Application Of UPLC-UV-MS/MS Technology On Determination And Degradation Of Pesticides Residues In Soil And Water

Nowadays, environmental contamination caused by pesticide residuespersist in soil and water has become a global concern. It is increasinglyimportant to develop effective and green determination method anddegradation method for pesticide residues in soil and water. Present workdeveloped determination methods based on ultra performance liquidchromatography with UV detector (UPLC UV) and ultra performance liquidchromatography tandem with MS detector (UPLC MS/MS) methodologies forpesticides and herbicides in soil and water samples. Also, the UPLC UV andUPLC MS/MS thechnology were applied for the studies of micro biologicaldegradation and photocatalytic degradation of the pesticides. This thesisincludes the following three contents:1. A simultaneous quantification method based on UPLC UV for DDTsin soil samples was developed. The soil sample preparation involved an ultrasonic extraction and a solid phase extraction technology to extract cleanup and concentrate the extracts. Using a Waters ACQUITY UPLC UV systemwith an ACQUITY UPLC BEH C₁₈(50mm×2.1mm I.D.,1.7m) columnand a gradient elution, the run time of four DDTs was within3min under thewavelength of238nm. The external standard method was used forquantification, and the calibration curves showed good linear relations in theconcentration range of12.5500μg kg^-1(R²>0.999). The average recoveries ofthree spiked concentration levels of25,50and100g kg^-1were between7095%, and the RSD values were less than12%. The limit of detection (LOD)in soil ranged from2.5μg kg^-1(S/N=3). A primary micro biologicaldegradation study for DDTs was investigated and the degradation rate was upto65%.2. A simultaneous UPLC MS/MS determination method for atrazine,phenmedipham, mefenacet, metholachlor, and hexazinone five currently usedherbicides residues in soil five herbicides in soil was established. The samplepreparation using the modified QuEChERS (quick, easy, cheap, effective,rigged, and safe) method completed the extraction and clean up steps in oneprocedure. The sample preparation method investigated and optimized theextraction procedures, absorbents and matrix effect. The optimizedQuEChERS sample pretreatment used none buffer, extracted with acetonitrile,and cleaned up with primary secondary amine (PSA) and C18sorbent, thencentrifugated and filtrated before determination. The UPLC MS/MS method was performed on Waters ACQUITY UPLCTM BEH C18(50mm×2.1mmi.d.,1.7m) and the column temperature was30, the gradient elution withmethanol and pure water as the mobile phase and the flow rate was0.25mL min1. The analysis used positive electro spray ionization (ESI⁺) sourceunder the multiple reaction monitoring (MRM) mode and external standardmethod for quantification. The results showed that the correlation coefficientsup to0.9984were obtained across a concentration range of0.5200g kg^-1.The method was validated with soil samples spiked at two fortification levels(4and40g kg^-1) and recoveries were in the range of75.4%98.5%withrelative standard deviations (RSD) of3.2%11.8%. Limits of detection (LOD)ranged0.0050.020g kg^-1(S/N=3), the limits of quantification (LOQ) were0.0170.067g kg^-1(S/N=10).3. Degradation of hexazinone has been investigated by means ofphotocatalysis of mixed phase crystal nano TiO₂. Influences of adsorption,amount of nano TiO₂, pH and irradiation time on the photocatalytic processare studied. Results show that hexazinone is totally degraded within40min ofirradiation under pH neutral conditions. This compares favorably withDegussa P25TiO₂when conducted under the same experimental conditions.Preliminary photocatalytic kinetic information for hexazinone degradation isproposed. First order kinetics is obtained for the adsorption and photocatalyticdegradation reactions, which fit the Langmuir–Hinshelwood model. A rapid,sensitive and accurate UPLC–MS/MS technique is developed and utilized to determine the level of hexazinone in water in support of the degradationkinetics study. The results indicate a limit of detection (LOD) at0.05g L^-1and the recoveries between90.2and98.5%with relative standard deviations(RSD) lower than12%. A LC–MS/MS technique is used to trace thedegradation process. Complete degradation is achieved into final productsincluding nontoxic water, carbon dioxide and urea. A probable pathway for thetotal photocatalytic degradation of hexazinone is proposed.