New development of trace element analysis technolo

2022-08-26
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New progress in the analysis technology of trace elements in food

for food research, the determination of trace elements is very important. To study the toxicological properties and nutritional properties of various elements, and to control the element pollution in food or in the process of production and packaging, we need to widely investigate the content level of trace elements in various foods and the existing forms of elements in food

in the research on the effects of various elements in food on human health, trace element analysis technology has made great progress. The instruments used are mainly microwave digestion device, atomic absorption spectrometer (AAS), inductively coupled plasma atomic emission spectrometer (ICP-OES), inductively coupled plasma mass spectrometer (ICP-MS), etc, Based on the demand of trace element analysis in food, this paper focuses on the development status of these technologies since the 1990s

sample processing for trace element analysis in food

in most food analysis, the sample preparation time is often 20 times longer than the analysis time. For example, the digestion of biological materials takes about 2 ~ 24h according to different digestion procedures, while the determination of each element by Graphite Furnace AAS takes only 3min, including drying, ashing, atomization and cooling. If multi-element analysis is carried out, a longer sample preparation time is required, but ICP-OES and ICP-MS can also be used to analyze the samples in about 3min. In order to further improve the number of samples analyzed every day, it is necessary to have a faster sample preparation speed

although high salt resistant atomizers such as V-slot atomizer and cross atomizer, as well as injection devices such as electrothermal evaporation (ETV) and laser ablation (LA) can be used to directly analyze high salt or solids in food, it is difficult to obtain good quantitative results. At present, the most successful method is still to analyze the food after it is completely digested into aqueous solution. Its main advantages are: low pollution, preservation of volatile elements, less reagent consumption, fast digestion, and small sample size

microwave digestion device solves the "bottleneck" problem of dissolving samples in food analysis. It can be used together with AAS, ICP-OES and ICP-MS to digest various organic and inorganic samples that are difficult to digest. The main technical index of the microwave digestion device is the working pressure. When using quartz container, the maximum bearing pressure is 120bar, the control pressure is 80bar, and the working temperature can reach 300 ℃. When using polytetrafluoroethylene container, the maximum bearing pressure is 120bar, the control pressure can reach 60bar, and the working temperature can reach 260 ℃. It can process 16 samples at the same time in many important industrial fields such as aerospace and ordnance industry. Generally, the samples are processed for 25min, cooled for 15min, and then cooled for 40min. The key is to use them correctly and properly to complete a sample processing cycle

The latest development of AAS

atomic absorption spectrometry (AAS) is one of the main detection technologies in food analysis. It can adopt electrothermal atomization (graphite furnace), flame atomization or hydride generation. These methods enable an element to be accurately determined at the trace level. The main disadvantage of AAS is that it is essentially a single element analysis technology, which cannot be transformed into multi-element simultaneous analysis or sequential analysis. The simultaneous preheating of multiple lamps makes the operating cost of the instrument increase exponentially, the stray light increases greatly, and the stability of the instrument decreases, but its analysis speed has little improvement, let alone compared with the analysis speed of ICP-OES and ICP-MS

since the 1990s, the development of AAS is mainly reflected in the following aspects:

■ CCD solid-state detectors are widely used; Photomultiplier tubes (PMT) have been eliminated by mainstream AAS manufacturers

■ the degree of automation is greatly improved, and the flame and graphite furnace can be switched automatically. After switching, the light path is automatically aligned

■ the control software of the instrument is unprecedentedly powerful. It can not only comprehensively control all parameters of the instrument and comprehensively monitor the safety interlock, but also recommend the best operating parameters for all analysis task software. Even people who have never used AAS can get accurate results. Flame AAS also has a touch-screen design, and the operation of the whole instrument is getting closer to the operation of daily household appliances such as washing machines

■ the biggest highlight of AAS technological progress is the birth of longitudinal magnetic field Zeeman effect AAS, which is gradually replacing the older design of transverse magnetic field Zeeman effect AAS

the latest progress of ICP-OES

with the development of technology, the ICP-OES determination method has been popularized. The digested samples can directly enter the high-temperature plasma (the typical ICP temperature is 5000 ~ 7000K), and the analysis can be carried out at the same time through multicolor observation. The advantage of this technology is that it can analyze more than 70 elements, each element has high sensitivity, and its detection limit is usually ppb (ng/ml), The linear range of the standard curve is more than 6 orders of magnitude, and the interference is very small

the full spectrum direct reading ICP-OES launched in the early 1990s is a revolutionary leap, which enables ICP-OES to obtain the spectral line and its background information at the same time, that is, the direct reading of all the information of a spectral line, which is precisely the ability that the traditional single channel scanning and fixed multi-channel ICP-OES do not have. Therefore, whether the background can be measured at the same time has become the dividing line between the full spectrum direct reading instrument and the traditional instrument

but full spectrum direct reading ICP-OES does not mean that this technology has come to an end. On the contrary, after more than ten years of practical application, it is found that its shortcomings still exist. Its shortcomings are mainly manifested in the following aspects, and the improvement is also mainly aimed at these aspects:

