Key Words: QuEChERS; Tobacco; Pesticide residues; On-line gel permeation chromatography; Gas chromatography/tandem mass spectrometry

Transcription

1 CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 43, Issue 10, October 2015 Online English edition of the Chinese language journal Cite this article as: Chin J Anal Chem, 2015, 43(10), RESEARCH PAPER Determination of Pesticide Residues in Tobacco Using Modified QuEChERS Procedure Coupled to On-line Gel Permeation Chromatography-Gas Chromatography/ Tandem Mass Spectrometry LUO Yan-Bo 1, ZHENG Hao-Bo 2, JIANG Xing-Yi 1, LI Xue 1, ZHANG Hong-Fei 1, ZHU Feng-Peng 1, PANG Yong-Qiang 1, *, FENG Yu-Qi 2 1 China National Tobacco Quality Supervision and Test Center, Zhengzhou , China 2 College of Chemistry and Molecular Science, Wuhan University, Wuhan , China Abstract: Magnetic graphitized carbon black/primary secondary amine/magnetite (GCB/PSA/Fe 3 O 4 ) composite material was used as modified QuEChERS adsorbent for the clean-up of tobacco extract. A method for the determination of ten pesticide residues in tobacco was proposed by coupling the modified QuEChERS procedure to on-line gel permeation chromatography-gas chromatography/tandem mass spectrometry (GPC-GC-MS/MS). Several parameters affecting clean-up efficiency were investigated. Under optimized conditions including min of on-line GPC collection time, 80 mg of adsorbent amount, and 1.0 min of clean-up time, the limits of detection for target analytes ranged from ng L 1 to 100 ng L 1. The linear regression data of the proposed method were obtained with correlation coefficients for all target analytes. The relative standard deviations for both intra- and inter-day were less than 15.1% and 19.8%, respectively. The recoveries were in the range of 68.8% 132.2% for real samples. Finally, the method was successfully applied to the analysis of pesticide residues in real samples, and the results were in agreement with those obtained from the current standard method. Key Words: QuEChERS; Tobacco; Pesticide residues; On-line gel permeation chromatography; Gas chromatography/tandem mass spectrometry 1 Introduction Pesticides are routinely used to control pests and diseases during the breeding, cultivation, curing and storage of tobacco. However, pesticide residues not only reduce the quality of tobacco leaf, but also bring about potential risks to human health, animals and environment [1]. So it is important for determination of pesticide residues in tobacco. However, the concentration level of pesticide residues in tobacco was not high (ng g 1 level), and matrix interference was often occurred. Therefore, sample preparation procedures are usually essential [2]. Up till now, liquid-liquid extraction [3], solid-phase extraction [4] and solid-phase microextraction [5] were reported for determination of pesticide residues in tobacco. However, the above sample preparation techniques have some disadvantages more or less, such as large volume of organic solvent consumption, tedious operation, time-consuming and low recovery. As an alternative technique, QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) method was proposed by Anastassiades and Lehotay in 2003 [6]. QuEChERS technique shows good relative standard deviations (RSDs) values in real samples, as well as higher Received 3 February 2015; accepted 12 May 2015 * Corresponding author @qq.com This work was supported by the National Natural Science Foundation of China (No ) and the Science Innovation Foundation of China National Tobacco Quality and Supervision & Test Center (No CA0110). Copyright 2015, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: /S (15)

2 and more consistent recoveries of target analytes than liquid-liquid extraction. So it was widely applied to the determination of pesticide residues [3]. The traditional QuEChERS technique involves a troublesome and tedious centrifugation or filtration procedure to separate or retrieve the adsorbents. To overcome the aforementioned shortcoming, a modified QuEChERS procedure was reported by using magnetic graphitized carbon black/primary secondary amine/ magnetite (GCB/PSA/Fe 3 O 4 ) as adsorbent [7]. The retrieval of adsorbent could be realized by external magnetic field, so it has the advantages such as simple operation and high efficiency. As a powerful analytical method, on-line gel permeation chromatography-gas chromatography-mass spectrometry (on-line GPC-GC-MS) was an automatic analysis technique by combining gel permeation chromatography with gas chromatography-mass spectrometry. The use of on-line GPC-GC-MS for the analysis of pesticide residues was reported with excellent advantages such as strong antiinterference ability, high sensitivity and continuous automatic on-line analysis [8 10]. In this study, the tobacco was cleaned-up by the modified QuEChERS method, and then determined by on-line GPC-GC-MS/MS. By selecting ten widely used pesticides including herbicide, bud inhibitor and insecticide as model analytes, a rapid and accurate method was proposed for the determination of pesticide residues in tobacco. 