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Novel Zeolitic Imidazolate Frameworks Based on Magnetic Multiwalled Carbon Nanotubes for Magnetic Solid-Phase Extraction of Organochlorine Pesticides from Agricultural Irrigation Water Samples

Novel Zeolitic Imidazolate Frameworks Based on Magnetic Multiwalled Carbon Nanotubes for Magnetic... applied sciences Article Novel Zeolitic Imidazolate Frameworks Based on Magnetic Multiwalled Carbon Nanotubes for Magnetic Solid-Phase Extraction of Organochlorine Pesticides from Agricultural Irrigation Water Samples 1 2 2 2 2 Xiaodong Huang , Guangyang Liu , Donghui Xu , Xiaomin Xu , Lingyun Li , 2 2 1 , Shuning Zheng , Huan Lin and Haixiang Gao * Department of Applied Chemistry, China Agricultural University, Beijing 100193, China; huangxiaodong@caas.cn Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, Key Laboratory of Vegetables Quality and Safety Control, Ministry of Agriculture and Rural Affairs of China, Beijing 100081, China; liuguangyang@caas.cn (G.L.); xudonghui@caas.cn (D.X.); hsuixiaomin@gmai.com (X.X.); lilingyun@caas.cn (L.L.); zhengshuning@caas.cn (S.Z.); linhuan03@caas.cn (H.L.) * Correspondence: hxgao@cau.edu.cn; Tel.: +86-10-62731991 Received: 18 May 2018; Accepted: 5 June 2018; Published: 12 June 2018 Abstract: Magnetic solid-phase extraction is an effective and convenient sample pretreatment technique that has received considerable interest in recent years. A lot of research indicated that magnetic nanocarbon-material-based composites have good application prospects as adsorbents for magnetic solid-phase extraction of pesticides. Herein, a novel zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes (M-M-ZIF-67) has been prepared as an adsorbent for magnetic solid-phase extraction of nine organochlorine pesticides from agricultural irrigation water samples. The obtained M-M-ZIF-67 material possessed porous surfaces and super-paramagnetism due to the utilization of magnetic multiwalled carbon nanotubes as the magnetic kernel and support. To evaluate the extraction performance of the M-M-ZIF-67, the main parameters that affected the extraction efficiency were researched. Under the optimal conditions, a good linearity for the nine organochlorine pesticides was achieved with the determination coefficients (R ) higher than 0.9916. The limits of detection (signal/noise = 3:1) were in the range 0.07–1.03 g L . The recoveries of all analytes for the method at spiked levels of 10 and 100 g L were 74.9–116.3% and 75.1–112.7%, respectively. The developed M-M-ZIF-67 based magnetic solid-phase extraction method has a potential application prospect for the monitoring of trace level of organochlorine pesticides in environmental water samples. Keywords: zeolitic imidazolate framework; multi-walled carbon nanotubes; magnetic solid phase extraction; organochlorine pesticides; agricultural irrigation water 1. Introduction Sample pretreatment is a crucial step in analysis of trace or ultra-trace amounts analytes in complex matrices. Solid-phase extraction (SPE) is a type of widely used pretreatment for effective concentration of analytes in complex matrices before instrumental analysis [1]. A variety of pretreatment methods has been developed based on this technique, including solid-phase microextraction (SPME) [2], micro-SPE (-SPE) [3], and stir-bar sorptive extraction (SBSE) [4]. Magnetic solid-phase extraction (MSPE), as a new type of SPE, is a pretreatment method that has received considerable interest in recent years. In this technique, magnetic adsorbents are directly Appl. Sci. 2018, 8, 959; doi:10.3390/app8060959 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 959 2 of 15 dispersed into sample solutions, and this dispersive extraction mode can enhance the contact area between adsorbents and analytes [5]. Notably, magnetic adsorbents can be separated from the sample solutions under an external magnetic field without the need of traditional centrifugation or filtration, thereby simplifying the extraction process. Furthermore, magnetic adsorbents can be recycled and reused easily, which is cost effective and environmentally friendly [6]. Therefore, the MSPE technique shows comprehensive advantages of simplicity, time, reagent and labor savings, and excellent extraction efficiency, which meet the principles of green analytical chemistry [7]. The diversity of the materials used in MSPE is the main factor that has led to extensive development of this technique in recent decades [8]. Multiple magnetic sorbents have been synthesized by embedding magnetic cores in different organic or inorganic materials, such as chitosan [9], ionic liquids [10], polymers [11], silica [12], metallic oxides [13], molecularly imprinted polymers [14], and carbon nanomaterials [15,16]. Multiwalled carbon nanotubes (MWCNTs) are formed by seamless rolling up of several layers of graphite sheets. Because of their excellent properties, such as high surface area and inner volume, stability, mechanical strength, ability to establish – interactions, and capacity for functionalization, MWCNTs have the possibility of acting as good sorbents [17]. MWCNTs have recently attracted considerable interest as adsorbents in MSPE for extracting different analytes, such as antibiotics [18], estrogens [19], mycotoxins [20], metal ions [21], environmental pollutants [22], and pesticides [23,24]. The use of magnetic MWCNTs combined with other materials has attracted great interest. Metal–organic frameworks (MOFs) are microporous inorganic–organic crystalline structure materials. They are formed by self-assembly of metal ions (clusters or secondary building units) and organic ligands (linkers) by coordination bonds, and they have a highly ordered and three-dimensional structure [25]. MOFs are promising sorbent materials, and using them for extraction could have the advantages of enhanced selectivity and stability, permeable channels and coordination nanospace, framework flexibility and dynamics, easy tunability, and modification [26]. However, application of MOFs is limited in certain cases owing to the lack of water and thermal stability, for example, MOF-199 and MOF-5 lose their extraction efficiency when they are exposed to moisture for a long time [27]. Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs that are composed of tetrahedral transition metal ions (e.g., Zn and Co) and imidazolate-type organic linkers, and they exhibit high water stability in aqueous media [28]. Owing to their features of microporosity, uniform structured cavities, and a high surface area, ZIFs have many applications, such as chemical pollutant removal [29,30], chromatographic separation [31,32], and drug delivery [33]. ZIF-67 is a recently developed ZIF compound that has the formula Co(Hmim) (mim = 2-methylimidazole) with a sodalite-related zeolite type structure [34]. Because of the low coordination of the Co cation, ZIF-67 has three times higher adsorption capacity for dye from aqueous solutions than ZIF-8 [35]. Owing to the hydrogen bonding and – electron donor–acceptor interactions between the adsorbent and analytes, Fe O –MWCNTs–OH@poly-ZIF67 shows good selective extraction of aromatic acids [36]. This research 3 4 indicates that magnetic nanocarbon-material-based ZIFs have good application prospects for adsorbing pesticides because of their good adsorptive properties derived from the magnetic carbon nanocomposite. Organochlorine pesticides (OCPs) are ubiquitous in the environment due to their extensive application and persistent organic pollutants characteristics, which pose great risks to human health and ecosystems [37]. On account of their bioaccumulation, degradation resistance and carcinogenesis, tetratogenesis, and mutagenesis to human, 15 OCPs, including hexachlorocyclohexane (HCHs) and dichlorodiphenyltrichloroethane (DDTs), were banned with the issue of the Stockholm Convention in 2004. However, the current study showed that the concentration level of OCPs in aqueous environment 1 1 around Beijing were ranged from 9.81 to 32.1 ng L (average 15.1  7.78 ng L ) [38]. Therefore, it is necessary to develop sensitive and accuracy analytical methods for continuous monitoring trace level of the OCPs in water samples. Inspired by the abovementioned studies, a novel magnetic Co-based ZIF composite was synthesized by organic–inorganic coordination. The obtained M-M-ZIF-67 composite possessed porous surfaces and super-paramagnetism due to the utilization of Fe O –MWCNTs as the magnetic 3 4 Appl. Sci. 2018, 8, 959 3 of 15 kernel and support. In addition, due to the large surface area and excellent adsorption capacity of MWCNTs, the prepared hybrid material could be good adsorbent for MSPE of OCPs. In the end, an M-M-ZIF-67 based MSPE method was established and applied for the extraction of OCPs from agricultural irrigation water samples prior to gas chromatography–tandem triple quadrupole mass spectrometry (GC–MS/MS). 2. Materials and Methods 2.1. Reagents and Materials The standard liquid pesticides -HCH (CAS number: 319-84-6), -HCH (CAS number: 319-85-7), -HCH (CAS number: 58-89-9), -HCH (CAS number: 319-86-8), p,p -DDD (CAS number: 72-54-8), o,p-DDE (CAS number: 3424-82-6), p,p -DDE (CAS number: 72-55-9), o,p-DDT (CAS number: 789-02-6), and p,p -DDT (CAS number: 50-29-3) were obtained from the Agro-Environmental Protection Institute, Ministry of Agriculture (Tianjin, China) at concentrations of 1000 mg L . A standard mixture containing 20 mg L of each of the nine OCPs was prepared in methanol and stored at 20 C in the dark. High-performance liquid chromatography grade acetonitrile, methanol, and n-hexane were purchased from Sigma-Aldrich (St. Louis, MO, USA). Anhydrous sodium sulfate was supplied by Agilent (CA, California, USA). The MWCNTs (8–15 nm inner diameter (id), 10–30 m long, 95% purity), analytical grade ferric chloride hexahydrate (FeCl 6H O), ferrous 3 2 chloride tetrahydrate (FeCl 4H O), 2-methylimidazole, cobalt nitrate hexahydrate (Co(NO ) 6H O), 2 2 3 2 2 and ammonium hydroxide (mass fraction 28%) were provided by Aladdin Co. (Shanghai, China). Ethanol and all of the other reagents were of analytical grade and acquired from the Beijing Chemical Reagents Co. (Beijing, China). 2.2. Preparation of Fe O –MWCNTs–ZIF-67 3 4 2.2.1. Synthesis of Fe O –MWCNTs 3 4 The Fe O –MWCNTs were prepared according to the authors’ previously reported method with 3 4 slight modification [39]. In brief, MWCNT powder (0.2 g) was suspended in ultrapure water (240 mL) by sonication for 1 h and then transferred to a three-necked flask. After a solution of FeCl 6H O (1.8 g) 3 2 and FeCl 4H O (0.8 g) dissolved in ultrapure water (25 mL) was added to the flask, the mixture was 2 2 vigorously stirred with a mechanical stirrer (THZ-82A, Youlian instrument research institute, Jintan, China) under protection of N at 150 rpm and 80 C conditions for 30 min. Ammonium hydroxide (28%, 10 mL) was then added and the mixture was vigorously stirred for another 30 min. After cooling to room temperature, the sediments were collected by magnetic separation and washed three times with ethanol and ultrapure water to eliminate unreacted chemicals. The obtained Fe O –MWCNT 3 4 nanoparticles were dried in a vacuum oven at 60 C for 24 h. 2.2.2. Synthesis of ZIF-67 and M-M-ZIF-67 The ZIF-67 material was fabricated following a reported method [40]. The preparation procedure for M-M-ZIF-67 was as follows. First, the Fe O –MWCNTs were dispersed in ultrapure water (20 mL) 3 4 and mixed with 3 mL of an aqueous solution of Co(NO ) 6H O (0.45 g) with consistent stirring for 3 2 2 30 min under 150 rpm condition. An aqueous solution of 2-methylimidazole (20 mL, 0.45 g) was then added to the solution and the solution was stirred for 6 h. All of these synthetic processes were performed at room temperature. Finally, the M-M-ZIF-67 product was obtained by magnetic separation and washed three times with ethanol and ultrapure water. The synthesized material was dried in a vacuum oven at 60 C for 24 h. Appl. Sci. 2018, 8, 959 4 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 14 2.3. MSPE Procedure The workflow of MSPE using M-M-ZIF-67 is shown in Figure 1. First, M-M-ZIF-67 (6 mg) was The workflow of MSPE using M-M-ZIF-67 is shown in Figure 1. First, M-M-ZIF-67 (6 mg) was placed in a 10 mL centrifuge tube containing 5.0 mL of the aqueous standard solution or sample solution placed in a 10 mL centrifuge tube containing 5.0 mL of the aqueous