Electronic Supplementary Material
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Polymer-coated magnetic nanospheres for preconcentration of organochlorine and
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pyrethroid pesticides prior to their determination by gas chromatography with electron
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capture detection
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Genggeng Yang, Zeying He, Xueke Liu, Chang Liu, Jing Zhan, Donghui Liu, Peng
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Wang, Zhiqiang Zhou*
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Beijing Advanced Innovation Center for Food Nutrition and Human Health, Department of
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Applied Chemistry, China Agricultural University, Beijing 100193, P.R. China
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*Corresponding
author:
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zqzhou@cau.edu.cn
Tel:
+8610-62733547;
Fax:
+8610-62733547;
E-mail:
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Optimization of method
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Single-factor experiment and Plackett–Burman design with two levels of each factor
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(Table S1) were used to optimize the parameters. After the experimental factors and levels
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were established, Box–Behnken design was conducted to investigate the optimum conditions
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of the extraction process. In the all optimization experiments, the amounts of OCPs and
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pyrethroids in the spiked water were 50 ng and 100 ng, respectively. Recovery was used to
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assess the extraction efficiency. Minitab16 software was used to design the experiment and
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analyze data.
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Table S1 Experimental variables and levels of the Plackett–Burman design.
Variable
NaCl concentration (%)
Sample volume (mL)
Extraction time (min)
Volume of desorption solvent (mL)
Desorption time (s)
Key
A
B
C
D
E
Low
0
30
5
0.5
15
Level
High
20
200
30
1.5
60
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Selection of eluting solvent
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Eluting solvent is a vital factor that affects desorption efficiency. Because of the
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discontinuity of the variate, selection of eluting solvent was considered separately. Five kinds
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of solvents, including acetone, methanol, ethyl acetate, methylene chloride and acetonitrile
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were investigated. The results showed ethyl acetate had the best recoveries compared with
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other solvents, especially for OCPs (Fig. S1), and the interferences on the chromatographic
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analysis were found not disturbing the signal of target analytes. Thus, ethyl acetate was
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selected as eluting solvent.
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Screening experiment
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Plackett–Burman design was used to screen the factors that affected the recoveries of
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target analytes, including NaCl concentration, sample volume, extraction time, volume of
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desorption solvent and desorption time. Each factor was set at two levels. The adsorption
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mechanism between this adsorbent and target analytes are hydrophobic interaction and π-π
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interaction. According to the octanol-water partition (Kow) and chemical structure (whether
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has benzene ring or not), the target analytes were divided into two groups: group 1, beta-HCH,
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delta-HCH, heptachlor, trans-chlordane, cis-chlordane, and group 2, p,p'-DDE, bifenthrin,
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beta-cypermethrin, deltamethrin, lambda-cyhalothrin, esfenvalerate. As shown in Fig. S2, the
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absolute value of the t value of each factor was used as the ordinate. When the absolute value
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was greater than the critical value (2.306), it meant the effect was significant. All the factors
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had impact on the extraction efficiency were investigated. For OCPs, extraction time and
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sample volume affected the extraction significantly. For pyrethroids, NaCl concentration,
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sample volume and extraction time were more important. So, NaCl concentration, sample
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volume and extraction time were optimized in the following experiments. Volume of the
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desorption solvent was set at 1mL, and desorption time was set at 30 s.
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Optimization design
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Box–Behnken design was used for the optimization of selected factors (NaCl
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concentration, sample volume and extraction time). An experimental scheme was designed by
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Minitab16 software, including 12 tests and three repetitions at the central point for three
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independent variables. A polynomial equation was used to express the design:
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Y= a + b·X1 + c·X2 + d·X3 + e·X1·X2 + f·X1·X3 + g·X2·X3 + h·X12 + i·X22 + j·X32
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Where X1, X2, X3 are the independent factors, a-j are the coefficients of the polynomial
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equation, and Y is the response. delta-HCH and beta-cypermethrin were used as representative
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analytes for the optimization experiments (Table S2). Analysis of variance (ANOVA) was
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employed to assess the data of optimization. The lack of fit test showed that p-value of
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delta-HCH (0.060) and beta-cypermethrin (0.471) were greater than the level of significance
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(0.05) suggesting the model can predict the recoveries of delta-HCH and beta-cypermethrin.
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The coefficient (R2) was similar to the adjusted R-square, indicating the model was reliable.
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The coefficients of equations were evaluated with p-value. If p-value was less than 0.05, it
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meant the parameter had significant effect on the recoveries. After optimizing, the modified
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equations were acquired and listed in Table S2.
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Table S2 Coefficient of determination (R2), adjusted R-square and second-order coded model
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equations for the recoveries of delta-HCH and beta-cypermethrin.
R2
Adjusted
(%)
R-square
97.25
92.30
Y=116.4-0.50X2+2.1X3-0.055X32
beta-cypermethrin 91.85
77.17
Y=71.3-0.38X1-0.032X2+1.39X3
Analyte
delta-HCH
Second-order coded model equations
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The response surface methodology was used to intuitively analyze the optimization data
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and the results were shown in Fig. S3-8. In case of delta-HCH, the response surface for NaCl
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concentration versus extraction time was shown in Fig. S3. When the extraction time was in
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the range of 15-25 min with NaCl concentration in the range of 15-20 %, the recovery was the
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best. Fig. S4 showed the response surface for water volume versus extraction time. Best
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recovery was obtained when the extraction time was in the range of 15-30 min with sample
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volume less than 75 mL. Fig. S5 showed the response surface for NaCl concentration versus
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volume of water. Good recoveries were obtained when the sample volume was in the range of
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50-75 mL. NaCl concentration was not important for the recovery. In conclusion, the optimal
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conditions for extraction of delta-HCH form water sample were: NaCl concentration was
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15-20 %; sample volume was 50-75 mL; extraction time was 15-30 min. Similarly, the
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optimal conditions for beta-cypermethrin were: NaCl concentration was 15 %; sample volume
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was 50-75 mL; extraction time was 25-30 min. In order to consider the recoveries of both
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analytes, the optimal condition weres: NaCl concentration was 15 %; sample volume was 50
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mL; extraction time was 30 min.
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Fig. S1 Effect of eluting solvent on the extraction effficiency of organochlorine and
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pyrethroid pesticides.
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Fig. S2 Pareto charts of the main effects of (a) organochlorine and (b) pyrethroid pesticides in
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the Plackett–Burman design. The black bar means the variable is negative correlation and the
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gray bar means the variable is positive correlation.
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Fig. S3 The response surface of delta-HCH for NaCl concentration versus extraction time.
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Fig. S4 The response surface of delta-HCH for water volume versus extraction time.
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Fig. S5 The response surface of delta-HCH for NaCl concentration versus water volume.
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Fig. S6 The response surface of beta-cypermethrin for NaCl concentration versus extraction
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time.
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Fig. S7 The response surface of beta-cypermethrin for water volume versus extraction time.
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Fig. S8 The response surface of beta-cypermethrin for NaCl concentration versus water
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volume.