Relationship between the Structure of Ionic Liquid and Its Enrichment

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Agricultural and Environmental Chemistry

Relationship between structure of ionic liquid and its enrichment ability to trace fungicides from environmental water sample Jiale Yang, Chen Fan, Gang Tang, Wenbing Zhang, Hongqiang Dong, You Liang, Yanfei Wang, Mingqiang Zou, and Yongsong Cao J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b03244 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 26, 2018

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Journal of Agricultural and Food Chemistry

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Relationship between structure of ionic liquid and its enrichment

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ability to trace fungicides from environmental water sample

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Jiale Yang1, Chen Fan1, Gang Tang1, Wenbing Zhang1, Hongqiang Dong1, You Liang1,

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Yanfei Wang2, Mingqiang Zou2, Yongsong Cao1*

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1

College of Plant Protection, China Agricultural University, Beijing, China

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Institute of Equipment Technology, Chinese Academy of Inspection and Quarantine,

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Beijing, China

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*Corresponding author: NO.2 Yuanmingyuan West Road, China Agricultural University,

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Beijing, China

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Zip/Postal code: 100193

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Telephone number: 86-10-62734302 (O), 86-10-62734302 (FAX)

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Email: [email protected], [email protected]

1

ACS Paragon Plus Environment


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Abstract

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In order to elucidate the relationship between structure of ionic liquid (IL) and its

16

enrichment ability to trace pesticides from environmental water sample, a series of

17

imidazole-based ILs were synthesized to used for extracting four fungicides (boscalid,

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cyprodinil, fluazinam, pyrimethanil) though in-situ IL-DLLME method. The results

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showed that aromatic heterocyclic monocation ionic liquids (MILs) had better extraction

20

ability to fungicides than other three alicyclic heterocyclic MILs. Dication ionic liquids

21

(DILs) with the four carbons at the side chain had better ability to extract fungicides

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than MILs and DILs with long bridge carbon chain had better recoveries of fungicides

23

with low Kow values. The proposed method showed high mean enrichment factors and

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high recoveries of the fungicides from real water samples. The rules of relationship

25

between structure of ionic liquid and enrichment ability are instructive to the application

26

of ILs in pretreatment of complex substances.

27 28

Keywords: Ionic liquid, In-situ IL-DLLME, Pretreatment, Fungicide, Enrichment ability,

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Environmental water sample

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2


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1. Introduction

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Due to direct impacts on the ecological safety, water environmental pollution has

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become the hot issues in recent years. As one of the major sources of water

34

contaminations, pesticides are being detected in the aquatic environment, and have the

35

potential to cause known or suspected adverse ecological or human health effects

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Boscalid, cyprodinil, fluzinam and pyrmethinail are broad-spectrum fungicides and

37

widely used for controlling plant pathogens in various vegetables and fruits, and have

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been frequently found from nature water in many countries such as Northern Greece,

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South America, China and Cameroon

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other three are classified as moderate toxicity to fish 7. Boscalid, fluzinam and

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pyrmethinail are considered as possible human carcinogen, boscalid has the concern

42

about the reproduction of human, and pyrmethinail has the adverse effects on internal

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organs, special to kidney 8. All four heterocyclic fungicides have the concern of

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bioaccumulation, which may cause long-term problems 9. According to the drinking

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water standard of European Union (EU), the concentration of single pesticide should not

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more than 0.1 µg L-1, and for total pesticides, 0.5 µg L-1 is the highest allowed value 10.

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The standard is very strict to detecting instrument and pretreatment method. Even some

48

sensitive equipments like ultra-performance liquid chromatography-mass spectrometry

49

(UPLC-MS) and gas chromatography-mass spectrometry (GS-MS) are hard to achieve

50

such high level standard. During detecting trace pollutants, the process of pretreatment

51

takes the most time and produces the maximum error. Hence enhancing the sensitivity of

52

pretreatment method is the best practicable means.

1, 2

.