2 Experimental 2.1 Instruments and reagents On-line GPC-GC-MS/MS system (Shimadzu, Japan) consisted of GPC, 2010-plus GC and TQ 8030 MS/MS. The GPC consisted of binary LC-20AD pumps, a SIL-20A autosampler, a CTO-20AC column oven, a DGU-20A 3R degasser and a SPD-20A ultraviolet detector. GC-MS/MS consisted of QP2010 Plus gas chromatograph and a TQ8030 tandem mass spectrometer. FCV-12AH flow channel selection valves were used for on-line connection of GPC and GC-MS/MS. Scanning electron microscopy (SEM) images were taken using a Quanta 200 scanning electron microscope (FEI, Holland). Cyclohexane, acetone and toluene were of chromatographic grade and purchased from Duksan Pure Chemicals Co., Ltd. (Gyeonggi-do, Korea). Pure water used throughout the study was purified through a Milli-Q system (Milford, MA, USA). Certified standards and internal standard (triphenyl phosphate, TPP) were purchased from Dr. Ehrenstorfer (Augsburg, Germany). Individual pesticide stock standard solutions (100 μg ml 1 ) and TPP standard solution (20 μg ml 1 ) were prepared in toluene (acetone or methanol was added when necessary). A multistandard mixture was prepared by dilution of each pesticide stock standard solution with toluene. The working calibration standard solutions were prepared by appropriate dilution of multistandard mixture with toluene in 10-mL flasks containing 100 μl of TPP standard solution. The all standard solutions were stored at 20 C in the dark till be used. Fe 3 O 4 nanoparticles were synthesized via a solvothermal process [7]. Briefly, Ferric chloride hexahydrate (5.0 g) was dissolved in ethylene glycol (100 ml). Sodium acetate (15.0 g) and ethylene diamine (50 ml) were added to the solution. After being vigorously stirred for 30 min, the homogeneous mixture was sealed in a Teflon lined stainless-steel autoclave (200-mL capacity). The autoclave was heated to 200 C and maintained for 8 h, and then allowed to cool to room temperature. The product was magnetically collected, and washed with water/ethanol for several times. The washed product was then vacuum-dried at 60 C for 6 h. The magnetic adsorbent was fabricated via a simple co-mixing method. Briefly, Fe 3 O 4 (1000 mg), PSA (500 mg) and GCB (400 mg) were placed in a 15-mL vial. After adding 5 ml of acetonitrile, the magnetic adsorbent dispersive solution was obtained by vortexing vigorously for 1 min. An external magnet was attached to the outside bottom of the vial and the magnetic adsorbent was gathered to the bottom of the vial. After the supernatant was discarded, the product was washed with acetonitrile for several times. The washed product was then vacuum-dried at 60 C overnight. 2.2 Instrumental conditions GPC column of Shodex CLNpak EV-200 (150 mm 2.0 mm i.d., 16 μm) was purchased from Shoko Co., (Tokyo, Japan). The column oven temperature was maintained at 40 C. Cyclohexane/acetone mixing solvent (70:30, V/V) was used as the GPC mobile phase, and the flow rate was set at 0.1 ml min 1. The GPC sample injection volume was 10 μl. GPC eluent fraction (200 μl, collection time was from 3.30 to 5.30 min) containing all target analytes was totally online transferred into GC injector for GC-MS/MS analysis. For the GC-MS/MS analysis, the GC system was equipped with a large volume programmed temperature vaporizer injector (PTV-2010 injector), a retention gap (5 m 0.53 mm i.d., deactivated capillary column), a retention pre-column (DB-35 ms, 5 m 0.25 mm i.d., film thickness 0.25 μm), a separation column (DB-35 ms, 25 m 0.25 mm i.d., film thickness 0.25 μm) and a solvent vapor exit (SVE). The oven temperature was programmed at 82 C for 5 min, increased to 300 C at a rate of 8 C min 1 and held for 7.75 min, and the total GC analysis time was 40 min. Splitless injection mode was used and sampling time was set as 7.0 min. Carrier gas (helium) pressure was initially set at 120 kpa, and increased to 180 kpa at a rate of 100 kpa min 1 and held for 4.4 min, and then decreased to 120 kpa at a rate of 49.8 kpa min 1 and held

3 LUO Yan-Bo et al. / Chinese Journal of Analytical Chemistry, 2015, 43(10): for 33.8 min. The ion source and interface temperatures were 200 and 300 C, respectively. MS was operated in EI mode with standard electron energy of 70 ev. The solvent cut time was 15 min. Argon was used as collision gas with a pressure of 200 kpa. Target analytes were quantified by multiple reaction monitoring (MRM) mode. The parameters of MRM mode and retention time (tr) for the target analytes and TPP were shown in Table 1. The morphology of PSA and GCB/PSA/Fe3O4 were examined by SEM. Figure 1a showed a SEM image of PSA with irregular shape and clear and smooth surface. Figures 1b and 1c were morphology images of magnetic adsorbent with different magnifications, which indicated the components of materials were dispersed