Enantioselective Degradation of Tebuconazole in Wheat and Soil under Open Field Conditions

Xiaolan Ye1,a, Anguo Peng1,2,b*, Jing Qiu3,c*,Tingting Chai3,d, Hualin Zhao3,e , Xinghua Ge3,f

1School of Chemistry and Chemical Engineering, University of South China, Hengyang, 421000, China

2School of Nuclear Science and Technology, University of South China, Hengyang, 421000, China

3Institute of Quality Standards & Testing Technology for Agro-Products, Key Laboratory of Agro-product Quality and Safety, Chinese Academy of Agricultural Sciences, Beijing 100081, China

ayexiaolan920@163.com, b*agpeng@xum.edu.cn, c*qiujing@caas.cn,

dgedac@126.com,ezhao3362699@126.com,fTianhedi888@163.com

Keywords: Tebuconazole, Enantioselectivity, Degradation, Wheat, soil.

Abstract. This study assesses enantioselectivity on the degradation of tebuconazole in wheat grain,

straw, and soil in Beijing and Zhejiang under open field conditions. After agricultural application,

the analytes were extracted from soil and grain with acetonitrile, and from straw with acetonitrile

containing 1% acetic acid through ultrasonic extraction. The extracts were cleaned by

dispersive-solid phase extraction, and determined by chiral liquid chromatography-tandem mass

spectrometry with a Lux amylose-2 column. The results of field trials indicated that the degradation

of tebuconazole enantiomers followed first-order kinetics in straw and soil at the two sites. Their

half-lives in straw ranged from 3.88 to 4.93 days, which were shorter than those in soil ranging

from 40.76 to 43.86 days. The (-)-tebuconazole showed faster degradation in straw from Beijing

and Zhejiang. In Zhejiang soil, preferential degradation of (+)-tebuconazole was observed, whereas

(-)-tebuconazole was preferential in Beijing soil. The terminal residues of (-)-tebuconazole in most

grains were higher than those of its antipode, indicating significant enantioselective residues.

Introduction

At present, chiral pesticides used in agriculture mainly include aryloxy carboxylic acid and amide

weeding agent, organophosphate and pyrethroid insecticides, triazole fungicides, and other varieties.

Triazole fungicides represent the most important category of fungicides to date, and are used to

protect from, cure, and eradicate a wide spectrum of crop diseases. They are used to inhibit

ergosterol biosynthesis and often have one or two chiral carbons [1-2]. They often differ in

bioactivity, toxicity, metabolism, excretion, environmental behavior [3-4], and so on despite their

enantiomers having identical physiochemical properties [5-6]. Therefore, it is necessary to

investigate and clarify the specific environmental behaviors of chiral triazole fungicides, in order to

provide more information for improving efficiency, evaluating food safety, and environmental risk.

In recent years, more and more studies have been published that focus on enantioselectivity of

ecotoxicity, degradation, and environmental behavior of several chiral triazole fungicides including

tebuconazole [7], diniconazole [8], triadimenol [5,9], and triadimefon [10], etc.

Tebuconazole, [(RS)-1-p-chlorophenyl-4,4-dimethyl-3-(1H-1,2,4-triazol-1-ylmethyl)

pentan-3-ol] (Fig. 1), is a relatively broad spectrum triazole fungicide, which has a stereogenic

center in the alcohol moiety and consists of a pair of enantiomers. Tebuconazole was commonly

used on various cereals, fruits, as well as vegetable crops due to its excellent curative and protective

effect against numerous pathogens [11]. As a seed dressing, tebuconazole is an effective treatment

against various smut and bunt diseases of cereals; also, as a spray, tebuconazole controls numerous

pathogens in various crops. However, tebuconazole has been cited as a potential neurotoxicant that

might result in functional endocrine and immune alterations [12]. As WHO reported, even if it has

low acute toxicity, tebuconazole has been proved to potentially provoke adrenal gland hypertrophy

in chronic dog studies and teratogenic effects in mice [13].

