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Li Liu Xin Zhou Chun Wang Qiuhua Wu Zhi Wang Department of Chemistry, College of Science, Agricultural University of Hebei, Baoding, China Received October 1, 2014 Revised November 29, 2014 Accepted December 8, 2014
Research Article
Extraction and enrichment of polycyclic aromatic hydrocarbons by ordered mesoporous carbon reinforced hollow fiber liquid-phase microextraction A novel microextraction method, ordered mesoporous carbon reinforced hollow fiber liquidphase microextraction coupled with high-performance liquid chromatography and fluorescence detection, was developed for the determination of some organic pollutants in water samples. Four polycyclic aromatic hydrocarbons (fluorene, anthracene, fluoranthene, and pyrene) were selected to validate this new method. Main parameters that could influence the extraction efficiency such as extraction time, fiber length, stirring rate, the type of the extraction solvent, pH value, the concentration of ordered mesoporous carbon, and salt effect were optimized. Under the optimal extraction conditions, good linearity was observed in the range of 2–1000 ng/L, with the correlation coefficients of 0.9954–0.9986. The recoveries for the spiked samples were in the range of 88.96–100.17%. The limits of detection of the method were 0.4–4 ng/L. The relative standard deviations varied from 4.2–5.9%. The results demonstrated that the newly developed method was an efficient pretreatment and enrichment procedure for the determination of polycyclic aromatic hydrocarbons in environmental water samples. Keywords: Hollow fibers / Liquid-phase microextraction / Mesoporous carbon / Polycyclic aromatic hydrocarbons / Water samples DOI 10.1002/jssc.201401071
Additional supporting information may be found in the online version of this article at the publisher’s web-site
1 Introduction Polycyclic aromatic hydrocarbons (PAHs) are important environmental persistent pollutants containing two or more fused aromatic rings [1]. They are formed during the incomplete combustion or high-temperature pyrolysis of oil and gas, coal, smoked meat, and other organic matter containing carbon and hydrogen [2]. Owing to the aerodynamic character of smoke, PAHs have been widely distributed in the environment [3], which could pose a potential hazard to human health and the environment. PAHs can threaten the safety of food chains for both humans and animals [4] and find their way into human organs through skin exposure, diet, and breathing, which may cause lung cancer, upper digestive tract cancer, skin cancer, bladder cancer, etc. [5,6]. Therefore, it is highly desirable to develop simple and sensitive analytical methods for the determination of trace level of PAHs in different samples. Correspondence: Dr. Qiuhua Wu, College of Science, Agricultural University of Hebei, Baoding 071001, Hebei, China E-mail: [email protected] Fax: +86-312-7528292
Abbreviations: ANT, Anthracene; BET, Brunauer–Emmett– Teller; FLD, fluorescence detection; FLT, Fluoranthene; FLU, Fluorene; OMC, ordered mesoporous carbon; PAH, polycyclic aromatic hydrocarbon; PYR, Pyrene C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
PAHs commonly exist in very low concentrations in the aquatic environment because they are hydrophobic compounds with low water solubility [7]. So before an instrumental analysis, a sufficient and effective sample pretreatment method is usually required and is sometimes even a key step in the whole analytical procedure. So far, various sample preconcentration techniques have been developed and subsequently applied for the monitoring of PAHs from different samples, such as LLE [8], SPE [9–11], SPME [12,13], ultrasonic extraction [14,15] PLE [16], MAE [17], and LPME based on the solidification of a floating organic microdrop [18, 19]. Among them, LLE, SPME, and SPE have become widely accepted and increasingly used for the preconcentration of PAHs from different samples. However, these methods suffer from one or more drawbacks. LLE requires a large amount of toxic organic solvents. SPE is time consuming and relatively expensive. SPME is a simple, organic-solvent-free and efficient extraction technique, but the number and variety of the commercially available fibers are limited. To overcome these drawbacks, a relatively new sample pretreatment technique called hollow fiber (HF) LPME has attracted considerable attention in analytical chemistry in recent years [20–23]. HF-LPME, introduced by Pedersen-Bjergaard and Rasmussen [24], is a miniaturization of traditional LLE and represents a recent advancement in the field of sample preseparation and preconcentration techniques. The www.jss-journal.com
