| Vegetables and fruits are the most valuable foods containing a high amount of vitamins,their consumption being a necessary condition for maintaining human health and physical development.At present,pesticides including insecticides and fungicides are widely applied in the agricultural production of vegetables and fruits to improve the quantity and quality of the yield as well as to protect foods against pests and diseases.Over 1500 pesticides with different chemical properties and toxicological characteristics are used worldwide.The persistent and irrational use of pesticides not only leads to environmental pollution,but also endangers human health.At present,the problem of pesticide residues in tropical fruits is more prominent and has received widespread attention.Therefore,the establishment of multi-residue detection techniques for pesticides in tropical fruits and the study of degradation rules of pesticides of key concern are of great significance in improving the quality and safety of tropical fruits and protecting the dietary safety of consumers.In the study,a method based on a modified Qu ECh ERS coupled with liquid chromatography-quadrupole-time-of-flight mass spectrometry(LC-Q-TOF/MS)was developed for the rapid detection of 33 pesticides and fungicides in tropical fruits(mangoes,bananas,sweetsop and wax apple),while the dynamic patterns of degradation of pesticide residues of key concern were investigated under three different temperature conditions(25°C,4°C and-18°C).The main studies were as follows:(1)The pretreatment parameters were optimized by comparing the spiked recoveries obtained with different extraction solvent volumes,buffer salts,and clean-up adsorbent types.The method could effectively reduce the matrix effect.Under the optimized conditions,the 33pesticides showed good linearity in their respective linear ranges,with their correlation coefficients(R~2)more than 0.99.The screening detection limits(SDLs)for the target pesticides in four tropical fruits were in the range of 0.5-20μg/kg,and the limits of quantitation(LOQs)were in the range of 0.5-50μg/kg.The accuracy and precision of the method were verified by spiked recovery experiments.The recoveries for 33 pesticides at three spiked levels ranged from 68.8%to 120.8%,with relative standard deviations(RSDs)less than 20%.(2)The method was applied to screen and quantify 102 batches of tropical fruit samples(47 for mango,43 for banana,6 for sweetsop and 6 for wax apple).The pesticides detected in the mango samples were mainly difenoconazole(59.6%),tebuconazole(51.1%),pyraclostrobin and azoxystrobin(38.3%and 34.0%,respectively).The most frequently detected pesticide in the banana samples was pyraclostrobin(67.4%),followed by imidacloprid(34.9%)and prochloraz(30.2%),of which the three batches detected were all neonicotinoids(clothianidin,imidacloprid and thiamethoxam).The target pesticides in the samples of sweetsop and wax apple were below the national MRLs and were relatively safe.(3)The degradation rules of eight neonicotinoid pesticides in tropical fruits(mango and banana)under different storage conditions(25℃,4℃and-18℃)were studied by applying the developed Qu ECh ERS combined with LC-Q-TOF/MS method,and the degradation half-lives of eight pesticide residues was predicted by fitting first-order kinetic model.The results showed that the half-lives of eight neonicotinoid pesticides in mango and banana ranged from 7.6 d to 12.8 d and 3.8 d to 10.3 d at room temperature,8.3 d to 22.4 d and 7.3 d to 21.0 d under cold storage,and 33.0 d to 115.5 d and 36.5 d to 63.0 d under freezing condition,respectively.Among them,it was tentatively determined that thiamethoxam showed longer half-lives and relatively stable chemical properties under various storage conditions,while acetamiprid presented relatively short half-lives and was easily degraded.The half-lives of eight neonicotinoid pesticides in mango and banana under freezing condition were obviously longer than that under normal temperature and cold condition. |
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