Stereoselective Analysis and Dissipation of Propiconazole in Wheat

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Stereoselective Analysis and Dissipation of Propiconazole in Wheat, Grapes, and Soil by Supercritical Fluid Chromatography Tandem Mass Spectrometry Youpu Cheng, Yongquan Zheng, Fengshou Dong, Jing Li, Yaofang Zhang, Shuhong Sun, Ning Li, Xin-Yi Cui, Yuanhong Wang, Xinglu Pan, and Weilong Zhang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.6b04623 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016

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Journal of Agricultural and Food Chemistry

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Stereoselective Analysis and Dissipation of Propiconazole in Wheat, Grape s, and

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Soil by Supercritical Fluid Chromatography Tandem Mass Spectrometry

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Youpu Cheng,*,‡ Yongquan Zheng,*,† Fengshou Dong,† Jing Li,§ Yaofang Zhang,‡

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Shuhong Sun,‡ Ning Li,‡ Xinyi Cui,‡ Yuanhong Wang,‡ Xinglu Pan,† and

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Weilong Zhang‡

6 7 8 9

†

State Key Laboratory for Biology of Plant Diseases and Insect Pests, Institute of

Plant Protection, Chinese Academy of Agricultural Sciences, Beijing, China ffi

Tianjin Agricultural University, Tianjin, China

§

Institute of Quality Standard and Testing Technology for Agro-products, Tianjin

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Academy of Agricultural Sciences, Tianjin, China

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* Corresponding author (Tel: +86-22-23781301; Fax: +86-22-23781315; E-mail:

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[email protected])

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ABSTRACT: An efficient and sensitive chiral analytical method was established for

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the determination of propiconazole stereoisomers by supercritical fluid

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chromatography/tandem mass spectrometry (SFC-MS/MS). Stereoisomeric separation

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was performed on a Chiralpak AD-3 column with CO2/ethanol (93:7) as the mobile

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phase. The four propiconazole stereoisomers were well separated in 4.7 min with

19

resolutions above 2.0. The specificity, linearity, matrix effects, accuracy, precision,

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and stability of the developed method were evaluated. The stereoselective dissipation

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of propiconazole in wheat straw, grape, and soil samples was investigated by the

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proposed method. The results indicated that significant stereoselective degradation

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occurred in wheat straw and grapes, with preferential degradation of (-)-propiconazole

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A and (+)-propiconazole B in wheat straw and the opposite case in grapes. No

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enantioselectivity was observed in soil, although diastereoisomer A degraded faster

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than diastereoisomer B. These results could contribute to a more accurate assessment

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of the environmental risk and food safety of propiconazole.

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KEYWORDS: propiconazole, stereoselective analysis, SFC-MS/MS, wheat, grapes

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INTRODUCTION

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Propiconazole

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(cis-trans-1-[[2-(2,4-dichlorophenyl)-4-propyl-1,3-dioxolan-2-yl]methyl]-1H-1,2,4-tri

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azole) is a broad-spectrum systemic triazole fungicide with prominent protective and

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curative properties. This fungicide has been widely used to control powdery mildew,

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rusts, and leaf spot on wheat, grapes, turf and hardwoods, among others.1 However,

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this compound is associated with a higher incidence of liver tumors (benign and

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malignant) in long-term fed male mice,2, 3 and the U.S. EPA has identified it as a

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possible human carcinogen.4

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Propiconazole has two asymmetrically substituted C atoms and thus exists as two

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pairs of enantiomers. The four stereoisomers are separated into cis [(2R, 4S) and (2S,

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4R)], 1 and 2, and trans [(2R, 4R) and (2S, 4S)], 3 and 4, configurations (Figure 1).5

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Similar to most chiral pesticides, propiconazole has been produced and used to date in

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the form of a mixture of the four stereoisomers. Commonly, the enantiomers of the

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same chiral compound have identical physicochemical properties and abiotic