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basic principle of HF-LPME has been depicted clearly in previously reported literature [25, 26]. The advantages of HF-LPME are as follows: Firstly, the HF-LPME equipment is very simple and inexpensive [27]. Secondly, HF-LPME has high enrichment factors, low consumption of organic solvents, and excellent sample cleanup function. Thirdly, the HF is disposable after each use due to its cheapness, environmentally friendly and thus the carryover problems can be overcome [28]. Besides, because the large molecules and particles are prevented from entering the small pore size of the fiber, HF-LPME has good selectivity [29]. In recent years, mesoporous carbon materials have sparked considerable research interest for potential applications as adsorbents [30, 31], supercapacitors [32], and catalyst supports [33], because of its tunable pore size, high specific surface area, low density, high chemical stability, good electrical conductivity, and thermal conductivity [34,35]. Various templates including silica [36], metal–organic frameworks [37], and molecular sieves [38, 39] have been successfully used for the synthesis of mesoporous carbon materials [40, 41]. Among them, the molecular sieve MCM-41 also has been successfully employed as the template for the preparation of mesoporous carbon materials [42–44]. There are some reports about the applications of MCM-41-based materials as the adsorbent for the extraction or removal of organic pollutants. For example, Shao et al. have prepared Mn/MCM41 composites for methyl blue removal [45]. Parida et al. have applied amine-functionalized MCM-41 (NH2 -MCM-41) for the sorption of copper(II) [46]. Mello et al. have used aminemodified MCM-41 mesoporous materials for the sorption of CO2 [47]. However, the application of MCM-41-based ordered mesoporous carbon as an adsorbent for the extraction or removal of organic pollutants has not been reported in the literature. The main aim of this study was to prepare an ordered mesoporous carbon (OMC) using MCM-41 as template and explore its application potentials in the extraction of organic pollutants when it was incorporated into HF-LPME. To the best of our knowledge, this is the first report that OMCreinforced HF-LPME (OMC-HF-LPME) has been used for the preconcentration of four polycyclic aromatic hydrocarbons in environmental water before their determination by HPLC with fluorescence detection (FLD). Several important experimental parameters that could affect the extraction efficiency were investigated and optimized. The results proved that developed OMC-HF-LPME method integrated the high adsorption capacity of the OMC and clean-up effect of HF-LPME, and had a good performance in the determination of four polycyclic aromatic hydrocarbons in environmental water.
2 Materials and methods 2.1 Reagents and materials MCM-41 template was obtained from Boaixin Chemical Reagents (Baoding, China). Pesticide standards of the polycyclic aromatic hydrocarbons (Fluorene (FLU), C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Anthracene (ANT), Fluoranthene (FLT), and Pyrene (PYR)) were purchased from Aladdin-Reagent (Shanghai, China). 1-Octanol, tetrahydrofuran, ethyl acetate, n-hexane, chromatography-grade acetonitrile, and methanol were purchased from Huaxin Chemical Reagent (Baoding, China). Sodium chloride (NaCl), hydrochloric acid (HCl), H2 SO4 (98%), ethanol, and all other reagents were purchased from Beijing Chemical Reagents (Beijing, China). The water used throughout the work was double-distilled on a SZ-93 automatic double-distiller purchased from Shanghai Yarong Biochemistry Instrumental Factory (Shanghai, China, http://yarong.instrument.com.cn/). The Accurel Q 3/2 polypropylene hollow fiber membrane (200 m thick wall, 600 m inner diameter, and 0.2 m average pore size) was purchased from Membrana (Wuppertal, Germany).
2.2 Apparatus The HPLC system, assembled from modular components (Waters, Milford, MA, USA), consisted of a 600E pump, and a fluorescence detector. The mobile phase was a mixture of acetonitrile/water (75: 25 v/v) at a flow rate of 1 mL/min. A Centurysil C18 column (4.6 id × 250 mm, 5.0 m) from Dalian Jiangshen Separation Science (Dalian, China) was used for separations. A Millennium32 workstation (Waters) was utilized to control the system and process the chromatographic data. The fluorescence detection was performed by the following excitation (Ex)/emission (Em) wavelength program: 254/323 nm from 0 to 10.5 min for the determination of FLU, 252/402 nm from 10.5 to 11.0 min for ANT, 280/460 nm from 11.5 to 15 min for FLT, and 270/390 nm from 15 to 30.00 min for PYR, respectively. The size and morphology of the MC were observed by TEM using a JEOL model JEM-2011(HR) (Tokyo, Japan) at 5 kV and SEM using an S-3000N microscope (Hitachi, Japan). The Brunauer–Emmett–Teller (BET) surface areas were determined from the N2 adsorption at 77 K using a Tristar II 3020 instrument (USA).
2.3 Preparation of OMC OMC was prepared according to the literature method [48] with some modifications. Firstly, 1 g of MCM-41 template, 1.5 g of sucrose and 5 mL of distilled water were added into a 50 mL of beaker. After magnetic stirring the mixtures for 50 min, 0.19 g of H2 SO4 (98 wt%) was added into the solution, followed by magnetic stirring the mixtures for 10 min. After that, the mixtures were heated at 100⬚C for 6 h and at 160⬚C for 6 h in oven. Secondly, the mixture was cooled to room temperature and the resultant black precipitate was ground to a fine powder. After the addition of 1 g of sucrose, 0.1 g of H2 SO4 (98 wt%) and 5 mL of distilled water, the samples were treated again at 100⬚C for 6 h and at 160⬚C for 6 h. The obtained MCM-41/sucrose composites were carbonized in a conventional furnace at 900⬚C for 2 h in nitrogen flow. Then, www.jss-journal.com
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Figure 1. TEM (A) and SEM (B) images of the OMC composite.
the MCM-41 template was removed by mixing the composite with 20 mL of HF (25 wt%) for 10 h and the obtained porous carbon was rinsed with ethanol and distilled water, respectively, to neutralize the material surface. Finally, the products were dried in an oven and 0.3 g MCM-41-based mesoporous carbon material was obtained.
