Fourier-transform infrared (FTIR) is based on the idea of the interference of radiation between two beams to produce an interferogram.
The latter is a signal produced as a function of the change of path length between the two beams. The two domains of distance and frequency are interconvertible by the mathematical method of Fourier-transformation.
The basic components of an FTIR spectrometer are shown in images:
• FTIR spectrometers use a Glower or Nernst source for the mid-infrared region.
• If the far-infrared region is to be examined, then a mercury lamp can be used.
• For the near-infrared, tungsten–halogen lamps are used.
• The radiation emerging from the source is passed through an interferometer to the sample before reaching a detector.
• Upon amplification of the signal, in which high-frequency contributions have been eliminated by a filter, the data are converted to digital form by an analog-to-digital converter and transferred to the computer for Fourier-transformation.
• Michelson interferometer consists of two perpendicular plane mirrors ‘m1’ and ‘m2’ and a beam splitter ‘M’.
• One of which can travel in a direction perpendicular to the plane. A semi-reflecting film, the beam splitter, divides the planes of these two mirrors.
• The beam splitter material has to be taken according to the region to be examined.
Materials such as germanium or iron oxide are coated onto an infrared transparent substrate such as potassium bromide or cesium iodide to produce beam splitters for the mid- or near-infrared regions.
There are two commonly used detectors for the mid-infrared region;
• The normal detector for routine use is a pyroelectric device with deuterium triglycine sulfate (DTGS) in a temperature-resistant alkali halide window.
• Golay cell detector
• Photoconductive cell and photovoltaic cell
If the sample is soluble it may be dissolved and handled as for liquid. Solid samples for which no solvent is suitable can be prepared for analysis by incorporating them into a pressed pellet of alkali halide like potassium bromide.
Liquid samples are sandwiched using liquid sample cells of highly purified alkali halides such as sodium chloride and other salts such as potassium bromide and calcium fluoride can also be used.
Gas sample is made up of sodium chloride and potassium bromide. It is similar to the liquid sample cell.
For numerous reasons, Fourier transform infrared spectroscopy is favoured over dispersive or filter methods of infrared spectral analysis:
• It is a non-destructive technique
• It offers an accurate measurement approach that does not require external calibration.
• It has the ability to boost speed by gathering a scan every second.
• It can improve sensitivity by combining one-second scans to reduce random noise.
• It has improved optical throughput.
• It has a simple mechanical design with only one moving part.
• Analysis of drug compounds.
• Analysis of ink and toner particles.
• Analysis of hair and fiber.
• Analysis of alcohol compounds.
• Analysis of pigments present in paint samples.
• FTIR instruments have several significant advantages over older dispersive instruments.
• A large number of resolution elements are being monitored simultaneously.
• Another strength of FTIR is its speed advantage.
• The mirror can move short distances quite rapidly and, it makes it possible to obtain spectra on a millisecond timescale.
• By using a helium-neon laser as a reference, the mirror position is known with high precision.