evenly. The presence of Fe3O4 endows the adsorbent magnetic merit, leading to the convenient separation or retrieval from dispersion Sample preparation The purpose of this study was to establish a method for the determination of pesticide residues in tobacco, so the optimized extraction method of tobacco reported in previous study was directly adopted[2]. Tobacco sample (2.00 g) was weighed into centrifuge tube, and 10 ml of pure water was added. It was shaken for 30 s and then left to stand for 10 min. After that, 10 ml of acetonitrile and 100 μl of 20 μg ml 1 TPP standard solution were added, and the tube was vortexed for 2 min and frozen at 20 C for 10 min. Then 4 g of anhydrous magnesium sulfate, 1 g of sodium chloride, 1 g of trisodium citrate dehydrate and 0.5 g of disodium hydrogen citrate were simultaneously added and the mixtures was immediately hand shaken for 30 s. After 5 ml of toluene was added, the tube was vortexed for 2 min. The upper layer was collected after the centrifugation step (4000 rpm, 5 min). For the following clean-up step, 0.5 ml of extract was added into an Eppendorf vial (1.5 ml) containing magnetic adsorbent, and then the mixture was shaken vigorously for 1 min. The supernatant was collected with the aid of an external magnet and was supplied to on-line GPC-GC-MS/MS for analysis. For the preparation of matrix-matched working standard solution, 0.5 ml of blank tobacco (pesticide-free tobacco) supernatants were evaporated to dryness under a gentle stream of nitrogen at 35 C, and then reconstituted with 0.5 ml of working standard solutions with different concentrations Results and discussion Characterization of magnetic adsorbent GPC collection time optimization Figure 2 shows the schematic diagram of the GPC-GC-MS/ MS system. The working principle was as following: the sample was injected into the GPC column by autosampler, and then the target analytes were separated by the GPC column. The portion of GPC eluent containing all target analytes was totally transferred into GC PTV injector for GC-MS/MS analysis. The GPC eluent (200 μl in this study) was introduced to retention column at the temperature (82 C in this study) below the solvent boiling point. The target analytes were distributed throughout the sample layer to form a flooded zone. The solvent started to evaporate at the rear end of the flooded zone and was released to an activated carbon absorption tube through opening solvent vapor exit. The analytes would be finally refocused by the stationary phase of Table 1 Parameters of MRM mode and retention time (tr) for target analytes and TPP Trifluralin Benfluralin Tefluthrin Butralin Isopropalin Pendimethalin Flumetralin Nitrofen Bifenthrin Fenpropathrin TPP tr (min) Ion pair, m/z, (collision energy, ev) Quantitative Qualitative 306.1>264.1 (8) 292.1>264.0 (8) 177.0>127.1 (16) 266.1>190.1 (8) 280.0>238.0 (10) 252.1>162.1 (10) 143.0>107.0 (20) 282.9>162.0 (24) 181.1>166.1 (12) 265.1>210.1 (12) 326.0>325.0 (10) 306.1>206.1 (14) 292.1>206.0 (12) 177.0>137.1 (16) 266.1>236.1 (12) 280.0>180.0 (15) 252.1>191.1 (8) 143.0>108.0 (20) 282.9>253.0 (12) 181.1>153.1 (8) 265.1>172.1 (14) 326.0>233.0 (10) Fig.1 SEM images of (a) primary secondary amine (PSA) and (b, c) GCB/PSA/Fe3O4 with different magnifications

4 Fig.2 Schematic diagram of on-line GPC-GC-MS/MS system pre-column. After the solvent was evaporated, this procedure would be terminated. At the same time, the GC oven temperature began to increase and the target analytes were transferred to the GC separation column for analysis. It is essential for GPC separation to obtain a fraction containing all target pesticides. To investigate the optimal t R of target pesticides, fluvalinate and chinomethionate were selected as the marker molecules because the molecular weights (M W ) of almost all target pesticides fall in the M W range of them. The retention time of them was 3.50 and 5.08 min, respectively. To ensure all target analytes were contained in the GPC fraction, GPC eluent from 3.30 min to 5.30 min was collected by the sample loop and online delivered to the GC-MS/MS system for analysis. 3.3 Optimization of modified QuEChERS procedure To assure the clean-up efficiency towards the target analytes, the adsorbent amount (mass ratio of GCB:PSA:Fe 3 O 4 was 4:5:10) and clean-up time were investigated. As shown in Fig.3a, with the adsorbent amount increased, the color of final tobacco sample became lighter. When the amount of adsorbent was increased to 80 mg, the color has no significant change with the increasing adsorbent amount. In addition, no obvious change of peak areas of the target analytes was observed in the tested range. Considering the clean-up efficiency and consumption of magnetic adsorbent, 80 mg of magnetic adsorbent was adopted as the optimum amount. As the clean-up time increased from 0.5 min to 6.0 min, there was no obvious difference in the clean-up efficiency and peak areas of target analytes. It