Advances in pretreatment technology for pesticide residue detection in food

After the pesticide is applied to the crop, most of it is converted for a variety of reasons, but there will be a small amount of residue in the crop. Prolonged feeding of residual pesticides can affect human health. In recent years, in the cultivation of tea, grain, vegetables and fruits, many farmers have neglected the correct and rational use of pesticides. Pesticide pollution problems often occur, and pesticide residues exceed the standard, which is quite serious and is increasing year by year. In order to monitor the pesticide residues in food , it is necessary to carry out pretreatment technology research on pesticide residue detection .

1 sample pretreatment technology and its progress

With the development of science and technology, more and more kinds of environmental pollutants need to be analyzed, and the content of the components to be tested is getting lower and lower, which poses a new challenge to the pretreatment method of the sample. The quality of the sample directly affects the test results. The following is an overview of the sample pretreatment techniques.

1.1 Common sample preparation technology

1.1.1 Solvent Extraction (LLE) One of the most common extraction techniques for liquid samples is solvent extraction. Separation, extraction or purification is achieved by different solubility or partition ratios of different components in the sample in two immiscible solvents. The purpose, usually called liquid-liquid extraction. Depending on the substrate, it can be divided into liquid-liquid extraction, liquid-solid extraction and liquid-gas extraction (solution absorption). The current liquid-liquid extraction technology has been developed to continuous extraction and countercurrent extraction, which is beneficial for the treatment of samples with low partition coefficient materials; micro-extraction technology is beneficial to improve sensitivity and reduce solvent usage; extraction cartridge technology mimics traditional liquids - - Liquid extraction technology, and makes sample collection very easy, while avoiding sample emulsification problems; online extraction and automatic liquid-liquid extraction can reduce human error and facilitate the processing of large volume samples.

1.1.2 Solid phase extraction (SPE) Solid phase extraction is the use of a solid adsorbent to adsorb a target compound in a liquid sample, separating it from the matrix and interfering compound of the sample, and then eluting with an eluent or desorbing it by heating. The purpose of separating and enriching the target compound is achieved. Compared with traditional methods such as liquid-liquid extraction, solid phase extraction has the following advantages: (1) high recovery and enrichment multiples. (2) The use of high-purity toxic organic solvents is small, reducing environmental pollution, and is an environmentally friendly separation and enrichment method. (3) No phase separation operation, easy to collect analyte components, can handle small volume samples. (4) Easy to operate, fast, and easy to automate. Solid phase extraction can be used to analyze active ingredients or harmful ingredients in foods, as well as various pollutants in environmentally friendly water samples.

1.1.3 Solid Phase Microextraction (SPME) Solid phase microextraction technology is developed on the basis of solid phase extraction. Compared with liquid-liquid extraction or solid phase extraction, it has short operation time, small sample volume and no extraction. Solvent, suitable for the analysis of volatile and non-volatile substances, good reproducibility. There are many factors affecting the sensitivity of solid phase microextraction, but the type and thickness of the extraction head coating are the most critical. The use of SPME in food and biological samples is increasing, such as the detection of chloropropanol in soy sauce and the detection of organochlorine compounds in the blood.

1.1.4 Headspace Technology (HS) Analytical determination of trace high volatility in samples can be performed using gas extraction or headspace technology. Headspace technology can be divided into static headspace and dynamic headspace. They have the following characteristics: (1) easy to operate, just fill the sample into the headspace bottle, and then seal and store until chromatographic analysis; (2) can be automated, already Many gas chromatograph manufacturers are able to provide integrated gas chromatograph headspace samplers; (3) many variable factors, static headspace only need to determine the equilibrium time and temperature of the sample in the headspace bottle, while dynamic headspace still needs Determine the type and filling amount of the adsorbent in the trap; (4) The dynamic term has a high sensitivity, and the detection limit can reach 10-12. Headspace technology combined with chromatography as a widely used reliable and efficient analytical measurement technology has become the standard method in many countries and organizations.

