Applications Of Novel Sample Pre-treatment Technologies Coupled With High Performance Liquid Chromatography For The Determination Of Some Pesticide Residues

Sample preparation is one of the most important and crucial steps in the whole analytical process of pesticide residues. The primary objective of the sample preparation is to reduce or eliminate the interferences originally present in the samples and concentrate the analytes of interest so that they can be determined at low levels. Traditional extraction techniques, such as liquid-liquid extraction （LLE）, solid-phase extraction （SPE）, are laborious, time-consuming and need large volumes of samples and toxic organic solvents. In order to overcome these shortcomings, much attention is being paid to the development of miniaturized, more efficient and environment-friendly extraction techniques that could greatly reduce the solvent consuming. For this purpose, solid-phase microextraction （SPME） and liquid-phase microextraction （LPME） techniques have emerged for sample preparations. Due to their high adsorption capacity, some nano materials including carbon nanotubes and graphene have been applied to sample preparation and good results are achieved.In this thesis work, several analytical methods were developed for the determination of some pesticides in environmental water and agricultural samples by carbon nanotube reinforced hollow fiber liquid-phase microextraction （HF-LPME）, graphene-coated fiber solid-phase microextraction and graphene magnetic solid-phase extraction （MSPE） coupled with high performance liquid chromatography （HPLC）. The thesis is mainly concerned with the following aspects:1. A new method was developed for the determination of five carbamate pesticides （metolcarb, carbofuran, carbaryl, isoprocarb, diethofencarb） in water and fruit samples by using carbon nanotubes reinforced hollow fiber-liquid phase microextraction （HF-LPME） prior to high performance liquid chromatography-diode array detection （HPLC-DAD）. Parameters that could affect the extraction efficiencies, such as the type of the organic extraction solvent and its volume, the type and concentration of multi-walled carbon nanotube （MWCNTs）, fiber length, extraction time, the pH of the sample solution, stirring rate and ionic strength, were investigated and optimized. Under the optimal extraction conditions, the linearity was observed in the range of 0.5-300 ng/mLfor water samples and 1-300 ng/g for fruit samples, with the correlation coefficients （r） of 0.9992-0.9999 and 0.9996-0.9998, respectively. The enrichment factors （EFs） obtained were between 75 and 119, and the limits of detection （LODs） were 0.1-1.0 ng/mL for water samples and 0.2-2.0 ng/g for fruit samples. The recoveries for the spiked samples were in the range of 77.5%-104.4%, and the precision values, expressed as the relative standard deviations （RSDs）, were in the range between 2.9% and 9.2%. The results demonstrated that the newly developed method was an efficient pretreatment and enrichment procedure for the determination of carbamate pesticides in water and fruit samples.2. A novel graphene nanocomposite was synthesized and used as a solid-phase microextraction （SPME） fiber coating material. The structure of the graphene-coated fiber was characterized by scanning electron microscopy and X-ray diffraction. The application of the graphene-coated fiber was evaluated through the extraction and determination of the four carbamate pesticides （carbofuran, carbaryl, pirimicarb, diethofencarb） in water samples by high performance liquid chromatography-diode array detection （HPLC-DAD）. Parameters that affect the extraction and adsorption efficiencies, such as the extraction time, stirring rate, extraction temperature, and salt addition, were investigated and optimized. Compared with the two commonly used commercial fibers （CW/TPR, 50μm; PDMS/DVB, 60μm）, the graphene-coated fiber showed higher extraction efficiency. Under the optimum conditions, the LODs, based on a signal-to-noise ratio （S/N） of 3, were 0.1-0.8 ng/mL. The limits of quantification （LOQ, S/N = 9） were 0.3-2.4 ng/mL. The linearity was observed in the range from their corresponding LOQs to 400.0 ng/mL with the correlation coefficients （r） ranging from 0.9994 to 0.9998. The recoveries of the carbamate pesticides from water samples at spiking levels of 10.0, 50.0 and 100 ng mL-1 were in the range of 83.8% - 95.4%, and the relative standard deviations （RSDs） were between 2.2% and 6.6%.3. A graphene-based Fe₃O₄ magnetic nanoparticles （G-Fe₃O₄ MNPs） was synthesized and used as the adsorbent for the extraction and determination of triazine herbicides in environmental water samples followed by high performance liquid chromatography-diode array detection （HPLC-DAD）. After extraction, the adsorbent can be conveniently separated from the aqueous samples by an external magnet. The main factors influencing the extraction efficiency including the amount of MNPs, the extraction time, the pH of sample solution and the desorption conditions were optimized. Under the optimized experimental conditions, good linearity was observed in the range of 0.1-50.0 ng/mL for all the analytes, with the correlation coefficients （r） ranging from 0.9996 to 0.9999. The limits of detection of the method ranged between 0.025 and 0.040 ng/mL. Good reproducibility was obtained with the relative standard deviations below 5.2%. The developed method was applied to the analysis of some triazine herbicides in different water samples （lake, river and reservoir）. The recoveries of the method were in the range between 89.0% and 96.2%. 4. A new method for the determination of five sulfonylurea herbicides （metsulfuron methyl, chlorsulfuron, tribenuron methyl, pyrazosulfuron ethyl and chlorimuron-ethyl） in water samples was developed by magnetic graphene solid-phase extraction coupled with high performance liquid chromatography. Parameters that affect the extraction efficiency, such as the pH of sample solution, extraction time, the amount of the adsorbent and the desorption conditions were investigated and optimized. Under the optimum conditions, the linearity of the method was obtained in the range of 0.05⁵0 ng/mL with the correlation coefficients （r） ranging from 0.9989 to 0.9995. The enrichment factors were in the range between 2712³120, the method detection limits were 0.005-0.04 ng/mL （S/N = 3）. The proposed method has been successfully applied to the analysis of target sulfonylurea herbicides in rreservoir, sea and lake water samples with satisfactory results. The spiking recoveries ranged from 81.6% to 97.0% with the relative standard deviations fall between 2.10%⁶.3%.