Advanced Materials Research Vols. 726-731 (2013) pp 348-356Online available since 2013/Aug/16 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.726-731.348

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 128.118.88.48, Pennsylvania State University, University Park, United States of America-30/05/14,12:24:34)

Some studies have also shown that tebuconazole can exist in the environment for a long period

of time, in Tifton loamy sand during laboratory incubation [14], greenhouse-grown lettuces [15],

and Boronia [16], etc. Tebuconazole is the main final product according to its wheat metabolism

research. The degradation of tebuconazole in soil outdoors was faster than that in laboratory studies.

Based on long-term studies (3-5 y) under field conditions, the residue did not accumulate in soil.

At present, tebuconazole is widely used on wheat to control powdery mildews and gray mold in

China. As is common with many chiral pesticides, tebuconazole is often marketed and released into

the environment as a racemate. In order to investigate the enantioselectivity on kinetics, degradation,

or bioaccumulation of tebuconazole, it is important to determine relative concentrations of

tebuconazole enantiomers after using a racemic mixture. Previous reports showed enantioselective

degradation of tebuconazole in cabbage, cucumber and soil [17], strawberries [18], and in rabbit

plasma [7]. The S-enantiomer degraded faster than the R-enantiomer when incubating tebuconazole

racemates with rat liver microsomes [19].

In this study, a liquid chromatography-triple quadrupole mass spectrometry (LC−MS/MS)

analytical method using an amylose-2 chiral column was explored to determine tebuconazole

enantiomers successfully. On the amylose-2 column, the (-)-tebuconazole was eluted first in the

acetonitrile/water mobile phase [17]. Then the enantioselectivity were investigated in the

degradation process of tebuconazole enantiomers in wheat and soil after agricultural application

under open field conditions. These results may provide clues for better understanding of

environmental and human risk assessments of chiral triazole fungicides.

Fig. 1 Structures of tebuconazole enantiomers.

Experimental

Materials, Reagents and Apparatus. The analytical standard and commercial product of

rac-tebuconazole (97.0%, purity) was provided by Shanghai Shenglian Chemical LTD, China. A

stock solution of rac-tebuconazole was prepared in acetonitrile and stored at -4˚C. Working

standard solutions were obtained by diluting the appropriate amount of the stock solution in

acetonitrile. Water was purified by a Milli-Q system.

Acetonitrile (HPLC grade) was purchased from Fisher Scientific (Fair Lawn, NJ, USA). Acetic

acid, sodium chloride, anhydrous magnesium sulphate, and anhydrous acetate sodium were all

analytical grades and obtained from Beijing Chemical Plant (Beijing, China). Primary secondary

amine (PSA), Bondesil-C18 and Florisil were purchased from Agilent Technologies (California,

USA). LD5-2B centrifuge, KQ-800 ultrasonic cleaner, and PL303 balance were purchased from

Beijing Jingli Centrifuge Co., Ltd. (Beijing, China), Kunshan Ultrasonic Instrument Co., Ltd.

(Kunshan, China), and Mettler Toledo Instrument Co., Ltd. (Shanghai, China), respectively.

Sample Preparation. In this study, the field trials were conducted in Beijing and Zhejiang

Provinces in 2011, which included the dissipation and terminal residues experiment, they were

performed according to the “Guidelines for Pesticide Residue Field Trials” (NY/T 788-2004),

issued by the Ministry of Agriculture, the People’s Republic of China. Each treatment site consisted

of three 30 m2 replicate plots that were separated by irrigation channels.

