A spectrometer is a scientific instrument originally used to split light into an array of separate colors, called a spectrum. Spectrometers were developed in early studies of physics, astronomy, and chemistry. The capability of spectroscopy to determine chemical composition drove its advancement and continues to be one of its primary uses. Spectrometers are used in astronomy to analyze the chemical composition of stars and planets, and spectrometers gather data on the origin of the universe.
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Spectrophotometer is a kind of very important analytical instrument, in the field of physics, chemistry, biology, medicine, materials science, environmental science and other scientific research, or in the chemical, pharmaceutical, environmental testing, metallurgy and other modern production and Management, spectrophotometer has a wide range of important applications.
Spectrophotometers were used by Duboscq and Nessler et al. In 1854 to apply Lambert-Beer's law to quantitative analytical chemistry and to design the first colorimeter. By 1918, the United States National Bureau of Standards made the first UV-visible spectrophotometer. Since then, the UV-Vis spectrophotometer has been continuously improved, and automatic recording, automatic printing, digital display, computer control and other types of instruments, so that the sensitivity and accuracy of photometric method is also continuously improved, and its scope of application is also expanding.
Since the advent of UV-Vis spectrophotometry has been in the application of great development, especially in the development of related disciplines, prompting the continuous development of spectrophotometer instruments, functions more complete, making the application of photometric method broaden range.
At present, spectrophotometry has been widely used in various fields of industry and agriculture and in all fields of scientific research and has become a powerful test tool for people to engage in production and scientific research. There are two types of mainstream products on the market: scanning grating spectrophotometers and fixed grating spectrophotometers.
UV-visible spectrophotometer has a long history, the main theoretical framework has long been established, production technology is relatively mature. However, the new technologies that make up the UV, visible spectrophotometer, such as light, electricity, electricity, etc., may further promote the overall performance of the UV-visible spectrophotometer. In the pursuit of accurate, fast and reliable at the same time, miniaturization, intelligent, online, network has become a modern UV-visible spectrophotometer new growth point.
Spectrophotometry in the field of analysis has been decades of history, is still one of the most widely used analytical methods. With the development of spectroscopic components and spectroscopy, detection devices and detection technology, large-scale integrated manufacturing technology and the widespread application of single chip microcomputer, microprocessor, computer and DSP technology, the performance of spectrophotometer has been continuously improved. Automation, intelligence, speed and miniaturization of the direction of development.
In the spectroscopic components, through the process of prisms, engraved gratings and holographic gratings, the commercial holographic blazed grating has rapidly replaced the general grated grating. In instrument control, with the advent of single-chip microcomputer, microprocessor and the combination of hardware and software technology, the automation from early manual control to automatic control.
In the display, recording and drawing, the early use of meter (potentiometer) instructions plotted plotter, and later digital voltmeter digital display, now more use of LCD screen or computer screen display. In the area of detectors, early use of photovoltaic cells and phototubes was followed by the more common use of photomultiplier tubes and even photodiode arrays. The combination of array detector and concave grating makes the measurement speed of the instrument have a qualitative leap, and the performance is more stable and reliable, which is favored by instrument users.
Instrument software features can greatly enhance the performance and value of the instrument, modern spectrophotometer manufacturers attach great importance to the development of equipment supporting software. In addition to instrument control software and general data analysis and processing software, many instruments have been developed for specialized applications in different industries and have brought great convenience to instrument users.
In general, UV-visible spectrophotometer mainly by the light source system, monochromator system, sample room, the detection system, as shown in Figure 1. The composite light emitted by the light source is decomposed into monochromatic light by the monochromator. When the monochromatic light passes through the sample chamber, a part of the light is absorbed by the sample and the remaining unabsorbed light reaches the detector and is converted into an electrical signal. After amplification and data processing, the measurement results are given by the display system.
Figure 1. UV-Vis spectrophotometer structure--source
The main components of the spectrophotometer are described below.
Light source: emits a continuous spectrum in the desired wavelength range, with sufficient light intensity and stability. Visible light: tungsten, tungsten (320 ~ 2500nm) UV: hydrogen, deuterium lamp (180 ~ 375nm); Xenon: UV, visible light can be used as a light source.
Monochromator: A device that decomposes a continuous spectrum of light emitted by a light source into monochromatic light.
Prism: According to different wavelengths of light through the prism when the refractive index is different.
Grating: A large number of equally-spaced streaks (600,1200,2400 bars / mm) engraved on the aluminized glass surface. Use of light through the grating diffraction and interference occurs when the phenomenon of light.
Absorption pool: for the pending test and reference solution. Visible light: optical glass pool; UV: quartz pool.
Detector: The use of photoelectric effect, the light energy into a current signal. Photovoltaic cells, phototubes, photomultiplier tubes.
Galvanometer (indicator): scale display or digital display, automatic scanning record.
The absorption spectrum of a substance essentially means that the molecules and atoms in the substance absorb the light energy of certain wavelengths in the incident light, and accordingly the results of the molecular vibration level transition and the electron level transition occur.
Since each substance has its own distinct molecule, atom, and different spatial structure of the molecule, its absorption of light energy is not the same, so each substance has its own unique, fixed curve of the absorption spectrum that can be absorbed.
Discrimination or determination of the content of this substance at the absorbance of certain characteristic wavelengths in the spectrum is the basis for qualitative and quantitative spectrophotometric analysis. Spectrophotometric analysis is based on the absorption spectrum of the material to study the composition of matter, structure and substance of the effective means of interaction.