3-6

. Fluzinam is classified as high toxicity and

53

In-situ ionic liquid dispersive liquid-liquid microextraction (in-situ IL-DLLME) is a

54

novel pretreatment method to extract trace target determinand in complex substances. In

55

this approach, a hydrophilic IL is dissolved in an aqueous sample solution, and then an 3



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anion exchange reagent for example lithium bis[(trifluoromethyl)sulfonyl]imide (LiNTf2)

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is added to the solution to form fine droplets of the hydrophobic IL extraction solvent

58

which can be easily separated from the aqueous solution by the process of centrifugation

59

or adsorption and desorption with magnetic nanoparticles (MNPs) Fe3O4

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method has many advantages like short processing time, high enrichment factor, high

61

sensitivity and the very few amount of organic solvent usage. Therefore, in-situ

62

IL-DLLME has been widely used for pretreatment methods such as drug purification 13,

63

14

11, 12

. This

, pesticide residues testing 15 and toxic substances detecting in cosmetics 16.

64

Ionic liquid itself is constituted by cation and anion, any variation of these two parts

65

will change its physicochemical characteristic, which differentiate it from conventional

66

organic solvent such as negligible vapor pressure, wide liquid range, excellent thermal,

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chemical and electrochemical stability, wide electrochemical potential window, and

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superb solubility for a wide range of organic, inorganic, organometallic and biological

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substances

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Dication IL (DIL) consists of a doubly charged cation linked by a spacer which is

71

usually an alkyl chain

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widely used in pretreatment field where ILs act as a green extraction solvent. Generally,

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the cations of MILs used as extraction solvents are 1-alkyl-3-methylimidazolium,

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1-alkyl-1-methylpyrrolidinium, 1-alkyl-1-methylpiperidinium, quaternary phosphonium,

75

and quaternary ammonium and their anions mostly are hexafluorophosphate,

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bis[(trifluoromethane)sulfonyl]imide, and dicyanamide. These MILs have been

77

successfully used to extract trace contaminant such as carbamate, pyrethroid and

78

benzoylurea pesticides, aliphatic hydrocarbons, polycyclic aromatic hydrocarbons,

79

phenols, esters, lead(II), sulfonamides 15, 21-28. Similarly, DILs are also developed to use

80

as extraction solvents, for example, 1,3-(propyl-1,3-diyl)bis(3-methylimidazolium)

17-19

. Monocation IL (MIL) is composed by one cation and one anion.

20

. The excellent physicochemical properties of ILs make them

4


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bis[(trifluoromethane)sulfonyl]imide has a good efficiency on extracting lead(II) from

82

the water samples

83

analytes. However, the relationship between structure of IL and enrichment ability to

84

analytes is still unclear.

11

. Various structures of ILs showed various abilities to extract

85

In this work, in order to explore the relationship between IL structure and enrichment

86

ability to trace pesticides from environmental water sample, an in-situ IL-DLLME

87

combined ultrasmall superparamagnetic Fe3O4 was established as a pretreatment method

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by using the newly synthetic imidazole-based ILs as the extraction solvent. Besides,

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some basic factors like cationic IL amount, the ratio of cationic IL to LiNTf2,

90

temperature, and the amount of Fe3O4 were investigated.

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2. Materials and Methods

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2.1 Chemical and reagents

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Cyprodinil was purchased from Sigma-Aldrich (St. Louis, MO, USA). Pyrimethanil

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(98%), boscalid (98.5%), fluazinam (≥ 98%), 1-butyl-3-methylimidazolium bromide

95

[BMIm][Br],

96

1-butyl-1-methylpiperidinium

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[BPyri][Br], LiNTf2, and Fe3O4 (20-30 nm) were purchased from Aladdin Chemical

98

Reagent Corporation (Shanghai, China). 1,2-Dibromoethane, 1,3-dibromopropane,

99

1,4-dibromobutane,

1-butyl-1-methylpyrrolidinium bromide

[BMPi][Br],

1,6-dibromohexane,

bromide

[BMPyrr][Br],

n-butylpyridinium

1,8-dibromooctane,

bromide

1,10-dibromodecane,

100

1-bromobutane, 1-methylimidazole, 1-ethylimidazole, 1-propyl-1H-imidazole and

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4-methylmorpholine were obtained from Tianjin Heowns biochemical Technology Co.