A wettable powder (75%, m/m) containing 12.5% of tebuconazole and 62.5% of chlorothalonil

was dissolved in water and sprayed at the dosage of 900 g ai/ha for straw and soil application when

wheat was being grown for the first time at that site. Tebuconazole or products with similar

structures had not been applied to the plots previously. Wheat straw and soil samples were collected

randomly at 0 (2 h), 1, 3, 5, 7, 10, 14, 21, 30, 45 and 60 days after treatment. Meanwhile,

tebuconazole terminal residues in grains were sprayed at the tillering-earing stage. There were two

treatments: one was low dosage (600 g ai/ha) that was applied two (L2) and three (L3) times, and a

Advanced Materials Research Vols. 726-731 349

high dosage (900 g ai/ha) that was applied two (H2) and three (H3) times. Then grains were

randomly collected from the harvest at intervals of 7, 14, and 21 days after the last spraying. The

straw (>1 Kg) sample consisted of all of the straw that was above the ground. Soil samples (>2 kg),

with no stones or plant debris, were obtained from each plot (0–15cm depth). The wheat grain (2

Kg) was sampled randomly. The chopped straw and crushed grains were divided into subsamples,

and then the subsamples were stored at -20 ºC until analysis.

Soil Sample. Ten grams of each soil sample were weighed in a 100 mL polypropylene

centrifuge tube, followed by the addition of 20 mL acetonitrile. The tube was then agitated for 1

min on a vortex mixer followed by ultrasonic extraction for 20 min (first heating water to 30 ºC and

then ultrasonic extraction). Five grams of sodium chloride was added into the extract, shaken

vigorously for 1 min, and centrifuged at 5000 r min-1

for 5 min. All of the supernatant was

transferred to a 100 mL round-bottomed flask, and then the above procedure was repeated for a

second time before extraction. All the supernatant was concentrated to near dryness using a vacuum

rotatory evaporator at 35 ºC. The residue was redissolved in 2 mL acetonitrile for the next clean-up.

Straw Sample. Five grams of each triturated straw sample were weighed in a 100 mL

polypropylene centrifuge tube followed by the addition of 20 mL acetonitrile containing 1% acetic

acid. The tube was then agitated for 2 min on a vortex mixer. Two grams of anhydrous magnesium

sulphate and 2 g anhydrous acetate sodium were added into the tube and then it was shaken

vigorously for 1 min. The straw samples were then extracted ultrasonically for 20 min (first heating

water to 30 ºC then ultrasonic extraction). The extract was filtered into a 100 mL round-bottomed

flask. Then the above procedure was repeated before extraction; however, this time only anhydrous

acetate sodium was added into the tube. All the supernatant was concentrated to near dryness using

a vacuum rotatory evaporator at 35˚C. The residue was redissolved in 5 mL acetonitrile for the next

clean-up.

Grain Sample. Five grams of threshed and triturated grain were weighed in a 100 mL

polypropylene centrifuge tube, followed by the addition of 10 mL purified water and 15 mL

acetoneitrile. The tube was then agitated for 1 min on a vortex mixer, followed by ultrasonic

extraction for 20 min (first heating water to 30˚C then ultrasonic extraction). Then sodium chloride

(2.5 g) was added into the extract, shaken vigorously for 1 min, and centrifuged at 5000 r min-1

for

5 min. The supernatant was transferred to a 100 mL round-bottomed flask, the above procedure was

repeated again before extraction, but this time the purified water was not added into the tube. The

supernatant was concentrated to near dryness using a vacuum rotatory evaporator at 35 ˚C. The

residue was redissolved in 2 mL acetonitrile for the next clean-up.

Dispersive-solid phase extraction was used to clean up the sample extract. For this, 100 mg

Florisil was added into 2 mL of the extract from soil; while, 50 mg Florisil and 50 mg C18 were

added into 2 mL of the extract from straw. For grain, a low-temperature clean step was used. The

grain sample extract (2 mL) was transferred into a 10 mL polypropylene centrifuge tube and stored

for 2 h in a freezer (-20˚C). Then 1 mL of the supernatant was transferred to a 5 mL polypropylene

centrifuge tube containing 50 mg C18. Then the sample extracts, which had been cleaned up, were

vortexed for 0.5 min and centrifuged at 5000 r min-1

for 2 min. The supernatants were filtered into

auto-sampler vials using 0.22 µm syringe filters for LC-MS/MS analysis.