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degradation rates.6, 7 However, a large difference may be exhibited between their

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individual bioactivities, toxicities, or metabolism when the stereoisomers are exposed

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to asymmetric chemical or biological systems, and the dissipation or bioaccumulation

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of the stereoisomers in organisms or the environment is often stereoselective.8, 9 Thus,

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the data from conventional analyses that have treated stereoisomers as one compound

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are incomplete and nonspecific. It is essential to study stereoselectivity of a chiral

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pesticide in plants and the environment to accurately evaluate food safety and -3ACS Paragon Plus Environment


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environmental risk.10 For propiconazole, the biological activity of each stereoisomer

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was shown to vary with pathogens.11 Stereoisomers with absolute configuration 2S

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were more efficient inhibitors of ergosterol biosynthesis in Ustilago maydis than the

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corresponding 2R stereoisomers. In many cases, the fungicidal activity of

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cis-propiconazole was more efficient than that of trans-propiconazole.12, 13 Although

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the bioactivity of each propiconazole stereoisomer differs, few studies have examined

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their fates individually under field conditions possibly due to the lack of effective

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chiral analytical methods.

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In our previous study, an analytical method for the stereoisomeric separation and

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determination of propiconazole by normal phase HPLC with UV detection was

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established.14 However, this method requires a long analysis time and a large amount

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of organic solvent. Furthermore, because of an inability to remove certain endogenous

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interferences and the lack of high selectivity for UV detection, it was challenging to

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apply the methodology to some complex matrices, such as wheat. Mass spectrometric

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detection, with its advantages of high selectivity, high sensitivity, and simple sample

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pretreatment, has become the first choice in many laboratories.15 A study using

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reversed-phase HPLC-MS/MS revealed that the four propiconazole stereoisomers

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could be separated on a Lux Cellulose-2 column, but the retention time was too long

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(greater than 50 min) to be suitable for routine analysis.16 Another study attempted to

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separate propiconazole stereoisomers using HPLC-MS/MS on an AGP column, but

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the resolutions among the eluted peaks of propiconazole under optimal conditions

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were only 0.8, 0.6, and 1.2.17 In recent years, supercritical fluid chromatography -4ACS Paragon Plus Environment

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(SFC), which possesses the advantages of speed, efficient separation and

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environmental friendliness, has received increasing attention in chiral separation. 18

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Toribio et al.19 successfully separated propiconazole stereoisomers using SFC with a

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diode-array detector. Nevertheless, simultaneous determination of propiconazole

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stereoisomers in wheat, grapes and soil by SFC has not been reported to date.

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SFC-MS/MS is an effective alternative technique that overcomes many of the

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shortcomings inherent to current methods.20 It combines the advantages of SFC with

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HPLC-MS/MS technology. In addition, the use of post-column polar solvent

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compensation technology has greatly improved the sensitivity of this technique.21

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The objective of this work was to develop and validate an SFC-MS/MS method for

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the determination of propiconazole stereoisomers in plants and soil and apply the

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proposed method to study the stereoselective behavior of propiconazole in wheat

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straw, grapes, and soil. The results will not only improve our understanding of chiral

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pesticides but also provide data for the rational production and use of chiral

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pesticides.

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MATERIALS AND METHODS

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Reagents and Materials. A propiconazole stereoisomer mix standard (purity:

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96.5%), with the ratio of the four stereoisomers shown in Figure 2A close to 2:2:3:3,

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was purchased from China Standard Material Center (Beijing, China). The

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commercial 20% propiconazole micro-emulsion was obtained from the Langfang

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pesticide pilot plant (Langfang, China). Four stereoisomers of propiconazole with

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purity≥99.9% were prepared by Chiralway Biotech Co., Ltd. (Shanghai, China). -5ACS Paragon Plus Environment


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Chromatography-grade acetonitrile, methanol, ethanol, 2-propanol, and formic acid

