Basic principle of Fourier transform infrared spectrometer

Fourier Transform Infrared Spectrometer (abbreviated as FTIR Spectrometer) is called Fourier Transform Infrared Spectrometer for short. It is different from the principle of dispersive infrared spectroscopy. It is an infrared spectrometer developed based on the principle of Fourier transform of infrared light after interference. It is mainly composed of infrared light source, diaphragm, interferometer (beam splitter, moving mirror, fixed mirror), sample chamber, detector and various infrared mirrors, lasers, control circuit board and power supply. It can conduct qualitative and quantitative analysis on samples, and is widely used in medicine and chemical industry, geology and mining, petroleum, coal, environmental protection, customs, gem identification, criminal investigation and identification and other fields.

working principle:

Like visible light, infrared light is an electromagnetic wave, and infrared light is an electromagnetic wave with a wavelength between visible light and microwave. The infrared light can also be divided into near infrared, mid infrared and far infrared wave regions according to the wavelength range, of which the mid infrared region (2.5~25 μ m; 4000~400cm-1) can well reflect the various physical processes carried out inside the molecule and the characteristics of the molecular structure, and is most effective in solving various problems in the molecular structure and chemical composition. Therefore, the mid infrared region is the most widely used region in the infrared spectrum. Generally speaking, the infrared spectrum refers to this range.

The infrared spectrum belongs to the absorption spectrum, which is generated by the absorption of infrared light of a specific wavelength when the compound molecule vibrates. The wavelength of the infrared light absorbed by the chemical bond vibration depends on the chemical bond constant and the converted mass of the atoms connected at both ends, that is, the structural characteristics of. This is the theoretical basis for determining the structure of compounds by infrared spectroscopy.

As a “molecular fingerprint”, infrared spectroscopy is widely used in the study of molecular structure and chemical composition of substances. The spatial configuration of molecules can be determined according to the position, strength and shape of the spectral band frequency obtained after the molecules absorb infrared light, as well as the relationship between the absorption spectral band and temperature, aggregation state, etc. The force constant, bond length and bond angle of the chemical structure can be calculated. From the perspective of spectral analysis, it is mainly to infer the existence of a certain group or bond in the molecule by using the frequency of the characteristic absorption band, infer the adjacent group or bond by the change of the frequency of the characteristic absorption band, and then determine the chemical structure of the molecule. Of course, it is also possible to quantitatively analyze mixtures and compounds by changing the intensity of the characteristic absorption band. In view of the extensive application of infrared spectrum, the infrared spectrometer that plots the infrared spectrum has also become the key research object of scientists.

The Fourier transform infrared (FT-IR) spectrometer is designed according to the principle of light coherence, so it is an interference spectrometer. It is mainly composed of light source (silicon carbon rod, high-pressure mercury lamp), interferometer, detector, computer and recording system. Most Fourier transform infrared spectrometers use Michelson interferometer, so the original spectrogram measured in the experiment is the interference diagram of the light source, Then, the computer performs fast Fourier transform calculation on the interferogram to obtain the spectrogram with wavelength or wave number as a function. Therefore, the spectrogram is called Fourier transform infrared spectrum, and the instrument is called Fourier transform infrared spectrometer.

Optical principle:

It is a typical optical path system of the Fourier transform infrared spectrometer. The radiation from the infrared light source enters the Michelson interferometer after being made into a parallel light through the concave lens. The pulsating beam leaving the interferometer projects onto a swinging mirror B, making the beam alternately pass through the sample pool or reference pool, and then focus the beam on the detector through the swinging mirror C (synchronous with B).

The Fourier transform infrared spectrometer has no dispersive elements and no cracks, so the light from the light source has enough energy to shine on the sample after interference and then arrives at the detector. The main core component of the measurement part of the Fourier transform infrared spectrometer is the interferometer. Figure 3 is the working principle diagram when a single beam of light shines on the Michelson interferometer. The interferometer is composed of a fixed reflector M1 (fixed mirror), Movable mirror M2 (movable mirror) and beam splitter B are composed. M1 and M2 are mutually perpendicular plane mirrors. B is placed between M1 and M2 at an angle of 45 °. B can divide the light beam from the light source into two equal parts. Half of the light beam is reflected after B, and the other half is transmitted through B In the Michelson interferometer, when the incident light from the light source is divided into two beams by the optical beam splitter, which are reflected by two mirrors and then converged, and then projected onto the detector, due to the movement of the moving mirror, the two beams of light produce optical path difference. When the optical path difference is an even number of times of the half wavelength, mutual interference occurs to produce an open line; When it is an odd number of times the half wavelength, destructive interference will occur and dark lines will be generated. If the optical path difference is neither an even number of times nor an odd number of times the half wavelength, the coherent light intensity is between the first two cases. When the moving mirror moves in contact, the cosine of the signal recorded on the detector changes. Every quarter of the wavelength distance moved, the signal will change periodically from light to dark.

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