Enantiomeric Separation Of Three Typical Chiral Pesticides Under Reversed Phase And Enantioselective Degradation Of Profenofos In Citrus

In the present study,three typical chiral pesticides(Malathion,Fenvalerate,Profenofos)which have been extensively used on citrus production,were chiral resolved on polysaccharide chiral colun by reversed-phase liquid chromatography.The influence of the different chiral stationary phases,mobile phase composition and column temperature on enantioselective separations of these target chiral compounds were also investigated.Among three chiral pestides,profenofos is relatively more persistent in environment and the detectable rates of its residue in citrus products is maintained in a very high level.Given this situation,a chiral residue analytical method of profenofos in citrus mantrix was established and the study on the enantioselective degradation of profenofos in cirtrus was also conducted.It was the first report about the enanmeric separation of profenofos with ND(2)-RH(amylose-tris(3,5-dimethylphenyl carbamate))chiral column in the reverse-phase mode as well as the study of enantioselective degradation of profenofos enantiomers in citrus.The final results of the present study is helpful to provide a data support for a complete risk assessment of profenofos used in citrus production.The major results could be summarized as follows:1.An high performance liquid chromatography-tandem mass spectrometry(HPLC-MS/MS)method based on polysaccharide-based chiral stationary phases(CSPs)for chiral separation of malathion was established.The effects of different chiral stationary phases,mobile phase composition and column temperature on the separation of 2 stereoisomers of malathion were investigated in the reversed-phase mode.And the same time,the thermodynamic mechanism of chromatographic retention and separation between the stereoisomers of malathion and the stationary phase was discussed using thermodynamic methods.The results showed that the baseline separation of the 2stereoisomers of malathion could be achieved on a MJ(2)-RH(Cellulose-tris(4-methylbenzoate))chiral column using the mobile phase of methanol-water=70/30 at a flow rate of 0.2 m L/min and a column temperature of 30°C;In the experimental temperature range(20-40?),the chiral resolution of the stereoisomers of malathion is mainly driven by enthalpy,indicating a decrease in column temperature often results in a better chiral resolution.2.An high performance liquid chromatography-tandem mass spectrometry(HPLC-MS/MS)method based on polysaccharide-based chiral stationary phases(CSPs)for chiral separation of fenvalerate was established.The effects of different chiral stationary phases,mobile phase composition,column temperature and flow rate on the separation of 4 stereoisomers of fenvalerate were investigated in the reversed-phase mode.At the same time,the thermodynamic mechanism of chromatographic retention and separation between the stereoisomers of fenvalerate and the stationary phase was discussed using thermodynamic methods.The results showed that the baseline separation of the 4 stereoisomers of fenvalerate could be achieved on a Lux Cellulose-3chiral column using the mobile phase of acetonitrile-water(5 m M ammonium formate)=55/45 at a flow rate of 0.4 m L/min and a column temperature of 30°C within 14 mins.In the experimental temperature range(10-38?),the chiral resolution of(+)-Distereomer A and(+)-Distereomer B and(-)-Distereomer B and(-)-Distereomer A is mainly driven by entropy,indicating a increase in column temperature often results in a better chiral resolution;the chiral resolution process of(+)-Distereomer B and(-)-Distereomer B is mainly driven by enthalpy,indicating a decrease in column temperature often results in a better chiral resolution.The present investigation expands the application of HPLC-MS/MS in the separation and determination of chiral pyrethroid pesticides.3.An high performance liquid chromatography-tandem mass spectrometry(HPLC-MS/MS)method based on polysaccharide-based chiral stationary phases(CSPs)for chiral separation of profenofos was established.The effects of chiral immobilization,mobile phase composition,column temperature and flow rate on the separation of 2stereoisomers of profenofos were investigated under reversed-phase liquid chromatographic conditions.And the same time,the thermodynamic mechanism of chromatographic retention and separation between the stereoisomers of profenofos and the stationary phase was discussed using thermodynamic methods.The results showed that the baseline separation of the 2 stereoisomers of profenofos could be achieved on a ND(2)-RH(amylose-tris(3,5-dimethylphenylcarbamate))chiral column using the mobile phase of methanol-water=70/30 at a flow rate of 0.2 m L/min and a column temperature of 35°C within 25 mins.In the experimental temperature range(20-40?),the chiral resolution process of the profenofos isomer is mainly driven by entropy,indicating a increase in column temperature often results in a better chiral resolution.4.An analytical method was developed for the determination of the enantiomers of profenofos in whole citrus fruits by high performance liquid chromatography-tandem mass spectrometry with QuEChERS.Citrus samples were extracted with vertical shaking(acetonitrile),and purified with multi-walled carbon nanotubes(inner diameter specification:10-20 nm).The enantiomers of profenofos were separated by a ND(2)-RH chiral column utilizing isocratic elution with the mobile phase of methanol-water=70/30,and finally detected by tandem mass spectrometry in positive ESI mode(ESI~+).Identification and quantitation were achieved by HPLC-MS/MS with multi-reaction monitoring(MRM)and matrix-matched standard curve method.Good linearity was obtained in the analytes concentration range of 2.5-250?g/L with correlation coefficients(r)larger than 0.9996.The LOQs were 5?g/kg.At three different spiked levels(monomer concentrations of 5,100,500?g/kg),the average recovery of the two enantiomers of profenofos in whole citrus fruits was 76.14%-82.75%,relative standard deviation(RSD)was 3.21%-5.95%.The method is simple,sensitive,reproducible,and has a small matrix effect.It is suitable for the quantitative detection of the enantiomers of profenofos in citrus.5.An one-year field experiment was carried out in Si Chuan and Chong Qing on Miyagawa Satsuma Mandarin.The results of dynamic experiments showed that the degradation of the enantiomers of profenofos in citrus conformed to the first-order kinetic equation,and the degradation half-life of(+)-Profenofos is 12.83-14.14 d,the degradation half-life of(-)-Profenofos is 14.14-16.11 d.By comparing the enantiomeric scores(EF values),it was found that the EF values were less than 0.50 during the field test;the EF value of the profenofos enantiomer in the Chong Qing citrus field experiment was the lowest at 0.43 after 30 d of application;the EF value of profenofos in the field experiment of citrus in Si Chuan was the lowest at 0.44 after 40 d of application.The result indicates that there were obviously selective degradation of profenofos in citrus.The degradation rate of(+)-Profenofos is faster than(-)-Profenofos,and(-)-Profenofos is easily enriched in citrus.