Study On Rapid Analysis Methods Of Aflatoxins And Pesticides Residues In Grain Based On Dynamic Microwave-assisted Extraction Technology

Grain is an indispensable food in people’s daily life.From the cultivation of a food crop to the entry into the market as a commodity,it needs to go through many links.Some pesticides are usually used to prevent and control diseases and insect pests in the process of grain planting,harmful substances can be introduced into the process of processing due to the bad environment and improper use of food additives,mildew is easy to occur in the process of storage and transportation.The use of chemicals in storage and the pollution in the production and marketing links can easily lead to the quality and safety problems of grain.At present,China’s food market security issues are becoming increasingly prominent.In recent years,problems such as excessive health indicators,pesticide residues,and mycotoxins in food have become frequent,causing great concern.Therefore,it is of great significance to analyze the residues of harmful substances in grain.The first and most important step in the residues analysis of harmful substances in grain is sample pretreatment.At present,physical and chemical methods are mainly used to analyze the harmful substances in grain,the grain samples should be pretreated properly to meet the requirements of the analytical method before analysis.However,there are many problems in the traditional sample pretreatment methods.They often have complicated operation steps,long operation time,and a large amount of toxic organic reagents used in the experiment process,which brings a certain burden to the environment.Therefore,it is extremely important to explore some novel,fast,reliable,effective and environmentally friendly analysis methods of harmful substances in grain.Microwave-assisted extraction is a rapid and efficient extraction technology,which has the advantages of low consumption of organic solvents,short extraction time and high extraction efficiency.It is widely used in food,medicine and environmental analysis.Compared with static microwave-assisted extraction,dynamic microwave-assisted extraction has more obvious advantages.Under microwave irradiation,fresh and continuous extraction solvent passes through the sample,which increases the extraction speed of target compounds from the sample,and can improve the degradation of some thermally unstable targets caused by long-time microwave irradiation.At the same time,the dynamic microwave-assisted extraction technology can realize the on-line combination with other new sample pretreatment technologies,therefore,the time of sample pretreatment is greatly reduced and the analytical efficiency of target compounds is effectively improved.In this paper,a dynamic microwave-assisted extraction device was self-assembled.This thesis focuses on grain as the research object.In view of the complexity,slowness,and incompatibility of the traditional sample preparation methods for harmful substances in grain,microwave-assisted extraction online combined with studies such as QuEChERS,solidification of floating organic drop liquid phase microextraction,matrix dispersive solid phase extraction were conducted to simplify the sample pretreatment,reduce the use of organic solvents,and establish a series of rapid,efficient,sensitive,reliable and new analytical methods for harmful substances in grain.The research contents and results are as follows:1.A reliable,safe and rapid method for the determination of aflatoxins in grain was established.Four common aflatoxins（B₁,B₂,G₁,G₂）in grain were analyzed by high performance liquid chromatography combined with dynamic microwave-assisted extraction and improved QuEChERS method.In the process of this method,the microwave power,extraction solvent,extraction time,salt quality,purification agent quality and other experimental conditions were optimized,and the best experimental conditions were evaluated by t-test method.Under the optimal experimental conditions,the lowest detection limits of aflatoxin B₁,B₂,G₁,and G₂were between 0.010 and 0.012μg/kg,and the lowest limit of quantifications were between 0.025 and 0.039μg/kg.The proposed method was successfully applied to the determination of four aflatoxins（B₁,B₂,G₁,G₂）in grain.The recoveries of spiked samples ranged from 80.6%to 96.0%,and the relative standard deviations were between 2.9%and 9.4%.This method can effectively save the experimental cost of aflatoxins analysis in grain,reduce the use of organic solvent,and can process 12 samples at the same time,improve the sample processing efficiency,and establish a new technology for aflatoxin analysis in grain samples,which is of great significance for ensuring the quality and safety of food,and provides new ideas and reference basis for aflatoxins analysis.2.A rapid and low-cost analytical method for the extraction of dimethoate,diazinon,methyl parathion,malathion,chlorpyrifos and ethiophos in grain samples was proposed for the first time.The method used dynamic microwave-assisted extraction and improved QuEChERS method to detect organophosphorus pesticides in grain samples.The extraction and purification process can be completed in one step,and the pretreatment time of grain samples is effectively shortened.During the experiment,a series of factors affecting the extraction efficiency of organophosphorus pesticides were investigated,including microwave power,extraction solvent,extraction flow rate,purification agent quality and salt quality.The whole process can be completed in 4 minutes,greatly reducing the sample pretreatment time.The detection limit was0.43-1.31μg/kg,the relative standard deviation was in the range of 2.0-9.6%,and the recoveries of target analytes were 77.9-99.0%.The experimental results showed that the sample pretreatment process was simple and the amount of organic solvent was small.At the same time,the whole extraction process was completed in a closed environment,which effectively reduced the health threat of organic solvents to operators,so it was very suitable for the daily analysis of organophosphorus pesticides in grain.3.A dynamic microwave-assisted extraction combined with a solidification of floating organic drop liquid phase microextraction method was developed for the first time to analyze organochlorine pesticides（HCHs,DDT and their isomers）in grain samples by gas chromatography.Organochlorine pesticides were extracted with acetonitrile aqueous solution containing trace n-hexadecane under microwave irradiation.After extraction,the organochlorine pesticides could be directly analyzed by gas chromatography without further filtration and purification.The microwave power,extraction solvent,solvent flow rate,volume of n-hexadecane and the quality of salt were optimized in detail.The detection limits of organochlorine pesticides（HCH and DDT）were 0.097-0.101μg/kg,and the intra-day and inter-day precision were in the range of 2.6-8.5%.In order to investigate the applicability of the method,the method was successfully applied to the analysis of organochlorine pesticides in six kinds of grain samples.The recoveries of target compounds with different spiked concentrations ranged from 72.2%to 94.3%.The experimental results showed that the method can reduce the use of toxic organic solvents,simplify the sample pretreatment process,and has the advantages of low cost and high enrichment ratio.It is an environmentally friendly sample analysis method,and has great reference value for rapid analysis of organochlorine pesticides in complex solid samples.4.Dynamic microwave-assisted extraction combined with matrix solid phase dispersive extraction was frist developed for the determination of cypermethrin and deltamethrin in grain samples by gas chromatography.The effects of dynamic microwave-assisted extraction and matrix solid-phase dispersion extraction on the extraction of cypermethrin and deltamethrin were studied.Three closely related experimental parameters（microwave power,extraction solvent volume and extraction solvent flow rate）were investigated by Box-Behnken software.Under the optimal experimental parameters,grain samples obtained good purification effect.The detection limits of cypermethrin and deltamethrin were 0.64 ng/g,the relative standard deviations were 3.1-8.4%and 2.9-7.8%,respectively.The recoveries were in the range of90.3-99.2%and 87.9-102.2%,respectively.The extraction and purification process was completed in one step,which effectively simplified the experimental operation process.It can be used for the analysis of cypermethrin and deltamethrin in different types of grain samples.5.Based on the classical chemical kinetic equation,the kinetic characteristics of microwave-assisted extraction were studied.The relationship among extraction temperature,extraction time and target analytes recoveries was analyzed.Combining with Arrnihus formula,the reaction order,rate constant and activation energy which can represent the reaction rate of target analytes were obtained by analyzing the specific experimental process.