China Education Equipment Purchasing Network News: At present, trace element detection instruments popular in the market, one is graphite furnace atomic absorption, and the other is electrochemical instruments. Spectrophotometry, which is widely used in analysis, can only be used for biochemical analysis and constant analysis with high content because the sensitivity cannot meet the requirements. Atomic absorption has higher precision and detection speed, which should be the first, but due to the complexity of the equipment, high price, long investment return period, various carrier gas and other factors are required, generally more than three units are equipped, and the county Most of the units below the level choose electrochemical analysis instruments.
There are many types of trace element analyzers (trace element detectors), each with a specific market. There is a way to use a camera to look at nails and a TV to look at textures and colors to measure trace elements. This is pure flicker. This kind of equipment can not enter the hospital, and most of them deceive people in pharmacies or mobile establishments.
No matter what the principle instrument is, it must meet the most basic requirements of analytical instruments before it can be used for actual detection.
The main indicators of the instrument are as follows:
1. Repeatability: The error of measuring the same sample multiple times or measuring multiple samples of the same content is generally expressed by the coefficient of variation. The smaller the better, this indicator mainly expresses the reliability, precision and stability of the instrument.
2. Lower detection limit: refers to the lowest detection limit of the specified content, which is generally based on three times the noise, and is generally detected with elements with low pollution and high sensitivity, such as cadmium ion detection for electrochemistry, and copper ion expression for atomic absorption. This index indicates the sensitivity of the instrument.
3. Correlation coefficient: the regression coefficient of different concentration elements and linear equation, the maximum is 1, and it should be at least 0.99 or more.
4. Calibration curve deviation: use a known quantity to compare the deviation with a certain point of the standard curve, the smaller the better, generally less than 10%.
Note: The two items 3 and 4 express the accuracy of the instrument
Different types of analytical instruments have their strengths and weaknesses, and any kind of instrument can not cover the world. The advantages and disadvantages are relatively speaking. For example: atomic absorption metal cadmium has high sensitivity, while lead sensitivity is extremely low. The analysis speed of electrochemical analysis instruments is not as good as atomic absorption, but traditional atomic absorption requires a specific element lamp for each element, which cannot be determined jointly.
Electrochemical analysis instruments use electrodes as sensors, especially solid electrodes cannot be used stably for a long time. A certain number of samples need to be reprocessed such as mercury plating (referring to lead, cadmium, etc.), but the measurement of lead elements can be quantified from 0.1 micrograms. Detection, this can't be done by atomic absorption anyway.
Atomic absorption measurement is fast and accurate. However, it cannot be used for the detection of anions and organic substances. In addition, the use cost is high, and the environmental requirements are high. It is difficult to promote the application of users in the grassroots units.
Electrochemical instruments use metal mercury as the working electrode, which is suspected of polluting the environment, but can be used for the analysis of cations, anions, organics, etc. The purchase, maintenance, and operating costs are low, the return on investment is high, and it is easy to promote. There are many national standards and industry standards that use electrochemical polarography and dissolution methods. Therefore, high-end users use more atomic absorption, and county-level and below users use more electrochemical instruments.
How to judge the performance of an instrument?
The average user lacks professional knowledge or little understanding, but only a few key points are required. First look at whether the instrument is stable. This is the most basic requirement of the instrument. It is the basis for analysis and testing. If this indicator is not good, there is no other way to talk about it. It is purely fooling. If the same sample is used, the results obtained will vary from high to low, which means that there is a problem with the instrument. When the content of the measured substance is high, the stability of the determination is generally good, but this does not indicate a problem, because the purchase of such instruments is used for trace element detection, not for high content detection. Because it is trace, the element peak obtained is not high, which is a test of the true performance of the instrument. For high content detection, the coefficient of variation can reach 1%, but it is good for low content to reach below 5%. If you look at the actual measurement of the instrument, some manufacturers avoid the convenience of observing the repeatability, prevent the operator from conveniently superimposing the curve, and do not set the procedure for calculating the coefficient of variation. This is because the performance of the instrument is not good, and it is afraid that others will see it at a glance. The problem is that the performance of the instrument is not critical, and manufacturers are avoiding this problem. In fact, the coefficient of variation of the instruments of some manufacturers is between 10-20%, and the error greatly exceeds the relevant regulations. The results have low credibility. The internal coefficient of variation of our company's products is less than 5%, which can be tested by the instrument. At a glance.
The electrode used in polarography in electrochemistry is extremely small (referring to the mercury electrode), and the sensitivity is related to the specific surface area of ​​the electrode. The large area has a high signal-to-noise ratio (refers to the ratio of signal to noise), high sensitivity, and strong anti-interference ability, but Since the area of ​​the mercury-dropping electrode depends on the volume, it will fall off uncontrollably when it is large. Therefore, the surface area of ​​the electrode is severely restricted. This determines that the instrument made by the conventional method has a large noise and poor repeatability. Electrode-like instruments are a common problem. Only our company's products solve this problem by adopting a series of technologies such as static mercury electrodes. In this technical field, it is better than any similar instrument, which can be confirmed by on-site comparison. Another type of electrochemical instrument avoids this kind of problem, using solid electrodes to measure all elements, and attacking instruments that use polarographic methods from the perspective of environmental protection use mercury electrodes. In fact, in the test, mercury is working in a closed environment, not in contact with the atmosphere. Just like a sphygmomanometer, the current measurement is still mercury sphygmomanometer, which is still widely used. The polarographic analysis method using the mercury drop electrode is the national standard method and industry standard in many fields. For example, the determination of zinc in serum is a method prescribed by the Ministry of Health's inspection regulations, and there are more polarographic analysis standards in food. It is good to use solid electrodes (such as glassy carbon electrodes) to measure certain specific elements such as lead, but it is impossible to use it to solve the detection of all elements without changing, whether in theory or in practice There is no supporting evidence. Those who spend a lot of money to buy such instruments are either lack of knowledge or another reason. Due to the low content and small useful signal, trace element analysis has high technical requirements. If you do well, the repeatability is high, the sensitivity is high, and the performance of the instrument is stable. If you do not do well, the repeatability is poor, and the entire instrument foundation is poor. You must use various means to deal with the user and hide the truth. The easiest way to judge is to see whether the instrument is set up to facilitate the comparison of multiple curves, directly observe the repeatability function, and whether it is convenient to find the coefficient of variation. Further measure the zinc in the blood. Repeat the test to see whether the repeatability is good or bad. Because the content of zinc is low, the performance of the instrument can be evaluated. Before doing this, you need to check the blank to prevent the reagent blank from being too high and causing the artifacts.
It is only possible to examine other performance indicators on the basis of good repeatability. Objectively speaking, as long as the repeatability is done well, other performance indicators should have little problem. But the kind of instrument that uses one electrode as all the elements, making blank plus standard solution and measured sample is very different. That is to say, if you don't make a sample and only make a standard solution, it may be very good. However, when the sample is added, the sample brings various interferences and the repeatability, linearity, and sensitivity have very large changes. At present, it is relatively mature to use solid glassy carbon electrode to measure blood lead. Others should look at whether the country or industry has standards in this regard, is there any literature in this regard, and is there any examples of actual samples used in the literature. If not, some companies have developed a set of analysis methods using this electrode in a short period of time. Whether it is super capable or another reason, it needs to be considered by those interested.
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