I. Classification of Pesticides Pesticides are the collective name for chemical agents used to control pests, weeds and other harmful organisms that endanger agricultural crops and agricultural and sideline products. There are many ways to classify them. Common categories are as follows:
1, according to the different control objects, divided into insecticides, tinctures, fungicides, herbicides, rodenticides and so on. Often called pest control insecticides, control of red spider known as acaricides, control of crop fungi, bacteria and viruses known as fungicides, control of weeds called herbicides, known as the killing of rodents Rat agent.
2, according to the mode of action of pesticides divided into stomach agents, contact poisons, fumigants, inhalants, attractants, repellents, antifeedants, infertility agents.
3, according to the chemical composition and structure of pesticides can be divided into two categories of organic pesticides and inorganic pesticides. Apart from a small part of inorganic substances, most of pesticides are organic compounds, including elemental organic compounds, metal organic compounds and general organic compounds. The organophosphates we tested belong to elemental organic compounds and the like.
There are also classified according to the source of pesticides, the use of pesticides, prevention and control principles.
Second, biochemical analysis applied to what pesticides The reason why the vast majority of chemical pesticides can kill insects, cockroaches, or effective is that they have different degrees of inhibition of enzymes within the organism. Therefore, pesticides that are tested using biochemical analysis should be inhibitors (activators) of certain enzymes in the organism, causing these enzymes to lose their normal physiological effects, thus causing the organism to exhibit symptoms of poisoning. Of course, based on the current research level, although some pesticides are known to have an adverse effect on an enzyme, the reasons for the extraction and preservation of the enzyme also constrain the use of biochemical methods to test the pesticide. Therefore, although there are many types of enzymes in living organisms, the principle of chemical pesticides is mostly based on the action of enzymes in living organisms, but there are not many kinds of pesticides tested by biochemical analysis.
Third, test pesticide residues What methods pesticide residue test methods, in general, can be divided into conventional test methods and rapid test methods. The conventional test methods include gas chromatography, gel chromatography and thin layer chromatography. These methods are all based on the use of different partition coefficients of pesticides in different carriers to determine the type and content of pesticides qualitatively and quantitatively. There are two methods for rapid measurement: rapid card method and enzyme inhibition rate method (also called spectrophotometry). Both the rapid card assay and the enzyme inhibition method use the principle of inhibition of enzyme activity.
Fourth, the role of acetylcholinesterase on acetylcholine The role of acetylcholinesterase and acetylcholine, the first to form a complex of enzyme substrate, and then the acetylcholine acyl transfer to the enzyme molecules, the formation of acetylated esterase. Acetyl esterase is very unstable and easily hydrolyzed. Its half-life is about 0.1 milliseconds. After the hydrolysis of acetylated esterase, the enzyme molecules are released, so that the enzyme is revived and recycled.
Fifth, organophosphorus and carbamate pesticides on the inhibition of acetylcholinesterase, what is reversible inhibition of organophosphorus and carbamate pesticides is a structural analog of cholinesterase, the substrate of acetylcholinesterase. When they interact with acetylcholinesterase, they first form a non-covalent intermediate complex, which is then hydrolyzed. At the same time, the phosphoryl group reacts with the serine hydroxyl of the cholinesterase active center to form covalently bound phosphorylase. Phosphorylation of phosphorylase is difficult to hydrolyze, so phosphorylase is very stable and its stability is more than 107 times that of acetylase. Therefore, it can be said that the inhibition of acetylcholinesterase by organic phosphorus is irreversible. Irreversible inhibition means that the inhibitor inactivates the enzyme with a strong covalent bond with certain groups on the enzyme molecule. It is characterized by a gradual increase in inhibition over time, and finally complete inhibition. Reversible inhibition means that the binding of the inhibitor to the enzyme molecule is reversible and does not produce irreversible covalent modifications.
6. Meaning of the Michaelis constant The Michaelis constant km represents the dissociation constant of the enzyme-substrate complex. Each enzyme has its own specific km for a specific substrate. 1/km is called the affinity constant for binding of the enzyme to the substrate. The Michaelis-Menten constant can also be quantified as the concentration of the substrate when the enzymatic reaction rate is half of the maximum reaction rate at a certain enzyme concentration.
VII. Factors influencing the inhibitory rate of acetylcholinesterase in the pre-reaction process The inhibitory rate of acetylcholinesterase in pesticides depends mainly on the molecular structure of the pesticide. When the pesticide molecule contains a functional strong electron-withdrawing group, the more easily the phosphoryl group and the acetylcholinesterase serine hydroxyl group are combined, the greater the inhibition rate in a certain period of time. Of course, for a certain pesticide, in the pre-reaction, the inhibition of the environmental temperature and the inhibition time and the concentration of the pesticide are all factors that affect the inhibition rate. Therefore, in the pre-reaction, the environmental temperature, the inhibition time, and the consistency of the pesticide concentration should be ensured as much as possible. The inhibitory rate of some pesticides when they are just prepared is large, and the inhibitory rate may be reduced when the same pesticides are tested again after a few days. This is because some pesticides have been degraded and do not cause inhibition.
8. After the pre-reaction is over, the factors that affect the determination of inhibition rate In the process of testing the pesticide residue by the enzyme inhibition rate method, after the pre-reaction is completed, the substrate is added for color reaction. The test determines the rate of inhibition based on the color change in the system. Therefore, when testing a batch of samples to be tested, the process of adding the substrate and testing on the machine should be as fast as possible, and the amount of the added substrate must be the same, and there must be no loss of any solution, otherwise the measured data is not reliable. .
Nine, the sample extraction should pay attention to what some fruits and vegetables such as onions, garlic, radish, leeks, celery, parsley, Poria, mushrooms, tomatoes, etc. contain plant secondary substances that affect the enzyme, some vegetables in the chlorophyll content is too High, affect the colorimetric reaction. Therefore, for the above types of fruits and vegetables, the whole plant (body) extraction method should be adopted as far as possible to eliminate the interference. In addition, due to the same strain (sample) of the sample to be tested, the pesticide content in different parts of the sample is different. Therefore, when the repeatability test is performed, it is better to measure the different times of the same test solution, otherwise there may be weight due to different samples. The current situation is not good.
X. How to preserve the enzyme inhibitory rate of the reagents Most of the reagents used in the assay method are biochemical reagents, so their storage conditions are more severe. Powdered enzymes and substrates should be stored at -20°C. For agents that have been formulated as solutions, such as enzyme solutions, substrate solutions, developer solutions, etc., should be stored at 0-4°C. The extraction agent can be sealed and stored under normal conditions, whether formulated as a solution or not. XI. What are the national targets for rapid pesticide residue testing and what are the test methods? The current national standards for the rapid detection of pesticide residues include the "National Standard of the People's Republic of China" and the "Agricultural Industry Standard of the People's Republic of China". . These two standards stipulate the criteria for the determination of test results and the minimum detection limits for common pesticides such as methamidophos, omethoate, carbofuran, and dichlorvos. Different standards, based on the type of enzyme used and methods, the limits of the various pesticides and the criteria for determination are also different. For example, the agricultural industry standard judges that the rejection rate is greater than 70%, and the national standard has a predetermined inhibition rate of more than 50% as unqualified, and also stipulates that the absorbance value greater than 0.9 is unqualified. The methods used by both standards are based on the principle that the enzyme is inhibited by the pesticide and the activity is reduced. Among the national standards, there are acetylcholinesterase and phytoesterase-flourase, while the industry standard uses butyrylcholinesterase.
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