Instruments and Chromatographic Conditions. An API 2000 (Applied Biosystems /MDS

Sciex, CA, USA) triple quadrupole mass spectrometry with turbo spray source, was connected to an

Agilent 1200 HPLC system (Agilent, CA, USA) equipped with a G1311A Quatpump, a G1322A

vacuum degasser, and a G1329A auto-sampler. LC-MS/MS system control, data acquirement, and

process were achieved with the Analyst 1.4.2 software. The enantioseparation of tebuconazole was

achieved on a Lux amylose-2 column (150×2 mm, 3 µm, Phenomenex) at 25˚C. The injection

volume was 10 µl. The mobile phase was acetonitrile-water (60/40, v/v) and the isocratic elution

was performed with a flow rate of 0.3 mL min-1

.

The electron spray ionization in positive ion (ESI+) with multiple reaction monitoring (MRM)

mode was applied. The optimized parameters were as follows: curtain gas (CUR) 10, collision gas

(CAD) 5, ion source temperature (TEM) 450˚C, ion source gas 1 (GS1) 30, ion source gas 2 (GS2)

350 Advances in Environmental Technologies

70, and ion spray voltage +4500 V. Quantification and qualititation were performed for the

enantiomers using the transitions of m/z 308 > 70 (CE, 50 V), m/z 308 > 125 (CE, 50 V), and m/z

308 > 151(CE, 35 V).

Calibration Curves and Assay Validation. A series of matrix-match rac-tebuconazole standard

solutions for determining the linearity of the two enantiomers (0.01-10mg L-1

) were prepared with a

blank extract for LC-MS/MS analysis. Peak area of each enantiomer was measured and plotted

against its concentration. The standard deviation (SD) and the relative standard deviation (RSD)

were calculated for the entire calibration range. A series of blank samples fortified with

rac-tebuconazole at 0.01, 0.1, and 0.5 mg/kg for soil and straw; and 0.02, 0.2, 1 mg kg-1

for grain

were prepared for method validation and determined as described previously. The peak area for the

extract analyte and the equivalent matrix-matched standard were compared to estimate the recovery.

The limit of detection (LOD) was considered to be the concentration that produced a signal to noise

(S/N) ratio of 3, and the limit of quantification (LOQ) was defined as the lowest concentration in

the calibration curve with acceptable accuracy and precision.

It was assumed that the degradation of the enantiomers in these samples followed a first-order

kinetics model. From the linear range of logarithmic plots, corresponding rate constants k for (-)-

and (+)-tebuconazole were determined. The half-time (T1/2) was obtained according to following

equations:

0

kxc c e−= . （1）

12

0.6932lnTk k

= = . （2）

The enantiomeric fraction (EF) was employed to measure the degradation of tebuconazole

enantiomers in wheat straw, grain, and soil samples. This descriptor was more easily employed than

the enantiomeric ratio (ER) in mathematical fate expressions [20]. EF was defined as follows: EF =

peak areas of the (-)-enantiomer/peak areas of ((-)-enantiomer + (+)-enantiomer). The range of EF

values was from 0 to 1, with EF = 0.5 representing the racemic mixture.

Results and Discussion

Method Validation. Typical chromatograms of the blank and fortified samples of the straw and

soil are shown in Fig. 2. The two enantiomers were separated completely, and there were no

interference peaks at their retention times. The summary of calibration data in Table 1 showed good

linear ranges of 0.01-0.5 mg L-1

in grain and 0.01-10 mg L-1

in straw and soil, for analysis of the

two enantiomers. Table 2 shows the mean recoveries of the two enantiomers in straw and soil

determined at the three fortification levels of 0.005, 0.05, and 0.25 mg kg-1

; and in grain at the three

fortification levels of 0.01, 0.1, and 0.5 mg kg-1

. Recoveries of both tebuconazole enantiomers

ranged from 73.4%±1.2% to 112.4%±3.1% in straw, from 91.7%±11.0% to 111.6%±6.1% in grain,

and from 79.5±3.1% to 103.4%±9.8% in soil.