The basis of quantitative analysis of UV-Vis spectrophotometry is Lambert-Beer's law. That is, the absorbance of a substance at a certain concentration is proportional to the thickness of its absorbing medium, and its mathematical expression is as follows
A-absorbance; a- molar absorption coefficient; b- thickness of the absorbing medium; c- concentration of the absorbing material.
The composite light emitted by the tungsten lamp and the deuterium lamp of the light source drives the mirror M1 through the stepper motor control, reflects through the incident slit and enters the monochromator, and the monochromatic light diffracted by the grating is focused by the collimating mirror M2 to converge Through the exit slit, the light beam reaches the chopper, the light of a period of time becomes the reference light path, and the light of another period of time becomes the sample light path. The last two lights illuminate the detector (photomultiplier tube) alternately, as shown in Figure 2.
Figure 2. Optical system schematic--source
Photomultiplier tube signal detected by the preamplifier, the driver card is passed to the computer controller, the computer controller to promote the drive card to coordinate the various parts of the center, as shown in Figure 3.
Figure 3. Electrical schematic--source
Spectrophotometry is one of the most common and effective tools for analysts. Almost every analytical laboratory is inseparable from the UV-visible spectrophotometer. Spectrophotometry has the following main features.
Due to the large number of new color developers synthesized and the progress made in applied research, the sensitivity of the elemental determination has been advanced. In particular, the application of the multi-complex and various surfactants has led many The molar extinction coefficient of the element increased from tens of thousands to hundreds of thousands.
At present, some elements can be determined directly by the photometric method just by controlling the appropriate color developing conditions. For example, cobalt, uranium, nickel, copper, silver, iron and other elements have been measured satisfactorily.
For the general spectrophotometry, the relative error of the concentration measurement in the range of 1 to 3%, such as the use of differential spectrophotometry to measure the error can be reduced to 0.X%.
The trace (10-8 ~ 10-6%) (after pre-enrichment) can be used from the constant (1% to 50%) (especially with the differential method).
As a variety of inorganic and organic matter in the UV-visible region are absorbed, so can be measured by this method. So far, almost all elements of the periodic table of the chemical (except for a few radioactive elements and inert elements) can use this method. Of the total number of papers published in the world analyzed by spectrophotometry, 28% are accounted for, and our country accounts for 33% of the total number of published papers.
Spectral range
Wavelength accuracy
Resolution
Scanning rates
Absorbance
Stray light
According to the absorption spectrum of some of the characteristics of absorption, especially the maximum absorption wavelength λ max and molar absorption coefficient ε, is a commonly used physical parameters of the test substance.
The analytical sample and the standard sample were prepared in the same solvent at the same concentration, and the UV-Vis absorption spectra were measured under the same conditions. If both are the same substance, the spectra of the two should be exactly the same. If there is no standard, it can be compared with the ready-made standard spectrum control. This method requires accurate instruments, high precision, and the determination of the same conditions.
As the UV absorption spectrum contains only 2 to 3 broad absorption bands, and the UV spectrum is mainly the absorption of the chromophore molecules in the UV region, and the molecular and other parts of little. Different molecular structures with the same chromophore do not affect the UV absorption spectrum of the chromophore in larger molecules. Different molecular structures may have identical UV absorption spectra, but their absorption coefficients are different. If the analytical sample and the standard sample have the same absorption wavelength and the same absorption coefficient, the analytical sample and the standard sample can be regarded as the same substance.
With the help of spectrophotometry, some chemical reaction rate constants can be drawn and the reaction activation energy can be obtained from the speed data obtained from two or more temperature conditions.
Ultraviolet Absorption Spectrometry can detect compounds containing traces of UV-absorbing impurities. If the compound in the UV-visible region has no obvious absorption peak, and its impurities in the ultraviolet region has a strong absorption peak, you can detect impurities in the compound.
Different polar solvents produce hydrogen bonds of different intensities, which allows the use of UV spectroscopy to determine the hydrogen bond strength of the compounds in different solvents to determine which solvent to choose.
Metal ions often form complexes with organic compounds, most of the complexes in the UV-visible region is absorbed, we can use spectrophotometry to study its composition.
UV-Vis spectrophotometer is a type of analytical instrument with a long history, but every time it absorbs new technological achievements, it gives it a new vitality. Scanning grating spectrophotometer relies on a mature design and manufacturing process, combined with computer control and other new technological achievements, there is still a strong vitality. In many ways, scanning products still represent the highest technical level.
The production of array detectors has led directly to the design of fixed grating spectrophotometers, making it a big step toward faster, more stable and adaptable measurements. In addition, judging from the future development, the miniaturization and on-line measurement of instruments will become one of the major directions in real-time measurement. Getting your spectrophotometer out of the lab can become a more versatile and popular measurement and analysis device, array detector, and other solid-state designs that can play an important role.
Fiber optics will also be an important technology that has made the use of UV-visible spectrophotometers more convenient and also made the spectrophotometer configuration more flexible. The combination of fiber optic design and modularity may allow the spectrophotometer to break through a completely fixed, static composition and become a freely configurable, self-contained instrument. Optical fiber is also an important means of online measurement. The impact of computer technology will be even more enhanced.
Spectrophotometer automation, intelligence is one aspect. Software, on the other hand, has to some extent brought real instruments close to virtual processes, and the combination of networks and information technologies can have a further impact. In addition to the traditional spatial dispersive spectroscopic methods, acousto-optic modulation filtering and Fourier transform spectroscopy also show their potential for application in the UV-visible band.
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