2.4 Sample collection Lake water samples were collected from Qiandaohu (Hangzhou, China); sea water was collected from Rushan (Shandong, China). All the solvents and water samples were filtered through a 0.45 m membrane to eliminate particulate matter before analysis.
3 Results and discussion 3.1 Characterization of OMC The morphology and microstructure of the OMC were examined by TEM and SEM. The TEM image (Fig. 1A) and the SEM image (Fig. 1B) show that the morphology of the MCM41 silica particles and their structural characteristics are preserved in the as-prepared templated carbon. The OMC like the MCM-41 molecular sieve was nanoporous carbon with interconnected hierarchical pore and parallel mesoporous channel structures. Further evidence of the porous structure of the OMC is provided by the N2 sorption isotherm (Supporting Information Fig. S1). The OMC exhibits a porosity made up of mesopores with size 4.5 nm and it has a BET surface area of 302 m2 /g and a pore volume of 0.34 cm3 /g.
2.5 OMC-HF-LPME procedure The hollow fiber was cut into segments of 6 cm long. Then, the fiber was washed with acetone in an ultrasonic bath for 5 min to remove any impurities in the fiber and dried in air. After that, the fiber was immersed entirely in 1-octanol for 30 s to impregnate the fiber pores. Then, the excess of 1-octanol was carefully removed by washing the surface and the inner wall of the hollow fiber with double-distilled water under ultrasonication for 1 min. Then, 20 L acceptor phase (1.0 mg/mL OMC in 1-octanol) was injected into the lumen of the fiber. Then both ends of the fiber were sealed with heated tweezers. Extractions were performed according to the following procedure: the prepared hollow fiber was immersed in 20 mL of sample solution containing 10 L 1-octanol and a magnetic stir bar in a 25 mL screw cap glass vial. The extraction process was performed at 800 rpm stirring rate for 30 min. After extraction, the fiber was taken out from the glass vial carefully and the outside of the fiber was dried with filter paper. Then, the fiber was transferred into a 0.5 mL microvial and 100 L methanol was added for desorption by vortexing for 1 min. Finally, 20 L desorption solution was injected for HPLC analysis. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3.2 Optimization of extraction conditions To achieve a high enrichment and extraction efficiency, several parameters, including desorption solution, sample pH, extraction time, stirring rate, salt concentration, the amount of OMC, and fiber length were optimized.
3.2.1 Selection of organic extraction solvent and its volume The choice of organic extraction solvents plays a key role in HF-LPME since both extraction recovery and speed are affected by the partition coefficients between the sample and the organic phase. There are several factors that should be considered: (1) The organic extraction solvent should have a good affinity with the hollow fiber and should be insoluble in sample solution, (2) it should have a low volatility, low toxicity, and an appropriate viscosity, (3) it should have a high partition coefficient for the analytes, and a good dispersion capability for OMC. Based on these considerations, several organic extraction solvents (1-octanol, tetrahydrofuran, ethyl www.jss-journal.com
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acetate, and n-hexane) were chosen to evaluate their influence on the extraction efficiency. As a result, 1-octanol gave a higher sensitivity than the other solvents for all the analytes. Therefore, 1-octanol was chosen as the extraction solvent for the acceptor phase. In HF-LPME, adding small amount of the extraction solvent into the sample solution could increase the contact area between the extraction solvent and the sample. In this study, the effect of the added volume of 1-octanol to the sample solution was also investigated in the range from 0 to 30 L. The result indicated that the extraction efficiency reached the maximum when the volume of 1-octanol was increased to 10 L. Hence, 10 L of 1-octanol was selected for subsequent studies. 3.2.2 Effect of the concentration of OMC To evaluate the effect of the addition of OMC into the acceptor phase on the extraction capability, the concentration of OMC was investigated in the range from 0 to 4 mg/mL. As shown in Supporting Information Fig. S2, the peak areas of the four PAHs increased with increasing concentration of OMC from 0 to 1 mg/mL, and the peak areas of the four PAHs remained almost constant when the concentration of OMC exceeded 1 mg/mL. Considering the detection sensitivity and consumption of OMC, 1 mg/mL of OMC was chosen. 3.2.3 Fiber length In this study, the influence of the hollow fiber length was examined by changing its length in the range from 2 to 8 cm. As is illustrated in Supporting Information Fig. S3, the peak areas of the four PAHs increased by increasing the hollow fiber length from 2 to 6 cm, and after that, the signals kept almost constant. So, a 6 cm long hollow fiber was used for the further experiments. 