The LOQ and LOD for the two enantiomers were respectively found to be 0.005 mg/kg and

0.0035 mg kg-1

in straw, 0.01 mg kg-1

and 0.003 mg kg-1

in grain, and 0.003 mg kg-1

and 0.001 mg

kg-1

in soil. These results show that the developed method could be applied to the following

enantioselective studies as it was accurate, sensitive, and repeatable.

In recent years, chiral analysis methods with reversed-phase LC-MS/MS showed significant

advantages, including nimble use, higher sensitivity, and less interference, when compared to the

chiral HPLC method [21-22]. A previous report showed that the LOQ and LOD of tebuconazole

enantiomers were respectively found to be 0.025 mg kg-1

and 0.01 mg kg-1

in soil and cabbage

using HPLC, but 0.003 mg kg-1

and 0.001 mg kg-1

in cucumber using LC-MS/MS [17].

Advanced Materials Research Vols. 726-731 351

Fig. 2 Representative chromatograms of extracts from Zhejiang grain fortified with

rac-tebuconazole (0.02 mg kg-1

) (A) and at 21 d (L3 plots) after treatment (B).

Table 1 Linear calibration curves of tebuconazole enantiomers

Test samples Enantiomer Calibration curves Relative

coefficient

Concentration

range (mg L-1

)

Beijing grains (-) y=1027067x - 7767 R2 = 0.996 0.01-0.5

(+) y=2925677x+6847 R2 = 0.998 0.01-0.5

Zhejiang grains (-) y=1779228x + 10433 R² = 0.996 0.01-0.5

(+) y=3880195 x - 24670 R² = 0.998 0.01-0.5

Beijing straw (-) y = 71784x + 9544. R² = 0.996 0.01-10

(+) y = 69390x + 10472 R² = 0.996 0.01-10

Zhejiang straw (-) y = 90625x + 15530 R² = 0.994 0.01-10

(+) y = 10761x + 9496. R² = 0.998 0.01-10

Beijing soil (-) y = 94429x + 13089 R² = 0.994 0.01-10

(+) y = 20260x + 29025 R² = 0.995 0.01-10

Zhejiang soil (-) y=1641149x+311883 R² = 0.991 0.01-10

(+) y=1917179x+199006 R² = 0.992 0.01-10

Table 2 Recovery of tebuconazole enantiomers in wheat grain, straw and soil

Test samples Fortification level

(mg kg-1

)

Recovery (%, ±SD) (n=5)

(-)-tebuconazole (+) - tebuconazol

grains 0.01 103.6±7.4 97.1±7.6

0.1 111.6±6.1 91.7±6.0

0.5 109.8±2.5 82±4.3

straw 0.005 96.9±6.0 112.4±3.1

0.05 86.6±4.9 82.4±3.2

0.25 73.4±1.2 84.5±2.9

soil 0.005 103.4±7.0 96.7±6.4

0.05 79.5±3.1 80.2±4.5

0.25 88.8±4.0 85.8±4.9

Enantioselective Degradation of Tebuconazole in Straw. The degradation parameters of

tebuconazole enantiomers in straw and soil are summarized in Table 3. Enantiomeric degradation

curves and EF values for straw from Beijing and Zhejiang are shown in Fig. 3. The degradation of

both enantiomers in straw in the two locations followed first-order kinetics under open field

conditions. In the Beijing straw, the concentrations of (-)-tebuconazole and (+)-tebuconazole

352 Advances in Environmental Technologies

decreased respectively from 10.08 to 0.05 mg kg-1

and 4.86 to 0.07 mg kg-1

in 30 days after

application and were undetectable after 30 days. In the Zhejiang straw, their concentrations

decreased respectively from 51.92 to 0.43 mg kg-1

and 49.38 to 0.39 mg kg-1

in 21 days. This

suggests that higher enantiomeric concentrations were found in Zhejiang than in Beijing.

