| Sample preparation is one of the most important and crucial steps in the whole analyticalprocess for the determination of some contaminants in food samples. It is not only toisolate the target compounds from the samples, thus reducing or eliminating theinterferences originally present in the sample, but also simultaneously to concentrate theanalytes to facilitate their determinations at low levels. Since conventional extractiontechniques, such as solid phase extraction, soxhlet extraction and liquid liquid extraction,are laborious, time-consuming, and need large volumes of samples and toxic organicsolvents, they could not meet the need of some analytical methods. Therefore, it isnecessary to develope some new sample preparation techniques, which are time-saving,effective, and environment-friendly. At present, solid phase microextraction and magneticsolid phase extraction methods have undergone a rapid development and have shown anextremely extensive application prospect in food, environmental, material, biological andpharmaceutical aspects.Graphene (G), with high theoretical surface area, rich stacking π-electron system, bothchemical and thermal stability, and ease of modification, has a potential to become anexcellent adsorbent for analytical purpose.In this dissertation, several analytical methods were developed for the sensitivedetermination of some contaminants in food samples by both solid-phase microextractionand magnetic solid phase extraction techniques with grapheme-related composite asadsorbent, coupled with gas chromatography (GC). This dissertation is mainly concernedwith the following aspects:(1) A graphene-based Fe3O4magnetic nanoparticles (G-Fe3O4MNPs) was synthesizedby chemical coprecipitation method. The G-Fe3O4MNPs was used as the adsorbent for theextraction and determination of eight pyrethroid pesticides in water samples followed bygas chromatography-electron capture detection (GC-ECD). The main experimentalparameters such as the amount of G-Fe3O4MNPs, extraction time and desorptionconditions were optimized. Under the optimum conditions, the limits of detection of themethod for the eight pyrethroid pesticides ranged from0.04to0.40μg/L. The method wassuccessfully applied to the determination of the pesticides in water samples. The recoveries of the method for the analytes were in the range of80.7~105.3%, and the relative standarddeviations were in the range of3.8~6.0%.(2) A sensitive analytical method for the four fungicides (procymidone, folpet,vinclozolin and ditalimfos) in water and juice samples was developed by using magneticsolid-phase extraction (MSPE) with magnetic graphene nanocomposite (G-Fe3O4) as theadsorbent followed by the determination with GC-ECD. Some parameters which affect theextraction efficiency such as the amount of G-Fe3O4MNPs, extraction time, ionic strengthand pH of the sample solution, and desorption conditions were optimized. Under theoptimum conditions, the enrichment factors of the method for the analytes were in therange of1495~1849. The limits of detection for the fungicides ranged from1.0to7.0ng/L.The recoveries of the method for the analytes were in the range from79.2%to102.4%.The developed G-Fe3O4-MSPE method was simple and efficient for the extraction anddetermination of the four fungicides in water and grape juice samples.(3) A sensitive method for the determination of the chloroacetanilide herbicides alachlor,acetochlor, pretilachlor, butachlor and metolachlor in water samples was developed. It isbased on solid-phase extraction using magnetic graphene nanocomposite (G-Fe3O4MNPs)as the adsorbent, followed by GC-ECD. The main experimental parameters such as theamount of G-Fe3O4MNPs, extraction time, ionic strength, the pH of the sample solutionand desorption conditions were optimized. Under the optimum conditions, the enrichmentfactors of the method for the analytes were in the range from649to1078. A good linearresponse was achieved in the range of0.20~20.00μg/L, with correlation coefficients (r)varying from0.9964to0.9998. The limits of detection of the method ranged from0.02to0.05μg/L and the relative standard deviations were below4.5%. The method wassuccessfully applied to the determination of the herbicides in water samples. Therecoveries of the method for the analytes in reservoir, tap and rain water samples were inthe range of80.7%~105.3%.In order to further improve the sensitivity, a method for the determination of thechloroacetanilide herbicides, alachlor, acetochlor, pretilachlor, butachlor and metolachlorin water samples was developed by coupling the MSPE with dispersive liquid-liquidmicroextraction followed by GC-FID. The limits of detection for the herbicides rangedfrom0.01to0.03μg/L. The enrichment factors of the method were in the range of3399~4002.