was attributed to that the magnetic adsorbent was dispersed in the sample solution, so the clean-up balance could be reachied in a quite short time. Considering the clean-up efficiency and time consumed in sample preparation process, 1.0 min of clean-up time was adopted. To testify the clean-up efficiency of the proposed QuEChERS procedure, it was compared with traditional QuEChERS method. As shown in Fig.3b, the initial color of tobacco extract was yellow, and the color was changed to light yellow after treated by traditional QuEChERS method, however, the color looked almost transparent after treated by the proposed QuEChERS method. It is concluded that the proposed QuEChERS method displayed a better clean-up performance than traditional QuEChERS method to remove pigment in tobacco. In addition, the centrifugation or filtration step which was used in traditional QuEChERS method was omitted in the proposed method, so it also was simple and rapid. Fig.3 Effects of adsorbent amount on clean-up efficiency (a); photographs of tobacco sample obtained by different clean-up methods: (b I) extract for tobacco without clean-up step, (b II) extract for tobacco treated with traditional QuEChERS method, (b III) extract for tobacco treated with the proposed QuEChERS method in this study

5 3.4 Validation of the method Under the optimized conditions, a serial of experiments with regard to the limit of detection (LOD), limit of quantification (LOQ), linearity and reproducibility were performed to validate the proposed QuEChERS method. The LODs and LOQs were calculated as the concentration corresponding to the signals of 3 and 10 times of the standard deviation of the baseline noise, respectively. The linear regression analysis was performed from the matrix-matched standard calibration solutions by plotting the mean peak area ratios (y, target analytes/tpp) versus concentrations of the respective analytes (x). The LODs, LOQs and linear regression data are listed in Table 2. The LODs and LOQs for the target analytes were found to be ng L 1 and ng L 1, respectively. Good linearity in linear ranges with correlation coefficient values (R 2 ) greater than are obtained. The extraction method in standard [11] was same to this study, and the LODs for 9 target analytes (except fenpropathrin) were in the range of ng L 1. Due to the reduced matrix interference and large injection volume of on-line GPC-GC-MS/MS, the sensitivity of the proposed method was significantly improved. The intra-day and inter-day RSDs were calculated with the target analytes spiked at three different concentration levels (the spiked amounts of low, medium and high concentration levels were corresponding to 2, 10 and 100 times of lowest concentration point of individual linear range) in tobacco. Four parallel extractions over a day gave the intra-day RSDs, and the inter-day RSDs were determined by extracting sample solutions that had been independently prepared for three contiguous days. The results are summarized in Table 3, the intra- and inter-day RSDs were less than 15.1% and 19.8%. Among them, the RSDs of target analytes at high and medium concentration levels were less than 8.5%, and the RSDs of target analytes at low concentration level were relatively higher. As described by Association of Official Analytical Chemists [12], acceptable RSDs values were increased with the decrease of concentration level. In this study, the RSDs values of target analytes at low concentration level ( ppb) were lower than 21% (the acceptable RSDs values when the concentration level of target analyte was 10 ppb), illustrating the acceptable reproducibility achieved by the proposed QuEChERS method. 3.5 Real samples analysis To demonstrate the applicability of the proposed QuEChERS method, three tobacco samples were tested. As a comparison, the samples were also treated by traditional QuEChERS method [11]. Tefluthrin, pendimethalin and bifenthrin were detected in sample A, and the concentrations were all lower than respective LODs by the two methods. Tefluthrin was detected in sample B, and the concentrations obtained by Table 2 Linear range, regression data, correlation coefficients (R 2 ), limits of detection (LODs) and limits of quantitation (LOQs) of target analytes Linear range Calibration curves (ng ml 1 ) Slope (1/10 3 ) Intercept (1/10 3 ) R 2 LOD (ng L 1 ) LOQ (ng L 1 ) Trifluralin Benfluralin Tefluthrin Butralin Isopropalin Pendimethalin Flumetralin Nitrofen Bifenthrin Fenpropathrin Table 3 Method precisions at three different concentrations for target analytes Intra-day precision RSD (%, n = 4) Inter-day precision RSD (%, n = 3) Low Medium High Low Medium High Trifluralin Benfluralin Tefluthrin Butralin Isopropalin Pendimethalin Flumetralin Nitrofen Bifenthrin Fenpropathrin The concentration of the targets in test solution at the low, medium and high levels were 2, 10 and 100 times in linear range of the lowest value as in Table 2, respectively.