3.2.4 Effect of extraction time Generally, extraction time is a very important factor in most extraction procedures since it influences the analytes’ partition between the sample solution and the organic phase. In the present work, to obtain the optimum extraction efficiency, a series of extraction time were evaluated to establish extraction–time profiles of target compounds. According to the results illustrated in Supporting Information Fig. S4, the peak areas of the four PAHs increased with increased extraction time before 30 min and after that there were no significant changes in the peak areas. Therefore, the optimum extraction time for the four PAHs was 30 min. 3.2.5 Sample agitation rate Fast agitation of the sample solution could continuously update the sample solution outside of the fiber and increase the contacting frequencies between the analytes and the fiber, C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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thus reducing the extraction equilibrium time. At the same time, a high stirring rate could help the dispersion of the OMC during the extraction. So stirring intensity is one of the important parameters that require to be optimized. In this work, different stirring speeds, from 200 to 1000 rpm were tested. The results in Supporting Information Fig. S5 revealed that the peak areas of the analytes increased with increased stirring speed from 200 to 800 rpm and then remained almost unchanged. Hence, 800 rpm was selected as the optimum agitation rate for the subsequent experiments. 3.2.6 Effect of ionic strength of the samples Generally, the presence of salt in sample solution could affect the solubility of the analytes and therefore the extraction efficiencies would be enhanced due to the salting-out effect, but on the other hand, it could also increase the viscosity of the solution, which would decrease the extraction capability. The influence of NaCl with the concentration ranging from 0–16% w/v was evaluated. The results showed that the addition of NaCl had a negligible effect on the peak areas of the analytes in the concentration range investigated. So no salt was added to the samples in the following experiments. 3.2.7 Effect of sample solution pH It is well known that the pH of sample solution plays a crucial factor in the extraction performance because the existing forms of the analytes in the sample solution can be affected by the pH value. Therefore, the influence of solution pH on the extraction efficiency of the four PAHs was investigated within the pH range of 2.0–12.0 by adding 1.0 mol/L sodium hydroxide or 1.0 mol/L hydrochloric acid to the sample solutions. The experimental results showed that no obvious peak area changes were observed for the PAHs when the pH of the sample solution was changed, which could be attributed to the fact that the PAHs exist as neutral compounds under ordinary conditions and the PAHs existing forms are unlikely influenced by the change of the sample solution pH. So, the pH of the sample solution was not adjusted.
3.3 Desorption solvent and desorption time After extraction, the hollow fiber was transferred to a centrifuge tube for desorption. In this study, three organic solvents including methanol, acetone, and acetonitrile were evaluated as the desorption solvent for the desorption of the analytes. Under the same extraction and elution conditions, the results showed that the three organic solvents provided similar results, but a better chromatographic peak shape was obtained when methanol was used. Thus, methanol was selected as the desorption solvent. The effect of desorption time and the methanol volume was also investigated. The desorption time was optimized by vortexing from 30 s to 4 min. As a result, four PAHs peak areas reached the highest values at desorption time of 1.0 min, and no obvious increase in peak www.jss-journal.com
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J. Sep. Sci. 2015, 38, 683–689 Table 1. Linear range, precision, correlation coefficients, and detection limits obtained for the proposed method to determine polycyclic aromatic hydrocarbons pesticides in water sample
PAHs
LRa) (ng/L)
r
LOD(ng/L)
Table 2. Determination of polycyclic aromatic hydrocarbons pesticides and recoveries in water samples
Sea water (n = 5)
RSD (%) (n = 5) PAHs
FLU ANT FLT PYR
5.0–1000.0 2.0–1000.0 10.0–1000.0 10.0–1000.0
0.9906 0.9913 0.9986 0.9954
1.0 0.4 4.0 4.0
5.3 4.8 4.2 5.9
FLU
ANT a) LR, linear range.
FLT
areas was observed when longer desorption time was applied. Therefore, 1 min desorption time was chosen. The volume of methanol was investigated in the range from 20 to 150 L, and as a result, 100 L methanol yielded the best desorption result.
687
PYR
Spiked (ng/L) 0.0 40.0 200.0 0.0 16.0 80.0 0.0 80.0 400.0 0.0 80.0 400.0
Found (ng/L) ndb) 35.56 197.8 ndb) 14.51 73.3 ndb) 80.03 400.6 ndb) 78.93 399.9
Lake water (n = 5)
Ra) (%)
RSD (%)
88.89 98.92
6.2 5.1
90.61 91.63
5.6 4.7
100.0 100.1
5.3 6.8
98.63 99.95
5.2 4.5
Found (ng/L) 6.25 41.03 200.1 2.81 18.50 82.01 ndb) 79.97 400.1 ndb) 79.30 399.7
Ra) (%)
RSD (%)
86.96 96.96
6.2 7.1
98.06 99.00
7.4 5.1
99.96 100.0
4.6 5.9
99.13 99.94
4.1 6.3
a) R: recovery of the method. b) nd, not detected.