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were purchased from Honeywell International (Morris Plains, NJ). Ultra-pure water

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was obtained from a Milli-Q water purification system (Bedford, MA). Analytic grade

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anhydrous magnesium sulfate, sodium chloride, and acetonitrile were from Sinopharm

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Chemical Reagent Beijing Company (Beijing, China). Primary secondary amine

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(PSA, 40 μm), octadecylsilane (C18, 40 μm), graphitized carbon black (GCB, 40 μm)

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sorbents, and Cleanert Pesti-Carb/PSA (500 mg/500 mg/6 mL) SPE cartridges were

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purchased from Agela Technologies, Inc. (Tianjin, China). Liquid CO2, Ar and N2

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with purity over 99.999% were acquired from Qianxi Jingcheng Gas Sales Center

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(Beijing, China).

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A standard stock solution of the propiconazole stereoisomer mix (1000 mg/L) was

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prepared in chromatography-grade acetonitrile. The pure solvent solutions required

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for standard curves (10-1000 μg/L propiconazole mix) were prepared in acetonitrile

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from the stock solution by serial dilution. All solutions were protected against light

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with aluminum foil and stored at -20 °C prior to use.

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Field Experiments. Working areas for wheat and grape were prepared in

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experimental fields at Tianjin Agricultural University (Tianjin, China) in 2015. The

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fields were divided into 30 m2-sized blocks, and a 1-m buffer zone was established to

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separate each plot. Four trial plots were prepared for each treatment, of which three

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plots were used as replicates, and the fourth one was used as the control. No plots had

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been treated with the targeted fungicide in the preceding five years, and the use of any

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other fungicide with a similar structure to that of propiconazole was forbidden during -6ACS Paragon Plus Environment

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the trial period. The temperature of the area was maintained in the range of 26±10 °C

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throughout the experiment. The propiconazole commercial product (20%

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micro-emulsion) dissolved in water was sprayed one time on each plot. The

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application rate was 180 g a.i./ha (grams of active ingredient per hectare, 1.5 times the

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recommended dose, hereinafter the same) for the plots of wheat and wheat soil and 90

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g a.i./ha for the grape plots. The foliar treatment was conducted on wheat and grape

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plants at growth stages 2.7 and 7.5, respectively (according to the BBCH scale, i.e. the

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Biologische Bundesanstalt, Bundessortenamt and Chemische Industrie scale).

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Representative wheat straw (including straw and leaves) and grape samples for the

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dissipation study were collected from each plot on day 0 (2 h), 1, 3, 5, 7, 10, 14, 21,

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28, and 35 after spraying; wheat grain samples were collected at harvest on day 50

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after treatment. Soil samples were collected at depths of 0-15 cm at 15 randomly

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selected points. The soil was sandy loam with 1.86% organic matter and pH 8.57

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(suspension of soil in 0.01 M CaCl 2, 1:2.5 w/w). All samples were separately minced

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(for wheat straw), mixed and stored at -20 °C.

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Instrumentation and SFC-MS/MS Analytical Conditions. Stereoselective

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analysis of propiconazole stereoisomers was performed using an Acquity ultra

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performance convergence chromatography system (Waters, Milford, MA), which

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included an auto back pressure regulator (ABPR), a sample manager, a binary solvent

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manager, a column manager, a Waters 515 compensation pump and a PDA detector.

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The optical rotation of the propiconazole stereoisomers was measured using an

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OR-2090 Plus detector (JASCO Corporation, Japan). A Chiralpak AD-3 chiral -7ACS Paragon Plus Environment


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column (150 mm×4.6 mm i.d., 3 μm particle size, Daicel Chemical Industries, Japan)

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was used for the resolution of analytes. The chiral stationary phase of the column was

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amylose tris-(3,5-dimethylphenyl-carbamate) coated on 3-μm silica gel. The mobile

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phase, consisting of CO2 and ethanol at a ratio of 93:7, was pumped at a flow rate of

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2.0 mL/min during the analysis process. The ABPR pressure, column oven

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temperature and sample room temperature were set at 2200 psi, 30 °C, and 25 °C,

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respectively. In each run, 2 μL analyte samples were injected and 0.1% formic

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acid/methanol (v/v), as a post-column compensation additive, was pumped at a flow

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rate of 0.1 mL/min. Under the above conditions, the four propiconazole stereoisomers

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were eluted within 4.7 min.