Table 3 Regressive equations of tebuconazole enantiomers in straw and soil

Test material Enantiomer Regressive equationsa R² Half-lives

b

(days)

Beijing sraw (-) y = 11.6743 e-0.1788 x

R² = 0.9818 3.88c

(+) y = 6.1452 e-0.1406 x

R² = 0.9690 4.93d

Zhejiang straw (-) y = 10.6641 e-0.1724 x

R² = 0.7302 4.02c

(+) y = 9.9908 e-0.1718 x

R² = 0.7282 4.03c

Beijing soil (-) y = 0.5833 e-0.0170 x

R² = 0.7619 40.76c

(+) y = 0.4886 e-0.0159 x

R² = 0.7097 43.58c

Zhejiang soil (-) y = 0.1611 e-0.0158 x

R² = 0.7943 43.86c

(+) y = 0.2396 e-0.0167 x

R² = 0.7884 41.50c

aThe regressive functions were obtained based on the mean value of three replicates.

bSignificant differences (P < 0.05, Student’s paired t-test) are indicated with different alphabets.

Fig. 3 Degradation curves and EF values of tebuconazole enantiomers in straw

As Table 3 shows, the degradation half-lives of (-)-tebuconazole and (+)-tebuconazole were 3.88

and 4.93 d, respectively, in Beijing straw, and 4.02 and 4.03 d in the Zhejiang straw. The

differences in the half-lives of two enantiomers were statistically significant (P＜0.05, Student’s

paired t-test) in Beijing straw, indicating that (-)-tebuconazole degraded faster than its antipode.

However, for the Zhejiang straw both enantiomers degraded at similar rates as the difference was

not significant.

As shown in Fig. 3, the EFs in the straw were about 0.700 on the first day, significantly declined

from 0.636 to 0.503 after 7 days, and slowly decreased to 0.432 in the following 23 days from

Beijing. This suggested significant enantioselectivity related to the degradation of tebuconazole in

Beijing straw in the experimental period, and that the enantioselectivity decreased with time after

application. However, the EFs in Zhejiang increased slightly from 0.513 to 0.519 in 21 days and no

significant change was found after that, indicating nonselectivity for both enantiomers in Zhejiang

straw under open field conditions.

Some researchers have reported that the plant enzyme system plays an important role in the

enantioselective degradation and bioactivity of chiral pesticide [5, 20, 23]. As mentioned earlier,

(-)-R-tebuconazole is more active against five phytopathogens than the (+)-S-form [24]. The

enrichment of the higher activity enantiomer in plants has been reported by previous research [23,

25 ]. (+)-tebuconazole degraded faster than (-)-tebuconazole in strawberry, and resulted in

strawberry enriched with (-)-tebuconazole [18]. The enantioselectivity on the dissipation of

Advanced Materials Research Vols. 726-731 353

tebuconazole in cabbage and cucumber were the opposite [17]. In this study, (-)-tebuconazole

degraded faster than (+)-tebuconazole in Beijing straw, but dissipated with similar rates to its

antipode in Zhejiang straw. This may be due to differences in climate and soil type between the two

locations, leading to different degradation rates for the two enantiomers. Due to lack of knowledge

about the enzyme systems in straw for tebuconazole biotransformation, further work should be done

to clarify degradation and bioactivity of tebuconazole enantiomers in straw.

Enantioselective Degradation of Tebuconazole in Soil. As Fig. 4 shows, both enantiomers of

tebuconazole showed slower degradation rates in soil than straw, and their degradation also

followed the first-order kinetics. The concentrations of (-)- and (+)-tebuconazole respectively

degraded from 0.869 to 0.223 mg/kg, and from 0.702 to 0.215 mg kg-1

within 60 days and digestion

rates were 74% and 69%, respectively, in Beijing soil. In Zhejiang soil, they respectively degraded

from 0.201 to 0.069 mg kg-1

, and from 0.318 to 0.098 mg kg-1

and the digestion rates were 66% and

69%, respectively, after 60 days.