3.4 Linearity, repeatability, and limits of detection The performance parameters of the OMC-HF-LPME technique, such as repeatability, linearity, and LOD were investigated to estimate the efficiency and the feasibility for its application to the analysis of environmental water samples. A series of PAH-free double-distilled water samples spiked with each of the PAHs at different concentration levels (0.5, 1.0, 2.0, 5.0, 10.0, 20.0, 50.0, 100.0, 1000.0 ng/L) were prepared for the establishment of the calibration curve. Each concentration level was carried out in triplicate, and results were presented in Table 1. Based on the S/N of 3, the LOD was 1.0 ng/L for FLU, 0.4 ng/mL for ANT and 4 ng/mL for both FLT and PYR. The linear concentration ranges of the method were from 5 to 1000.0 ng/L for FLU, from 2.0 to 1000.0 ng/L for ANT and from 2.0 to 1000.0 ng/L for both FLT and PYR, with the correlation coefficients of 0.9954– 0.9986. The characteristic calibration data obtained are listed in Table 1.
The repeatability study was carried out by performing five parallel analyses at two concentration levels (40.0 and 200.0 ng/L for FLU, 16.0 and 80.0 ng/L for ANT, 80.0 and 400.0 ng/L for FLT and PYR). The RSDs varied from 4.2–5.9% (Table 1). These results indicated that the developed method of OMC-HF-LPME had good repeatability, high sensitivity, and wide linear range.
3.5 Environmental water samples analysis The different environmental water samples were applied to validate the applicability of the method. The results shown in Table 2 indicate that no residues of the PAHs were detected in sea water samples; only a low concentration of FLU (6.25 ng/L) and ANT (2.81 ng/L) was found in lake water sample. To assess the accuracy of the method, water samples were spiked at two concentration levels (40.0 and
Figure 2. The typical chromatograms for the extracted polycyclic aromatic hydrocarbons from lake water (A), and the blank lake water sample spiked with 50 ng/L FLU, 10 ng/L ANT, and 100 ng/L FLT and PYR (B).
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200.0 ng/L for FLU, 16 and 80 ng/L for ANT, 80 and 400 ng/L for FLT and PYR) and the data showed that satisfactory recoveries for the FLU, ANT, FLT, and PYR were between 88.96 and 100.17% with RSDs between 4.2 and 5.9%, indicating a good accuracy of the method for the determination of the four PAHs in environmental water samples. Figure 2 shows typical chromatograms of the blank and spiked lake water sample.
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4 Conclusions In this study, OMC-HF-LPME combined with HPLC–FLD was developed for the first time and successfully applied for the determination of four polycyclic aromatic hydrocarbons in environmental water samples. After the optimization of the extraction conditions, an effective enrichment procedure with high sensitivity, good linearity, and good precision was obtained. The results proved that developed OMC-HFLPME method has excellent preconcentration and cleanup efficiency, and can be a good alternative approach for the determination of other environmental pollutants. Financial support from the National Natural Science Foundation of China (No. 31171698, 31471643), the Innovation Research Program of Department of Education of Hebei for Hebei Provincial Universities (LJRC009), the Natural Science Foundations of Hebei (B2012204028), the Scientific and Technological Research Foundation of Department of Education of Hebei Province (ZD20131033) and the Natural Science Foundation of Agricultural University of Hebei (ZD201405) are gratefully acknowledged.
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[43] Lin, C. F., Zhang, X., Lin, H., Wang, N., Li, J. B., Yang, X. Z., Adv. Mater. Res. 2006, 11–12, 543–546. [44] Song, H. H., Li, L. X., Chen, X. H., N. Carbon Mater. 2006, 21, 374–383. [45] Shao, Y., Wang, X., Kang, Y., Shu, Y., Sun, Q., Li, L., J. Colloid Interface Sci. 2014, 429, 25–33. [46] Parida, K., Mishra, K. G., Dash, S. K., Ind. Eng. Chem. Res. 2012, 51, 2235–2246. [47] Mello, M. R., Phanon, D., Silveira, G. Q., Llewellyn, P. L., Ronconi, C. M., Micropor. Mesopor. Mat. 2011, 143, 174–179. ˇ ´ M. Z., Ranđelovic, ´ M. S., Zarubica, A. R., [48] Momcilovi c, ˇ ´ B. Z., Chem. Eng. Onjia, A. E., Kokunesoski, M., Matovic, J. 2013, 220, 276–283.
