24/07/2024
【Real stuff】 What are the main detection technologies for pesticide residues?
1-Biological determination technology of pesticide residues
Use the physiological and biochemical reactions of indicator organisms to judge pesticide residues and their pollution. For example, sensitive houseflies raised in the laboratory can be used as test materials, and the degree of poisoning after contacting the sample to be tested can be used to indicate the pesticide residue in the sample; the degree of inhibition of pathogen growth can be used to detect fungicide residues, and the degree of inhibition of root growth of corn or other indicator plants can be used to detect sulfonylurea herbicide residues in the soil. This method does not require sample pretreatment, which is relatively simple and fast or does not require pretreatment, but has high requirements for indicator organisms. The test results cannot determine the type of pesticide, and false positives or false negatives may occur. This method can be used as a rapid test method for agricultural products causing poisoning or on-site use.
2-Physical and chemical detection of pesticide residues
Chemical detection methods for pesticide residues include spectrophotometry, polarography, atomic absorption spectroscopy, thin layer chromatography, gas chromatography, liquid chromatography, isotope labeling, nuclear magnetic resonance spectroscopy, chromatograph-mass spectrometry, etc. Since the 1990s, modern chemical analysis technology has been changing with each passing day, and many new technologies have entered the practical stage, such as capillary electrophoresis technology (CZE), chromatograph-mass spectrometry technology (GC-MS, HPLC-MS), supercritical fluid chromatography technology (SFC), direct spectral analysis technology, etc. The application of these new technologies has greatly improved the sensitivity of pesticide residue analysis, simplified the analysis steps, and improved the analysis efficiency. However, some of these analysis methods are not very sensitive, such as spectrophotometry and thin layer chromatography. Some require expensive instruments, such as chromatograph-mass spectrometry, nuclear magnetic resonance spectroscopy, etc. Others require special equipment, such as isotope labeling. Therefore, at present, gas chromatography and liquid chromatography are still commonly used, which have the advantages of simplicity, rapidity, sensitivity, good stability and reproducibility, wide linear range, and low cost.
(1) Gas chromatography (GC)
The chromatographic method using gas as mobile phase is used for the detection of volatile pesticides. It has high selectivity, high separation efficiency, high sensitivity, rapidity and characteristics. It is one of the most commonly used methods for the detection of pesticide residues. The detectors currently used for the detection of pesticide residues mainly include electron capture detector (ECD), micro-electron capture detector (u-ECD), flame photometric detector (FPD), pulse flame photometric detector (P-FPD), nitrogen phosphorus detector (NPD), etc.
(2) Liquid chromatography (HPLC)
A chromatographic method using liquid as mobile phase. It can separate and detect highly polar, large molecular weight and ionic pesticides. It can be used for the detection of pesticides that are not easy to vaporize or easily decomposed by heat. In recent years, the use of new high-efficiency stationary phases, high-pressure pumps and high-sensitivity detectors, pre-column and post-column derivatization technologies, and computer-assisted use have greatly improved the detection efficiency, sensitivity, speed and degree of operation automation. Currently, the most commonly used detectors for pesticide residue detection are ultraviolet absorption detectors (UV), diode array detectors (DAD) and fluorescence detectors (FLD).
(3) Chromatography-mass spectrometry (GC-MS, HPLC-MS)
The combination of gas phase or liquid phase and mass spectrometry has the advantages of high separation efficiency of chromatography and the characteristics of accurate identification of compound structure of mass spectrometry. It can achieve qualitative and quantitative detection purposes at the same time. It is particularly suitable for the detection of pesticide metabolites, degradation products and multi-residue detection. However, this method requires expensive instruments and is complicated and difficult to operate. It is not suitable for regular detection. It can generally be used for final confirmation.
