Article pubs.acs.org/JAFC
Influence of Uptake Pathways on the Stereoselective Dissipation of Chiral Neonicotinoid Sulfoxaflor in Greenhouse Vegetables Zenglong Chen, Fengshou Dong, Xinglu Pan, Jun Xu, Xingang Liu, Xiaohu Wu, and Yongquan Zheng* State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of Plant Protection, Chinese Academy of Agricultural Sciences, Beijing 100193, People’s Republic of China S Supporting Information *
ABSTRACT: Stereoselectivity is of vital importance in our environment and needs to be taken into account for comprehensive risk assessment and regulatory decisions of chiral neonicotinoid sulfoxaflor. However, little is known about the dissipation of sulfoxaflor stereoisomers with respect to stereoselectivity in plants under greenhouse cultivation. To bridge the knowledge gap, the current study was initiated to investigate the stereoselective degradation of sulfoxaflor in solar greenhouse cucumber and tomato from foliage and root uptake pathways. The stereoselective dissipation of sulfoxaflor was not statistically different between enantiomer pairs from foliage and root pathways of vegetables (P < 0.05). The persistence of sulfoxaflor stereoisomers was consistently prolonged under the foliage uptake pathway (t1/2, 3.38−14.09 days) compared to the root uptake pathway (t1/2, 2.65−5.07 days) in both vegetable fruits. Nevertheless, the concentrations of (+)-sulfoxaflor A and (−)-sulfoxaflor B were both slightly higher than that of their antipode. The tiny difference should be emphasized because it might be magnified to a significant difference by the high-potential bioaccumulation of sulfoxaflor in the food chain. KEYWORDS: sulfoxaflor, stereoselectivity, solar greenhouse, uptake pathways, ultrahigh performance supercritical fluid chromatography−tandem mass spectrometry
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INTRODUCTION Greenhouse cultivation is essential for vegetable production and quality, and pesticides are commonly used to protect vegetables from damaging effects.1,2 Chiral pesticides account for >30% of the total pesticides used worldwide, and this proportion is expected to increase as compounds with more complex molecular structures are registered.3,4 However, detailed knowledge of the stereoselective dissipation of chiral pesticides under solar greenhouse cultivation, especially for the emerging chiral neonicotinoid insecticides, has received little attention. In addition, the uptake pathway of vegetables can affect the chiral preference of pesticides, probably due to the endogenous plant constituents responsible for the uptake and transport of pesticides.4,5 To bridge this knowledge gap, a novel chiral neonicotinoid sulfoxaflor was chosen as a “chiral probe” to investigate the stereoselective dissipation pattern in greenhouse vegetables by two typical application modesfoliage spraying and root irrigation. Sulfoxaflor, [(E)-methyl(oxo){(1E)-1-[6-(trifluoromethyl)pyridin-3-yl]ethyl}-γ6-sulfanylidene]cyanamide, is the first commercial sulfoximine agrochemical in development with two chiral centers, a carbon atom and a sulfur atom, as a mixture of two pairs of enantiomers (Figure 1).6,7 According to the sign of optical rotations and peak areas, the four stereoisomers are named (+)-sulfoxaflor A, (−)-sulfoxaflor A, (+)-sulfoxaflor B, and (−)-sulfoxaflor B. A and B are diastereomers of sulfoxaflor. Sulfoxaflor has been considered to be the first fourth-generation neonicotinoid for the control of sap-feeding insects in efficacy.8,9 This is because the mode of action of sulfoxaflor belongs to subgroup 4C of the nicotinic acetylcholine receptor, which is separated from the conventional neonicotinoids 4A.10 Up to now, the majority of studies © 2016 American Chemical Society
Figure 1. Chemical structure of sulfoxaflor and chromatogram of the sulfoxaflor stereoisomers (∗ indicates chiral center).
regarding sulfoxaflor have focused on its stereoisomer mixture (mix-sulfoxaflor), such as the biological characterization,6,11,12 mode of action,8−10,13 metabolism,14 and residual detection.7,15 Stereoselective separation and pharmacokinetic degradation of sulfoxaflor was performed in soil in our earlier study, indicating a statistically nonsignificant enantioselective degradation.7 However, the overall trend of the stereoselective dissipation of sulfoxaflor is poorly understood in vegetables, as reflected by different metabolism and translocation mechanisms. For instance, no enantioselective dissipation and enantiomerspecific behaviors were shown between cycloxaprid enantiomers in soil;16,17 however, significant difference was observed in the stereoselective absorption of Youdonger (Brassica campestris subsp. chinensis).18 The studies of the stereoselective Received: Revised: Accepted: Published: 2655
December March 16, March 18, March 18,
15, 2015 2016 2016 2016 DOI: 10.1021/acs.jafc.5b05940 J. Agric. Food Chem. 2016, 64, 2655−2660
Article
Journal of Agricultural and Food Chemistry
method described in a previous study.7 Briefly, representative portions of 5 ± 0.10 g matrices were weighed into a 50 mL PTFE centrifuge tube. Next, 5 mL of acetonitrile was added and the tubes were shaken with an oscillation frequency of 1350 min−1 for 10 min (CK-2000 high-throughput grinder, TH Morgan, Beijing, China). Subsequently, 3 g of anhydrous magnesium sulfate and 0.75 g of sodium chloride were added followed by shaking for an additional 1 min and centrifugation for 5 min at 2588g relative centrifugal force (RCF). Then 1.5 mL of the acetonitrile layer was transferred into a single-use centrifuge tube containing 5 mg of MWCNTs and 150 mg of anhydrous magnesium sulfate. Afterward, the tubes were vortexed for 30 s (XW-80A vortex, Kirin Medical Instruments, Hangzhou, China) and were centrifuged for 5 min at 2188g RCF. The resulting supernatant was filtered through a 0.22 μm nylon syringe filter into an autosampler vial and analyzed using the ultrahigh-performance supercritical fluid chromatography system coupled with a triplequadrupole mass spectrometer (UHPSFC-MS/MS, Waters, Milford, MA, USA). Other details (e.g., chiral stationary phase, mobile phase, backpressure, column temperature, and multiple reaction monitoring parameters) can found in our earlier study.7 Quality Assurance/Quality Control. Cucumber and tomato samples cultivated in control plots were used as field blanks. Blanks spiked with working solution mixtures of sulfoxaflor stereoisomers at three concentration levels (50, 100, and 500 μg/kg) were used for the method detection limit and recovery assay. Both the blanks and spiked samples were treated the same as the samples throughout the study process. All samples were analyzed in quintuplicate. No target sulfoxaflor stereoisomers were detected in the blanks. The mean recoveries were in the range of 76.9−103.7, 72.9−99.2, 76.0−98.0, and 80.7−92.0% for (+)-sulfoxaflor A, (−)-sulfoxaflor A, (+)-sulfoxaflor B, and (−)-sulfoxaflor B, respectively, and the corresponding relative standard deviations were