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Li Liu Xin Zhou Chun Wang Qiuhua Wu Zhi Wang Department of Chemistry, College of Science, Agricultural University of Hebei, Baoding, China Received October 1, 2014 Revised November 29, 2014 Accepted December 8, 2014
Research Article
Extraction and enrichment of polycyclic aromatic hydrocarbons by ordered mesoporous carbon reinforced hollow fiber liquid-phase microextraction A novel microextraction method, ordered mesoporous carbon reinforced hollow fiber liquidphase microextraction coupled with high-performance liquid chromatography and fluorescence detection, was developed for the determination of some organic pollutants in water samples. Four polycyclic aromatic hydrocarbons (fluorene, anthracene, fluoranthene, and pyrene) were selected to validate this new method. Main parameters that could influence the extraction efficiency such as extraction time, fiber length, stirring rate, the type of the extraction solvent, pH value, the concentration of ordered mesoporous carbon, and salt effect were optimized. Under the optimal extraction conditions, good linearity was observed in the range of 2–1000 ng/L, with the correlation coefficients of 0.9954–0.9986. The recoveries for the spiked samples were in the range of 88.96–100.17%. The limits of detection of the method were 0.4–4 ng/L. The relative standard deviations varied from 4.2–5.9%. The results demonstrated that the newly developed method was an efficient pretreatment and enrichment procedure for the determination of polycyclic aromatic hydrocarbons in environmental water samples. Keywords: Hollow fibers / Liquid-phase microextraction / Mesoporous carbon / Polycyclic aromatic hydrocarbons / Water samples DOI 10.1002/jssc.201401071
Additional supporting information may be found in the online version of this article at the publisher’s web-site
1 Introduction Polycyclic aromatic hydrocarbons (PAHs) are important environmental persistent pollutants containing two or more fused aromatic rings [1]. They are formed during the incomplete combustion or high-temperature pyrolysis of oil and gas, coal, smoked meat, and other organic matter containing carbon and hydrogen [2]. Owing to the aerodynamic character of smoke, PAHs have been widely distributed in the environment [3], which could pose a potential hazard to human health and the environment. PAHs can threaten the safety of food chains for both humans and animals [4] and find their way into human organs through skin exposure, diet, and breathing, which may cause lung cancer, upper digestive tract cancer, skin cancer, bladder cancer, etc. [5,6]. Therefore, it is highly desirable to develop simple and sensitive analytical methods for the determination of trace level of PAHs in different samples. Correspondence: Dr. Qiuhua Wu, College of Science, Agricultural University of Hebei, Baoding 071001, Hebei, China E-mail: [email protected] Fax: +86-312-7528292
Abbreviations: ANT, Anthracene; BET, Brunauer–Emmett– Teller; FLD, fluorescence detection; FLT, Fluoranthene; FLU, Fluorene; OMC, ordered mesoporous carbon; PAH, polycyclic aromatic hydrocarbon; PYR, Pyrene C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
PAHs commonly exist in very low concentrations in the aquatic environment because they are hydrophobic compounds with low water solubility [7]. So before an instrumental analysis, a sufficient and effective sample pretreatment method is usually required and is sometimes even a key step in the whole analytical procedure. So far, various sample preconcentration techniques have been developed and subsequently applied for the monitoring of PAHs from different samples, such as LLE [8], SPE [9–11], SPME [12,13], ultrasonic extraction [14,15] PLE [16], MAE [17], and LPME based on the solidification of a floating organic microdrop [18, 19]. Among them, LLE, SPME, and SPE have become widely accepted and increasingly used for the preconcentration of PAHs from different samples. However, these methods suffer from one or more drawbacks. LLE requires a large amount of toxic organic solvents. SPE is time consuming and relatively expensive. SPME is a simple, organic-solvent-free and efficient extraction technique, but the number and variety of the commercially available fibers are limited. To overcome these drawbacks, a relatively new sample pretreatment technique called hollow fiber (HF) LPME has attracted considerable attention in analytical chemistry in recent years [20–23]. HF-LPME, introduced by Pedersen-Bjergaard and Rasmussen [24], is a miniaturization of traditional LLE and represents a recent advancement in the field of sample preseparation and preconcentration techniques. The www.jss-journal.com
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basic principle of HF-LPME has been depicted clearly in previously reported literature [25, 26]. The advantages of HF-LPME are as follows: Firstly, the HF-LPME equipment is very simple and inexpensive [27]. Secondly, HF-LPME has high enrichment factors, low consumption of organic solvents, and excellent sample cleanup function. Thirdly, the HF is disposable after each use due to its cheapness, environmentally friendly and thus the carryover problems can be overcome [28]. Besides, because the large molecules and particles are prevented from entering the small pore size of the fiber, HF-LPME has good selectivity [29]. In recent years, mesoporous carbon materials have sparked considerable research interest for potential applications as adsorbents [30, 31], supercapacitors [32], and catalyst supports [33], because of its tunable pore size, high specific surface area, low density, high chemical stability, good electrical conductivity, and thermal conductivity [34,35]. Various templates including silica [36], metal–organic frameworks [37], and molecular sieves [38, 39] have been successfully used for the synthesis of mesoporous carbon materials [40, 41]. Among them, the molecular sieve MCM-41 also has been successfully employed as the template for the preparation of mesoporous carbon materials [42–44]. There are some reports about the applications of MCM-41-based materials as the adsorbent for the extraction or removal of organic pollutants. For example, Shao et al. have prepared Mn/MCM41 composites for methyl blue removal [45]. Parida et al. have applied amine-functionalized MCM-41 (NH2 -MCM-41) for the sorption of copper(II) [46]. Mello et al. have used aminemodified MCM-41 mesoporous materials for the sorption of CO2 [47]. However, the application of MCM-41-based ordered mesoporous carbon as an adsorbent for the extraction or removal of organic pollutants has not been reported in the literature. The main aim of this study was to prepare an ordered mesoporous carbon (OMC) using MCM-41 as template and explore its application potentials in the extraction of organic pollutants when it was incorporated into HF-LPME. To the best of our knowledge, this is the first report that OMCreinforced HF-LPME (OMC-HF-LPME) has been used for the preconcentration of four polycyclic aromatic hydrocarbons in environmental water before their determination by HPLC with fluorescence detection (FLD). Several important experimental parameters that could affect the extraction efficiency were investigated and optimized. The results proved that developed OMC-HF-LPME method integrated the high adsorption capacity of the OMC and clean-up effect of HF-LPME, and had a good performance in the determination of four polycyclic aromatic hydrocarbons in environmental water.
