| The traditional sample pretreatment methods have not been able to meet the requirement of modern food and environmental analysis, because of they are time-consuming, tedious and often require a large volume of hazardous and expensive solvent. Therefore, it is imperative to develop efficient, simple, sensitive, fast and environmentally friendly sample pretreatment methods for the determination of contaminants in different food and environmental samples.For complex solid samples, microwave-assisted extraction（MAE） has great advantages, such as rapidity, simplicity, high extraction yield, low cost of operation and low solvent consumption, which make the method has been widely applied to food, pharmaceutical and environmental analysis. Compared with static microwave-assisted extraction, dynamic microwave-assisted extraction（DMAE） has more advantages. In DMAE, fresh extraction solvent was continuously supplied and the analytes were transferred out of the extraction vessel as soon as they were extracted. This is especially important to avoid degradation and contamination of analytes. Moreover, the extract could be online filtered and DMAE could be coupled with other sample pretreatment techniques, which could greatly simplify the operation, reduce the whole pretreatment time and then improve analytical efficiency.In this thesis, a system of DMAE was assembled in our laboratory. The system mainly consists of a household microwave oven, a peristaltic pump, a glass extractor designed by ourselves and PTFE tubes used to deliver the extraction solvent. Different kinds of herbicides and insecticides were selected as analytes. Gas chromatography-mass spectrometry（GC-MS） and high performance liquid chromatography（HPLC） were applied to separation and determination of the analytes. According to the characteristics of DMAE, several methods based on DMAE coupled with related liquid-phase extraction were established for the determination of pesticides in complex food and environment samples.In Introduction, the principle, classification, characteristic, influencing factors and application researches of microwave-assisted extraction were reviewed. The development and application of dynamic microwave-assisted extraction and liquid-phase extraction were summarized in detail.In Chapter 2, a fast and green sample pretreatment method, medium-assisted non-polar solvent dynamic microwave extraction, was first applied to the extraction of ten organophosphorus pesticides（OPPs） from cereal samples. Without adding any polar solvent, graphite powders（GP） were used as microwave absorption medium to transform microwave energy into heat energy. For recycling GP, an extractor consisting of inner and exterior tube was made by sealing GP inside the exterior tube of a glass sleeve. In dynamic microwave extraction process, hexane was used as extraction solvent, ten OPPs could be extracted completely within 3.5 min, and the extract was directly analyzed by GC-MS without any clean-up process. The effects of some experimental conditions on extraction efficiency were examined and optimized, including ratio of GP to silica gel, type and volume of extraction solvent, microwave power and flow rate of extraction solvent, and the Student t test was applied to the evaluation of the selected experimental conditions. When mass ratio of GP to silica gel was 1:3, microwave power was 150 W and the flow rate of extraction solvent was 2.0 m L min-1, ten OPPs were extracted completely using 7 m L hexane. The recoveries of OPPs by analyzing the four different spiked cereal samples were from 73.2% to 99.8%, and the relative standard deviations（RSDs） were from 1.0% to 6.7%, the limit detections were as low as 0.31 μg kg-1.In Chapter 3, a new method for determination of pesticide residues in fatty samples was established. Microwave absorption medium（MAM） assisted dynamic microwave extraction coupled with reversed-phase ionic liquid dispersive liquid–liquid microextraction（RP-DLLME） was developed and applied to the determination of six triazine herbicides in corn and soybean samples. Triazine herbicides in samples were extracted with hexane by the help of MAM assisted dynamic microwave extraction and then directly concentrated by RP-DLLME using IL. The direct extraction of analytes from the high-fat extract is a good alternative with the advantages of simple operation, efficient removal of fat, the good compatibility with DMAE and good HPLC behavior. Some experimental parameters which could affect the extraction recoveries were studied and optimized, and the most relevant parameters in DMAE（microwave power, microwave extraction time and flow rate of extraction solvent） were studied with a Box–Behnken Design model. Under optimal experimental conditions, the limits of detection for cyanazine, metribuzine, desmetryn, atrazine, ametryn, and terbuthylazine were 1.22, 2.21, 3.15, 2.15, 4.16 and 3.40 μg kg-1. RSDs of intra- and inter-day were in the range of 2.7-5.3% and 2.6-6.9%. Two corn and two soybean samples were analyzed, the recoveries obtained were in the range of 80.7-106.9%, and the RSDs were from 2.1% to 7.8%. The experiment results indicated that the present method was effective for the extraction of pesticides in high-fat content cereal samples.In Chapter 4, based on the improved continuous flow of single drop microextraction techniques, a new method for the analysis of seven OPPs was developed using dynamic microwave-assisted extraction online coupled with single drop microextraction prior to gas chromatographic mass spectrometry（GC-MS）. In