102

Ltd. (Tianjin, China). Methanol and acetonitrile were HPLC-grade from Fisher

103

Scientific Company (Leicestershire, UK). Ultrapure water was from the laboratory using

104

a Milli-Q water purification system (Millipore, Co. USA). River water, pond water, and

105

lake water samples were obtained from Nansha He branch, local village, and Cuihu 5



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national urban wetland park (Haidian District, Beijing), respectively. All water samples

107

were stored in -18 °C refrigerator and filtered through 0.22 µm membranes before use.

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2.2 Apparatus

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The HPLC equipment used was Shimadzu LC-20AT HPLC system (Kyoto, Japan),

110

mainly including two LC-20ATvp high-pressure pumps, a SPD-M20Avp UV-vis

111

photodiode array detector and LCsolution Lite workstation. 1H NMR spectra were

112

recorded on a BrukerAvance DPX 300 MHz NMR spectrometer (Bruker, Germany).

113

Vortex-Genie Mixer (Scientific Industries, USA) was used for eluting target compounds.

114

To determine the zeta potential of the in-situ synthetic DILs, a laser particle size

115

analyser (Mastersizer 3000, Malvern Instruments Co. UK) was employed.

116

2.3 Preparation of standard and sample solutions

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Four fungicides were dissolved by acetonitrile to the concentration of 1000 mg L-1.

118

Then, 1 mL fungicide solution was removed by transfer pipette to the 10 mL volumetric

119

flask and diluted with acetonitrile to obtain different concentrations standard solutions.

120

All these solutions were used to draw the calibration curves which were between the

121

peak areas of HPLC-DAD and the concentrations of fungicides.

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The working solution with the concentration of 100 µg L-1 was used to optimize

123

conditions, and diluted with pure water. For nature water working solution, the pesticide

124

concentrations were 5, 25, 50 µg L-1.

125

2.4 In-situ IL DLLME

126

Approximately 10 mL of working solution (100 µg L-1) or real water sample which

127

previously water-bath warmed at 40 °C was injected into a 15 mL conical-bottom glass

128

centrifuge tube, and 200 µL of brominated cation ionic liquid aqueous solution (0.2 M)

129

was added. Followed, 400 µL of LiNTf2 aqueous solution (0.2 M) was quickly injected

130

to the mixture to generate the hydrophobic liquid (Fig.1(3)). For a better dispersion, the 6


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tube was shaken for 30 sec by hand. Afterwards, 50 mg of Fe3O4 MNPs was added into

132

the tube and then the mixture was vortex oscillated 1.0 min. Fe3O4 MNPs adsorbing ILs

133

were isolated from the mixture by a neodymium cylinder magnet. Then removing

134

supernatant, 50 µL of acetonitrile was added to the tube using a pipette to elute target

135

components. Finally, 20 µL of analyte solution was subjected to the HPLC analysis. All

136

the addition steps should be operated in the water-bath pot with a stable temperature of

137

40 °C.

138

2.5 Analysis

139

The mobile phase was acetonitrile-water with 0.1% trifluoroacetic acid in gradient:

140

acetonitrile: 0-20 min, 20-90%, 20-25 min, 90%. The flow rate was 1.0 mL min-1. The

141

injection volume was 20 µL. Under the process of optimization, all four fungicides were

142

monitored at 254 nm. At the step of detecting method LODs, the pyrimethanil, boscalid,

143

cyprodinil and fluazinam were monitored at 270, 290, 254, 254 nm respectively.