standard solution or sample and shaken for 20 min for extraction. With aggregation of the adsorbent in the tube, the supernatant solution and shaken for 20 min for extraction. With aggregation of the adsorbent in the tube, the was discarded with the aid of an external ferrite magnet. Acetonitrile (2 mL) was then added into supernatant was discarded with the aid of an external ferrite magnet. Acetonitrile (2 mL) was then the tube, and ultrasonic elution of the analytes from the magnetic materials was performed for 5 min. added into the tube, and ultrasonic elution of the analytes from the magnetic materials was After the M-M-ZIF-67 composite was collected, the supernatant desorption solution was transferred performed for 5 min. After the M-M-ZIF-67 composite was collected, the supernatant desorption to another centrifuge tube. The same desorption procedures were performed one more time. Finally, solution was transferred to another centrifuge tube. The same desorption procedures were performed the combined desorbed elution was evaporated to dryness under a gentle stream of nitrogen at 40 C. one more time. Finally, the combined desorbed elution was evaporated to dryness under a gentle The residue was redissolved in 0.5 mL acetone, and 1 L of it was injected into the GC-MS/MS stream of nitrogen at 40 °C. The residue was redissolved in 0.5 mL acetone, and 1 μL of it was injected for analysis. into the GC-MS/MS for analysis. Figure 1. Schematic illustration of the synthetic route to prepare zeolitic imidazolate framework based Figure 1. Schematic illustration of the synthetic route to prepare zeolitic imidazolate framework on magnetic multiwalled carbon nanotubes and the magnetic solid-phase extraction steps for based on magnetic multiwalled carbon nanotubes and the magnetic solid-phase extraction steps for organochlorine pesticides analysis. organochlorine pesticides analysis. 2.4. Sample Preparation 2.4. Sample Preparation The river water sample was collected from the Liangshui River, Beijing, China. The tap water The river water sample was collected from the Liangshui River, Beijing, China. The tap water sample was obtained from the tap in the laboratory; and the underground water sample was obtained sample was obtained from the tap in the laboratory; and the underground water sample was obtained from Langfang City, Hebei Province, China. All of the samples were filtered through a 0.45 µ m from Langfang City, Hebei Province, China. All of the samples were filtered through a 0.45 m polytetrafluoroethylene membrane filter and stored at 4 °C in amber dark glass bottles. polytetrafluoroethylene membrane filter and stored at 4 C in amber dark glass bottles. 2.5. Apparatus and Gas Chromatography–Tandem Triple Quadrupole Mass Spectrometry Conditions 2.5. Apparatus and Gas Chromatography–Tandem Triple Quadrupole Mass Spectrometry Conditions The surface morphologies and particle sizes of the as-synthesized nanoparticles were observed The surface morphologies and particle sizes of the as-synthesized nanoparticles were observed by scanning electron microscopy (SEM, JSM-6300, JEOL, Tokyo, Japan) and transmission electron by scanning electron microscopy (SEM, JSM-6300, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-200CX, JEOL, Tokyo, Japan). The powder X-ray diffraction (XRD) patterns of microscopy (TEM, JEM-200CX, JEOL, Tokyo, Japan). The powder X-ray diffraction (XRD) patterns of the as-synthesized nanoparticles were obtained with an X-ray powder diffractometer (D8 Advance, the as-synthesized nanoparticles were obtained with an X-ray powder diffractometer (D8 Advance, Bruker, Karlsruhe, Germany). The Fourier-transform infrared (FT-IR) spectra of the as-synthesized Bruker, Karlsruhe, Germany). The Fourier-transform infrared (FT-IR) spectra of the as-synthesized nanoparticles were obtained with an FT-IR-8400 spectrometer (Shimadzu, Kyoto, Japan). A vibrating nanoparticles were obtained with an FT-IR-8400 spectrometer (Shimadzu, Kyoto, Japan). A vibrating sample magnetometer (VSM, Lake Shore 7410, Columbus, OH, USA) was used to investigate the sample magnetometer (VSM, Lake Shore 7410, Columbus, OH, USA) was used to investigate the magnetic properties of all of the synthetic materials. magnetic properties of all of the synthetic materials. Gas chromatography–tandem triple quadrupole mass spectrometry (GC–MS/MS) analysis was performed with a Shimadzu GC-2010 plus gas chromatograph coupled with an AOC-20s autosampler, a Shimadzu TQ8040 triple-quadrupole MS. The pesticides were separated on an Rtx- 5MS capillary column purchased from RESTEK (City, US State abbrev. if applicable, Country) (0.25 Appl. Sci. 2018, 8, 959 5 of 15 Gas chromatography–tandem triple quadrupole mass spectrometry (GC–MS/MS) analysis was performed with a Shimadzu GC-2010 plus gas chromatograph coupled with an AOC-20s autosampler, a Shimadzu TQ8040 triple-quadrupole MS. The pesticides were separated on an Rtx-5MS capillary column purchased from RESTEK (Bellefonte, PA, USA) (0.25 mm (id) 30 m, 0.25 m film thickness). Helium gas was used as the carrier gas at a constant flow rate of 1 mL min . The column temperature was programmed as follows: the initial temperature of 40 C was maintained for 4 min, the temperature 1   1 was increased to 125 C at 25 C min , the temperature was ramped to 300 C at 10 C min , and the temperature was maintained at 300 C for 6 min. The total run time was 30.9 min. The injector temperature was 250 C, and the injection volume was 1.0 L in splitless mode. The specific multiple reaction monitoring (MRM) transitions for all the nine OCPs and the other parameters are given in Table 1. Table 1. Acquisition and chromatographic parameters of the nine organochlorine pesticides. a b Pesticides Retention Time (min) MRM1 (m/z) CE1 (eV) MRM2 (m/z) CE2 (eV) -HCH 15.32 218.90 > 182.90 8 218.90 > 144.90 20 -HCH 15.87 218.90 > 182.90 8 218.90 > 144.90 20 -HCH 16.01 218.90 > 182.90 8 218.90 > 144.90 20 -HCH 16.59 218.90 > 182.90 10 218.90 >144.90 20 0 d 19.47 246.00 > 176.00 30 246.00 > 211.00 22 o,p -DDE p,p -DDE 20.09 246.00 > 176.00 30 246.00 > 211.00 22 0 e p,p -DDD 20.90 235.00 > 165.00 24 235.00 > 199.00 14 0 f o,p -DDT 20.95 235.00 > 165.00 24 235.00 > 199.00 16 p,p -DDT 21.61 235.00 > 165.00 24 235.00 > 199.00 16 a b c MRM means multiple reaction monitoring transitions; CE means collision energy; HCH means d e hexachlorocyclohexane; DDE means 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene; DDD means [1,1-dichloro-2, 2-bis(p-chlorophenyl)ethylene; DDT means 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethylene. 2.6. Quality Control and Quality Assurance Quality control and quality assurance (QA/QC) experiments, including a blank sample test, recovery test, repeatability test, and limits of detection (LOD) experiment were performed to evaluate the feasibility of the method. In addition, 5 L ultrapure water served as the water sample for the blank test, while the water samples for the recovery test and repeatability test were made by spiking 50 L 20 mg L working solution into 100 mL ultrapure water. The LOD experiment was undertaken following the USA Environmental Protection Agency method. 3. Results 3.1. Characterization of M-M-ZIF-67 The micro-morphologies of the Fe O –MWCNTs and M-M-ZIF-67 were observed by SEM and 3 4 TEM. As shown in Figure 2A, the Fe O nanoparticles are attached to the MWCNT surface. The SEM 3 4 image of M-M-ZIF-67 in Figure 2B shows that the composite has a rough surface, indicating that the material has good potential as an adsorbent [27]. VSM was performed to investigate the magnetic behavior of the magnetic materials, and the results are shown in Figure 3A. The magnetic hysteresis loops show that both the remanence and coercivity values of the three types of magnetic materials are zero, which indicates that they have typical supermagnetic properties and could be separated using an external magnet. The saturation magnetization values of Fe O , Fe O –MWCNTs, and M-M-ZIF-67 are 66.8, 59.6, and 53.1 emu g , 3 4 3 4 respectively. As shown in the insert of Figure 3A, well-dispersed M-M-ZIF-67 particles exist in the absence of an external magnet, and they are rapidly attracted to the walls of the vial in a short time (about 20 s) with application of a magnet. The powder XRD patterns of Fe O , Fe O –MWCNTs, 3 4 3 4 and M-M-ZIF-67 are shown in Figure 3B. The diffraction patterns of M-M-ZIF-67 are in very close Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 14 Appl. Sci. 2018, 8, 959 6 of 15 agreement with the materials of Fe O and Fe O –MWCNTs. This indicates that Fe O is well retained 3 4 3 4 3 4 in the Fe O –MWCNTs and M-M-ZIF-67. The FT-IR spectra are shown in Figure 3C. For M-M-ZIF-67, 3 4 the adsorption band at 577 cm corresponds to the stretching vibrations of Fe–O, and the band at 1566 cm corresponds to in-plane bending of CH of the MWCNTs. This suggests that M-M-ZIF-67 is composed of Fe O and MWCNTs. The bands in the region 673 to 1454 cm are attributed to 3 4 complete ring stretching or bending vibration of 2-methylimidazole, and the main adsorption bands Figure 2. (A) transmission electron microscopy image of magnetic multiwalled carbon nanotubes and are preserved in the spectrum of the composite, which indicates that ZIF-67 is successfully synthesized (B) scanning electron microscopy image of zeolitic imidazolate framework based on magnetic on Appthe l. Scisurface . 2018, 8, xof FO M-M-ZIF-67. R PEER REVIEW 6 of 14 multiwalled carbon nanotubes. VSM was performed to investigate the magnetic behavior of the magnetic materials, and the results are shown in Figure 3A. The magnetic hysteresis loops show that both the remanence and coercivity values of the three types of magnetic materials are zero, which indicates that they have typical supermagnetic properties and could be separated using an external magnet. The saturation −1 magnetization values of Fe3O4, Fe3O4–MWCNTs, and M-M-ZIF-67 are 66.8, 59.6, and 53.1 emu g , respectively. As shown in the insert of Figure 3A, well-dispersed M-M-ZIF-67 particles exist in the absence of an external magnet, and they are rapidly attracted to the walls of the vial in a short time (about 20 s) with application of a magnet. The powder XRD patterns of Fe3O4, Fe3O4–MWCNTs, and M-M-ZIF-67 are shown in Figure 3B. The diffraction patterns of M-M-ZIF-67 are in very close agreement with the materials of Fe3O4 and Fe3O4–MWCNTs. This indicates that Fe3O4 is well retained in the Fe3O4–MWCNTs and M-M-ZIF-67. The FT-IR spectra are shown in Figure 3C. For M-M-ZIF- −1 67, the adsorption band at 577 cm corresponds to the stretching vibrations of Fe–O, and the band at −1 1566 cm corresponds to in-plane bending of CH2 of the MWCNTs. This suggests that M-M-ZIF-67 Figure 2. (A) transmission electron microscopy image of magnetic multiwalled carbon nanotubes and −1 is composed of Fe3O4 and MWCNTs. The bands in the region 673 to 1454 cm are attributed to Figure 2. (A) transmission electron microscopy image of magnetic multiwalled carbon nanotubes (B) scanning electron microscopy image of zeolitic imidazolate framework based on magnetic compl and ete (B r) in scanning g stretchi electr ng o on r bmicr endoscopy ing vibra image tion of of zeolitic 2-methy imidazol limidaate zole framework , and the m based ain aon dso magnetic rption bands multiwalled carbon nanotubes. multiwalled carbon nanotubes. are preserved in the spectrum of the composite, which indicates that ZIF-67 is successfully synthesized on the surface of M-M-ZIF-67. VSM was performed to investigate the magnetic behavior of the magnetic materials, and the results are shown in Figure 3A. The magnetic hysteresis loops show that both the remanence and coercivity values of the three types of magnetic materials are zero, which indicates that they have typical supermagnetic properties and could be separated using an external magnet. The saturation −1 magnetization values of Fe3O4, Fe3O4–MWCNTs, and M-M-ZIF-67 are 66.8, 59.6, and 53.1 emu g , respectively. As shown in the insert of Figure 3A, well-dispersed M-M-ZIF-67 particles exist in the absence of an external magnet, and they are rapidly attracted to the walls of the vial in a short time (about 20 s) with application of a magnet. The powder XRD patterns of Fe3O4, Fe3O4–MWCNTs, and M-M-ZIF-67 are shown in Figure 3B. The diffraction patterns of M-M-ZIF-67 are in very close agreement with the materials of Fe3O4 and Fe3O4–MWCNTs. This indicates that Fe3O4 is