(4) Graphene oxide and hydroxyl-terminated polydimethylsiloxane was mixed andcoated on the surface of etched stainless steel wire by sol-gel technology, which wasinstalled in the modified microsyringe and used for the solid phase microextraction fiber.The GO-PDMS coating fiber was used for the direct immersion extraction of polycyclicaromatic hydrocarbons (PAHs) from water samples. The surface structure and thermalstability of the coating was characterized. Several important experimental parameters that could influence the SPME efficiency such as desorption time, desorption temperature,extraction time, extraction temperature, ionic strength and stirring rate, were investigatedand optimized. Under the optimized conditions, the limits of detection were in the rangefrom0.2to0.8μg/L. The linear range was0.001~0.250μg/mL for naphthalene andfluoranthene and were0.005~0.250μg/mL for others. The linear correlation coefficientswere in the range of0.992~0.998. The results indicated that the homemade fiber had theadvantages of high extraction efficiency, good thermal stability and reproducibility. Thecoating was successfully applied to the analysis of PAHs in lake, tap and mineral watersamples.(5) The etched stainless steel wire was first coated with sol-gel poly (3-aminopropyl)triethoxysilane, and then with the sol-gel mixture of graphene oxide andhydroxyl-terminated polydimethylsiloxane, which was installed in the modifiedmicrosyringe for solid phase microextraction. Then, the fiber was used for the directimmersion extraction of four triazole fungicides from tap, lake and rain water samples. Thecoating was characterized by scanning electron micrographs. The results showed that thecoating had a highly porous structure. Several important experimental parameters thatcould influence the SPME efficiency such as desorption mode, extraction time, extractiontemperature, sample pH and stirring rate, were investigated and optimized. Under theoptimized conditions, the limits of detection were in the range from0.01to0.03μg/L. Thelinear range was0.300~100μg/L. The linear correlation coefficients were in the range of0.9920~0.9970. The relative standard deviations for fiber-to-fiber variations were between8.5and10.7%. The lab-made fiber was successfully applied to the analysis of the fourtriazole fungicides in water samples with satisfactory results.(6) Poly (ethylene glycol)-grafted graphene (PEG-g-G) was prepared by sol-geltechnology and coated on the surface of etched stainless steel wire, which was installed inthe modified microsyringe. Then, the fiber was used for the extraction of seven volatilearomatic compounds (VACs) from tap, river and mineral water samples followed by thedetermination with gas chromatography-flame ionization detection (GC-FID). ThePEG-g-G coating was characterized by both the thermo-gravimetric analysis and scanningelectron micrographs. The results showed that the coating had a highly porous structureand good thermal stability. Several important experimental parameters that could influencethe SPME efficiency were investigated and optimized. Under the optimized conditions, thelimits of detection were in the range from1.0to6.0ng/L. The relative standard deviationsfor intraday and interday variations were in the range of1.8%~5.8%and6.5%~8.3%, andfor fiber-to-fiber variations, were between6.5%and11.9%. It can be reused more than200times without a significant loss of extraction efficiency. The results indicated that thePEG-g-G fiber had the advantages of high extraction efficiency and good thermal stabilityand durability. Then, the extraction performance of both the lab-made PEG-g-G and GO-PDMScoatings was compared by using them for the headspace solid phase microextraction offive VACs (chlorobenzene, bromobenzene, o-dichlorobenzene, m-dichlorobenzene and1,2,4-trichlorobenzene) from water samples followed by the determination with GC-FID.The results showed that The limits of detection by PEG-g-G coating (1.0~6.0ng/L) werelower than that by GO-PDMS coating (0.5~1.0μg/L), and the linear range by PEG-g-Gcoating (0.01~10.00μg/L) was wider than that by GO-PDMS coating (2.5~500.0μg/L). |
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