144

2.6 Validation of method

145

Enrichment factor, recovery, LODs were calculated using the formulas below 27, 29: EF=

C1 C0

(1)

Recovery=

LODs=

V1 ×C1 V0 ×C0

(2)

Sb m

(3)

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C1: the concentration of the fungicide in the elution phase

147

C0: the original concentration of the fungicide in the water sample

148

V0: the volume of the water sample (10 mL)

149

V1: the volume of the elution phase (50 µL)

150

Sb: the standard deviation of the blank signal

151

m: the slope of the calibration curve after extraction 7



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2.7 Synthesis of ILs

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Brominated monocation ionic liquid: In brief, 5.0 mmol bromobutane was added to

154

6.0 mmol 4-methylmorpholine dissolved in acetonitrile, and stirred for 12 h at 80 °C to

155

yield 1-butyl-1-methylmorpholinium bromide ([BMmo][Br])

156

recrystallized in acetonitrile/ethylacetate mixture, and dried in vacuo for 10 h at 35 °C

157

(Fig 1(1)).

158 159

30

. The salt was

Brominated dication ionic liquid: Based on previous works, DILs were synthesized 29, 31

. [C8(BIM)2][Br2]: Acetonitrile (50 mL) as solvent was added into a 100-mL flask,

160

and then mixed 1-butylimidazole (26.0 mmol) and 20.0 mmol of the 1,8-dibromooctane.

161

The mixture was heated at reflux for 24 h. After the reaction was completed, the

162

acetonitrile was evaporated off under reduced pressure and the residue was dissolved in

163

pure water. The water layer was washed with diethyl ether and concentrated. The residue

164

was dried under vacuum at 35 °C. All the brominated DILs were prepared by the

165

combination of imidazoles with different length of side chain and dibromoalkanes with

166

different length of bridge chain by this method (Fig 1(2)).

167

All ILs structures in this work are listed in the Table 1, and 1H NMR information of

168

synthetic ILs is listed in Fig S1-S12.

169

3. Results and discussion

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3.1 The effect of IL structure on enrichment ability

171

3.1.1 Monocation ILs

172

Five MILs with different rings of cation, including [BMPyrr][Br], [BMIm][Br],

173

[BMPi][Br], [BMmo][Br] and [BPyri][Br] were used to study the effect of IL structure

174

on enrichment ability. In this step, only three fungicides, boscalid, cyprodinil and

175

fluazinam can be extracted with five kinds of MILs. The enrichment efficiency of

176

fungicides was the highest using [BMIm][NTf2] followed by [BPyri][NTf2], 8


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[BMPi][NTf2], [BMPyrr][NTf2], and [BMmo][NTf2]. The results showed that all

178

recoveries of three fungicids from water were not excess 80% and imidazole-based MIL

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and pyridine-based MIL both had a better enrichment ability than other three MILs (Fig

180

2b). Accoding the ‘like dissolves like’ theory, aromatic heterocyclic [BMIm][NTf2] and

181

[BPyri][NTf2] had more similar structure to the fungicides than other three alicyclic

182

heterocyclic MILs thus showing better microextraction performance (Table S1).

183

Pyrimethanil couldn’t be extracted, and the recovery order of other three fungicides

184

from high to low was fluazinam, cyprodinil and boscalid (Fig 2b). In a similar trend, the

185

n-octanol-water partition coefficient (Kow) values of pyrmethinail, boscalid, cyprodinil

186

and fluazinam are 6.92*102, 9.12*102, 1.00*104, 1.07*104 respectively

187

[BMIm][NTf2] has been used to extract the compounds with high Kow values like

188

4-nonylphenol (Kow =4.57*105), pyrene (Kow =1.15*105) 33-35. Hence, the analytes with

189

high Kow values were preferably extracted by ILs 23.

190

3.1.2 Dication ILs

191

32

. Usually,

Eleven novel synthetic imidazolium-based DILs were used to evaluate the influence

192

of DIL structure on enrichment ability. When the bridge chain was fixed at three carbons,

193

the recoveries of fungicides directly increased from 0 to 111.7% with the length of side

194

carbon chain increasing from one to four (Fig 2a). [C3(MIm)2][Br2] couldn't enrich

195

fungicides and [C3(EIm)2][Br2] could only enrich cyprodinil and fluazinam at low

196

recoveries. [C3(PIm)2][Br2] and [C3(IpIM)2][Br2] showed the low enrichment efficiency

197

to four fungicides. While [C3(BIm)2][Br2] showed the best enrichment ability to four

198

fungicides from water. The kinds of fungicides which could be extracted by in-situ

199

IL-DLLME increased with the carbon number at side chain of DILs increasing.