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Quantitative analysis of the propiconazole stereoisomers was performed using a

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triple quadrupole Xevo-TQD mass spectrometer (Waters Corp.) equipped with an

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electrospray ionization (ESI) source. The nebulizer gas was 99.999% N 2, and the

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collision gas was 99.999% Ar with a pressure of 2×10 -3 mbar in the T-wave cell. The

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optimal monitoring conditions for target compounds were as follows: the capillary

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voltage, source temperature and desolvation temperature were 3.0 kV, 150 °C and

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500 °C, respectively, and a 50 L/h cone gas flow and 900 L/h desolvation gas flow

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were applied. ESI+ and multiple reaction monitoring mode were adopted. The cone

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voltage was 43 V, m/z 342 was selected as the precursor ion, and m/z 69 and m/z

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158.9 were set as the product quantitative and qualitative ions, respectively, with the

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corresponding collision energy settings at 22 and 28 V. The dwell time was 197 ms.


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Masslynx NT v.4.1 SCN 882 (Waters Corp.) software was used to collect and analyze

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the data.

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Sample Preparation. Samples of wheat straw, wheat grain, and grapes were

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thoroughly homogenized using a food processor. Soils were air-dried at room

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temperature and passed through a 2-mm sieve.

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An aliquot of homogenized sample of 5 g of wheat straw, 10 g of wheat grain, 10 g

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of grape or 10 g of soil was weighed into a 50-mL PTFE centrifuge tube with screw

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cap. For the recovery study, appropriate volumes of the propiconazole stereoisomer

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mix standard solution were added. Blank samples were treated similarly to the other

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samples, but no standard solution was added. Then, the tubes were vortexed for 3 min

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and kept at room temperature for 2h. Next, 5 mL of ultra-pure water was added to

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each matrix, except for grape, to which no water was added. Next, 10 mL of

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acetonitrile was added. The tubes were capped and vortexed for 3 min, ensuring the

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solvent interacted thoroughly with the sample, and then 4 g of anhydrous magnesium

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sulfate and 1 g of sodium chloride were added to each tube. After the tubes were

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capped, they were immediately vortexed intensively for 1 min and centrifuged for 5

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min at a relative centrifugal force (RCF) of 2811×g. Then, a portion of supernatant

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was transferred for further clean-up procedures.

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For the wheat straw, wheat grain, and soil samples, 1.5 mL of the upper layer

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(acetonitrile) was transferred into a 2-mL single-use centrifuge tube containing 150

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mg of anhydrous magnesium sulfate and a fixed amount of sorbent (50 mg PSA+40

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mg GCB for wheat straw and 50 mg PSA for wheat grain or soil). The tubes were -9ACS Paragon Plus Environment


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capped, vortexed again for 1 min and centrifuged for 5 min at 2811×g RCF. The

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obtained supernatants were filtered through a 0.22-μm nylon syringe for SFC-MS/MS

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injection.

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For the grape sample, 2 mL of the supernatant acetonitrile was transferred to a

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Cleanert Pesti-Carb/PSA SPE cartridge, which was preconditioned by 2×5 mL of

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elution solvent (acetonitrile/toluene, 3:1, v/v). After that, the target compound was

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eluted by 2×3 mL of elution solvent. All eluates were collected and evaporated to

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dryness at 40 °C. The extract was reconstituted in 2 mL of acetonitrile and filtered

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through a 0.22-μm syringe filter for SFC-MS/MS analysis.

