Abstract This paper introduces the development, principle, characteristics and application of UV-Vis spectrophotometry, and lists several examples of the application of UV-Vis spectrophotometry in various fields.
Key words organic analysis absorption spectrum ultraviolet-visible spectrophotometry
1. Development
People have already concluded in practice that different colored substances have different physical and chemical properties. According to these characteristics of the substance, it can be effectively analyzed and judged. Because the color has attracted attention, the content of the substance can be estimated based on the color depth of the substance, which can be traced back to ancient and medieval times. In 1852, Beer referred to the articles published by Bouguer in 1729 and Lambert in 1760, and proposed the basic law of spectrophotometry, that is, when the thickness of the liquid layer is equal, the intensity and color of the color The concentration of the color solution is proportional, which lays the theoretical foundation of spectrophotometry, which is the famous Bill Lambert law. In 1854, Duboscq and Nessler applied this theory to the field of quantitative analytical chemistry, and designed the first colorimeter. By 1918, the US National Bureau of Standards made the first ultraviolet-visible spectrophotometer. Since then, the UV-Vis spectrophotometer has been continuously improved, and various types of instruments such as automatic recording, automatic printing, digital display, and microcomputer control have appeared, which has continuously improved the sensitivity and accuracy of the photometric method, and its application scope has also continued to expand.
Since the advent of ultraviolet-visible spectrophotometry, there have been great developments in application, especially on the basis of the development of related disciplines, which has prompted the continuous innovation of spectrophotometer instruments and more complete functions, which has broadened the application of spectrophotometry. range. At present, spectrophotometry has been widely adopted by various departments of industry and agriculture and various fields of scientific research, and has become a powerful test method for people to engage in production and scientific research. China has a solid foundation in the field of analytical chemistry, and has reached a certain level in the world in terms of spectrophotometric analysis methods and instrument manufacturing.
2. Principle
The absorption spectrum of a substance is essentially that the molecules and atoms in the substance absorb the light energy of certain specific wavelengths in the incident light, and the molecular vibrational energy level transition and the electronic energy level transition occur accordingly. Since various substances have their own different molecules, atoms and different molecular space structures, their absorption of light energy will not be the same. Therefore, each substance has its own unique and fixed absorption spectrum curve. The determination of the absorbance at certain characteristic wavelengths on the spectrum to determine or determine the content of the substance is the basis of qualitative and quantitative analysis of spectrophotometry. Spectrophotometric analysis is an effective means to study the composition, structure and interaction of substances based on their absorption spectra.
The basis of quantitative analysis of ultraviolet-visible spectrophotometry is Lambert-Beer law. That is, the absorbance of a substance at a certain concentration is proportional to the thickness of its absorption medium.
The color of a substance is closely related to its electronic structure. When the electron transition caused by radiation (photons) causes the molecule (or ion) to rise from the ground state to the excited state, the molecule (or ion) will absorb light in the visible region or ultraviolet. The occurrence or change of color is related to the deformation of the normal electronic structure of the molecule. When a molecule contains one or more chromogenic genes (ie, atomic groups with unsaturated bonds), the radiation will cause changes in the energy of the electrons in the molecule. Common chromophores are:
CO, -N = N-, -N = O, -CN, CS
If there is a carbon atom between the two chromophores, a conjugated group is formed, which shifts the absorption band to a longer wavelength (ie, red shift), and the intensity of the absorption band increases significantly. When molecules contain chromophore groups (groups with unshared electron pairs), a red-shift effect can also occur. Common chromophore groups are: -OH -NH2, -SH, -Cl, -Br, -I.
3. Features
Spectrophotometry is one of the most useful tools for analysts. Almost every analytical laboratory is inseparable from the UV-Vis spectrophotometer. The main features of spectrophotometry are:
(1) Widely used
Because a variety of inorganic and organic substances are absorbed in the ultraviolet-visible region, they can be determined by this method. So far, almost all elements on the periodic table of chemical elements (except for a few radioactive elements and inert elements) can use this method. Of the total number of analysis papers published internationally, photometry accounts for about 28%, and China accounts for about 33% of the total number of published papers.
