Only by understanding the basic structure of domestic spectrophotometer, in order to make better use of a spectrophotometer. The instrument composition of the spectrophotometer is relatively simple, and the main components include a light source, a monochromator, an absorption cell, a detector, and a data processing and recording system.
(1) Light source The light source in the spectrophotometer provides continuous radiation to the instrument. The ideal light source should emit continuous spectrum in the entire ultraviolet spectrum region, with sufficient radiation intensity, good stability and long life.
Since the wavelength ranges of different light sources are different, different light sources are used in the visible light region and the ultraviolet light region in the spectrophotometer. At present, there are two kinds of light sources commonly used in instruments: tungsten lamps, hydrogen lamps or xenon lamps. A tungsten filament lamp is commonly used as a light source in the visible light region, the near-ultraviolet region, and the near-infrared region, and has a wavelength range of about 320-500 nm. Since the energy output of the tungsten lamp varies approximately at the fourth power of the operating voltage, in order to stabilize the light source, the voltage must be strictly controlled. In addition, in order to prevent the tungsten vapor from sinking in the inner wall of the cold bulb when working at high temperature, a small amount of iodine vapor, that is, a iodine tungsten lamp, is introduced into the tungsten bulb, and the iodine tungsten lamp has a longer service life than the ordinary tungsten lamp.
(2) Monochromator The monochromator is the core part of the spectrophotometer. The main optical characteristics and working characteristics of the spectrophotometer are basically determined by the monochromator. Its function is to disperse the continuous spectrum emitted by the light source into various three. The monochromator in the spectrophotometer is a complete piece - outside the prism or grating, there are also human and exit slits and a set of mirrors. According to the requirements of performance spectrum, dispersion rate, resolution and other performance indicators, prisms or grating splitting monochromators can be used, and filter monochromators can also be used.
The domestic spectrophotometer light sheet is the simplest and cheapest dispersive element, but the monochromaticity is not good, so the measurement accuracy is greatly limited, which is generally used to eliminate the stray light of the monochromator. Prisms and gratings are currently widely used dispersion elements. The ribs can be used as suitable dispersive elements from the ultraviolet to the mid-infrared region, the main disadvantage of which is the non-linear distribution of the dispersion wavelength; and the grating can be used in the ultraviolet, visible, near-infrared range. The main disadvantage of gratings is that there is secondary spectral interference analysis, and the effect of stray light is larger than that of prisms, so filters are often used to remove stray light.
(3) Absorption cell The absorption cell (cuvette) is a container for holding a sample solution, and has two planes which are parallel to each other, transparent, and thick. There are mainly quartz pools and glass pools. A glass cell is used in the visible region, and a quartz cell is used in the ultraviolet region.
The absorption cell may be available in a variety of sizes, typically 1 cm thick. In addition, the material and thickness of each set of absorption tanks should be identical to avoid errors. Fingerprints, oil stains or deposits on the walls of the absorption tank can significantly affect their light transmission, so be sure to wash them thoroughly before use.
(4) Detector The detector is a photoelectric conversion device that displays the optical signal transmitted through the absorption cell into a measurable electrical signal. Commonly used are photovoltaic cells, photocells or photomultiplier tubes. The photomultiplier tube uses secondary electron emission to amplify the photocurrent, and the magnification can be as high as 108 times. It is the most widely used but high-priced detector. More advanced instruments use diode arrays as detectors.
The diode array detector does not use an exit slit, and a linear array of diodes is placed at its position, and monochromatic light of different wavelengths is simultaneously detected after splitting. Mann's response speed is fast, but the sensitivity is not as good as the photomultiplier tube, because the latter has a high magnification.
(5) Data processing and recording system The data processing and recording system includes an amplifier, an A/D conversion unit, a microcomputer, etc.; and an automatic control of the instrument.
The photocurrent generated by the domestic spectrophotometer detector is converted into an analog signal in some way, and linearly amplified, the amplified analog signal is output, and the feedback A/D conversion unit finally performs appropriate data processing through the microcomputer, and The spectrum of the sample to be tested is displayed or printed by the terminal device.
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