Fourier Transform Infrared Spectrometer (FTIR Spectrometer), referred to as Fourier Infrared Spectrometer, and the principle of Dispersive Infrared Spectroscopy, is based on the principle of Fourier transform of infrared light after interference. The developed infrared spectrometer is mainly composed of infrared light source, diaphragm, interferometer (beam splitter, moving mirror, fixed mirror), sample chamber, detector and various infrared mirrors, laser, control circuit board and power supply. It can be used for qualitative and quantitative analysis of samples, and is widely used in the fields of pharmaceutical and chemical industry, geology and minerals, petroleum, coal, environmental protection, customs, gem identification, and criminal investigation.
working principle:
Infrared rays are electromagnetic waves like visible light, and infrared rays are electromagnetic waves with a wavelength between visible light and microwave. Infrared light can be divided into three regions: near-infrared, mid-infrared and far-infrared depending on the wavelength range. The mid-infrared region (2.5-25 μm; 4000-400 cm-1) can well reflect various physical processes inside the molecule. As well as the molecular structure, it is most effective in solving various problems in molecular structure and chemical composition. Therefore, the mid-infrared region is the most widely used region in the infrared spectrum, and the infrared spectrum generally refers to this range.
Infrared spectroscopy belongs to the absorption spectrum and is generated by the absorption of specific wavelengths of infrared light when the compound molecules vibrate. The wavelength of the infrared light absorbed by the chemical bond vibration depends on the chemical bond constant and the atomic weight of the atoms connected at both ends, which is also dependent. Structure. This is the theoretical basis for the determination of the structure of compounds by infrared spectroscopy.
Infrared spectroscopy is widely used as a "molecular fingerprint" for the study of molecular structure and chemical composition of matter. According to the relationship between the position, intensity, shape, absorption band and temperature, aggregation state of the band obtained by the absorption of infrared light by the molecule, the spatial configuration of the molecule can be determined, and the chemical constant, bond length and bond of the chemical structure can be determined. angle. From the point of view of spectral analysis, it is mainly to use the frequency of the characteristic absorption band to infer that a certain group or bond exists in the molecule, and to estimate the adjacent group or bond by the change of the frequency of the characteristic absorption band, thereby determining the chemical structure of the molecule, of course. The mixture and compound can also be quantitatively analyzed by changes in the intensity of the characteristic absorption band. In view of the wide application of infrared spectroscopy, infrared spectrometers with infrared spectroscopy have become the focus of scientists.
Fourier transform infrared (FT-IR) spectrometer is designed according to the principle of coherence of light, so it is an interference spectrometer, which is mainly composed of light source (silicon carbon rod, high pressure mercury lamp), interferometer, detector, computer and record. System composition, most of the Fourier transform infrared spectrometer uses Michelson interferometer, so the original spectrum of the experimental measurement is the interference diagram of the light source, and then the fast Fourier transform calculation of the interferogram is performed by computer to obtain the wavelength or wave number. For the spectral map of the function, therefore, the spectrum is called Fourier transform infrared spectroscopy, and the instrument is called a Fourier transform infrared spectrometer.
Optical principle:
The typical optical path system of the Fourier transform infrared spectrometer, the radiation from the infrared source, enters the Michelson interferometer after passing through the concave mirror, and the pulsating beam leaving the interferometer is projected onto a oscillating mirror B, so that the beam alternates through the sample. The pool or reference cell is then oscillated by mirror C (synchronized with B) to focus the beam onto the detector.
Fourier transform infrared spectrometer has no dispersive components, and there is no gap. Therefore, the light from the light source has enough energy to interfere with the sample and then reach the detector. The main core component of the measurement part of the Fourier transform infrared spectrometer is the interferometer. Figure 3 is a single The working principle diagram of the beam irradiated by the Michelson interferometer, the interferometer is composed of a fixed mirror M1 (fixed mirror), a movable mirror M2 (moving mirror) and a beam splitter B, M1 and M2 are Plane mirrors that are perpendicular to each other. B is placed between M1 and M2 at an angle of 45°. B can split the beam from the source into two equal parts, half of which is reflected by B and the other half of which is transmitted through B. In the Michelson interferometer, when the incident light from the light source is split into two beams by the optical beam splitter, reflected by the two mirrors and then gathered together, and then projected onto the detector, the two beams are moved due to the movement of the moving mirror. Light produces an optical path difference. When the optical path difference is an even multiple of a half wavelength, constructive interference occurs to produce an open line; when it is an odd multiple of a half wavelength, destructive interference occurs, and a dark line is generated. If the optical path difference is neither half When the even-numbered times of the wavelength are not an odd multiple, the intensity of the coherent light is between the first two cases. When the moving mirror is moved, the cosine of the signal recorded on the detector changes, and the distance per quarter of the wavelength is shifted. The signal changes periodically from light to dark.
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