200

DILs with four carbons at the side chain could efficiently extract four heterocyclic

201

fungicides from water samples (Fig 2c). DILs with longer bridge chain possessed 9



202

stronger positive charge led to stronger affinity with negative Fe3O4 MNPs than MILs

203

(Table S2). Fig.2c indicates that fungicides (cyprodinil and fluazinam) with high Kow

204

values have better recoverives than fungicides (pyrimethanil and boscalid) with low

205

Kow values. The recoveries of fungicides with low Kow values increased with the

206

increase of the number of bridge carbon reaching the maximum at 8 and then decreased.

207

DILs showed high enrichment ability to fungicides with high Kow values (at least 92%),

208

and exhibited a decreasing trend with the length of bridge chain increasing. The

209

enrichment ability of [C10(BIM)2][NTf2]2 was less than [C8(BIM)2][NTf2]2. The

210

increasing amount of hydrophobic DILs negatively affected the subsequent

211

microextraction, and resulted in an increase of the ionic strength affecting to the final

212

metathesis reaction 36. DILs with short bridge chain had better efficiency on extracting

213

fungicides with high Kow values, and DILs with long bridge chain were more suitable to

214

extract fungicides with low Kow values.

215

3.2 The influence of [C8(BIm)2][Br2] amount and the molar ratio of [C8(BIm)2][Br2]

216

to LiNTf2 on enrichment ability

217

According to the relationship between structure of ionic liquid and enrichment ability

218

to trace fungicides, [C8(BIM)2][Br2] was used as the optimal IL. To investigate the

219

influence of [C8(BIm)2][Br2] amount, different volumes (50, 100, 150, 200, 300 µL) of

220

[C8(BIm)2][Br2] aqueous solution (0.2 M) were tested when keeping the molar ratio of

221

[C8(BIm)2][Br2] to LiNTf2 at 1:2. The recoveries of four fungicides from water raised

222

with the amount of [C8(BIm)2][Br2] increasing from 50 to 200 µL, then decreased

223

(Fig.3a). As seen in Fig.3b, the optimal amount of [C8(BIm)2][Br2] aqueous solution

224

(0.2 M) was 200 µL, which corresponding to adding 20 mg of [C8(BIm)2][Br2]. Higher

225

amounts of [C8(BIm)2][Br2] resulted in decreasing of enrichment ability which could be

226

attributed to the insufficient dispersion of excessive [C8(BIm)2][Br2] in the limited 10


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sample volume 27.

228

Six different molar ratios of [C8(BIm)2][Br2] to LiNTf2 (namely 1:1, 1:1.25, 1:1.5,

229

1:1.75, 1:2 and 1:4) were examined to explore the effect of LiNTf2 on the enrichment

230

ability. The ratio of [C8(BIm)2][Br2] to LiNTf2 at 1:2 showed the best recovery and was

231

selected as the optimal ratio (Fig.3b). A possible reason for this observation could be

232

that the addition of LiNTf2 increased the ionic strength of the sample solution, resulting

233

in an increased volume of the sedimented DIL phase. Moreover, the increased viscosity

234

of the sample solution may decrease the diffusion rate of analytes into the DIL phase 32.

235

3.3 The influence of MNPs amount and temperature on extraction ability

236

The effect of MNPs amount on the enrichment ability was studied in the range of

237

20-90 mg. As can be seen in Fig 4a, the recoveries of fungicides increased with the

238

increase of the amount of Fe3O4 MNPs, which reached to a quantitative value at 50 mg

239

then decreased. This phenomenon most likely attributed to the rapid aggregation of

240

excessive Fe3O4 MNPs in water which resulted in ineffective surface charge 27, 37, 38.

241

The effect of temperature on the enrichment ability of DILs showed the recovery of

242

heterocyclic fungicide was as order of 40, 50, 30, 17 and 60 °C. The analytes need time

243

and energy to distribute into the hydrophobic DILs, and the high viscosities of these

244

DILs at low temperatures limit their real potential applications 20. Hence a relative high

245

temperature is useful to achieve good enrichment ability to fungicides

246

when the temperature was over 50 °C, the enrichment ability of DILs decreased.

