Fourier transform infrared spectrometer (FT-IR) is the third generation of infrared absorption spectrometer introduced in the 1970s.
Main Components of FT-IR
Fourier transform infrared spectrometer consists of five parts: light source, Michelson interferometer, detector and recording system.
(1) Light source
Infrared light source is an object that can emit high-intensity continuous infrared radiation. Commonly used ones are Nernst lamp and silicon carbon rod.
Nernst lamp is made of sintered zirconium, yttrium, cerium or thorium oxide. Most of them are round, about 30mm long and 2.5mm in diameter, with platinum wire wrapped at both ends as conductors. Nernst lamp is non-conductive at low temperature, but becomes a conductor when heated to about 800℃ and begins to emit light. The operating temperature is about 1500℃ and the power is 50~200W, so a preheating device is required. The characteristics of Nernst lamp are high emission intensity, especially in the area above 1000 cm-1; but it is brittle and has poor mechanical strength. It will be damaged by slight pressure or twisting, and frequent switching will shorten its life.
Silicon carbon rod is made of sintered silicon carbide. It is generally made into a solid rod with thick ends and thin middle, with a diameter of about 5mm and a length of about 50mm. The middle part is the light-emitting part. The operating temperature is about 1300℃ and the power is 200~400 W. It emits strong light in the low wavenumber region and has a wide wavenumber range, that is, 400~4000 cm-1. It is characterized by being strong, having a long life and a large light-emitting area.
(2) Michelson interferometer
The light emitted by the light source is converted into interference light, providing a basis for subsequent analysis. By adjusting the interferometer, interference fringes of equal thickness and equal inclination can be generated. It is mainly used for the measurement of length and refractive index. If one interference fringe is observed to move, the arm movement of M2 is λ/2, which is equivalent to the change of the thickness of the air film between M1 and M2 by λ/2.
(3) Detector
Due to the low energy of infrared photons, which is not enough to trigger photoelectron emission, photoelectric tubes in UV-visible detectors are not suitable for infrared light detection. Currently, there are three commonly used infrared detectors: vacuum thermocouples, thermoelectric calorimeters and photoelectric tubes.
Vacuum thermocouples are the most commonly used detectors in dispersive infrared spectrometers. A layer of gold black or platinum black is plated on one side of a small piece of gold foil or platinum foil as the receiving surface (target) of infrared radiation, and two different metals or semiconductors are welded on the other side to form a thermocouple. In order to avoid heat loss and thermal interference from the environment, the thermocouple is placed in a vacuum chamber with the front of the target facing the infrared transmission window. When infrared radiation is irradiated on the target surface, the temperature of the thermocouple welding point rises, generating a temperature potential, and then a current that changes with the intensity of the infrared light is generated in the circuit. A good thermocouple detector can respond to a temperature change of 10-6℃.
The sensitive element of the thermoelectric calorimeter is a substance with a temperature-sensitive dipole moment, such as trisaccharide sulfate (TGS for short). TGS can show a strong polarization effect below 49℃, and the polarization degree decreases with increasing temperature. On one side of the TGS sheet, galvanic acid is plated and on the other side, gold is plated to form two electrodes. When infrared light is irradiated on the sheet, the temperature of the sheet rises, the polarization degree changes, and the two poles generate induced charges, which can be measured in the form of current or voltage after amplification. TGS has a fast response speed and is widely used as a detector for Fourier transform infrared spectrometers.
(4) Recording system
Infrared spectrometers generally record spectra automatically by recorders. New instruments are also equipped with microprocessors to control instrument operation, spectrum inspection, etc.
FT-IR Workflow
The interferometer receives infrared light from the light source at the initial stage. The interferometer includes a stationary reflector M paired with a movable reflector M that meet at a right angle and a beam splitter positioned at a 45-degree angle to these reflectors. When light strikes the beam splitter it divides into two beams with equal intensity where half of the light passes through and half bounces back. After reflecting off M and M', the two beams of light interact with the beam splitter where they either reflect or transmit before finally arriving at the detector. Constructive interference occurs at the detector when the optical path difference between the two beams is an even multiple of λ/2 while destructive interference occurs at the detector when the optical path difference is an odd multiple of λ/2. By moving the movable reflector M within the interferometer and recording the light intensity at the detector against the reflector's movement distance you can generate an interference pattern. The interference pattern will alter when the sample absorbs specific infrared frequencies while being placed on the optical path. The electronic computer performs complex Fourier transform processing on the interference pattern to produce a conventional infrared absorption spectrum.
Fourier transform infrared spectrometers offer quick scanning times with full spectrum scans completed in 1 second together with high resolution between 0.1 and 0.005 cm-1 as well as broad spectral coverage and precise measurement results.
FT-IR Application Cases
FT-IR spectroscopy serves numerous purposes across various scientific disciplines. This section demonstrates the real-world uses of FT-IR spectroscopy.
1. Functional group analysis
Functional group analysis can be conducted effectively through FT-IR spectroscopy. The measurement of infrared absorption peaks from various functional groups enables researchers to determine functional group types and quantities within the sample which facilitates the inference of chemical properties and structural characteristics.
2. Polymer characterization
The characterization of polymers is another application of FT-IR spectroscopy. The infrared spectrum measurement of a polymer reveals crucial data about the polymer's molecular structure along with its segment motion and phase transition properties.
3. Drug development and quality control
FT-IR spectroscopy serves as a fundamental tool for both drug development and quality assurance processes. The infrared spectrum measurement of the drug enables verification of its molecular structure along with purity and detection of any impurities. The drug's safety and effectiveness depend on this process while it also supports technical development of new drugs.
4. Environmental monitoring and governance
FT-IR spectroscopy serves as a standard tool for environmental monitoring and governance applications. Through real-time measurement of atmospheric toxins and water pollutants we gain an understanding of environmental conditions that supports scientific environmental protection efforts.
5. Food safety testing
FT-IR spectroscopy technology finds one of its important application areas in food safety testing. Food quality and safety assessment through ingredient measurement enables consumers to eat food products with assurance.