well retained Figure 3. (A) magnetic hysteresis loops for (a) Fe3O4; (b) magnetic multiwalled carbon nanotubes; and Figure 3. (A) magnetic hysteresis loops for (a) Fe O ; (b) magnetic multiwalled carbon nanotubes; 3 4 in the Fe3O4–MWCNTs and M-M-ZIF-67. The FT-IR spectra are shown in Figure 3C. For M-M-ZIF- c) zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes. The insert shows and c) zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes. The insert −1 67, the adsorption band at 577 cm corresponds to the stretching vibrations of Fe–O, and the band at magnetic separation and dispersion of zeolitic imidazolate framework based on magnetic shows magnetic separation and dispersion of zeolitic imidazolate framework based on magnetic −1 1566 cm corresponds to in-plane bending of CH2 of the MWCNTs. This suggests that M-M-ZIF-67 multiwalled carbon nanotubes; (B) X-ray diffraction patterns of (a) Fe3O4; (b) magnetic multiwalled multiwalled carbon nanotubes; (B) X-ray diffraction patterns of (a) Fe O ; (b) magnetic multiwalled 3 4 −1 is composed of Fe3O4 and MWCNTs. The bands in the region 673 to 1454 cm are attributed to carbon nanotubes; and c) zeolitic imidazolate framework based on magnetic multiwalled carbon carbon nanotubes; and c) zeolitic imidazolate framework based on magnetic multiwalled carbon complete ring stretching or bending vibration of 2-methylimidazole, and the main adsorption bands nanotubes; (C) Fourier-transform infrared spectra of (a) Fe3O4; (b) magnetic multiwalled carbon nanotubes; (C) Fourier-transform infrared spectra of (a) Fe O ; (b) magnetic multiwalled carbon 3 4 are preserved in the spectrum of the composite, which indicates that ZIF-67 is successfully nanotubes; (c) zeolitic imidazolate framework-67; and (d) zeolitic imidazolate framework based on nanotubes; (c) zeolitic imidazolate framework-67; and (d) zeolitic imidazolate framework based on synthesized on the surface of M-M-ZIF-67. magnetic multiwalled carbon nanotubes. magnetic multiwalled carbon nanotubes. 3.2. Optimization of the MSPE Parameters 3.2. Optimization of the MSPE Parameters The MSPE parameters affect extraction of the OCPs and the desorption performance. The amount of M-M-ZIF-67, extraction time, type of desorption solvent, desorption time, and frequency of desorption were investigated by single-factor experiments following a step by step procedure at a spiked level of 50 g L . Figure 3. (A) magnetic hysteresis loops for (a) Fe3O4; (b) magnetic multiwalled carbon nanotubes; and c) zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes. The insert shows magnetic separation and dispersion of zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes; (B) X-ray diffraction patterns of (a) Fe3O4; (b) magnetic multiwalled carbon nanotubes; and c) zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes; (C) Fourier-transform infrared spectra of (a) Fe3O4; (b) magnetic multiwalled carbon nanotubes; (c) zeolitic imidazolate framework-67; and (d) zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes. 3.2. Optimization of the MSPE Parameters Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 14 The MSPE parameters affect extraction of the OCPs and the desorption performance. The amount of M-M-ZIF-67, extraction time, type of desorption solvent, desorption time, and frequency of desorption were investigated by single-factor experiments following a step by step procedure at a −1 spiked level of 50 µ g L . 3.2.1. Optimization of the Extraction Process The present study aims to achieve satisfactory extraction performance for analytes in water samples. The effect of the amount of M-M-ZIF-67 on extraction was investigated using different amounts of the sorbent ranging from 2 to 10 mg. The experimental results (Figure 4A) show that the extraction recoveries of the OCPs rapidly increase when the amount of M-M-ZIF-67 is increased from Appl. Sci. 2018, 8, 959 7 of 15 2 to 6 mg, and the recoveries then slightly decrease when the amount of sorbent is increased from 6 to 10 mg. Based on these results, 6 mg of M-M-ZIF-67 was chosen for the following experiments. The extraction time is an important factor for MSPE because it influences the adsorption 3.2.1. Optimization of the Extraction Process equilibrium of the analytes between the sample solution and adsorbent. Therefore, the extraction time The present study aims to achieve satisfactory extraction performance for analytes in water was varied in the range 5–90 min to investigate its influence on the extraction efficiency. As shown samples. The effect of the amount of M-M-ZIF-67 on extraction was investigated using different in Figure 4B, the recoveries of the target OCPs increase with increasing extraction time from 5 to 20 amounts of the sorbent ranging from 2 to 10 mg. The experimental results (Figure 4A) show that the min, and they then remain almost constant until 30 min. The extraction recoveries for most of the extraction recoveries of the OCPs rapidly increase when the amount of M-M-ZIF-67 is increased from analytes then decrease with a further increase of the extraction time. This is probably because of 2 to 6 mg, and the recoveries then slightly decrease when the amount of sorbent is increased from 6 to desorption of the analytes from the adsorbent, especially when the adsorption equilibrium is reached. 10 mg. Based on these results, 6 mg of M-M-ZIF-67 was chosen for the following experiments. Thus, 20 min was chosen as the extraction time for the following experiments. −1 Figure 4. Optimization of the extraction process for 50 µ g L1 organochlorine pesticides using zeolitic Figure 4. Optimization of the extraction process for 50 g L organochlorine pesticides using zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes: (A) amount of sorbent and imidazolate framework based on magnetic multiwalled carbon nanotubes: (A) amount of sorbent (B) extraction time. Desorption conditions: 2 mL of acetonitrile; ultrasonication time, 5 min; and and (B) extraction time. Desorption conditions: 2 mL of acetonitrile; ultrasonication time, 5 min; and ultrasonication performed one more time. The error bars show the standard deviation of the mean (n ultrasonication performed one more time. The error bars show the standard deviation of the mean = 3). (n = 3). 3.2.2. Optimization of the Desorption Process The extraction time is an important factor for MSPE because it influences the adsorption equilibrium The deso of rp the tio analytes n solvent between is cruci the al sample and it soluti can si on gnand ificaadsorbent. ntly affect Ther the efor MSPE e, the effextraction iciency. Thre time e different solvents were investigated as desorption solvents to investigate their effect on the extraction was varied in the range 5–90 min to investigate its influence on the extraction efficiency. As shown in Figur efficiency: e 4B, the aceto recoveries nitrile, ace ofto the ne,tar an get d nOCPs -hexan incr e. Fi ease gure with 5A incr shoeasing ws that extr the action extracti time on fr reom cov5 erto ies 20 of min, the target analytes eluted by acetonitrile are better than those with the other solvents. Therefore, and they then remain almost constant until 30 min. The extraction recoveries for most of the analytes then acetodecr nitriease le wa with s used a furt as th her e d incr eso ease rptio of n the solvextraction ent for the time. followi This ng e isx pr pe obably rimentbecause s. of desorption of The desorption time is another important factor that influences the efficiency of the MSPE the analytes from the adsorbent, especially when the adsorption equilibrium is reached. Thus, 20 min was proce chosen ss. To ias nvthe estiextraction gate the inf time luence for othe f thfollowing e desorptio experiments. n time on the MSPE efficiency, experiments were performed with ultrasonic desorption times of 2, 5, 10, and 15 min. As shown in Figure 5B, the 3.2.2. Optimization of the Desorption Process extraction recoveries of the analytes are satisfactory when the sample solution is ultrasonicated for 5 and 10 min. Considering the efficiency, an ultrasonic desorption time of 5 min was chosen for the The desorption solvent is crucial and it can significantly affect the MSPE efficiency. Three different subsequent experiments. solvents were investigated as desorption solvents to investigate their effect on the extraction efficiency: acetonitrile, acetone, and n-hexane. Figure 5A shows that the extraction recoveries of the target analytes eluted by acetonitrile are better than those with the other solvents. Therefore, acetonitrile was used as the desorption solvent for the following experiments. The desorption time is another important factor that influences the efficiency of the MSPE process. To investigate the influence of the desorption time on the MSPE efficiency, experiments were performed with ultrasonic desorption times of 2, 5, 10, and 15 min. As shown in Figure 5B, the extraction recoveries of the analytes are satisfactory when the sample solution is ultrasonicated for 5 and 10 min. Considering the efficiency, an ultrasonic desorption time of 5 min was chosen for the subsequent experiments. To investigate the effect of the frequency of desorption, ultrasonic desorption of the nine OCPs was performed one to four times using 2 mL of the desorption solvent and a desorption time of 5 min. Appl. Sci. 2018, 8, 959 8 of 15 The results are shown in Figure 5C. The optimum recoveries are obtained when the analytes are eluted two times. Thus, the frequency of desorption was set to two for the following experiments. Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 14 −1 Figure 5. Optimization of the desorption process for 50 µ g L1 organochlorine pesticides using zeolitic Figure 5. Optimization of the desorption process for 50 g L organochlorine pesticides using zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes: (A) desorption solvent; (B) imidazolate framework based on magnetic multiwalled carbon nanotubes: (A) desorption solvent; desorption time; and (C) frequency of desorption. Extraction conditions: sample volume, 5 mL; (B) desorption time; and (C) frequency of desorption. Extraction conditions: sample volume, 5 mL; amount of sorbent, 6 mg; and extraction time, 20 min. The error bars show the standard deviation of amount of sorbent, 6 mg; and extraction time, 20 min. The error bars show the standard deviation of the mean (n = 3). the mean (n = 3). To investigate the effect of the frequency of desorption, ultrasonic desorption of the nine OCPs 3.3. Method Characterization was performed one to four times using 2 mL of the desorption solvent and a desorption time of 5 A series of experiments was performed under the optimized conditions (Table 2) and the min. The results are shown in Figure 5C. The optimum recoveries are obtained when the analytes are analytical characteristics, such as the linear ranges (LRs), LOD, and relative standard deviations eluted two times. Thus, the frequency of desorption was set to two for the following experiments. (RSDs), were determined to validate the developed method. Working solutions containing the target analytes at concentrations of 1, 2, 5, 10, 20, 50, 100, and 200 g L were prepared to construct the 3.3. Method Characterization working curves. Good linearities for the nine OCPs were achieved with determination coefficients (R ) A series of experiments was performed under the optimized conditions (Table 2) and the ranging from 0.9916 to 0.9989. The LOD for all of the analytes were calculated at signal/noise (S/N) analytical characteristics, such as the linear ranges (LRs), LOD, and relative standard deviations ratios of 3. The LODs range from 0.07 to 1.03 g L . The repeatability of the method was evaluated by (RSDs), were determined to validate the developed method. Working solutions containing the target performing six replicate analyses of spiked samples with 10 g L of each of the OCPs, and the RSDs −1 analytes at concentrations of 1, 2, 5, 10, 20, 50, 100, and 200 µ g L were prepared to construct the are in the range 1.0–8.5%. All of these results indicate that the proposed method has high sensitivity working curves. Good linearities for the nine OCPs were achieved with determination coefficients and good repeatability. (R ) ranging from 0.9916 to 0.9989. The LOD for all of the analytes were calcul ated at signal/noise −1 (S/N) ratios of 3. The LODs range from 0.07 to 1.03 µ g L . The repeatability of the method was Table 2. Analytical parameters of zeolitic imidazolate framework based on magnetic multiwalled −1 evaluated by performing six replicate analyses of spiked samples with 10 µ g L of each of the OCPs, carbon nanotubes as an adsorbent for magnetic solid-phase extraction of nine organochlorine pesticides and the RSDs are in the range 1.0–8.5%. All of these results indicate that the proposed method has in an ultrapure water sample. high sensitivity and good repeatability. 