the developed method, analytes in tea samples were extracted with 25% ethanol aqueous solution and purified with acidic alumina at the same time, and then the analytes were concentrated into microextraction solvent. When the extraction was completed, the solvent microdrop containing the enriched analytes was retracted into the microsyringe and directly analyzed by GC-MS without any filtration or clean-up step. Several experimental conditions, including type of extraction solvent, type and amount of dispersant, type and volume of microextraction solvent, microwave irradiation power, extraction time and flow rate of extraction solvent were investigated and optimized by a univariate method and Box-Behnken Design. In tne method, the standard curves showed good linear relationship with the correlation coefficient ranging from 0.9973 to 0.9998. The limits of detection were in the range of 0.36-1.67 μg kg-1. The recoveries of OPPs obtained by analyzing the four tea samples were in the range of 84.9-106.4%, and RSDs were lower than 6.1%. Besides, compared with traditional methods, when the present method was applied, the whole consumption of organic solvent was reduced, the pretreatment time was shortened, and the operation was simplified. Therefore, it can be concluded that the present method is suitable for the analysis of OPPs in complex solid samples.In Chapter 5, a green and simple method, dynamic microwave assisted extraction coupled with cloud point extraction, was developed for the analysis of triazine herbicides in soil samples. The method combines the advantages of dynamic microwave-assisted extraction and cloud point preconcentration, which could greatly simplify the operation and reduce the whole pretreatment time. In the present method, 3.0% non-ionic surfactant Triton X-114 aqueous solution used as extraction medium was continuously pumped through the extraction vessel by the rate of 2.0 m L min-1, and analytes were transferred from the soil sample into the extraction solvent under 250 W microwave power. After that, Na Cl was added into the resulting extract, and the mixture was heated to 60℃ and held for 20 min. After centrifugation, the analytes were enriched into the surfactant-rich phase and directly analyzed by HPLC. None of hazardous organic solvents were used in extraction process, and no filtration or cleanup step was needed. Under optimal experimental conditions, the limits of detection for cyanazine, metribuzine, desmetryn, atrazine, terbumeton, and terbuthylazine were 0.72, 1.71, 0.66, 0.26, 0.84, and 0.72 μg kg-1. RSDs of intra- and inter-day were in the range of 1.8-4.5% and 2.1-6.5%. The present method was successfully used in the analysis of four agricultural soil samples, the recoveries were from 80.3% to 98.3 % and the RSDs were lower than 6.6 %.In Chapter 6, a novel extraction method, dynamic microwave-assisted extraction coupled with ionic liquid homogeneous liquid liquid microextraction, was developed for the determination of triazine herbicides, including desmetryn, terbumeton, propazine, terbuthylazine, dimethametryn, and dipropetryn in fresh vegetable samples by high performance liquid chromatography. In the developed method, a small amount of a hydrophilic IL was dissolved completely in the water, and the IL aqueous solution was used as extraction solvent. By dynamic microwave-assisted extraction, the fresh extraction solvent was continuously pumped through the extraction vessel. The target analytes could be extracted completely within few minutes and collected easily by adding [NH4][PF6] to form a water insoluble ionic liquid via a simple metathesis reaction. The effects of some experimental conditions, including type and volume of ionic liquid, salt concentration, amount of ion-pairing agent [NH4][PF6], microwave power and flow rate of extraction solvent on the extraction efficiency were studied and evaluated in detail. The linearities of the six triazines were ranging from 2.50 μg kg-1 to 250.00 μg kg-1, and the correlation coefficients were greater than 0.9989. The limits of detection were in the range of 0.45-1.81 μg kg-1. When four vegetable samples were analyzed by the present method, satisfactory recoveries were obtained ranging from 76.8% to 106.9 % with RSDs in the range of 2.7-9.8 %.In Chapter 7, dynamic microwave-assisted extraction coupled with ionic liquid-based aqueous two-phase extraction was developed for determination of triazines in fresh vegetables. The analytes in samples were successively extracted with hydrophilic IL aqueous solution under the action of microwave energy. Subsequently, ammonium sulfate was added into the obtained extract to form aqueous two-phase system. The IL-rich phase containing the analytes was analyzed by high performance liquid chromatography. The process has such advantages as no emulsification, short time in phase separation, low labor intensity and low toxicity. Several experimental conditions, including type and amount of ionic liquid, type and concentration of salt, p H of the extraction solvent, microwave power and the flow rate of extraction solvent were studied. Good linearity was obtained with the correlation coefficients greater than 0.9983. The limits of detection and quantitation of the analytes were in the range of 0.90-2.88 μg kg-1 and 3.00-9.59 μg kg-1, respectively. The present method was successfully used in the analysis of triazines in vegetable samples, and the recoveries of analytes were from 70.8% to 108.5% with RSDs in the range of 1.8-6.7%. |
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