6 the proposed QuEChERS method and traditional QuEChERS method were 1.06 and 1.03 ng ml 1, respectively; the concentrations of flumetralin and bifenthrin were all lower than respective LODs by the two methods. Butralin, pendimethalin and flumetralin were detected in sample C, and the concentrations obtained by the proposed QuEChERS method and traditional QuEChERS method were 92.1 and 92.7 ng ml 1, and ng ml 1, 3.68 and 3.70 ng ml 1, and the concentration of tefluthrin was lower than its LOD by the two methods. The results indicated that there was no difference between the two methods. Compared with other real samples, the highest variety of target analytes was detected for sample C, so the typical extracted ion chromatogram obtained by the proposed QuEChERS method was shown in Fig.4. The interferences from the matrix were removed from the target analytes. Meanwhile, the tobacco extract without clean-up was also directly tested by the on-line GPC-GC-MS/MS, and the concentration of tefluthrin in sample B was 1.01 ng ml 1, and the concentrations of butralin, pendimethalin and flumetralin in sample C were 92.8, and 3.81 ng ml 1, respectively, which were consistent with the result obtained by the methods abovementioned. Furthermore, the recoveries of target analytes in three real samples were determined by comparing the calculated amounts of target analytes from the spiked tobacco samples with the total spiked amounts. The results listed in Table 4 showed that the recoveries of the target analytes from the three real samples were in the range of 68.8% 132.2%. Due to the different matrix in various tobaccos, the recoveries of target analytes in real samples were different. The matrix enhancement effect and matrix suppression effect were all investigated for the determination of pesticide residues in tobacco by GC-MS/MS [13], in which matrix enhancement effect was observed for polar amino compounds (such as pendimethalin and bifenthrin, the highest recovery was 132.2%). Due to the different physicochemical properties, the matrix effects of analytes with same functional group were different (such as tefluthrin, the lowest recovery was 68.8%). In addition, the matrix effects of analytes at different concentration levels in samples were also different [14] (such as fenpropathrin). 4 Conclusions Magnetic adsorbent was used as modified QuEChERS adsorbent for the clean-up of tobacco extract. Under the optimized GPC-GC-MS/MS and clean-up parameters, a method for the determination of 10 pesticide residues in tobacco was established by coupling the modified QuEChERS procedure to on-line GPC-GC-MS/MS. Finally, the method was successfully applied to the analysis of pesticide residues in real tobacco samples. Magnetic adsorbents could be rapidly retrieved under external magnetic field; meanwhile, the on-line GPC-GC-MS/MS could increase injection volume, so the proposed method has advantages of simple operation, satisfactory clean-up performance and high sensitivity. Taken together, the proposed method may serve as a promising alternative to the present pesticide residues determination methods available. Fig.4 Typical extracted ion chromatogram of real sample Table 4 Recoveries at three different concentration levels for determination of target analytes in real samples Sample A (%) Sample B (%) Sample C (%) Low Medium High Low Medium High Low Medium High Trifluralin Benfluralin Tefluthrin Butralin Isopropalin Pendimethalin Flumetralin Nitrofen Bifenthrin Fenpropathrin The concentration of the targets in test solution at the low, medium and high levels were 2, 10 and 100 times in linear range of the lowest value as in Table 2, respectively.

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