Residue and dissipation dynamics of 16% polyoxin B in watermelon and soil

Abstract: 【Objective】In order to evaluate the safety of 16%polyoxin B soluble granule in watermelon and soil.We proposed an ultra-performance liquid chromatography coupled with photodiode array detection (UPLC-PDA) method to detect polyoxin B.Decline and residue of 16%polyoxin B were investigated in field experiments in Shandong and Zhejiang for two years (2016 and 2017) .The results would provide scientific guidance on the proper and safe use of polyoxin B for watermelon.【Methods】The test plot was designed according to guideline on pesticide residue trials.Each plot area was 30 m2and each experiment was conducted in triplicate with random arrangement.There was a protective zone between the treatments and a blank control area.In degradation dynamics experiment, 16%polyoxin B with high dose (127.5 g per 666.7 m2) were sprayed to watermelon and soil, respectively.The representative samples (watermelon and soil) were collected at 2 h, 1, 3, 5, 7, 10, 14, 21 d after 16%polyoxin B treatment.In the residue experiment, 16%polyoxin B with high dose (127.5 g per 666.7 m2) and low dose (80 g per 666.7 m2) were sprayed to watermelon in the early stages of cirsoid blight infestation.The representative samples (watermelon and sarcocarp) were collected at 5, 7, 10 d after the last spray.Field samples were collected and brought to the laboratory within 8 h for cryopreservation.The watermelon sample was cut into 4-8 petals (even number) vertically and uniformly, then the non-adjacent watermelon petals were divided into two groups.First group of watermelons with peel were cut into 2 cm blocks which were mixed by the homogenizer.Another group of watermelons without peel were mixed by the homogenizer.Two 150 g samples were taken into the labeled sample box, respectively.The soil sample was ground up, sieved and mixed, and two samples of 200 g were obtained by quartering method.After sample preparation we kept the samples in the fridge at-20℃.A 5.00 g watermelon, sarcocarp and soi was accurately weighed and placed in a 50 m L centrifuge tube.A 10.0 ml water:acetonitrile (30:70) was added into centrifuge tube with sample.After 10 min, the sample was shaken by the oscillator with350 r·min-1for 30 min, and then separated by centrifuge with 9 500 r·min-1.1 mL of supernatant solution filtered with 0.22μm syringe filter was taken for UPLC-PDA analysis.Polyoxin B was detected a262nm and retention time (RT) was about 1.4 min.【Results】The concentration of polyoxin B was calculated with external standards.Linear regression analysis was performed.polyoxin B showed good linearity with correlation coefficients 0.999 9 in the concentration range of 0.05-10 mg·L-1.The standard curve was y=348 927x-6 918.The limit to quantification (LOQ) and the limit of detection (LOD) were calculated based on the concentration or content of the target when the target produced double baseline noise signal on the detector.The results showed that LOQs of polyoxin B in watermelon, sarcocarp and soil were 0.01 mg·kg-1and the LODs of polyoxin was 5.0×10-11g.The average recoveries of polyoxin in the watermelon and soil were found at the five spiking levels from 0.1 to 1.0 mg·kg-1.The mean recoveries of polyoxin were in the acceptable ranges of 88.7%-100.6%for watermelon, 90.2%-94.6%for sarcocarp, and 92.8%-100.0%for soil, respectively.The RSDs of the method ranged from 9.5%to13.9%, 2.6%to11.8%and 2.9%to 6.5%, respectively.In degradation dynamics experiment, the concentration of polyoxin vestigital was detected, which ranged from 0.319 to 0.398 mg·kg-1for watermelon and 0.829 to 0.927 mg·kg-1for soil on the day of spraying.After 21 d, the concentration of polyoxin vestigital was all lower than 0.100 mg·kg-1for the watermelon and soil.The degradation dynamics of polyoxin were accorded with the firstgrade dynamic equations in watermelon and soil.The average levels of polyoxin degradation rate 3 days after treatment were 56.3%for the bwatermelon and 73.4%for the soil in Shangdong and Zhejiang (2016 and 2017) .The half-lives of polyoxin in the watermelon and soi were 1.9-3.4 days and 1.6-2.2 days, respectively.In the residue experiment, following the recommended dosage of polyoxin B (127.5 g per 666.7 m2and 80 g per 666.7 m2) which were sprayed 3 and 4 times the watermelon were collected on the 5th, 7th, 10th day after the last application.The final residues of polyoxin B were not found (polyoxin B content<0.1 mg·kg-1) .【Conclusion】Polyoxin B was easily degraded in the watermelon and soil.Therefor, it was an environmental friendly pesticide.The safety interval of 16%polyoxin B soluble granule for the watermelon was 5 days.The experiment results would provide data support for the safe use of polyoxin B in watermelon and agricultural products.