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Method Validation. Five blank samples of each matrix (wheat straw, wheat grain,

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grape and soil) were analyzed to verify the absence of interfering species surrounding

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the retention time of each stereoisomer. The linearity of the method was determined

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by linear regression analysis of both the standard solution and matrix-matched

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calibration curves. Parameters of the linear regression equations, including slope,

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intercept and the correlation coefficient (R2), were calculated. The matrix-dependent

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LOD and LOQ of the method were determined using the blank and calibration

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standards of wheat straw, wheat grain, grape, and soil. The LOD for each

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propiconazole stereoisomer is the concentration that produces a signal-to-noise (S/N)

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ratio of 3, whereas the LOQ is defined based on S/N=10, estimated from the

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chromatogram corresponding to the lowest concentration of the matrix-matched

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calibration.

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The matrix effect (ME, %) was calculated by - 10 ACS Paragon Plus Environment

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Sin matrix - Sin solvent ×100% Sin solvent

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ME =

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where S denotes the slope of the calibration curves.

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The accuracy and precision of the method were investigated by performing

Eqn. 1

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recovery assays. Five replicates of samples spiked with the propiconazole

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stereoisomer mix at three concentration levels (0.01, 0.05, and 0.5 mg/kg for wheat

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grain, grape and soil and 0.02, 0.05, and 0.5 mg/kg for wheat straw) were prepared on

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three different days by different operators. Analytes were extracted and purified

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according to the procedure mentioned above. The precision for repeatability and

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reproducibility was expressed as the intra-day and inter-day relative standard

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deviation (RSD).

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The stability of propiconazole in the solvent and matrices was tested monthly by

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injection of a newly prepared working solution. All of the samples used in the stability

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test were stored at -20 °C.

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Data Analysis. The separation parameters of the propiconazole stereoisomers,

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including retention factor (k'), selectivity factor (α) and resolution (Rs), were

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calculated as follows

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t  t0 t0 k2 α k1

k' 

Eqn. 2 Eqn. 3 t 2  t1 w1  w1

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Rs  1.177

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where t was the retention time, t0 was the void time under the above mentioned

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conditions (determined using 1,3,5-tri-tert-butylbenzene), and w was the peak at half

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height.

Eqn. 4

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The degradation of the propiconazole stereoisomers in plants and soil was assumed

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to accord with first-order kinetics. The corresponding degradation rate constant, k, and

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the half-life (T1/2, day) were calculated by

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C  C0 e - kt

Eqn. 5

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T1 / 2  ln 2 /k  0.693/k

Eqn. 6

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where C0 denotes the initial concentration and C denotes the concentration at

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sampling time t. Regression functions were obtained on the basis of the mean value of

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three replicates.

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The enantiomer fraction (EF) was used to measure the enantioselectivity of the

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dissipation of the propiconazole enantiomers, and the diastereoisomer fraction (DF)

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was calculated to fully describe the stereoselectivity of dissipation. The EF and the

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DF of the propiconazole stereoisomers were defined by 22, 23

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EFA 

( ) A (  ) A  ( ) A

Eqn. 7

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EFA 

( ) B (  ) B  ( ) B

Eqn. 8

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DF =

Da Da + Db

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where (+)A, (−)A, (+)B, and (−)B denote the peak areas of the four specified

Eqn. 9

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stereoisomers and Da and Db denote the peak areas of diastereoisomers A and B of

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propiconazole, respectively.

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RESULTS AND DISCUSSION

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Optimization of Chromatographic Conditions. Screening the chiral stationary

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phase (CSP). Four polysaccharide-based CSPs, Chiralpak AD-3, Chiralpak IA-3,

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Chiralpak IB-3, and Chiralpak IC-3, were employed to examine their ability to - 12 ACS Paragon Plus Environment

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discriminate propiconazole stereoisomers. The screening assays were performed using

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different organic solvents of acetonitrile, methanol, ethanol, and 2-propanol as

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modifiers. Typical SFC-MS/MS chromatograms and the related separation conditions

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are presented in Figure 2. The results showed that the best chromatographic separation

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of the four propiconazole stereoisomers could be achieved on the Chiralpak AD-3

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column, with resolutions above 2.0 and relatively short retention times (Figure 2A).