1 a 1 OCPs Determination Coefficients Linear Range (g L ) LOD (g L ) RSD (%) (n = 6) Table 2. Analytical parameters of zeolitic imidazolate framework based on magnetic multiwalled -HCH 2–200 0.9953 0.12 8.5 carbon nanotubes as an adsorbent for magnetic solid-phase extraction of nine organochlorine -HCH 1–200 0.9946 0.13 1.0 -HCH 1–200 0.9919 0.15 6.2 pesticides in an ultrapure water sample. -HCH 1–200 0.9947 0.07 4.0 a b o,p -DDE 1–200 0.9951 0.17 1.5 Linear Range LOD RSD (%) 0 OCPs Determination Coefficients p,p -DDE 1–200 0.9989 0.45 1.3 −1 −1 (µg L ) (µg L ) (n = 6) p,p -DDD 2–200 0.9934 0.41 3.1 0 α-HCH 2–200 0.9953 0.12 8.5 o,p -DDT 2–200 0.9932 0.74 3.9 p,p -DDT β-HCH 2–200 1–200 0.9916 0.9946 1.03 0.13 13.1 .0 a b γ-HCH 1–200 0.9919 0.15 6.2 LOD means limit of determination; RSDs means relative standard deviations, and were determined by performing six replicate analyses of the spiked samples with 10 g L of each of the organochlorine pesticides. δ-HCH 1–200 0.9947 0.07 4.0 o,p′-DDE 1–200 0.9951 0.17 1.5 3.4. Comparison p,p′-DDE 1–200 0.9989 0.45 1.3 p,p′-DDD 2–200 0.9934 0.41 3.1 To investigate the extraction performance of the prepared materials, 4 mg of Fe O –MWCNTs 3 4 o,p′-DDT 2–200 0.9932 0.74 3.9 and 6 mg of M-M-ZIF-67, which was synthesized based on the same amount of the former material, p,p′-DDT 2–200 0.9916 1.03 3.1 were applied to extraction of the nine OCPs using the proposed method. The results are shown in a b LOD means limit of determination; RSDs means relative standard deviations, and were determined Figure 6 based on three replicate analyses (n = 3). The peak areas of all of the analytes for M-M-ZIF-67 −1 by performing six replicate analyses of the spiked samples with 10 µ g L of each of the organochlorine pesticides. Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 14 3.4. Comparison To investigate the extraction performance of the prepared materials, 4 mg of Fe 3O4–MWCNTs and 6 mg of M-M-ZIF-67, which was synthesized based on the same amount of the former material, were applied to extraction of the nine OCPs using the proposed method. The results are shown in Appl. Sci. 2018, 8, 959 9 of 15 Figure 6 based on three replicate analyses (n = 3). The peak areas of all of the analytes for M-M-ZIF- 67 are 2.7–3.0 times higher than those for Fe3O4–MWCNTs, indicating that ZIF-67 plays a greater role arein 2.7–3.0 the extimes tractio higher n proce than ss. those for Fe O –MWCNTs, indicating that ZIF-67 plays a greater role in 3 4 the extraction process. Figure Figure 6. Ef 6.fect Effec oft the of th sorbent e sorben on t o the n th adsorption e adsorptio capacities n capacitie for s fo the r th nine e nin or e ganochlorine organochlorin pesticides. e pesticides. Extraction conditions: sample volume, 5 mL; amount of sorbent, 6 mg zeolitic imidazolate framework Extraction conditions: sample volume, 5 mL; amount of sorbent, 6 mg zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes and 4 mg magnetic multiwalled carbon nanotubes; based on magnetic multiwalled carbon nanotubes and 4 mg magnetic multiwalled carbon nanotubes; and extraction time, 20 min. Desorption conditions: 2 mL of acetonitrile; ultrasonication time, 5 min, and extraction time, 20 min. Desorption conditions: 2 mL of acetonitrile; ultrasonication time, 5 min, and ultrasonication performed one more time. The error bars show the standard deviation of the mean and ultrasonication performed one more time. The error bars show the standard deviation of the mean (n = 3). (n = 3). The performance of the current method was compared with some recently reported methods The performance of the current method was compared with some recently reported methods used for analysis of OCPs in water. The comparison data (Table 3) show that the proposed method used for analysis of OCPs in water. The comparison data (Table 3) show that the proposed method based on the M-M-ZIF-67 adsorbent exhibits good sensitivity (as the LOD), less use of sample and based on the M-M-ZIF-67 adsorbent exhibits good sensitivity (as the LOD), less use of sample and adsorbent. However, the Fe3O4–MWCNTs based adsorbent can make the phase separation process adsorbent. However, the Fe O –MWCNTs based adsorbent can make the phase separation process 3 4 easier and faster without additional centrifugation or filtration procedures, and also can avoid the easier and faster without additional centrifugation or filtration procedures, and also can avoid the time-consuming column passing operations encountered in SPE. Therefore, the developed M-M-ZIF- time-consuming column passing operations encountered in SPE. Therefore, the developed M-M-ZIF-67 67 based magnetic solid-phase extraction method showed agreement in accordance with the based magnetic solid-phase extraction method showed agreement in accordance with the principles of principles of green analytical chemistry [41]. green analytical chemistry [41]. Appl. Sci. 2018, 8, 959 10 of 15 Table 3. Comparison of different methods for analysis of organochlorine pesticides. Sample Number of Volume of Elution Sorbent Extraction Method Sorbent LOD RSD (%) Spiked Level Ref. Amount (mL) OCPs Solvent Amount (mg) Time (min) MSPE-GC-MS/MS BMZIF-derived 1 1 10 8 Dichloromethane, 2.00 mL 6 10 0.39–0.70 ng L 5.5–9.1 5–500 ng L [42] a b carbon c 1 1 -SPE-GC-MS ZnO-CF 10 15 Toluene, 0.30 mL 15 30 0.19–1.64 g L 2.3–10.2 1–50 g L [43] Cyclohexane-ethyl acetate e f 1 1 PT-SPE-GC-ECD GUF-MIR 1 3 5 - 0.24–0.66 ng g 3.5–6.7 2.2–220 ng g [44] (9:1, v/v), 0.60 mL g 1 1 MSPE-GC-ECD RGO/Fe O @Au 10 6 Acetonitrile, 0.25 mL 20 10 0.4–4.1 g L 1.7–7.3 100 g L [45] 3 4 h 1 1 MSPE-GC-ECD 20 5 Acetonitrile, 0.20 mL 20 10 1.0–1.9 ng L 6.2–8.3 100 g L [46] Fe O @MAA@IBL 3 4 rGO-amino-HNT@PT i 1 1 d-SPE -GC-MS 10 6 Acetonitrile, 0.50 mL 5 5 2–13 ng L 6.1–9.7 5–70 g L [47] k 1 1 -SPE-GC-MS MIL-101 10 5 Ethyl acetate, 0.20 mL 4 40 2.5–16 ng L 4.2–11.0 10 g L [48] acetonitrile-dichloromethane l 1 1 MSPE-GC/ECD -CD/MRGO 50 16 15 3 0.5–3.2 ng Kg 3.3–7.8 50 ng kg [49] (4:1, v/v), 1.00 mL m 1 1 MSPE-GC-MS/MS M-M-ZIF-67 5 9 Acetonitrile, 4.00 mL 6 20 0.07–1.03 g L 1.0–8.5 10 g L This work a b magnetic solid-phase extraction followed by gas chromatography–tandem triple quadrupole mass spectrometry determination; micro solid-phase extraction followed by gas c d e chromatography–mass spectrometry determination; Bimetallic MOF; Zinc oxide nanoparticles incorporated in carbon foam; Miniaturized pipette tip solid-phase extraction followed f g by gas chromatography combination with electronic capture detector determination; Glyoxal–urea–formaldehyde molecularly imprinted resin; Reduced graphene oxide/Fe O @gold 3 4 h i j nanocomposite; Fe O @mercaptoacetic acid@imine-based ligand; dispersive micro solid-phase extraction; Reduced graphene oxide–amino-halloysite nanotubes@polythiophene; 3 4 k l m metal-organic framework; -Cyclodextrin/iron oxide-reduced graphene oxide hybrid nanostructure; zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes. Appl. Sci. 2018, 8, 959 11 of 15 3.5. Real Sample Analysis The proposed MSPE method was applied to determination of OCPs in real agricultural irrigation water samples, including tap water, river water, and underground water. The results are summarized in Table 4 and the total ion chromatograms (TICs) of the analytes acquired from the tap water samples are shown in Figure 7. There are no OCPs in the selected water samples and the recovery results indicate that the developed method has good utility for analysis of OCPs in real water samples. Table 4. Analytical results for determination of organochlorine pesticides in real water samples. Spiked Concentration (g L , n = 3) Analyte Matrix 0 10 100 Found Recovery (%) RSD (%) Recovery (%) RSD (%) -HCH <LOD 83.4 7.4 84.7 0.7 -HCH <LOD 92.5 9.6 103.7 0.6 -HCH <LOD 93.3 5.5 91.9 0.4 -HCH <LOD 94.6 6.6 111.1 1.0 Tap water o,p -DDE <LOD 85.4 3.4 99.6 5.3 p,p -DDE <LOD 78.1 1.5 107.5 3.1 p,p -DDD <LOD 76.0 2.5 97.0 2.5 o,p -DDT <LOD 83.1 5.4 89.2 2.1 p,p -DDT <LOD 93.7 2.1 105.7 1.6 -HCH <LOD 76.8 5.7 80.5 2.4 -HCH <LOD 91.3 3.1 102.5 0.8 -HCH <LOD 84.1 8.5 91.2 3.5 -HCH <LOD 74.9 5.9 112.7 2.6 River water o,p -DDE <LOD 110.8 4.7 94.3 0.1 p,p -DDE <LOD 102.5 4.8 100.9 1.2 p,p -DDD <LOD 101.9 3.4 92.2 0.7 o,p -DDT <LOD 108.6 4.3 86.9 1.2 p,p -DDT <LOD 110.8 3.5 100.9 0.8 Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 14 -HCH <LOD 81.0 8.7 80.3 3.3 -HCH <LOD 83.9 7.8 97.2 2.1 3.5. Real Sample Analysis -HCH <LOD 90.1 7.5 80.4 5.2 -HCH <LOD 79.5 15.3 112.7 2.0 The proposed MSPE method was applied to determination of OCPs in real agricultural irrigation Underground water o,p -DDE <LOD 101.5 8.4 82.9 1.9 water samples, including tap water, river water, and underground water. The results are summarized p,p -DDE <LOD 95.3 7.6 81.0 1.4 p,p -DDD <LOD 92.8 8.9 75.1 2.2 in Table 4 and the total ion chromatograms (TICs) of the analytes acquired from the tap water samples o,p -DDT <LOD 111.1 6.4 110.3 3.2 are shown in Figure 7. There are no OCPs in the selected water samples and the recovery results p,p -DDT <LOD 116.3 3.7 90.5 2.3 indicate that the developed method has good utility for analysis of OCPs in real water samples. Figure 7. Total ion chromatograms of the nine organochlorine pesticides in water samples obtained Figure 7. Total ion chromatograms of the nine organochlorine pesticides in water samples obtained by gas chromatography–tandem triple quadrupole mass spectrometry: (a) tap water; (b) river water; by gas chromatography–tandem triple quadrupole mass spectrometry: (a) tap water; (b) river water; −1 −1 (c) underground water; (d) tap water spiked at 10 µ g L;1 and (e) tap water spiked at 100 µ g L . 1 (c) underground water; (d) tap water spiked at 10 g L ; and (e) tap water spiked at 100 g L . Table 4. Analytical results for determination of organochlorine pesticides in real water samples. −1 Spiked Concentration (µg L , n = 3) Matrix Analyte 0 10 100 Found Recovery (%) RSD (%) Recovery (%) RSD (%) α-HCH <LOD 83.4 7.4 84.7 0.7 β-HCH <LOD 92.5 9.6 103.7 0.6 γ-HCH <LOD 93.3 5.5 91.9 0.4 δ-HCH <LOD 94.6 6.6 111.1 1.0 Tap water o,p′-DDE <LOD 85.4 3.4 99.6 5.3 p,p′-DDE <LOD 78.1 1.5 107.5 3.1 p,p′-DDD <LOD 76.0 2.5 97.0 2.5 o,p′-DDT <LOD 83.1 5.4 89.2 2.1 p,p′-DDT <LOD 93.7 2.1 105.7 1.6 α-HCH <LOD 76.8 5.7 80.5 2.4 β-HCH <LOD 91.3 3.1 102.5 0.8 γ-HCH <LOD 84.1 8.5 91.2 3.5 δ-HCH <LOD 74.9 5.9 112.7 2.6 River water o,p′-DDE <LOD 110.8 4.7 94.3 0.1 p,p′-DDE <LOD 102.5 4.8 100.9 1.2 p,p′-DDD <LOD 101.9 3.4 92.2 0.7 o,p′-DDT <LOD 108.6 4.3 86.9 1.2 p,p′-DDT <LOD 110.8 3.5 100.9 0.8 α-HCH <LOD 81.0 8.7 80.3 3.3 β-HCH <LOD 83.9 7.8 97.2 2.1 Underground water γ-HCH <LOD 90.1 7.5 80.4 5.2 δ-HCH <LOD 79.5 15.3 112.7 2.0 Appl. Sci. 2018, 8, 959 12 of 15 4. Conclusions Magnetic composites containing Fe O , MWCNTs, and ZIF-67 have been synthesized. 3 4 The synthesized materials have porous surfaces and exhibit super-paramagnetism. They were used as sorbents for MSPE to extract nine OCPs from agricultural irrigation water samples. For M-M-ZIF-67, the OCP adsorption capacities are nearly three times higher than those for Fe O –MWCNTs. 3 4 The developed method achieves high extraction efficiencies, good linearities, low detection limits, and good accuracies and precision. The results suggest that M-M-ZIF-67 is a simple and effective potential adsorbent for removal of OCPs from environmental water samples. Author Contributions: X.H. and G.L. conceived and designed the experiments; X.X. and L.L. performed the experiments; X.H. and S.Z. analyzed the data; D.X. and H.L. contributed reagents and materials; X.H. and H.G. wrote the paper. Authorship must be limited to those who have contributed substantially to the work reported. 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Micro-solid-phase extraction of organochlorine pesticides using porous metal-organic framework mil-101 as sorbent. J. Chromatogr. A 2015, 1401, 9–16. [CrossRef] [PubMed] 49. Mahpishanian, S.; Sereshti, H. One-step green synthesis of -cyclodextrin/iron oxide-reduced graphene oxide nanocomposite with high supramolecular recognition capability: Application for vortex-assisted magnetic solid phase extraction of organochlorine pesticides residue from honey samples. J. Chromatogr. A 2017, 1485, 32–43. [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Novel Zeolitic Imidazolate Frameworks Based on Magnetic Multiwalled Carbon Nanotubes for Magnetic Solid-Phase Extraction of Organochlorine Pesticides from Agricultural Irrigation Water Samples