2 Materials and methods 2.1 Reagents and materials MCM-41 template was obtained from Boaixin Chemical Reagents (Baoding, China). Pesticide standards of the polycyclic aromatic hydrocarbons (Fluorene (FLU), C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Anthracene (ANT), Fluoranthene (FLT), and Pyrene (PYR)) were purchased from Aladdin-Reagent (Shanghai, China). 1-Octanol, tetrahydrofuran, ethyl acetate, n-hexane, chromatography-grade acetonitrile, and methanol were purchased from Huaxin Chemical Reagent (Baoding, China). Sodium chloride (NaCl), hydrochloric acid (HCl), H2 SO4 (98%), ethanol, and all other reagents were purchased from Beijing Chemical Reagents (Beijing, China). The water used throughout the work was double-distilled on a SZ-93 automatic double-distiller purchased from Shanghai Yarong Biochemistry Instrumental Factory (Shanghai, China, http://yarong.instrument.com.cn/). The Accurel Q 3/2 polypropylene hollow fiber membrane (200 m thick wall, 600 m inner diameter, and 0.2 m average pore size) was purchased from Membrana (Wuppertal, Germany).
2.2 Apparatus The HPLC system, assembled from modular components (Waters, Milford, MA, USA), consisted of a 600E pump, and a fluorescence detector. The mobile phase was a mixture of acetonitrile/water (75: 25 v/v) at a flow rate of 1 mL/min. A Centurysil C18 column (4.6 id × 250 mm, 5.0 m) from Dalian Jiangshen Separation Science (Dalian, China) was used for separations. A Millennium32 workstation (Waters) was utilized to control the system and process the chromatographic data. The fluorescence detection was performed by the following excitation (Ex)/emission (Em) wavelength program: 254/323 nm from 0 to 10.5 min for the determination of FLU, 252/402 nm from 10.5 to 11.0 min for ANT, 280/460 nm from 11.5 to 15 min for FLT, and 270/390 nm from 15 to 30.00 min for PYR, respectively. The size and morphology of the MC were observed by TEM using a JEOL model JEM-2011(HR) (Tokyo, Japan) at 5 kV and SEM using an S-3000N microscope (Hitachi, Japan). The Brunauer–Emmett–Teller (BET) surface areas were determined from the N2 adsorption at 77 K using a Tristar II 3020 instrument (USA).
2.3 Preparation of OMC OMC was prepared according to the literature method [48] with some modifications. Firstly, 1 g of MCM-41 template, 1.5 g of sucrose and 5 mL of distilled water were added into a 50 mL of beaker. After magnetic stirring the mixtures for 50 min, 0.19 g of H2 SO4 (98 wt%) was added into the solution, followed by magnetic stirring the mixtures for 10 min. After that, the mixtures were heated at 100⬚C for 6 h and at 160⬚C for 6 h in oven. Secondly, the mixture was cooled to room temperature and the resultant black precipitate was ground to a fine powder. After the addition of 1 g of sucrose, 0.1 g of H2 SO4 (98 wt%) and 5 mL of distilled water, the samples were treated again at 100⬚C for 6 h and at 160⬚C for 6 h. The obtained MCM-41/sucrose composites were carbonized in a conventional furnace at 900⬚C for 2 h in nitrogen flow. Then, www.jss-journal.com
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Figure 1. TEM (A) and SEM (B) images of the OMC composite.
the MCM-41 template was removed by mixing the composite with 20 mL of HF (25 wt%) for 10 h and the obtained porous carbon was rinsed with ethanol and distilled water, respectively, to neutralize the material surface. Finally, the products were dried in an oven and 0.3 g MCM-41-based mesoporous carbon material was obtained.