Different dissipation rates of the two enantiomers in soils can be observed from Table 3. In

Beijing soil, (-)-tebuconazole (T1/2=40.76 d) degraded slightly faster than the (+)-tebuconazole

(T1/2=43.58 d). Conversely, it showed slight slower degradation rates than its antipode (T1/2=41.50 d)

in Zhejiang soil. However, these differences between the degradation half-lives of two enantiomers

were not statistically significant at both locations. The EF values in Beijing were always above 0.5,

indicating that preferential degradation of (+)-tebuconazole was observed in Beijing soil, which

resulted in enrichment with (-)-tebuconazole. While (-)-tebuconazole degraded preferentially in the

Zhejiang soil as shown by the EF values always being below 0.5.

Bending et al. [26] reported that tebuconazole was degraded very slowly by microorganisms in

soil. Wang et al. [17] studied the enantioselective degradation of tebuconazole in three different

types of soil, which showed different half-lives, (-)-R-tebuconazole (T1/2 = 81.53, 64.17, and 54.56

degraded slightly faster than the (+)-S-form (T1/2 = 83.50, 67.28, and 58.80). Compared to their

results, the half-lives of tebuconazole enantiomers in soil from this study were smaller. This

phenomenon may be related to the way the tebuconazole was applied to the soil. In the study of

Wang et al., tebuconazole was incubated in the test soil, while in our study tebuconazole was

sprayed on soil under open field conditions. Additionally, the incubated soil and the field soil may

have different types of microorganisms, which would result in different degradation behaviors of

tebuconazole enantiomers as microbial decomposition plays an important role in the

enantioselective metabolism of many chiral chemicals in soil [20, 27, 28].

Fig. 4 Degradation curves and EF values of tebuconazole enantiomers in soil.

Terminal Residues of Tebuconazole in Grains. The residues of tebuconazole enantiomers in

wheat grains were investigated at harvest time with the grains being collected from different

intervals (7, 14, 21 d) after harvest, and different kinds of dosage treatments including lower/higher

dosages and two/three spray times. As Fig. 5 shows, the concentrations of (-)-tebuconazole were

0.02∼0.05 mg kg-1

in Beijing and 0.006∼0.017 mg kg-1

in Zhejiang on the 7th day, 0.014∼0.042 mg

kg-1

in Beijing and 0.026∼0.042 mg kg-1

in Zhejiang on the 14th day, and 0.005∼0.01 mg kg-1

in

Beijing and 0.009∼0.021 mg kg-1

in Zhejiang on the 21st day. The concentrations of

(+)-tebuconazole were 0.013∼0.03 mg kg-1

in Beijing and 0.01∼0.016 mg kg-1

in Zhejiang on the

7th day, 0.007∼0.02 mg kg-1

in Beijing and 0.016∼0.025 mg kg-1

in Zhejiang on the 14th day, and

0.003∼0.008 mg kg-1

in Beijing and 0.011∼0.015 mg kg-1

in Zhejiang on the 21st day.

354 Advances in Environmental Technologies

There were some differences between enantiomeric residues in grains from Beijing and Zhejiang,

which may be related to the different climate conditions and soil types at the two sites. The highest

concentrations of (-)- and (+)-tebuconazole were found in the Beijing treatment of H2-7d and in the

Zhejiang treatment of H3-14d respectively, suggesting the higher dosage on wheat lead to the

highest enantiomeric residues in grains. The enantiomeric concentrations of (-)-tebuconazole were

higher than its antipode in all grain samples except for L2-21d and L3-7d in Zhejiang grains,

indicating enantioselective residues of tebuconazole with preferable enrichment of (-)-tebuconazole

in most grains.

Fig. 5 Terminal residues of tebuconazole enantiomers in wheat grains.

Conclusion

After developing and validating an enantioselective method, this study investigated the

enantioselective degradation of tebuconazole in wheat and soil in Beijing and Zhejiang under field

conditions. The degradation process of tebuconazole enantiomers followed first-order kinetics in

soil and straw. The enantiomeric half-lives in straw were much shorter than in soil, indicating a fast

degradation rate in straw. The changes in EF values showed that the degradation and residues of

tebuconazole in wheat straw, grain, and soil presented enantioselectivity under field conditions at

the two locations.

Acknowledgment

This work was funded by the National Natural Science Foundation of China (21177156).

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