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applied sciences Article Novel Zeolitic Imidazolate Frameworks Based on Magnetic Multiwalled Carbon Nanotubes for Magnetic Solid-Phase Extraction of Organochlorine Pesticides from Agricultural Irrigation Water Samples 1 2 2 2 2 Xiaodong Huang , Guangyang Liu , Donghui Xu , Xiaomin Xu , Lingyun Li , 2 2 1 , Shuning Zheng , Huan Lin and Haixiang Gao * Department of Applied Chemistry, China Agricultural University, Beijing 100193, China; huangxiaodong@caas.cn Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, Key Laboratory of Vegetables Quality and Safety Control, Ministry of Agriculture and Rural Affairs of China, Beijing 100081, China; liuguangyang@caas.cn (G.L.); xudonghui@caas.cn (D.X.); hsuixiaomin@gmai.com (X.X.); lilingyun@caas.cn (L.L.); zhengshuning@caas.cn (S.Z.); linhuan03@caas.cn (H.L.) * Correspondence: hxgao@cau.edu.cn; Tel.: +86-10-62731991 Received: 18 May 2018; Accepted: 5 June 2018; Published: 12 June 2018 Abstract: Magnetic solid-phase extraction is an effective and convenient sample pretreatment technique that has received considerable interest in recent years. A lot of research indicated that magnetic nanocarbon-material-based composites have good application prospects as adsorbents for magnetic solid-phase extraction of pesticides. Herein, a novel zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes (M-M-ZIF-67) has been prepared as an adsorbent for magnetic solid-phase extraction of nine organochlorine pesticides from agricultural irrigation water samples. The obtained M-M-ZIF-67 material possessed porous surfaces and super-paramagnetism due to the utilization of magnetic multiwalled carbon nanotubes as the magnetic kernel and support. To evaluate the extraction performance of the M-M-ZIF-67, the main parameters that affected the extraction efficiency were researched. Under the optimal conditions, a good linearity for the nine organochlorine pesticides was achieved with the determination coefficients (R ) higher than 0.9916. The limits of detection (signal/noise = 3:1) were in the range 0.07–1.03 g L . The recoveries of all analytes for the method at spiked levels of 10 and 100 g L were 74.9–116.3% and 75.1–112.7%, respectively. The developed M-M-ZIF-67 based magnetic solid-phase extraction method has a potential application prospect for the monitoring of trace level of organochlorine pesticides in environmental water samples. Keywords: zeolitic imidazolate framework; multi-walled carbon nanotubes; magnetic solid phase extraction; organochlorine pesticides; agricultural irrigation water 1. Introduction Sample pretreatment is a crucial step in analysis of trace or ultra-trace amounts analytes in complex matrices. Solid-phase extraction (SPE) is a type of widely used pretreatment for effective concentration of analytes in complex matrices before instrumental analysis [1]. A variety of pretreatment methods has been developed based on this technique, including solid-phase microextraction (SPME) [2], micro-SPE (-SPE) [3], and stir-bar sorptive extraction (SBSE) [4]. Magnetic solid-phase extraction (MSPE), as a new type of SPE, is a pretreatment method that has received considerable interest in recent years. In this technique, magnetic adsorbents are directly Appl. Sci. 2018, 8, 959; doi:10.3390/app8060959 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 959 2 of 15 dispersed into sample solutions, and this dispersive extraction mode can enhance the contact area between adsorbents and analytes [5]. Notably, magnetic adsorbents can be separated from the sample solutions under an external magnetic field without the need of traditional centrifugation or filtration, thereby simplifying the extraction process. Furthermore, magnetic adsorbents can be recycled and reused easily, which is cost effective and environmentally friendly [6]. Therefore, the MSPE technique shows comprehensive advantages of simplicity, time, reagent and labor savings, and excellent extraction efficiency, which meet the principles of green analytical chemistry [7]. The diversity of the materials used in MSPE is the main factor that has led to extensive development of this technique in recent decades [8]. Multiple magnetic sorbents have been synthesized by embedding magnetic cores in different organic or inorganic materials, such as chitosan [9], ionic liquids [10], polymers [11], silica [12], metallic oxides [13], molecularly imprinted polymers [14], and carbon nanomaterials [15,16]. Multiwalled carbon nanotubes (MWCNTs) are formed by seamless rolling up of several layers of graphite sheets. Because of their excellent properties, such as high surface area and inner volume, stability, mechanical strength, ability to establish – interactions, and capacity for functionalization, MWCNTs have the possibility of acting as good sorbents [17]. MWCNTs have recently attracted considerable interest as adsorbents in MSPE for extracting different analytes, such as antibiotics [18], estrogens [19], mycotoxins [20], metal ions [21], environmental pollutants [22], and pesticides [23,24]. The use of magnetic MWCNTs combined with other materials has attracted great interest. Metal–organic frameworks (MOFs) are microporous inorganic–organic crystalline structure materials. They are formed by self-assembly of metal ions (clusters or secondary building units) and organic ligands (linkers) by coordination bonds, and they have a highly ordered and three-dimensional structure [25]. MOFs are promising sorbent materials, and using them for extraction could have the advantages of enhanced selectivity and stability, permeable channels and coordination nanospace, framework flexibility and dynamics, easy tunability, and modification [26]. However, application of MOFs is limited in certain cases owing to the lack of water and thermal stability, for example, MOF-199 and MOF-5 lose their extraction efficiency when they are exposed to moisture for a long time [27]. Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs that are composed of tetrahedral transition metal ions (e.g., Zn and Co) and imidazolate-type organic linkers, and they exhibit high water stability in aqueous media [28]. Owing to their features of microporosity, uniform structured cavities, and a high surface area, ZIFs have many applications, such as chemical pollutant removal [29,30], chromatographic separation [31,32], and drug delivery [33]. ZIF-67 is a recently developed ZIF compound that has the formula Co(Hmim) (mim = 2-methylimidazole) with a sodalite-related zeolite type structure [34]. Because of the low coordination of the Co cation, ZIF-67 has three times higher adsorption capacity for dye from aqueous solutions than ZIF-8 [35]. Owing to the hydrogen bonding and – electron donor–acceptor interactions between the adsorbent and analytes, Fe O –MWCNTs–OH@poly-ZIF67 shows good selective extraction of aromatic acids [36]. This research 3 4 indicates that magnetic nanocarbon-material-based ZIFs have good application prospects for adsorbing pesticides because of their good adsorptive properties derived from the magnetic carbon nanocomposite. Organochlorine pesticides (OCPs) are ubiquitous in the environment due to their extensive application and persistent organic pollutants characteristics, which pose great risks to human health and ecosystems [37]. On account of their bioaccumulation, degradation resistance and carcinogenesis, tetratogenesis, and mutagenesis to human, 15 OCPs, including hexachlorocyclohexane (HCHs) and dichlorodiphenyltrichloroethane (DDTs), were banned with the issue of the Stockholm Convention in 2004. However, the current study showed that the concentration level of OCPs in aqueous environment 1 1 around Beijing were ranged from 9.81 to 32.1 ng L (average 15.1  7.78 ng L ) [38]. Therefore, it is necessary to develop sensitive and accuracy analytical methods for continuous monitoring trace level of the OCPs in water samples. Inspired by the abovementioned studies, a novel magnetic Co-based ZIF composite was synthesized by organic–inorganic coordination. The obtained M-M-ZIF-67 composite possessed porous surfaces and super-paramagnetism due to the utilization of Fe O –MWCNTs as the magnetic 3 4 Appl. Sci. 2018, 8, 959 3 of 15 kernel and support. In addition, due to the large surface area and excellent adsorption capacity of MWCNTs, the prepared hybrid material could be good adsorbent for MSPE of OCPs. In the end, an M-M-ZIF-67 based MSPE method was established and applied for the extraction of OCPs from agricultural irrigation water samples prior to gas chromatography–tandem triple quadrupole mass spectrometry (GC–MS/MS). 2. Materials and Methods 2.1. Reagents and Materials The standard liquid pesticides -HCH (CAS number: 319-84-6), -HCH (CAS number: 319-85-7), -HCH (CAS number: 58-89-9), -HCH (CAS number: 319-86-8), p,p -DDD (CAS number: 72-54-8), o,p-DDE (CAS number: 3424-82-6), p,p -DDE (CAS number: 72-55-9), o,p-DDT (CAS number: 789-02-6), and p,p -DDT (CAS number: 50-29-3) were obtained from the Agro-Environmental Protection Institute, Ministry of Agriculture (Tianjin, China) at concentrations of 1000 mg L . A standard mixture containing 20 mg L of each of the nine OCPs was prepared in methanol and stored at 20 C in the dark. High-performance liquid chromatography grade acetonitrile, methanol, and n-hexane were purchased from Sigma-Aldrich (St. Louis, MO, USA). Anhydrous sodium sulfate was supplied by Agilent (CA, California, USA). The MWCNTs (8–15 nm inner diameter (id), 10–30 m long, 95% purity), analytical grade ferric chloride hexahydrate (FeCl 6H O), ferrous 3 2 chloride tetrahydrate (FeCl 4H O), 2-methylimidazole, cobalt nitrate hexahydrate (Co(NO ) 6H O), 2 2 3 2 2 and ammonium hydroxide (mass fraction 28%) were provided by Aladdin Co. (Shanghai, China). Ethanol and all of the other reagents were of analytical grade and acquired from the Beijing Chemical Reagents Co. (Beijing, China). 2.2. Preparation of Fe O –MWCNTs–ZIF-67 3 4 2.2.1. Synthesis of Fe O –MWCNTs 3 4 The Fe O –MWCNTs were prepared according to the authors’ previously reported method with 3 4 slight modification [39]. In brief, MWCNT powder (0.2 g) was suspended in ultrapure water (240 mL) by sonication for 1 h and then transferred to a three-necked flask. After a solution of FeCl 6H O (1.8 g) 3 2 and FeCl 4H O (0.8 g) dissolved in ultrapure water (25 mL) was added to the flask, the mixture was 2 2 vigorously stirred with a mechanical stirrer (THZ-82A, Youlian instrument research institute, Jintan, China) under protection of N at 150 rpm and 80 C conditions for 30 min. Ammonium hydroxide (28%, 10 mL) was then added and the mixture was vigorously stirred for another 30 min. After cooling to room temperature, the sediments were collected by magnetic separation and washed three times with ethanol and ultrapure water to eliminate unreacted chemicals. The obtained Fe O –MWCNT 3 4 nanoparticles were dried in a vacuum oven at 60 C for 24 h. 2.2.2. Synthesis of ZIF-67 and M-M-ZIF-67 The ZIF-67 material was fabricated following a reported method [40]. The preparation procedure for M-M-ZIF-67 was as follows. First, the Fe O –MWCNTs were dispersed in ultrapure water (20 mL) 3 4 and mixed with 3 mL of an aqueous solution of Co(NO ) 6H O (0.45 g) with consistent stirring for 3 2 2 30 min under 150 rpm condition. An aqueous solution of 2-methylimidazole (20 mL, 0.45 g) was then added to the solution and the solution was stirred for 6 h. All of these synthetic processes were performed at room temperature. Finally, the M-M-ZIF-67 product was obtained by magnetic separation and washed three times with ethanol and ultrapure water. The synthesized material was dried in a vacuum oven at 60 C for 24 h. Appl. Sci. 2018, 8, 959 4 of 15 Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 14 2.3. MSPE Procedure The workflow of MSPE using M-M-ZIF-67 is shown in Figure 1. First, M-M-ZIF-67 (6 mg) was The workflow of MSPE using M-M-ZIF-67 is shown in Figure 1. First, M-M-ZIF-67 (6 