require a vacuum because infrared light is not effective on nitrogen or oxygen gas. Fourier transform also provides a Fellgett Advantage, where the results are obtained quickly because of simultaneous measurement. The Connes advantage offered from FTIR is also valuable because the machine is self-calibrating. This occurs because the wavenumber scale of the interferometer is taken from a helium-neon laser, which acts as an internal reference for every scan. Since these gases are noble gases and very stable, they serve as a very reliable and accurate standard. There are also no discontinuities in the spectrum because there are no filter changes. The FTIR spectrometer can also obtain the same signal-noise ratio in a much shorter period of time (Jacquinot advantage). The energy input for an interferometer can be higher than in dispersive spectroscopy, which is restricted by the use of slits to create spectroscopy. These advantages are primarily because an interferometer functions with less disruptive elements than a dispersive instrument. Disadvantages would include the fact that there is only a single beam, so the environment can easily affect it. This causes the results to sometimes be fickle if the machine is not in the appropriate environment.
Fourier Transform Infrared Spectroscopy is a wonderful technique that is now used everywhere to identify compounds. It’s characteristic ability to self-calibrate and because there is no dispersive light allows it to have many advantages over other types of spectroscopy. Many labs will now choose to use Fourier Transform Infrared Spectroscopy over other types of spectroscopy simply because it is faster and it is accurate. This ingenious combination of an interferometer and spectrometer has improved the technique of infrared spectrometry significantly.