first, the problems caused by the simultaneous measurement of strong light and weak light. Among the many spectral lines excited by ICP at high temperature, the intensity of visible light in the range of 400 ~ 800nm is far greater than that in the ultraviolet region. The problem caused by the simultaneous measurement of these spectral lines is that some pixel points on the detector are rapidly aged and damaged due to long-term strong light irradiation, while the analytical spectral lines in the ultraviolet region cannot be used due to insufficient exposure. In terms of detection limit, Li, Na, K, Rb and CS are often similar to or even inferior to flame AAS when analyzed by ICP-OES. However, the results obtained by ICP-OES analysis of Cl and Br are usually not in good agreement with other mature analysis methods (such as ion chromatography, X-ray fluorescence or ion selective electrode method). Not only ICP-OES, but also ICP-MS has this problem when determining actual samples. Based on these conditions, there are generally two improvement measures adopted. One is to purchase ICP-OES with only UV distinguishing light system and a flame AAS. The cost is more affordable, and both instruments can obtain the best performance. Second, two sets of spectroscopic systems and two detectors are used in ICP-OES to process visible light and ultraviolet light respectively, so as to realize the optimization of the instrument

second, the problems caused by the simultaneous measurement of useful information and junk information. At present, the best improvement method is to use a specially designed detector to selectively read the information of the elements to be analyzed, filter out the above garbage information, so that the operator can obtain the most needed analysis results in the shortest time, and protect the detector from the aging damage caused by the irradiation of garbage information

third, the problem caused by the uneven resolution of the prism in the beam splitting system. This problem is particularly obvious in the ultraviolet region, because there are few spectral lines in the visible region, less interference, and less requirements for resolution, and the spectral lines of most elements are concentrated in the range of 200 ~ 400nm. At present, the best method is to use two gratings, the medium step grating and the plane grating, for cross dispersion within this wavelength range, which is significantly improved than the medium step grating and the prism

fourth, the problems caused by the simultaneous analysis of organic samples, inorganic samples, high salt samples, samples containing HF acid and samples containing NaOH. For ICP-OES users, the samples that need to be analyzed are diverse, which requires a highly adaptive sampling system, and try to reduce the distance between the atomizer, fog chamber and torch tube to reduce the memory effect. At present, the orthogonal atomizer is commonly used, and the atomizer nozzle is equipped with a corrosion-resistant gem nozzle, while the fog chamber is made of corrosion-resistant Ryton material, which can directly analyze 50% HCl, HNO3, H2SO4, 20% HF and 30% NaOH samples

fifth, the problems caused by the simultaneous analysis of principal and trace components and trace and ultra trace components. At present, the most successful is ICP-OES with two-way observation. The detection limit of axial observation can be improved by 10 times than that of lateral observation. The disadvantage of traditional vertical observation ICP-OES is that different observation positions need to be optimized when determining different elements. The two-way observation technology enables users to obtain the advantages of two observation methods at the same time in one analysis. Whether it is axial observation or lateral observation, the observation position is automatically optimized by the computer, which not only improves the sensitivity, but also expands the linear range, greatly increases the flexibility of analysis, and improves the analysis performance

sixth, the progressiveness of the instrument and the ease of use of the actual operation. Modern ICP-OES has been more and more used for quality control in productive factories, unlike the previous focus on universities and research institutes, and the quality of operators is certainly not as good as professional researchers. This problem is mainly solved through the Chinese culture of operating software, the Chinese culture of instrument software and hardware instructions, and the multimedia of instrument maintenance

seventh, the high stability of the instrument and the problems caused by wavelength drift. In order to concentrate all tens of thousands of spectral lines on the detector of several square centimeters, the traditional full spectrum direct reading ICP spectrometer requires that the instrument must have extremely high thermal stability. When the instrument is cold turned on, it generally requires a constant temperature process of more than 1 hour, and the slit height must be very small, which weakens the light intensity of the most important ultraviolet region of ICP, and it is difficult to avoid the high-intensity spectral lines emitted by the sample matrix and AR, N, etc., and greatly shortens the service life of the detector

at present, the most successful method is to use a double monochromator optical system and a double detector with reference. The traditional prism grating cross dispersion mode of full spectrum direct reading ICP spectrometer is carried out in two monochromators respectively. By adjusting the angle of the incident light entering the prism, the spectral level of the spectral line to be measured enters the second monochromator through the middle slit, and the spectral line to be analyzed in the spectrum and a section of spectrum near it are projected onto the CCD detector. Because the cross dispersion is carried out in two monochromators respectively, and each time it is projected onto the CCD detector is only a section of spectrum, it completely avoids the shortcomings of the traditional full spectrum direct reading ICP spectrometer, such as long-time preheating, small incident light slit, short detector life and so on

therefore, the instrument can analyze and determine samples without constant temperature at all, which is the highest achievement in the development of full spectrum direct reading ICP spectrometer at present

The latest progress of ICP-MS

ICP-MS has a broad prospect in food science, because it can not only determine the concentration of all metal elements, but also give isotope information. This isotope measurement capability enables isotope dilution analysis (IDA) technology to be applied, and stable non radioactive isotopes can be used for tracer research

the development of ICP-MS has many similarities with ICP-OES, almost the same in the injection system. At present, the most remarkable development of ICP-MS is the dynamic reaction cell technology. Dynamic reaction cell (DRC) is a reaction cell with a quadrupole system. Similar to the medium step spectroscopic ICP-OES, the DRC-ICP-MS has a double quadrupole mass analyzer, that is, the icp-ms-ms and DRC parts carry out chemical reactions and synchronously scan with the main quadrupole to realize preliminary ion selection and

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