There are many types of near-infrared spectrometers, and there are many classification methods depending on the angle of use. From the perspective of application, it can be divided into online process monitoring instruments, special instruments and general instruments. From the spectral information obtained by the instrument, there are special instruments that measure only a few wavelengths, and research instruments that can measure the entire near-infrared spectral region; some are dedicated to measuring short-wavelength near-infrared spectra, and some are suitable for measuring long Near-infrared spectrum of the band. The more commonly used classification mode is classified according to the spectroscopic form of the instrument, and can be classified into a filter type, a dispersion type (grating, prism), and a Fourier transform type. The following are described separately.
1. Filter type near-infrared spectroscopy instrument:
The filter type near-infrared spectroscopy apparatus uses a filter as a spectroscopic system, that is, a filter as a monochromatic optical device. The filter type near-infrared spectroscopy instrument can be divided into two types: fixed filter and adjustable filter, among which the fixed filter type instrument is the earliest design form of near-infrared spectrometer. When the instrument is working, the light emitted by the light source passes through the filter to obtain a broadband monochromatic light, which acts on the sample and reaches the detector.
The advantage of this type of instrument is that the instrument is small in size and can be used as a dedicated portable instrument; it has low manufacturing cost and is suitable for large-area promotion.
The disadvantages of this type of instrument are: the band of monochromatic light is wider, the wavelength resolution is poor; it is sensitive to temperature and humidity; the continuous spectrum is not obtained; the spectrum can not be preprocessed, and the amount of information obtained is small. Therefore, it can only be used as a special instrument for lower gears.
Second, the dispersion type near-infrared spectroscopy instrument:
The spectroscopic element of the dispersive near-infrared spectroscopy apparatus may be a prism or a grating. In order to obtain higher resolution, holographic gratings are often used as spectroscopic components in modern dispersive instruments. Scanning instruments pass the rotation of the gratings, so that monochromatic light passes through the samples in turn according to the wavelength, and enters the detector for detection. Depending on the physical properties of the sample, different sample devices can be selected for projection or reflection analysis.
The advantage of this type of instrument is that the scanning type near-infrared spectrometer can be used to perform full-spectrum scanning of the sample. The repeatability and resolution of the scanning are greatly improved by the filter type instrument, and some high-end dispersion type near-infrared spectrometers are also Can be used as a research grade instrument. One of the characteristics of modern near-infrared analysis when chemometrics is applied in the near infrared. Using full-spectrum analysis, a large amount of useful information can be extracted from the near-infrared spectrum; the corresponding calibration model can be obtained by correlating the spectral data with the properties (composition, characteristic data) of the training set sample by a reasonable metrology method; The nature of the unknown sample.
Disadvantages of this type of instrument: mechanical bearings of gratings or mirrors are prone to wear over a long period of time, affecting the accuracy and reproducibility of wavelengths; due to the large number of mechanical components, the seismic performance of the instrument is poor; the spectrum is susceptible to interference from stray light. The scanning speed is slow and the expansion performance is poor. Due to the use of external standard sample calibration instruments, the resolution, signal-to-noise ratio and other indicators are greatly improved compared to the filter type instrument, but there is still a qualitative difference compared with the Fourier type instrument.
Third, Fourier transform type near-infrared spectroscopy instrument:
The Fourier transform near-infrared spectrophotometer is abbreviated as Fourier transform spectrometer. It uses the correspondence between the interferogram and the spectrogram to measure and study by measuring the interferogram and performing Fourier integral transform on the interferogram. Near infrared spectroscopy. The basic composition consists of five parts: 1 analysis of the light generation system, consisting of a light source, a beam splitter, a sample, etc., to generate analytical light loaded with sample information; 2 an interferometer represented by a conventional Michaelson interferometer, And various types of improved interferometers in the future, the function of which is to divide the light emitted by the light source into two beams, thereby causing a certain optical path difference for generating the analysis light expressed in the space (time) domain, that is, the interference light; Detector for detecting interference light; 4 sampling system, digitizing the interference light detected by the detector through a digital-to-analog converter, and importing it into a computer system; 5 computer system and display, respectively, respectively, the sample interference light function and the light source interference light function Fourier transforms into intensity ä¿º frequency distribution map, the ratio of the two is the near-infrared spectrum of the sample, and is displayed in the display.
In the Fourier transform near-infrared spectroscopy instrument, the interferometer is the heart of the instrument, and its quality directly affects the myocardial infarction of the instrument. Therefore, it is necessary to understand the working principle of the traditional Michelson interferometer and the improved interferometer. .
(1) Traditional Michelson interferometer: The traditional Michelson interferometer system consists of two plane mirrors, optical beam splitters, light sources and detectors that are at an angle of 90 degrees. In the plane mirror, a fixed mirror is fixed, and a moving mirror moves in parallel along the direction of the figure. The moving mirror should always maintain a 90 degree angle with the fixed mirror during the movement. In order to reduce friction and prevent vibration, the moving mirror is usually fixed to the air bearing to move. The optical beam splitter has a translucent property, placed between the moving mirror and the fixed mirror and at an angle of 45 degrees with them, so that the incident monochromatic light is 50% transmitted, 50% reflected, so that a beam of light emitted from the light source is The beam splitter is split into two beams: reflected light A and transmitted light B. The A beam is directed perpendicular to the mirror; it is reflected there and returns to the beam splitter along the original path; half of it passes through the beam splitter towards the detector and the other half is reflected back to the source. The B beam is incident on the moving mirror through the beam splitter in the same way; it is also reflected there, and returns to the beam splitter along the original optical path; it is reflected by the beam splitter, and is directed to the detector like the A beam, and the other half Then return to the original light path through the beam splitter. The two beams A and B meet here to form coherent light with interfering light characteristics. When the moving mirror moves to different positions, the interference light intensity with different optical path differences can be obtained.
(2) Improved interferometer: Interferometer is the most important component of Fourier spectrometer. Its performance determines the quality of Fourier spectrometer. Based on the classic Michelson interferometer, it has increased luminous flux in recent years. There have been many improvements in terms of increased stability and shock resistance, and simplified instrument structure.
During the operation of the traditional Michelson interferometer, when the moving mirror moves, it will inevitably have a certain degree of oscillation, so that the two plane mirrors are not perpendicular to each other, so that the incident light cannot be directly incident into the moving mirror or the reflected light is deviated from the original incident light. The direction is such that reflected light parallel to the incident light is not obtained, which affects the quality of the interference light. External vibrations will have the same effect. Therefore, in addition to the very precise adjustment, the classic interferometer also avoids vibration during use to maintain the precise vertical mirror of the moving mirror and obtain a good spectrum. In order to improve the anti-vibration capability of the instrument, Bruker developed a three-dimensional stereo angle mirror interferometer with two three-dimensional plane angle mirrors as moving mirrors, two non-friction bearings installed at the center of a double swing device. Stereo plane angle mirror connection.
The essence of the three-dimensional plane angle mirror interferometer is to replace the plane mirror on the two main arms of the traditional interferometer with a stereoscopic plane angle mirror. According to the optical principle of the cube-corner mirror, when there is a slight perpendicularity error between the reflecting surfaces and a small swing of the cube corner mirror along the axial direction, the direction of the reflected light does not change, and can still be strictly pressed. The incident rays are emitted in parallel directions. It can be seen that the use of the three-dimensional cube angle mirror can effectively eliminate the additional optical path difference caused by the swinging, external vibration or tilting of the moving mirror during the movement, thereby improving the vibration resistance and repeatability together. .
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