mg) was placed in a 10 mL centrifuge tube containing 5.0 mL of the aqueous standard solution or sample solution placed in a 10 mL centrifuge tube containing 5.0 mL of the aqueous standard solution or sample and shaken for 20 min for extraction. With aggregation of the adsorbent in the tube, the supernatant solution and shaken for 20 min for extraction. With aggregation of the adsorbent in the tube, the was discarded with the aid of an external ferrite magnet. Acetonitrile (2 mL) was then added into supernatant was discarded with the aid of an external ferrite magnet. Acetonitrile (2 mL) was then the tube, and ultrasonic elution of the analytes from the magnetic materials was performed for 5 min. added into the tube, and ultrasonic elution of the analytes from the magnetic materials was After the M-M-ZIF-67 composite was collected, the supernatant desorption solution was transferred performed for 5 min. After the M-M-ZIF-67 composite was collected, the supernatant desorption to another centrifuge tube. The same desorption procedures were performed one more time. Finally, solution was transferred to another centrifuge tube. The same desorption procedures were performed the combined desorbed elution was evaporated to dryness under a gentle stream of nitrogen at 40 C. one more time. Finally, the combined desorbed elution was evaporated to dryness under a gentle The residue was redissolved in 0.5 mL acetone, and 1 L of it was injected into the GC-MS/MS stream of nitrogen at 40 °C. The residue was redissolved in 0.5 mL acetone, and 1 μL of it was injected for analysis. into the GC-MS/MS for analysis. Figure 1. Schematic illustration of the synthetic route to prepare zeolitic imidazolate framework based Figure 1. Schematic illustration of the synthetic route to prepare zeolitic imidazolate framework on magnetic multiwalled carbon nanotubes and the magnetic solid-phase extraction steps for based on magnetic multiwalled carbon nanotubes and the magnetic solid-phase extraction steps for organochlorine pesticides analysis. organochlorine pesticides analysis. 2.4. Sample Preparation 2.4. Sample Preparation The river water sample was collected from the Liangshui River, Beijing, China. The tap water The river water sample was collected from the Liangshui River, Beijing, China. The tap water sample was obtained from the tap in the laboratory; and the underground water sample was obtained sample was obtained from the tap in the laboratory; and the underground water sample was obtained from Langfang City, Hebei Province, China. All of the samples were filtered through a 0.45 µ m from Langfang City, Hebei Province, China. All of the samples were filtered through a 0.45 m polytetrafluoroethylene membrane filter and stored at 4 °C in amber dark glass bottles. polytetrafluoroethylene membrane filter and stored at 4 C in amber dark glass bottles. 2.5. Apparatus and Gas Chromatography–Tandem Triple Quadrupole Mass Spectrometry Conditions 2.5. Apparatus and Gas Chromatography–Tandem Triple Quadrupole Mass Spectrometry Conditions The surface morphologies and particle sizes of the as-synthesized nanoparticles were observed The surface morphologies and particle sizes of the as-synthesized nanoparticles were observed by scanning electron microscopy (SEM, JSM-6300, JEOL, Tokyo, Japan) and transmission electron by scanning electron microscopy (SEM, JSM-6300, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-200CX, JEOL, Tokyo, Japan). The powder X-ray diffraction (XRD) patterns of microscopy (TEM, JEM-200CX, JEOL, Tokyo, Japan). The powder X-ray diffraction (XRD) patterns of the as-synthesized nanoparticles were obtained with an X-ray powder diffractometer (D8 Advance, the as-synthesized nanoparticles were obtained with an X-ray powder diffractometer (D8 Advance, Bruker, Karlsruhe, Germany). The Fourier-transform infrared (FT-IR) spectra of the as-synthesized Bruker, Karlsruhe, Germany). The Fourier-transform infrared (FT-IR) spectra of the as-synthesized nanoparticles were obtained with an FT-IR-8400 spectrometer (Shimadzu, Kyoto, Japan). A vibrating nanoparticles were obtained with an FT-IR-8400 spectrometer (Shimadzu, Kyoto, Japan). A vibrating sample magnetometer (VSM, Lake Shore 7410, Columbus, OH, USA) was used to investigate the sample magnetometer (VSM, Lake Shore 7410, Columbus, OH, USA) was used to investigate the magnetic properties of all of the synthetic materials. magnetic properties of all of the synthetic materials. Gas chromatography–tandem triple quadrupole mass spectrometry (GC–MS/MS) analysis was performed with a Shimadzu GC-2010 plus gas chromatograph coupled with an AOC-20s autosampler, a Shimadzu TQ8040 triple-quadrupole MS. The pesticides were separated on an Rtx- 5MS capillary column purchased from RESTEK (City, US State abbrev. if applicable, Country) (0.25 Appl. Sci. 2018, 8, 959 5 of 15 Gas chromatography–tandem triple quadrupole mass spectrometry (GC–MS/MS) analysis was performed with a Shimadzu GC-2010 plus gas chromatograph coupled with an AOC-20s autosampler, a Shimadzu TQ8040 triple-quadrupole MS. The pesticides were separated on an Rtx-5MS capillary column purchased from RESTEK (Bellefonte, PA, USA) (0.25 mm (id) 30 m, 0.25 m film thickness). Helium gas was used as the carrier gas at a constant flow rate of 1 mL min . The column temperature was programmed as follows: the initial temperature of 40 C was maintained for 4 min, the temperature 1   1 was increased to 125 C at 25 C min , the temperature was ramped to 300 C at 10 C min , and the temperature was maintained at 300 C for 6 min. The total run time was 30.9 min. The injector temperature was 250 C, and the injection volume was 1.0 L in splitless mode. The specific multiple reaction monitoring (MRM) transitions for all the nine OCPs and the other parameters are given in Table 1. Table 1. Acquisition and chromatographic parameters of the nine organochlorine pesticides. a b Pesticides Retention Time (min) MRM1 (m/z) CE1 (eV) MRM2 (m/z) CE2 (eV) -HCH 15.32 218.90 > 182.90 8 218.90 > 144.90 20 -HCH 15.87 218.90 > 182.90 8 218.90 > 144.90 20 -HCH 16.01 218.90 > 182.90 8 218.90 > 144.90 20 -HCH 16.59 218.90 > 182.90 10 218.90 >144.90 20 0 d 19.47 246.00 > 176.00 30 246.00 > 211.00 22 o,p -DDE p,p -DDE 20.09 246.00 > 176.00 30 246.00 > 211.00 22 0 e p,p -DDD 20.90 235.00 > 165.00 24 235.00 > 199.00 14 0 f o,p -DDT 20.95 235.00 > 165.00 24 235.00 > 199.00 16 p,p -DDT 21.61 235.00 > 165.00 24 235.00 > 199.00 16 a b c MRM means multiple reaction monitoring transitions; CE means collision energy; HCH means d e hexachlorocyclohexane; DDE means 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene; DDD means [1,1-dichloro-2, 2-bis(p-chlorophenyl)ethylene; DDT means 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethylene. 2.6. Quality Control and Quality Assurance Quality control and quality assurance (QA/QC) experiments, including a blank sample test, recovery test, repeatability test, and limits of detection (LOD) experiment were performed to evaluate the feasibility of the method. In addition, 5 L ultrapure water served as the water sample for the blank test, while the water samples for the recovery test and repeatability test were made by spiking 50 L 20 mg L working solution into 100 mL ultrapure water. The LOD experiment was undertaken following the USA Environmental Protection Agency method. 3. Results 3.1. Characterization of M-M-ZIF-67 The micro-morphologies of the Fe O –MWCNTs and M-M-ZIF-67 were observed by SEM and 3 4 TEM. As shown in Figure 2A, the Fe O nanoparticles are attached to the MWCNT surface. The SEM 3 4 image of M-M-ZIF-67 in Figure 2B shows that the composite has a rough surface, indicating that the material has good potential as an adsorbent [27]. VSM was performed to investigate the magnetic behavior of the magnetic materials, and the results are shown in Figure 3A. The magnetic hysteresis loops show that both the remanence and coercivity values of the three types of magnetic materials are zero, which indicates that they have typical supermagnetic properties and could be separated using an external magnet. The saturation magnetization values of Fe O , Fe O –MWCNTs, and M-M-ZIF-67 are 66.8, 59.6, and 53.1 emu g , 3 4 3 4 respectively. As shown in the insert of Figure 3A, well-dispersed M-M-ZIF-67 particles exist in the absence of an external magnet, and they are rapidly attracted to the walls of the vial in a short time (about 20 s) with application of a magnet. The powder XRD patterns of Fe O , Fe O –MWCNTs, 3 4 3 4 and M-M-ZIF-67 are shown in Figure 3B. The diffraction patterns of M-M-ZIF-67 are in very close Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 14 Appl. Sci. 2018, 8, 959 6 of 15 agreement with the materials of Fe O and Fe O –MWCNTs. This indicates that Fe O is well retained 3 4 3 4 3 4 in the Fe O –MWCNTs and M-M-ZIF-67. The FT-IR spectra are shown in Figure 3C. For M-M-ZIF-67, 3 4 the adsorption band at 577 cm corresponds to the stretching vibrations of Fe–O, and the band at 1566 cm corresponds to in-plane bending of CH of the MWCNTs. This suggests that M-M-ZIF-67 is composed of Fe O and MWCNTs. The bands in the region 673 to 1454 cm are attributed to 3 4 complete ring stretching or bending vibration of 2-methylimidazole, and the main adsorption bands Figure 2. (A) transmission electron microscopy image of magnetic multiwalled carbon nanotubes and are preserved in the spectrum of the composite, which indicates that ZIF-67 is successfully synthesized (B) scanning electron microscopy image of zeolitic imidazolate framework based on magnetic on Appthe l. Scisurface . 2018, 8, xof FO M-M-ZIF-67. R PEER REVIEW 6 of 14 multiwalled carbon nanotubes. VSM was performed to investigate the magnetic behavior of the magnetic materials, and the results are shown in Figure 3A. The magnetic hysteresis loops show that both the remanence and coercivity values of the three types of magnetic materials are zero, which indicates that they have typical supermagnetic properties and could be separated using an external magnet. The saturation −1 magnetization values of Fe3O4, Fe3O4–MWCNTs, and M-M-ZIF-67 are 66.8, 59.6, and 53.1 emu g , respectively. As shown in the insert of Figure 3A, well-dispersed M-M-ZIF-67 particles exist in the absence of an external magnet, and they are rapidly attracted to the walls of the vial in a short time (about 20 s) with application of a magnet. The powder XRD patterns of Fe3O4, Fe3O4–MWCNTs, and M-M-ZIF-67 are shown in Figure 3B. The diffraction patterns of M-M-ZIF-67 are in very close agreement with the materials of Fe3O4 and Fe3O4–MWCNTs. This indicates that Fe3O4 is well retained in the Fe3O4–MWCNTs and M-M-ZIF-67. The FT-IR spectra are shown in Figure 3C. For M-M-ZIF- −1 67, the adsorption band at 577 cm corresponds to the stretching vibrations of Fe–O, and the band at −1 1566 cm corresponds to in-plane bending of CH2 of the MWCNTs. This suggests that M-M-ZIF-67 Figure 2. (A) transmission electron microscopy image of magnetic multiwalled carbon nanotubes and −1 is composed of Fe3O4 and MWCNTs. The bands in the region 673 to 1454 cm are attributed to Figure 2. (A) transmission electron microscopy image of magnetic multiwalled carbon nanotubes (B) scanning electron microscopy image of zeolitic imidazolate framework based on magnetic compl and ete (B r) in scanning g stretchi electr ng o on r bmicr endoscopy ing vibra image tion of of zeolitic 2-methy imidazol limidaate zole framework , and the m based ain aon dso magnetic rption bands multiwalled carbon nanotubes. multiwalled carbon nanotubes. are preserved in the spectrum of the composite, which indicates that ZIF-67 is successfully synthesized on the surface of M-M-ZIF-67. VSM was performed to investigate the magnetic behavior of the magnetic materials, and the results are shown in Figure 3A. The magnetic hysteresis loops show that both the remanence and coercivity values of the three types of magnetic materials are zero, which indicates that they have typical supermagnetic properties and could be separated using an external magnet. The saturation −1 magnetization values of Fe3O4, Fe3O4–MWCNTs, and M-M-ZIF-67 are 66.8, 59.6, and 53.1 emu g , respectively. As shown in the insert of Figure 3A, well-dispersed M-M-ZIF-67 particles exist in the absence of an external magnet, and they are rapidly attracted to the walls of the vial in a short time (about 20 s) with application of a magnet. The powder XRD patterns of Fe3O4, Fe3O4–MWCNTs, and