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Additionally, baseline resolution could be obtained on the Chiralpak IA-3 column, but

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the smallest resolution under the optimal separation conditions was just above 1.6

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(Figure 2E). In the case of the other two cellulose-based chiral columns, i.e.,

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Chiralpak IB-3 and Chiralpak IC-3, only partial separation was obtained (Figure 2B

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and Figure 2C), indicating an insufficient propiconazole resolution. Thus, the

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Chiralcel AD-3 column was chosen in this study.

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Chiralpak AD-3 and Chiralpak IA-3 are two types of amylose-based columns;

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although both are based on the tris-(3,5-dimethylphenylcarbamate) of amylose (Figure

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2A, Figure 2E), their linkage types are different. The former is a coated type, while

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the latter is an immobilized type. Generally, the chiral recognition ability of the

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immobilized polysaccharide-based chiral stationary phase is lower than that of the

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corresponding coated chiral stationary phase under the same standard conditions.24, 25

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Chiralpak IB-3 and Chiralpak IC-3 were based on the

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tris-(3,5-dimethylphenylcarbamate) of cellulose and the

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tris-(3,5-dichlorophenylcarbamate) of cellulose, respectively. Of the three

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polysaccharide CSPs, amylose tris-(3,5-dimethylphenylcarbamate) has often exhibited - 13 ACS Paragon Plus Environment


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excellent chiral resolution.26 This finding may be due to the difference of backbone

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structures of the CSPs and their polar functional groups, by which different

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preferences in their interaction with analytes occur.27, 28

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Optimization of the mobile phase. In the SFC-MS/MS system, the main component

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of the mobile phase was compressed liquid CO2. However, pure CO2 often could not

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adequately elute the analytes, especially the polar analytes. Therefore, a certain

277

amount of an organic modifier was added to separate the propiconazole stereoisomers

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on a Chiralcel AD-3 column. Comparative analysis assays concerning the type and the

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proportion of organic modifier (acetonitrile, methanol, ethanol, and 2-propanol) were

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conducted. The results showed that only ethanol or 2-propanol as the modifier could

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achieve the baseline separation of the four propiconazole stereoisomers. However,

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when 2-propanol was used, the retention time of each stereoisomer was delayed, and

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the stereo-resolution was negatively affected by peak tailing (Figure 2D).

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Furthermore, because of the lower content and higher viscosity of 2-propanol in the

285

mobile phase, the nebulization in the electrospray ionization source of the mass

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spectrometer was often worse than when using ethanol. Comparatively, ethanol was

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more conducive to the stereoisomeric separation of propiconazole on the Chiralcel

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AD-3 column in terms of its excellent separation, short retention time, and good peak

289

shape. A pioneering study revealed that the four stereoisomers of propiconazole could

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be separated on a Chiralcel AD column at 200 bar, 35 °C, and 2 mL/min. 19 However,

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baseline separation of the four eluted peaks could not be achieved simultaneously

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with ethanol as the modifier. Although good separation could be obtained with

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2-propanol as the modifier, the retention times were too long (above 15 min).

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As shown by the results, the organic modifier content in the mobile phase has a

295

considerable effect on chiral separation. When the ethanol content increased from 5%

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to 11% (v/v), k' for the four stereoisomers decreased from 3.53, 4.47, 5.03, and 5.73 to

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1.22, 1.47, 1.73, and 1.93, respectively. Simultaneously, the Rs values decreased

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significantly, from 5.08, 2.09, and 2.16 to 2.45, 2.33, and 1.45. By comparison, a final

299

amount of 7% ethanol (v/v) was selected for its relatively better resolution and shorter

300

retention times.

