Study On The Application And Adsorption Mechanism Of Covalent Organic Framework Materials For The Determination Of Pesticide Multi-residues In Tea

Pesticide residues in tea have a negative impact on human health and its international trade.Now consumers are attaching much importance to the quality of tea;meanwhile,with the tea pesticide testing items increasing and the requirements for pesticide limits becoming stricter in domestic and foreign determination standards,it is necessary to establish a detecting technique for multi-pesticide residues,especially in developing efficient and environmentally-friendly sample pretreatment methods.In recent years,the sample pretreatment technique based on solid phase adsorption has developed rapidly.Traditional solid phase extraction materials have the problems of low adsorption performance and low reuse rate to varying degrees,but covalent organic framework(COF)and its composite materials exhibit the characteristics of good stability,high specific surface area and porosity,easy preparation and repeated use.In this paper,COF and its composite materials with excellent adsorption performance were synthesized,and then used in the sample pretreatment of solid phase extraction(SPE),magnetic solid phase extraction(MSPE),solid phase microextraction(SPME).Combined with chromatography-mass spectrometry,four methods for the determination of pesticide residues in tea and tea beverages were established.The synthesis methods of COF materials were also optimized.The sensitivity,accuracy and applicability of the established methods as well as the reusable performance of the materials were evaluated.The mechanism of adsorption was studied in detail.The main results are as follows:1.The synthesis of spherical TPB-DMTP-COF for the determination of benzoylurea insecticides in tea by SPE-LC-MS/MSIn this study,uniform spherical TPB-DMTP-COF was prepared by room temperature synthesis method.The type of solvent,the amount of catalyst and the reaction time were optimized.The experimental results showed that TPB-DMTP-COF spherical morphology was more easily formed in acetonitrile system than in mesitylene and 1,4-dioxane system.The amount of catalyst determined the uniformity and particle size of spherical morphology.The reaction time affected the sphere diameter and crystal type of TPB-DMTP-COF.The characterization results showed that TPB-DMTP-COF had a high specific surface area(1419m² g^-1)and good crystal structure.The spherical TPB-DMTP-COF was used as the adsorption packing of SPE column.The loading and elution conditions were optimized,and the SPE-LC-MS/MS method was established for the determination of benzoylurea insecticides(BUs).Six BUs showed good linearity in the concentration range of 0.1-100 ng g^-1,with the correlation coefficient ranging from 0.9951 to 0.9976.The minimum detection limits of the method were 0.003-0.018 ng g^-1.The contents of six BUs were determined in different kinds of tea sold on the market.The recoveries of green tea and black tea samples were tested.The recoveries ranged from 80.7%to 102.2%,with the relative standard deviation of less than6.3%.This study provided an idea for the synthesis and application of uniform spherical COF materials.2.The synthesis of magnetic NH₂-Fe₃O₄@COF for analyzing trace BUs in tea beverages by MSPE-LC-MS/MSIn this study,a novel magnetic covalent organic framework(NH₂-Fe₃O₄@COF)was prepared by room temperature synthesis method.As MSPE adsorbents,these magnetic particles exhibited high adsorption performance with a short adsorption time(10 min)for six BUs.Quantum chemistry calculation demonstrated that adsorption mechanism was primarily attributed to electrostatic interaction between methoxyl groups in COF and amide groups in benzoylurea pesticide molecules as well as the potential hydrophobic effect.In this study,a MSPE-LC-MS/MS method was established for the determination of BUs.Six BUs showed a good linear relationship in the concentration range of 10-1000 ng L^-1,and the correlation coefficient ranged from 0.9977 to 0.9999.The minimum detection limits of the method were0.06-1.65 ng L^-1.Accuracy and applicability of the proposed method was further evaluated by analyzing four kinds of original tea beverages.The recoveries of six BUs in spiked samples ranged from 80.1%to 108.4%,with the relative standard deviation of less than 7.4%.This study provided a reference for establishing a determination method for trace pesticide residues in beverages.3.The synthesis of keto-enamine Tp-Azo-COF for analyzing trace organochlorine pesticides in tea by SPME-GC-MS/MSIn this study,a keto-enamine covalent organic framework called Tp-Azo-COF with rich electronegative N atoms was prepared as SPME coating.The Tp-Azo-COF coating had a large surface area(1218 m² g^-1)and good thermal and chemical stability,and it can be applied for the extraction,enrichment and thermal desorption of organochlorine pesticides(OCPs).According to quantum chemistry calculations,the adsorption affinity of Tp-Azo-COF coating for five OCPs was primarily affected by halogen bond and hydrophobicity interaction.The extraction efficiencies of Tp-Azo-COF coating for five OCPs were higher than those of three commercial SPME fiber coatings,and the enrichment factors ranged from 1061 to 3693.The needle fiber could be recycled for hundreds of times,which exhibited its good economic practicability.In this study,an ultra-sensitive method was established for the determination of OCPs by SPME-GC-MS/MS.Five OCPs showed good linearity in the concentration range of0.1-1000 ng L^-1,and the correlation coefficient ranged from 0.9921 to 0.9963.The minimum detection limits of the method were 0.002-0.08 ng L^-1.Moreover,the applicability of the developed method was evaluated by analyzing green tea samples,and the recoveries were in the range of 85.1%-101.6%,with the relative standard deviation of less than 8.6%.This research extended the application of stabilized keto-enamine COF as a sample enrichment probe for OCP analysis.4.The synthesis of magnetic TAPT-DHTA-COF for the determination of amide herbicides in tea by MSPE-GC-MS/MSIn this study,a hydrophilic magnetic sphere was prepared which dispersed stably in water phase.DHTA aldehyde monomer with rich hydroxyl groups and TAPT amino monomer were selected for the COF synthesis.The combination method of TAPT-DHTA-COF and hydrophilic magnetic sphere was optimized.Magnetic TAPT-DHTA-COF materials with hydrophilic properties were synthesized by room temperature synthesis method.The characterization results showed that magnetic TAPT-DHTA-COF had good magnetic response,and can achieve rapid magnetic separation after uniformly dispersing in water phase.With magnetic TAPT-DHTA-COF as the adsorbent of MSPE,the influence parameters of MSPE were optimized,and a method for the determination of amide herbicides in tea was established by MSPE-GC-MS/MS.In green tea substrate,six amide herbicides showed a good linear relationship in the concentration range of 1-500 ng g^-1,with the correlation coefficient between 0.9910 and 0.9982.The minimum detection limits of the method were 0.25-0.73ng·g^-1.The recoveries of green tea and black tea were measured and were found in the range of 80.1%-94.8%,with the relative standard deviation of less than 6.2%.The study on the adsorption mechanism between TAPT-DHTA-COF and amide herbicides showed that intermolecular hydrogen bond interaction was the main force in the adsorption process.This method provided an idea for improving the adsorption of polar compounds on COF materials.