Rapid Detection Technologies And Sample Pretreatment Means In Determination Of Pesticide Residues

In agriculture production, various pesticides have been used to prevent plants from diseases and insect pests. At the same time, the potential hazard of pesticide toxicity to environment and human health has become more and more obvious. Monitoring the pesticide residues in farm products has brought the public concern. Many countries have established the maximum residue limits of pesticide residues in food. The use of various kinds of high efficiency and low toxicity pesticides with different chemical structures and characters has increased the difficulties in pesticide residue determination. There are two main difficultiess in pesticide residue determination, viz., rapid detection and sample pretreatment.The purpose of this thesis was to develop rapid detection technologies and the pretreatment means. A test paper using enzyme inhibition and a rapid analysis method based on electrokinetic sequential injection analysis - enzyme inhibition was introduced. A funnelform single-drop microextraction and cloud point extraction-organic solvent back extraction were developed for gas chromatography. The details of the contents and results of the thesis are as follows:1. A rapid test paper utilizing enzyme inactivation has been devised for the detection of organophosphorus and carbamate pesticides. The enzyme extracted from flour was immobilized on a pre-activated nylon 66 membrane using glutaraldehyde as crosslinking agent. The chemical structure was shown as "nylon - glutaraldehyde -spacer - glutaraldehyde - enzyme". The parameters affecting enzyme reaction and immobilization were studied. The pH value of the buffer solution for the esterase reaction was adjusted to pH 7.6. The maximal enzyme activity was obtained with the dilution factor of 60. The substrate solution was chosen as 1.0 mmol/L 1-naphthyl acetate in 70% (v/v) acetone solution and the concentration of fast blue B salt was chosen as 0.6 mmol/L. Under the crosslinking conditions of 8.75 % (v/v) glutaraldehyde, 30 min, 0℃, the optimal result could be obtained.2. A fast and convenient analysis method based on the flour esterase reaction inhibited by organophosphorus and carbamate pesticides was developed for the determination of the pesticide residues in vegetable and pond water samples. The proposed method was performed with an electrokinetic sequential injection analysis (ESIA) system, which consisted of a spectrophotometer, a homemade electroosmosis pump and four solenoid valves controlled by a Visual C program. The analytical throughput of the proposed method was about 24 samples per hour. The esterase extracted from fresh flour was adopted in the enzyme reaction. Several reaction parameters, such as sample volume, length of reaction coil, and halt time were discussed in detail. The linear range of logarithmic concentration was 0.05-0.5μg/mL dimethoate, which was used as a converted concentration of total organophosphorus pesticides. The detection limit of 0.025mg/kg dimethoate was achieved and much lower than the maximum residue limits (MRLs) of vegetable samples established by the Ministry of Agriculture (China), in which most of the MRLs for vegetables were 0.1 mg/kg. By increasing the treating temperature, the hydrolysis degrees between the organophosphorus and carbamate pesticides are different, thus two kinds of pesticides can be differentiated by this way. The inhibition rates of four kinds of organophosphorus were compared with their molecular structure. Three different washing methods were also investigated for Chinese cabbage samples in order to evaluate washing methods in our daily life.3. A funnelform single-drop microextraction was developed for gas chromatography - electron-capture detection. Tea and juice samples were analyzed by the proposed method. Tea soup was used as the analytical samples during the determination of the pesticide residues in tea samples. A solvent microdrop of 4μL was formed at the tip of a microsyringe needle assembled with a small brass funnel in the microextraction with a stirring speed of 900 rpm. In the funnel, the restricted microdrop was shaken gently by a circular motion in an aqueous sample solution. Eleven organochlorine and two pyrethroid pesticides were used as the model compounds for evaluating the microextraction. The parameters affecting the enrichment factor of the microextraction were investigated. Under the selected conditions, including the funnel inner angle of 90°, the mixed solvent of 2:1 (v/v) n-hexane and ethyl acetate as extracting solvent, the extracting temperature of 40℃, the NaCl concentration of 10mg/mL, the enrichment factors of organochlorines and pyrethroids reached 272-875 and 147-183, respectively, and were higher than that obtained by the conventional single-drop method. The detection limit was in the range from 3 to 36 ng/g (S/N = 3), which were much lower than the maximum residue limits establish by EU. The relative standard deviation of peak height was less than 10.2 % (n = 8). This proposed technique is simple, convenient and efficient.4. Cloud point extraction of nonionic surfactants was investigated as a preconcentration step prior to the separation of gas chromatography. The obtained surfactant phase was treated with organic solvent and the target analytes were back extracted by short-term ultrasonic application. Phenol was used as a test compound to investigate the preconcentration means. The parameters affecting the enrichment factor were investigated. Under the selected conditions, including NaCl concentration of 1mol/L, TritonX-100 concentration of 3%(w/w), sample volume of 20 mL, extracting temperature of 74℃and extracting time of 90 min, 0.3 mL surfactant phase could be obtained, and the enrichment factor of phenol reached 707.