1.1.5 Membrane Extraction Technology (ME) Membrane extraction is a sample preparation technique based on separation and enrichment of non-porous membranes. Membrane extraction mainly includes several modes such as supported liquid membrane extraction, continuous flow membrane extraction, microporous membrane liquid-liquid extraction, and polymer membrane extraction. The advantages of membrane extraction are mainly high enrichment multiple, high purification efficiency, low dosage of organic solvent, low cost and easy to be used in online with analytical instruments . Membrane extraction technology is considered to be the most selective and the most "clean" sample preparation technology after treatment. In terms of solvent dosage, polymer membrane extraction technology can be used without solvent, and the high boiling point of liquid membrane used in supported liquid membrane extraction technology. The amount of organic solvent is negligible. Although the organic phase is used in continuous flow membrane extraction and microporous membrane liquid-liquid extraction, only a small volume of conventional organic solvent is required.

1.2 Other pre-processing techniques

1.2.1 Microwave Extraction Technology (SAE) Microwave extraction technology is a pretreatment technology with fast extraction speed, low reagent usage, high recovery, sensitivity and easy automatic control. It uses the characteristics of microwave heating to selectively extract the target components in the material. Microwave extraction is to place the sample in a sample cup made of polytetrafluoroethylene material. After adding the extraction solvent, the sample cup is placed in an extraction tank which is sealed, high-pressure resistant and does not absorb microwave energy. Since the extraction tank is sealed, when the extraction solvent is heated, the pressure inside the tank is increased due to the volatilization of the extraction solvent. The increase in pressure also increases the boiling point of the extraction solvent, which increases the extraction temperature. At the same time, due to the sealing, the extraction solvent is not lost, and the amount of the extraction solvent is reduced. The extraction temperature is increased during microwave heating to greatly improve the extraction efficiency.

1.2.2 Supercritical Fluid Extraction (SFE) Supercritical fluid extraction is a separation and extraction technique that uses a supercritical fluid as an extractant to separate and extract the required components from various complex samples. Since the density of the supercritical fluid is close to that of the liquid, the viscosity is only slightly higher than that of the gas, and the surface tension is small, which brings together the advantages of gas and liquid, and the extraction process can be completed under efficient, rapid and relatively economical conditions. The commonly used extraction solvent is carbon dioxide. Because it is non-toxic and does not cause toxic solvent residue like organic solvent extraction, it is an ideal and clean sample preparation technology. Carbon dioxide is commonly used as the SFE fluid to extract non-polar and moderately polar materials. For the sample molecule to contain a polar group such as a hydroxyl group or a carboxyl group, it is necessary to add an appropriate amount of a polar solvent to the carbon dioxide to increase the polarity of the fluid, or to use a polar supercritical fluid such as ammonia. When carbon dioxide is used as the fluid, the operating temperature is low, which is favorable for the extraction of the thermally unstable compound, and at the same time, the fluid does not contain oxygen, and the oxidation of the components is avoided.

1.2.3 Derivatization Derivatization is the chemical conversion of a target compound that is difficult to analyze and detect in a sample into another compound that is easy to analyze and detect. The latter analysis is used to characterize the suspect compound and/or Or quantitative analysis. The purpose of derivatization is as follows: (1) converting some compounds that are not suitable for an analytical technique into derivatives that can be used in the technique; (2) improving detection sensitivity; and (3) changing the performance of the compound and improving sensitivity; (4) contribute to the identification of the structure of the compound.

2. Conclusion

The common application of pesticide residue analysis in China is traditional techniques such as extraction and separation technology, Soxhlet extraction, oscillatory extraction and ultrasonic. The sample requires large amount, long extraction time and large consumption of organic solvent, resulting in the production of a large number of toxic waste solvents. Fast and accurate analysis requirements. In the mid-to-late 1980s, internationally, the development of solid phase extraction (SPE), supercritical fluid extraction (SFE) and solid phase microextraction (SPME) technologies have not been comprehensive in China. Application, this is the bottleneck restricting the analysis speed and analysis efficiency of pesticide residues in China.