247

Because higher temperature promoted the dissolution of hydrophobic DILs into water,

248

the volume of hydrophobic ILs decreased, and then part of analytes released from the

249

DIL phase, leading to lower enrichment ability. Therefore, 40 °C was selected for further

250

experiments.

251

3.4 Validation of the method 11


39

. However,


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252

Under the optimized conditions, the proposed in-situ IL-DLLME was validated for

253

LODs, linear range, determination coefficients (r2), and enrichment factor, and the

254

results are shown in Table 2. Boscalid, cyprodinil, fluazinam and pyrimethanil exhibited

255

good linearity with r2 = 0.9991, 0.9995, 1.000, 0.9990 respectively, in the range of 1-100

256

µg L−1. The LODs ranged from 0.032 to 0.89 µg L-1, recoveries range in pure water was

257

89.4-96.4% and enrichment factors were in the range of 110-199. The intra-day and

258

inter-day relative standard deviations (RSDs) were 3.06%-8.34%, 6.90%-8.34% (three

259

days, n = 6) respectively.

260

Table S3 presents the advantages of DILs in this work by comparing the proposed

261

in-situ DIL-DLLME methodology with several reported methods for extracting four

262

heterocyclic fungicides

263

produce good LODs with low ILs consumption (20 mg) within a short time. These

264

LODs values (≤ 0.1 µg L-1) of boscalid, cyprodinil and fluazinam are compliant with EU

265

drinking water standard. Besides, DILs showed a better enrichment ability to

266

pyrimethanil than other studies by simple steps.

267

3.5 Analysis of real water samples

40-47

. It can be found that the present in-situ DIL-DLLME can

268

The matrix effects were evaluated by detecting the relative recoveries that were

269

defined as the ratio of the peak areas between the spiked water samples extracts and the

270

spiked ultrapure water extracts. As the results shown in Table 3, the relative recoveries

271

varied from 77.1% to 119.6% and were little affected by matrix

272

method was finally applied to the analysis of four real water samples. The analytes

273

detected were quantified by the standard addition method. No fungicide was detected in

274

the nature water this time. Fig. 5 shows the typical chromatograms of pond and lake

275

water with in-situ DIL-DLLME optimized in this study. And Table 3 also shows the

276

RSDs range from 0.84% to 10.27% in tap, pond, river, and lake water samples. 12


47, 48

. The validated

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277

3.6 Conclusions

278

In this work, eleven novel synthetic imidazolium-based DILs and five common MILs

279

were used to elucidate the relationship between structure of ionic liquid and ability to

280

trace pesticides from environmental water sample. The results showed that aromatic

281

heterocyclic imidazole and pyridinium based MILs had better ability on enriching

282

fungicides than other three alicyclic heterocyclic MILs. Dication ionic liquids (DILs)

283

showed better extraction efficiency of fungicides than MILs by the step of adjusting the

284

length of side or bridge chain. With increasing of the carbon number at side chain of

285

DILs, the kind of fungicide could be extracted by in-situ IL-DLLME increased. When

286

four carbons fixed at the side chain, DIL with short bridge chain had better efficiency on

287

extracting fungicides with high Kow values, and a DIL with long bridge chain was more

288

suitable for fungicides with low Kow values.

289

The optimization in-situ IL-DLLME process was as followed: [C8(BIM)2][Br2] (200

290

µL, 0.20 M, 20 mg) reacted in situ with LiNTf2 (400 µL, 0.20 M) forming hydrophobic

291

IL ([C8BIM][N(Tf)2]2) as the extraction agent, and 50 mg of Fe3O4 was added to water

292

samples, all addition steps were operated at 40 °C water bath. Finally, 50 µL acetonitrile

293

was injected as the diluent. Under these optimized conditions, high recoveries in pure

294

water (89.4-96.4%) and real water (77.1-119.6%), short pretreatment time (

Relationship between the Structure of Ionic Liquid and Its Enrichment

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