(4) Supercritical fluid chromatography (SFC)
It is a chromatographic system with supercritical fluid as the mobile phase. Supercritical fluid refers to the state of a substance at critical temperature and critical pressure, which is between gas and liquid, and has some physical properties of both gas and liquid. Therefore, supercritical fluid chromatography has the characteristics of fast, efficient and sensitive gas chromatography, and the characteristics of liquid chromatography that can detect thermally unstable and large molecular compounds.
(5) Capillary electrophoresis (CE)
This method uses capillaries and high voltage (15-30KW) to separate various pesticide residues. It is very suitable for the separation and analysis of some ionized samples that are difficult to separate using traditional chromatography. It has an analytical capability 10-1000 times higher than HPLC, and the required buffer is environmentally friendly. Qualitative and quantitative analysis can be completed in a short time (30 minutes).
3-Commonly used rapid detection methods for pesticide residues
(1) Enzyme inhibition method
Organophosphorus and carbamate pesticides are both inhibitors of acetylcholinesterase in the nervous system. Therefore, the degree of inhibition of the pesticide target enzyme-acetylcholinesterase (AChE) can be used to detect organophosphorus and carbamate pesticides. Various rapid test cards and rapid test instruments have been developed for this method. When this method is used for testing, water, carbohydrates, proteins, fats and other substances in vegetables will not interfere with the detection of pesticide residues. There is no need to separate and remove impurities, which saves a lot of pretreatment time, thus achieving the purpose of rapid detection. Therefore, this method is fast and convenient, with simple pretreatment and no need for instruments or relatively simple instruments. It is suitable for qualitative and semi-quantitative determination on site. The current rapid detection of pesticide residues uses this method and has been promoted to the industry standard of the Ministry of Agriculture. However, this method can only be used to determine organophosphorus and carbamate insecticides. Its sensitivity is closely related to the enzyme used, the color reaction time and temperature. After the enzyme method is positive, it needs to be further tested with standard instrumental testing methods to identify the type of residual pesticides and the accurate residual amount.
(2) Immunoassay
There are radioimmunoassay, enzyme immunoassay, multi-component analyte immunoassay, immunosensor analysis, etc. The most commonly used is enzyme-linked immunosorbent assay (ELISA), which is a pesticide residue detection method based on the specific and reversible binding reaction between antigen and antibody. This method uses the principle that chemical substances can produce immune antibodies in animals. First, small molecule pesticide compounds are combined with large molecule biological substances to form macromolecules, which are made into antigens, and antibodies are produced in animals. The antibody screening kit is made into a kit, and the enzyme-linked immunosorbent reaction between antigens and antibodies is used to determine the pesticide residues by colorimetry. It has the advantages of strong specificity, high sensitivity, rapidity, and simple operation. The kit can be widely used for rapid detection of field samples and a large number of samples, and can accurately determine the quality and quantity. However, due to the large number of pesticide types, the difficulty of antibody preparation, the certain blindness of detection when the type of pesticide residues in the sample cannot be determined, and the dependence of antibodies on foreign imports, the application scope of the enzyme-linked immunosorbent assay is greatly limited. At present, the finished enzyme-linked immunosorbent assay kits in the Chinese market rely on imports from abroad.
(3) Chemical method - Suceling method
The principle of the "Suceling" method is that metal ion catalysts with strong catalytic effect can hydrolyze various organophosphorus pesticides (phosphates, dithioesters, phosphoamines) into phosphoric acid and alcohol under the catalytic effect. The hydrolysis products react with the color developer, causing the purple-red color of the color developer to fade and become colorless. It is mainly used for the detection of organophosphorus pesticide residues, especially methyl parathion and parathion pesticides. This method adopts the principle of chemical reaction, avoids the shortcomings of the commonly used biochemical method (enzyme method) (enzyme preparation, storage and reaction require relatively strict conditions), and the sensitivity also meets certain requirements. However, this method is mainly used for the qualitative detection of organophosphorus pesticide residues with high toxicity such as methyl parathion and parathion.