M-M-ZIF-67 are shown in Figure 3B. The diffraction patterns of M-M-ZIF-67 are in very close agreement with the materials of Fe3O4 and Fe3O4–MWCNTs. This indicates that Fe3O4 is well retained Figure 3. (A) magnetic hysteresis loops for (a) Fe3O4; (b) magnetic multiwalled carbon nanotubes; and Figure 3. (A) magnetic hysteresis loops for (a) Fe O ; (b) magnetic multiwalled carbon nanotubes; 3 4 in the Fe3O4–MWCNTs and M-M-ZIF-67. The FT-IR spectra are shown in Figure 3C. For M-M-ZIF- c) zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes. The insert shows and c) zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes. The insert −1 67, the adsorption band at 577 cm corresponds to the stretching vibrations of Fe–O, and the band at magnetic separation and dispersion of zeolitic imidazolate framework based on magnetic shows magnetic separation and dispersion of zeolitic imidazolate framework based on magnetic −1 1566 cm corresponds to in-plane bending of CH2 of the MWCNTs. This suggests that M-M-ZIF-67 multiwalled carbon nanotubes; (B) X-ray diffraction patterns of (a) Fe3O4; (b) magnetic multiwalled multiwalled carbon nanotubes; (B) X-ray diffraction patterns of (a) Fe O ; (b) magnetic multiwalled 3 4 −1 is composed of Fe3O4 and MWCNTs. The bands in the region 673 to 1454 cm are attributed to carbon nanotubes; and c) zeolitic imidazolate framework based on magnetic multiwalled carbon carbon nanotubes; and c) zeolitic imidazolate framework based on magnetic multiwalled carbon complete ring stretching or bending vibration of 2-methylimidazole, and the main adsorption bands nanotubes; (C) Fourier-transform infrared spectra of (a) Fe3O4; (b) magnetic multiwalled carbon nanotubes; (C) Fourier-transform infrared spectra of (a) Fe O ; (b) magnetic multiwalled carbon 3 4 are preserved in the spectrum of the composite, which indicates that ZIF-67 is successfully nanotubes; (c) zeolitic imidazolate framework-67; and (d) zeolitic imidazolate framework based on nanotubes; (c) zeolitic imidazolate framework-67; and (d) zeolitic imidazolate framework based on synthesized on the surface of M-M-ZIF-67. magnetic multiwalled carbon nanotubes. magnetic multiwalled carbon nanotubes. 3.2. Optimization of the MSPE Parameters 3.2. Optimization of the MSPE Parameters The MSPE parameters affect extraction of the OCPs and the desorption performance. The amount of M-M-ZIF-67, extraction time, type of desorption solvent, desorption time, and frequency of desorption were investigated by single-factor experiments following a step by step procedure at a spiked level of 50 g L . Figure 3. (A) magnetic hysteresis loops for (a) Fe3O4; (b) magnetic multiwalled carbon nanotubes; and c) zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes. The insert shows magnetic separation and dispersion of zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes; (B) X-ray diffraction patterns of (a) Fe3O4; (b) magnetic multiwalled carbon nanotubes; and c) zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes; (C) Fourier-transform infrared spectra of (a) Fe3O4; (b) magnetic multiwalled carbon nanotubes; (c) zeolitic imidazolate framework-67; and (d) zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes. 3.2. Optimization of the MSPE Parameters Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 14 The MSPE parameters affect extraction of the OCPs and the desorption performance. The amount of M-M-ZIF-67, extraction time, type of desorption solvent, desorption time, and frequency of desorption were investigated by single-factor experiments following a step by step procedure at a −1 spiked level of 50 µ g L . 3.2.1. Optimization of the Extraction Process The present study aims to achieve satisfactory extraction performance for analytes in water samples. The effect of the amount of M-M-ZIF-67 on extraction was investigated using different amounts of the sorbent ranging from 2 to 10 mg. The experimental results (Figure 4A) show that the extraction recoveries of the OCPs rapidly increase when the amount of M-M-ZIF-67 is increased from Appl. Sci. 2018, 8, 959 7 of 15 2 to 6 mg, and the recoveries then slightly decrease when the amount of sorbent is increased from 6 to 10 mg. Based on these results, 6 mg of M-M-ZIF-67 was chosen for the following experiments. The extraction time is an important factor for MSPE because it influences the adsorption 3.2.1. Optimization of the Extraction Process equilibrium of the analytes between the sample solution and adsorbent. Therefore, the extraction time The present study aims to achieve satisfactory extraction performance for analytes in water was varied in the range 5–90 min to investigate its influence on the extraction efficiency. As shown samples. The effect of the amount of M-M-ZIF-67 on extraction was investigated using different in Figure 4B, the recoveries of the target OCPs increase with increasing extraction time from 5 to 20 amounts of the sorbent ranging from 2 to 10 mg. The experimental results (Figure 4A) show that the min, and they then remain almost constant until 30 min. The extraction recoveries for most of the extraction recoveries of the OCPs rapidly increase when the amount of M-M-ZIF-67 is increased from analytes then decrease with a further increase of the extraction time. This is probably because of 2 to 6 mg, and the recoveries then slightly decrease when the amount of sorbent is increased from 6 to desorption of the analytes from the adsorbent, especially when the adsorption equilibrium is reached. 10 mg. Based on these results, 6 mg of M-M-ZIF-67 was chosen for the following experiments. Thus, 20 min was chosen as the extraction time for the following experiments. −1 Figure 4. Optimization of the extraction process for 50 µ g L1 organochlorine pesticides using zeolitic Figure 4. Optimization of the extraction process for 50 g L organochlorine pesticides using zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes: (A) amount of sorbent and imidazolate framework based on magnetic multiwalled carbon nanotubes: (A) amount of sorbent (B) extraction time. Desorption conditions: 2 mL of acetonitrile; ultrasonication time, 5 min; and and (B) extraction time. Desorption conditions: 2 mL of acetonitrile; ultrasonication time, 5 min; and ultrasonication performed one more time. The error bars show the standard deviation of the mean (n ultrasonication performed one more time. The error bars show the standard deviation of the mean = 3). (n = 3). 3.2.2. Optimization of the Desorption Process The extraction time is an important factor for MSPE because it influences the adsorption equilibrium The deso of rp the tio analytes n solvent between is cruci the al sample and it soluti can si on gnand ificaadsorbent. ntly affect Ther the efor MSPE e, the effextraction iciency. Thre time e different solvents were investigated as desorption solvents to investigate their effect on the extraction was varied in the range 5–90 min to investigate its influence on the extraction efficiency. As shown in Figur efficiency: e 4B, the aceto recoveries nitrile, ace ofto the ne,tar an get d nOCPs -hexan incr e. Fi ease gure with 5A incr shoeasing ws that extr the action extracti time on fr reom cov5 erto ies 20 of min, the target analytes eluted by acetonitrile are better than those with the other solvents. Therefore, and they then remain almost constant until 30 min. The extraction recoveries for most of the analytes then acetodecr nitriease le wa with s used a furt as th her e d incr eso ease rptio of n the solvextraction ent for the time. followi This ng e isx pr pe obably rimentbecause s. of desorption of The desorption time is another important factor that influences the efficiency of the MSPE the analytes from the adsorbent, especially when the adsorption equilibrium is reached. Thus, 20 min was proce chosen ss. To ias nvthe estiextraction gate the inf time luence for othe f thfollowing e desorptio experiments. n time on the MSPE efficiency, experiments were performed with ultrasonic desorption times of 2, 5, 10, and 15 min. As shown in Figure 5B, the 3.2.2. Optimization of the Desorption Process extraction recoveries of the analytes are satisfactory when the sample solution is ultrasonicated for 5 and 10 min. Considering the efficiency, an ultrasonic desorption time of 5 min was chosen for the The desorption solvent is crucial and it can significantly affect the MSPE efficiency. Three different subsequent experiments. solvents were investigated as desorption solvents to investigate their effect on the extraction efficiency: acetonitrile, acetone, and n-hexane. Figure 5A shows that the extraction recoveries of the target analytes eluted by acetonitrile are better than those with the other solvents. Therefore, acetonitrile was used as the desorption solvent for the following experiments. The desorption time is another important factor that influences the efficiency of the MSPE process. To investigate the influence of the desorption time on the MSPE efficiency, experiments were performed with ultrasonic desorption times of 2, 5, 10, and 15 min. As shown in Figure 5B, the extraction recoveries of the analytes are satisfactory when the sample solution is ultrasonicated for 5 and 10 min. Considering the efficiency, an ultrasonic desorption time of 5 min was chosen for the subsequent experiments. To investigate the effect of the frequency of desorption, ultrasonic desorption of the nine OCPs was performed one to four times using 2 mL of the desorption solvent and a desorption time of 5 min. Appl. Sci. 2018, 8, 959 8 of 15 The results are shown in Figure 5C. The optimum recoveries are obtained when the analytes are eluted two times. Thus, the frequency of desorption was set to two for the following experiments. Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 14 −1 Figure 5. Optimization of the desorption process for 50 µ g L1 organochlorine pesticides using zeolitic Figure 5. Optimization of the desorption process for 50 g L organochlorine pesticides using zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes: (A) desorption solvent; (B) imidazolate framework based on magnetic multiwalled carbon nanotubes: (A) desorption solvent; desorption time; and (C) frequency of desorption. Extraction conditions: sample volume, 5 mL; (B) desorption time; and (C) frequency of desorption. Extraction conditions: sample volume, 5 mL; amount of sorbent, 6 mg; and extraction time, 20 min. The error bars show the standard deviation of amount of sorbent, 6 mg; and extraction time, 20 min. The error bars show the standard deviation of the mean (n = 3). the mean (n = 3). To investigate the effect of the frequency of desorption, ultrasonic desorption of the nine OCPs 3.3. Method Characterization was performed one to four times using 2 mL of the desorption solvent and a desorption time of 5 A series of experiments was performed under the optimized conditions (Table 2) and the min. The results are shown in Figure 5C. The optimum recoveries are obtained when the analytes are analytical characteristics, such as the linear ranges (LRs), LOD, and relative standard deviations eluted two times. Thus, the frequency of desorption was set to two for the following experiments. (RSDs), were determined to validate the developed method. Working solutions containing the target analytes at concentrations of 1, 2, 5, 10, 20, 50, 100, and 200 g L were prepared to construct the 3.3. Method Characterization working curves. Good linearities for the nine OCPs were achieved with determination coefficients (R ) A series of experiments was performed under the optimized conditions (Table 2) and the ranging from 0.9916 to 0.9989. The LOD for all of the analytes were calculated at signal/noise (S/N) analytical characteristics, such as the linear ranges (LRs), LOD, and relative standard deviations ratios of 3. The LODs range from 0.07 to 1.03 g L . The repeatability of the method was evaluated by (RSDs), were determined to validate the developed method. Working solutions containing the target performing six replicate analyses of spiked samples with 10 g L of each of the OCPs, and the RSDs −1 analytes at concentrations of 1, 2, 5, 10, 20, 50, 100, and 200 µ g L were prepared to construct the are in the range 1.0–8.5%. All of these results indicate that the proposed method has high sensitivity working curves. Good linearities for the nine OCPs were achieved with determination coefficients and good repeatability. (R ) ranging from 0.9916 to 0.9989. The LOD for all of the analytes were calcul ated at signal/noise −1 (S/N) ratios of 3. The LODs range from 0.07 to 1.03 µ g L . The repeatability of the method was Table 2. Analytical parameters of zeolitic imidazolate framework based on magnetic multiwalled −1 evaluated by performing six replicate analyses of spiked samples with 10 µ g L of each of the OCPs, carbon nanotubes as an adsorbent for magnetic solid-phase extraction of nine organochlorine pesticides and the RSDs are in the range 1.0–8.5%. All of these results indicate that the proposed method has in an ultrapure water sample. high sensitivity and good repeatability. 