Determination Of Pesticide Residues In Food Samples By The Use Of New Extraction Adsorbents And Gas Chromatography

The food safety problems caused by pesticide residues are one of the most concerned issues.Because of the various types and low concentrations of pesticide residues in food samples and the complexity of food sample matrix,sample pretreatment is necessary to enrich and separate the analytes from samples and to eliminate or reduce the negative effects of sample matrix,so as to realize the suitable instrumental detection at trace level of the analytes.Magnetic solid phase extraction(MSPE)and solid phase microextraction(SPME)have attracted more and more attention due to their simple,efficient and environment-friendly features.The key of the two techniques lies in the selection of adsorbents.And therefore the development of new adsorbents with strong adsorption capability,good stability and selectivity is one of the hot topics in the fields of SPME and MSPE.In this thesis work,three new materials were prepared and investigated either as the adsorbent for the MSPE or as the fiber coating material for the SPME of some analytes.With the help of gas chromatography(GC)or gas chromatography-mass spectrometer(GC-MS)detection,the analytical methods for the determination of various pesticides residues in some food samples were developed.The main studies involved are as follows:(1)A fibrous magnetic boron nitride nanocomposite was synthesized and used as an adsorbent for the MSPE of some pesticides(organochlorine and pyrethriod pesticides)prior to their gas chromatography-electron capture detection(GC-ECD).The optimum experimental conditions were obtained by the experiments with the help of both single factor optimization and response surface analysis(Box-Behnken design,BBD).Under the optimized conditions,the response to the ten pesticides(dicofol,?-endosulfan,p,p'-DDE,nitrofen,?-endosulfan,p,p'-DDD,p,p'-DDT,bifenthrin,permethrin and fenvalerate)is linear in the 0.03?40 ng mL-1 range with the coefficients of determination(r2)higher than 0.9970.The limits of detection(LODs,S/N=3)are between 0.01 and 0.05 ng mL-1.The relative standard deviations(RSDs)at the concentration levels of 0.5 ng mL-1,20 ng mL-1 and 40 ng mL-1 were below 8.7%.The recoveries of the analytes from spiked tea water and tea beverage samples varied between 84.5%and 122%,with RSDs ranging from 4.8 to 12%.The adsorbent can be reused over 50 times without a significant loss of the extraction efficiency.(2)A boron nitride(BN)modified multi-walled carbon nanotubes(MWCNTs)material(BN@MWCNTs)was synthesized by one-step method.Compared with other synthesis methods,the current method is simple,efficient and environment-friendly,and the BN@MWCNTs exhibit a high thermal stability and good adsorption capacity.The synthesis conditions that affect the properties of BN@MWCNTs(the carbonization temperature and the mass ratio of BN to MWCNTs)were investigated to obtain the optimal synthesis conditions.The SPME based on the use of the BN@MWCNTs fiber prepared by sol-gel technology was coupled with GC-ECD to determine eleven organochlorine pesticides(OCPs)in different fruit and vegetable samples.Under the optimal experimental conditions,the linear response for the determination of the analytes in apple,peach,Chinese cabbage and cucumber samples was in the range from 0.03 to 200 ng g-1 with the r2 higher than 0.9977.The LODs and the limits of quantitative(LOQs)were 0.01?0.20 ng g-1 and 0.03?0.60 ng g-1 based on the signal-to-noise ratios of 3 and 9,respectively.The method recoveries of the analytes for spiked samples under three concentration levels(1.0,10.0,100.0 ng g-1)were between 83.7%and 124%with the RSDs ranging from 2.1%to 10.9%.The established method was successfully applied for the analysis of fruits and vegetables samples.(3)A nitrogen-doped metal organic framework(MOF)based porous carbon(C-(C3N4@MOF))was fabricated by the carbonization of a graphitic carbon nitride(g-C3N4)templated MOF(NH2-MIL-125).The C-(C3N4@MOF)was then coated on stainless steel wires by sol-gel technique to serve as a SPME fiber coating.The coated fiber was studied for the extraction of fourteen organophosphorous pesticides(OPPs)from different fruit and vegetable samples followed by GC-MS detection.The C-(C3N4@MOF)coated fiber exhibited a high extraction capability for the OPPs.Both single factor optimization and Box-Behnken Design methods were implemented to help optimize the experiment conditions for the extraction.The results indicated that the linear response for the fourteen OPPs was in the range from 0.69 to 3000 ng g-1 and the r2 ranged from 0.9981 to 0.9998.The LODs(S/N=3)were between 0.23 and 7.5 ng g-1.The method recoveries of the fourteen OPPs for spiked fruit and vegetable samples were between 82.6%and 118%,with the RSDs varying from 2.8%to 11.7%.The fiber can be reused over 100 times without a significant loss of extraction efficiency.