(2) High sensitivity
Due to the large amount of synthesis of the new color developer and the gratifying progress in applied research, the sensitivity to element determination has been advanced, especially the application research on multi-component complexes and various surfactants has made many The molar absorption coefficient of the element is increased from the original tens of thousands to hundreds of thousands.
(3) Good selectivity
At present, some elements can be directly measured by photometry as long as the appropriate color development conditions are controlled, such as the determination of cobalt, uranium, nickel, copper, silver, iron and other elements, and there are already satisfactory methods.
(4) High accuracy
For the general spectrophotometric method, the relative error of the concentration measurement is in the range of 1 to 3%. If the differential photometric method is used for measurement, the error can be reduced to a lower value.
(5) Wide range of applicable concentration
It can be from a constant (1% to 50%) (especially using the differential method) to a trace amount (10-8 to 10-6%) (after pre-enrichment).
(6) Low analysis cost, easy operation and fast
Because spectrophotometry has the above advantages, it is still widely used in chemical, metallurgy, geology, medicine, food, pharmaceutical and other departments and environmental monitoring systems. It is widely used in water quality analysis alone. There are currently more than 70 kinds of metal and non-metal elements that can be measured by direct and indirect methods.
4. Application
4.1 Test substances
According to some characteristic absorption on the absorption spectrum, especially the maximum absorption wavelength and the molar absorption coefficient are commonly used physical parameters of the test substance. This has a wide range of applications in drug analysis. In the pharmacopoeia at home and abroad, the maximum absorption wavelength and molar absorption coefficient of the ultraviolet absorption spectrum of many drugs have been loaded into it, which provides a good means for drug analysis.
4.2 Comparison with standards and standard spectra
The analytical sample and the standard sample were prepared in the same solvent at the same concentration, and the ultraviolet-visible absorption spectra were measured under the same conditions. If the two are the same substance, the spectra of the two should be exactly the same. If there is no standard sample, it can also be compared with the existing standard spectrum. This method requires accurate instruments, high precision, and the same measurement conditions.
4.3 Compare the consistency of the absorption coefficient of the maximum absorption wavelength
Because the ultraviolet absorption spectrum contains only 2 to 3 wider absorption bands, the ultraviolet spectrum is mainly the absorption of the chromophores in the molecule in the ultraviolet region, and has little relationship with the molecule and other parts. Different molecular structures with the same chromophore do not affect the ultraviolet absorption spectrum of the chromophore in larger molecules. Different molecular structures may have the same ultraviolet absorption spectrum, but their absorption coefficients are different. If the analysis sample and the standard sample have the same absorption wavelength and the same absorption coefficient, the analysis sample and the standard sample can be considered to be the same substance.
4.4 Purity inspection
Example 1 Ultraviolet absorption spectroscopy can determine that the compound contains trace impurities with ultraviolet absorption. If the compound has no obvious absorption peak in the ultraviolet-visible region, and its impurities have a strong absorption peak in the ultraviolet region, the impurities in the compound can be detected.
Example 2 Detection of benzene impurities in ethanol samples. The maximum absorption wavelength of benzene is at 256nm, while ethanol does not absorb at this wavelength. It can be clearly seen in the ultraviolet absorption spectrum.
If the compound has absorption in the ultraviolet and visible, the purity can be measured by the absorption coefficient.
Example 3 You can also use the differential method to test the purity of the sample. Take the pure product of the same concentration and measure it in the same solvent as a blank control. The differential spectrum between the sample and the pure product is the spectrum containing impurities in the sample.
4.5 Inferring the molecular structure of compounds
(1) Speculate the conjugated system and partial skeleton of the compound
If a compound is transparent in the ultraviolet region, there is no absorption peak, it means that there is no conjugated system (refers to the absence of multiple interphase double bonds). It may be aliphatic hydrocarbons, amines, nitriles, alcohols and other compounds that do not contain double bonds or ring structures.
If there is strong absorption at 210-250nm, there may be two double bond conjugated systems (such as conjugated diene or mantle
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