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In addition, the flow rate of the mobile phase was also investigated for the

302

separation of propiconazole stereoisomers. Short retention times and low resolution

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with the increasing flow rate were observed. When the flow rate increased from 1.6 to

304

2.4 mL/min, k' decreased significantly from 3.17, 3.84, 4.42, and 4.90 to 1.64, 2.04,

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2.39, and 2.69, while α increased from 1.21, 1.15, and 1.11 to 1.24, 1.17, and 1.13.

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Meanwhile, Rs decreased from 3.66, 2.84, and 2.08 to 3.27, 2.44, and 1.85. The flow

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rate of 2.0 mL/min, which gave rise to better resolutions and relatively centered

308

retention times, was selected for subsequent experiments.

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Selecting the ABPR pressure and column temperature. The pressure of the

310

supercritical system influences the density of CO2 and alters the eluotropic strength of

311

the fluid.29, 30 The effect of the ABPR pressure of the convergence manager was

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investigated by changing it from 1,800 to 2,600 psi at intervals of 200 psi. The ABPR

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pressure had a large effect on the retention time but a minimal effect on the resolution - 15 ACS Paragon Plus Environment


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of the propiconazole stereoisomers. k' decreased from 2.63, 3.23, 3.74, and 4.18 to

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2.00, 2.47, 2.87, and 3.20 with the increase in ABPR pressure, while α values were

316

constant. Considering that lower pressure is a benefit to the usage lifetime of the

317

Chiralpak AD-3 column, 2,200 psi, a relatively lower ABPR pressure with better

318

resolutions, was selected for further study.

319

Column temperature is a complicated factor affecting chiral separation when

320

supercritical fluid is used as the mobile phase. On the one hand, with the increase in

321

temperature, the density and solvating power of CO2 decreases, leading to an increase

322

in retention time. On the other hand, high temperature increases the solubility of the

323

analytes, causing a decrease in retention time.31 In the current study, the effect of

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column temperature on resolution was investigated for the propiconazole

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stereoisomers at 26, 28, 30, 32, 34 and 36 °C. A Chiralpak AD-3 column was

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employed with a flow rate of 2.0 mL/min and 7% ethanol as the mobile phase

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modifier with the ABPR pressure at 2200 psi. The results showed that the temperature

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had a relatively slight impact on the separation of the propiconazole stereoisomers.

329

Overall, a higher temperature was favorable to higher resolution in this study. With

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the increase in temperature, the selectivity did not change, but k' increased from 2.19,

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2.69, 3.13, and 3.51 to 2.39, 2.93, 3.37, and 3.78. Clearly, the first of the two listed

332

effects might dominate possibly due to the low percentage (0.05, Student’s paired t-test) was observed.

418

Stereoselective Degradation. Dissipation in wheat straw and grapes. Under field

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conditions, concentrations of the four propiconazole stereoisomers in wheat straw and

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grapes almost achieved a maximum 2 h after foliar application (Figure 4A and B) and

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then decreased with time. Thus, the concentration of each stereoisomer at 2 h was - 20 ACS Paragon Plus Environment

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regarded as the starting point of the regressive degradation equation. The degradation

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of the propiconazole stereoisomers in wheat straw and grapes followed first-order

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kinetics, with R2 in the range of 0.8255-0.9879. The respective half-lives of

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(+)-propiconazole B, (-)-propiconazole B, (-)-propiconazole A, and (+)-propiconazole

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A were 3.37, 5.01, 7.43, and 10.45 days in wheat straw, and 3.12, 2.39, 8.71, and 7.03

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days in grapes. The half-lives of (±)-propiconazole B and (±)-propiconazole A in both

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wheat straw and grapes were all significantly different (P

Stereoselective Analysis and Dissipation of Propiconazole in Wheat

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