1 a 1 OCPs Determination Coefficients Linear Range (g L ) LOD (g L ) RSD (%) (n = 6) Table 2. Analytical parameters of zeolitic imidazolate framework based on magnetic multiwalled -HCH 2–200 0.9953 0.12 8.5 carbon nanotubes as an adsorbent for magnetic solid-phase extraction of nine organochlorine -HCH 1–200 0.9946 0.13 1.0 -HCH 1–200 0.9919 0.15 6.2 pesticides in an ultrapure water sample. -HCH 1–200 0.9947 0.07 4.0 a b o,p -DDE 1–200 0.9951 0.17 1.5 Linear Range LOD RSD (%) 0 OCPs Determination Coefficients p,p -DDE 1–200 0.9989 0.45 1.3 −1 −1 (µg L ) (µg L ) (n = 6) p,p -DDD 2–200 0.9934 0.41 3.1 0 α-HCH 2–200 0.9953 0.12 8.5 o,p -DDT 2–200 0.9932 0.74 3.9 p,p -DDT β-HCH 2–200 1–200 0.9916 0.9946 1.03 0.13 13.1 .0 a b γ-HCH 1–200 0.9919 0.15 6.2 LOD means limit of determination; RSDs means relative standard deviations, and were determined by performing six replicate analyses of the spiked samples with 10 g L of each of the organochlorine pesticides. δ-HCH 1–200 0.9947 0.07 4.0 o,p′-DDE 1–200 0.9951 0.17 1.5 3.4. Comparison p,p′-DDE 1–200 0.9989 0.45 1.3 p,p′-DDD 2–200 0.9934 0.41 3.1 To investigate the extraction performance of the prepared materials, 4 mg of Fe O –MWCNTs 3 4 o,p′-DDT 2–200 0.9932 0.74 3.9 and 6 mg of M-M-ZIF-67, which was synthesized based on the same amount of the former material, p,p′-DDT 2–200 0.9916 1.03 3.1 were applied to extraction of the nine OCPs using the proposed method. The results are shown in a b LOD means limit of determination; RSDs means relative standard deviations, and were determined Figure 6 based on three replicate analyses (n = 3). The peak areas of all of the analytes for M-M-ZIF-67 −1 by performing six replicate analyses of the spiked samples with 10 µ g L of each of the organochlorine pesticides. Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 14 3.4. Comparison To investigate the extraction performance of the prepared materials, 4 mg of Fe 3O4–MWCNTs and 6 mg of M-M-ZIF-67, which was synthesized based on the same amount of the former material, were applied to extraction of the nine OCPs using the proposed method. The results are shown in Appl. Sci. 2018, 8, 959 9 of 15 Figure 6 based on three replicate analyses (n = 3). The peak areas of all of the analytes for M-M-ZIF- 67 are 2.7–3.0 times higher than those for Fe3O4–MWCNTs, indicating that ZIF-67 plays a greater role arein 2.7–3.0 the extimes tractio higher n proce than ss. those for Fe O –MWCNTs, indicating that ZIF-67 plays a greater role in 3 4 the extraction process. Figure Figure 6. Ef 6.fect Effec oft the of th sorbent e sorben on t o the n th adsorption e adsorptio capacities n capacitie for s fo the r th nine e nin or e ganochlorine organochlorin pesticides. e pesticides. Extraction conditions: sample volume, 5 mL; amount of sorbent, 6 mg zeolitic imidazolate framework Extraction conditions: sample volume, 5 mL; amount of sorbent, 6 mg zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes and 4 mg magnetic multiwalled carbon nanotubes; based on magnetic multiwalled carbon nanotubes and 4 mg magnetic multiwalled carbon nanotubes; and extraction time, 20 min. Desorption conditions: 2 mL of acetonitrile; ultrasonication time, 5 min, and extraction time, 20 min. Desorption conditions: 2 mL of acetonitrile; ultrasonication time, 5 min, and ultrasonication performed one more time. The error bars show the standard deviation of the mean and ultrasonication performed one more time. The error bars show the standard deviation of the mean (n = 3). (n = 3). The performance of the current method was compared with some recently reported methods The performance of the current method was compared with some recently reported methods used for analysis of OCPs in water. The comparison data (Table 3) show that the proposed method used for analysis of OCPs in water. The comparison data (Table 3) show that the proposed method based on the M-M-ZIF-67 adsorbent exhibits good sensitivity (as the LOD), less use of sample and based on the M-M-ZIF-67 adsorbent exhibits good sensitivity (as the LOD), less use of sample and adsorbent. However, the Fe3O4–MWCNTs based adsorbent can make the phase separation process adsorbent. However, the Fe O –MWCNTs based adsorbent can make the phase separation process 3 4 easier and faster without additional centrifugation or filtration procedures, and also can avoid the easier and faster without additional centrifugation or filtration procedures, and also can avoid the time-consuming column passing operations encountered in SPE. Therefore, the developed M-M-ZIF- time-consuming column passing operations encountered in SPE. Therefore, the developed M-M-ZIF-67 67 based magnetic solid-phase extraction method showed agreement in accordance with the based magnetic solid-phase extraction method showed agreement in accordance with the principles of principles of green analytical chemistry [41]. green analytical chemistry [41]. Appl. Sci. 2018, 8, 959 10 of 15 Table 3. Comparison of different methods for analysis of organochlorine pesticides. Sample Number of Volume of Elution Sorbent Extraction Method Sorbent LOD RSD (%) Spiked Level Ref. Amount (mL) OCPs Solvent Amount (mg) Time (min) MSPE-GC-MS/MS BMZIF-derived 1 1 10 8 Dichloromethane, 2.00 mL 6 10 0.39–0.70 ng L 5.5–9.1 5–500 ng L [42] a b carbon c 1 1 -SPE-GC-MS ZnO-CF 10 15 Toluene, 0.30 mL 15 30 0.19–1.64 g L 2.3–10.2 1–50 g L [43] Cyclohexane-ethyl acetate e f 1 1 PT-SPE-GC-ECD GUF-MIR 1 3 5 - 0.24–0.66 ng g 3.5–6.7 2.2–220 ng g [44] (9:1, v/v), 0.60 mL g 1 1 MSPE-GC-ECD RGO/Fe O @Au 10 6 Acetonitrile, 0.25 mL 20 10 0.4–4.1 g L 1.7–7.3 100 g L [45] 3 4 h 1 1 MSPE-GC-ECD 20 5 Acetonitrile, 0.20 mL 20 10 1.0–1.9 ng L 6.2–8.3 100 g L [46] Fe O @MAA@IBL 3 4 rGO-amino-HNT@PT i 1 1 d-SPE -GC-MS 10 6 Acetonitrile, 0.50 mL 5 5 2–13 ng L 6.1–9.7 5–70 g L [47] k 1 1 -SPE-GC-MS MIL-101 10 5 Ethyl acetate, 0.20 mL 4 40 2.5–16 ng L 4.2–11.0 10 g L [48] acetonitrile-dichloromethane l 1 1 MSPE-GC/ECD -CD/MRGO 50 16 15 3 0.5–3.2 ng Kg 3.3–7.8 50 ng kg [49] (4:1, v/v), 1.00 mL m 1 1 MSPE-GC-MS/MS M-M-ZIF-67 5 9 Acetonitrile, 4.00 mL 6 20 0.07–1.03 g L 1.0–8.5 10 g L This work a b magnetic solid-phase extraction followed by gas chromatography–tandem triple quadrupole mass spectrometry determination; micro solid-phase extraction followed by gas c d e chromatography–mass spectrometry determination; Bimetallic MOF; Zinc oxide nanoparticles incorporated in carbon foam; Miniaturized pipette tip solid-phase extraction followed f g by gas chromatography combination with electronic capture detector determination; Glyoxal–urea–formaldehyde molecularly imprinted resin; Reduced graphene oxide/Fe O @gold 3 4 h i j nanocomposite; Fe O @mercaptoacetic acid@imine-based ligand; dispersive micro solid-phase extraction; Reduced graphene oxide–amino-halloysite nanotubes@polythiophene; 3 4 k l m metal-organic framework; -Cyclodextrin/iron oxide-reduced graphene oxide hybrid nanostructure; zeolitic imidazolate framework based on magnetic multiwalled carbon nanotubes. Appl. Sci. 2018, 8, 959 11 of 15 3.5. Real Sample Analysis The proposed MSPE method was applied to determination of OCPs in real agricultural irrigation water samples, including tap water, river water, and underground water. The results are summarized in Table 4 and the total ion chromatograms (TICs) of the analytes acquired from the tap water samples are shown in Figure 7. There are no OCPs in the selected water samples and the recovery results indicate that the developed method has good utility for analysis of OCPs in real water samples. Table 4. Analytical results for determination of organochlorine pesticides in real water samples. Spiked Concentration (g L , n = 3) Analyte Matrix 0 10 100 Found Recovery (%) RSD (%) Recovery (%) RSD (%) -HCH <LOD 83.4 7.4 84.7 0.7 -HCH <LOD 92.5 9.6 103.7 0.6 -HCH <LOD 93.3 5.5 91.9 0.4 -HCH <LOD 94.6 6.6 111.1 1.0 Tap water o,p -DDE <LOD 85.4 3.4 99.6 5.3 p,p -DDE <LOD 78.1 1.5 107.5 3.1 p,p -DDD <LOD 76.0 2.5 97.0 2.5 o,p -DDT <LOD 83.1 5.4 89.2 2.1 p,p -DDT <LOD 93.7 2.1 105.7 1.6 -HCH <LOD 76.8 5.7 80.5 2.4 -HCH <LOD 91.3 3.1 102.5 0.8 -HCH <LOD 84.1 8.5 91.2 3.5 -HCH <LOD 74.9 5.9 112.7 2.6 River water o,p -DDE <LOD 110.8 4.7 94.3 0.1 p,p -DDE <LOD 102.5 4.8 100.9 1.2 p,p -DDD <LOD 101.9 3.4 92.2 0.7 o,p -DDT <LOD 108.6 4.3 86.9 1.2 p,p -DDT <LOD 110.8 3.5 100.9 0.8 Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 14 -HCH <LOD 81.0 8.7 80.3 3.3 -HCH <LOD 83.9 7.8 97.2 2.1 3.5. Real Sample Analysis -HCH <LOD 90.1 7.5 80.4 5.2 -HCH <LOD 79.5 15.3 112.7 2.0 The proposed MSPE method was applied to determination of OCPs in real agricultural irrigation Underground water o,p -DDE <LOD 101.5 8.4 82.9 1.9 water samples, including tap water, river water, and underground water. The results are summarized p,p -DDE <LOD 95.3 7.6 81.0 1.4 p,p -DDD <LOD 92.8 8.9 75.1 2.2 in Table 4 and the total ion chromatograms (TICs) of the analytes acquired from the tap water samples o,p -DDT <LOD 111.1 6.4 110.3 3.2 are shown in Figure 7. There are no OCPs in the selected water samples and the recovery results p,p -DDT <LOD 116.3 3.7 90.5 2.3 indicate that the developed method has good utility for analysis of OCPs in real water samples. Figure 7. Total ion chromatograms of the nine organochlorine pesticides in water samples obtained Figure 7. Total ion chromatograms of the nine organochlorine pesticides in water samples obtained by gas chromatography–tandem triple quadrupole mass spectrometry: (a) tap water; (b) river water; by gas chromatography–tandem triple quadrupole mass spectrometry: (a) tap water; (b) river water; −1 −1 (c) underground water; (d) tap water spiked at 10 µ g L;1 and (e) tap water spiked at 100 µ g L . 1 (c) underground water; (d) tap water spiked at 10 g L ; and (e) tap water spiked at 100 g L . Table 4. Analytical results for determination of organochlorine pesticides in real water samples. −1 Spiked Concentration (µg L , n = 3) Matrix Analyte 0 10 100 Found Recovery (%) RSD (%) Recovery (%) RSD (%) α-HCH <LOD 83.4 7.4 84.7 0.7 β-HCH <LOD 92.5 9.6 103.7 0.6 γ-HCH <LOD 93.3 5.5 91.9 0.4 δ-HCH <LOD 94.6 6.6 111.1 1.0 Tap water o,p′-DDE <LOD 85.4 3.4 99.6 5.3 p,p′-DDE <LOD 78.1 1.5 107.5 3.1 p,p′-DDD <LOD 76.0 2.5 97.0 2.5 o,p′-DDT <LOD 83.1 5.4 89.2 2.1 p,p′-DDT <LOD 93.7 2.1 105.7 1.6 α-HCH <LOD 76.8 5.7 80.5 2.4 β-HCH <LOD 91.3 3.1 102.5 0.8 γ-HCH <LOD 84.1 8.5 91.2 3.5 δ-HCH <LOD 74.9 5.9 112.7 2.6 River water o,p′-DDE <LOD 110.8 4.7 94.3 0.1 p,p′-DDE <LOD 102.5 4.8 100.9 1.2 p,p′-DDD <LOD 101.9 3.4 92.2 0.7 o,p′-DDT <LOD 108.6 4.3 86.9 1.2 p,p′-DDT <LOD 110.8 3.5 100.9 0.8 α-HCH <LOD 81.0 8.7 80.3 3.3 β-HCH <LOD 83.9 7.8 97.2 2.1 Underground water γ-HCH <LOD 90.1 7.5 80.4 5.2 δ-HCH <LOD 79.5 15.3 112.7 2.0 Appl. Sci. 2018, 8, 959 12 of 15 4. Conclusions Magnetic composites containing Fe O , MWCNTs, and ZIF-67 have been synthesized. 3 4 The synthesized materials have porous surfaces and exhibit super-paramagnetism. They were used as sorbents for MSPE to extract nine OCPs from agricultural irrigation water samples. For M-M-ZIF-67, the OCP adsorption capacities are nearly three times higher than those for Fe O –MWCNTs. 3 4 The developed method achieves high extraction efficiencies, good linearities, low detection limits, and good accuracies and precision. The results suggest that M-M-ZIF-67 is a simple and effective potential adsorbent for removal of OCPs from environmental water samples. Author Contributions: X.H. and G.L. conceived and designed the experiments; X.X. and L.L. performed the experiments; X.H. and S.Z. analyzed the data; D.X. and H.L. contributed reagents and materials; X.H. and H.G. wrote the paper. Authorship must be limited to those who have contributed substantially to the work reported. 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Published: Jun 12, 2018

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