2.4 Sample collection Lake water samples were collected from Qiandaohu (Hangzhou, China); sea water was collected from Rushan (Shandong, China). All the solvents and water samples were filtered through a 0.45 m membrane to eliminate particulate matter before analysis.
3 Results and discussion 3.1 Characterization of OMC The morphology and microstructure of the OMC were examined by TEM and SEM. The TEM image (Fig. 1A) and the SEM image (Fig. 1B) show that the morphology of the MCM41 silica particles and their structural characteristics are preserved in the as-prepared templated carbon. The OMC like the MCM-41 molecular sieve was nanoporous carbon with interconnected hierarchical pore and parallel mesoporous channel structures. Further evidence of the porous structure of the OMC is provided by the N2 sorption isotherm (Supporting Information Fig. S1). The OMC exhibits a porosity made up of mesopores with size 4.5 nm and it has a BET surface area of 302 m2 /g and a pore volume of 0.34 cm3 /g.
2.5 OMC-HF-LPME procedure The hollow fiber was cut into segments of 6 cm long. Then, the fiber was washed with acetone in an ultrasonic bath for 5 min to remove any impurities in the fiber and dried in air. After that, the fiber was immersed entirely in 1-octanol for 30 s to impregnate the fiber pores. Then, the excess of 1-octanol was carefully removed by washing the surface and the inner wall of the hollow fiber with double-distilled water under ultrasonication for 1 min. Then, 20 L acceptor phase (1.0 mg/mL OMC in 1-octanol) was injected into the lumen of the fiber. Then both ends of the fiber were sealed with heated tweezers. Extractions were performed according to the following procedure: the prepared hollow fiber was immersed in 20 mL of sample solution containing 10 L 1-octanol and a magnetic stir bar in a 25 mL screw cap glass vial. The extraction process was performed at 800 rpm stirring rate for 30 min. After extraction, the fiber was taken out from the glass vial carefully and the outside of the fiber was dried with filter paper. Then, the fiber was transferred into a 0.5 mL microvial and 100 L methanol was added for desorption by vortexing for 1 min. Finally, 20 L desorption solution was injected for HPLC analysis. C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
3.2 Optimization of extraction conditions To achieve a high enrichment and extraction efficiency, several parameters, including desorption solution, sample pH, extraction time, stirring rate, salt concentration, the amount of OMC, and fiber length were optimized.
3.2.1 Selection of organic extraction solvent and its volume The choice of organic extraction solvents plays a key role in HF-LPME since both extraction recovery and speed are affected by the partition coefficients between the sample and the organic phase. There are several factors that should be considered: (1) The organic extraction solvent should have a good affinity with the hollow fiber and should be insoluble in sample solution, (2) it should have a low volatility, low toxicity, and an appropriate viscosity, (3) it should have a high partition coefficient for the analytes, and a good dispersion capability for OMC. Based on these considerations, several organic extraction solvents (1-octanol, tetrahydrofuran, ethyl www.jss-journal.com
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acetate, and n-hexane) were chosen to evaluate their influence on the extraction efficiency. As a result, 1-octanol gave a higher sensitivity than the other solvents for all the analytes. Therefore, 1-octanol was chosen as the extraction solvent for the acceptor phase. In HF-LPME, adding small amount of the extraction solvent into the sample solution could increase the contact area between the extraction solvent and the sample. In this study, the effect of the added volume of 1-octanol to the sample solution was also investigated in the range from 0 to 30 L. The result indicated that the extraction efficiency reached the maximum when the volume of 1-octanol was increased to 10 L. Hence, 10 L of 1-octanol was selected for subsequent studies. 3.2.2 Effect of the concentration of OMC To evaluate the effect of the addition of OMC into the acceptor phase on the extraction capability, the concentration of OMC was investigated in the range from 0 to 4 mg/mL. As shown in Supporting Information Fig. S2, the peak areas of the four PAHs increased with increasing concentration of OMC from 0 to 1 mg/mL, and the peak areas of the four PAHs remained almost constant when the concentration of OMC exceeded 1 mg/mL. Considering the detection sensitivity and consumption of OMC, 1 mg/mL of OMC was chosen. 3.2.3 Fiber length In this study, the influence of the hollow fiber length was examined by changing its length in the range from 2 to 8 cm. As is illustrated in Supporting Information Fig. S3, the peak areas of the four PAHs increased by increasing the hollow fiber length from 2 to 6 cm, and after that, the signals kept almost constant. So, a 6 cm long hollow fiber was used for the further experiments. 3.2.4 Effect of extraction time Generally, extraction time is a very important factor in most extraction procedures since it influences the analytes’ partition between the sample solution and the organic phase. In the present work, to obtain the optimum extraction efficiency, a series of extraction time were evaluated to establish extraction–time profiles of target compounds. According to the results illustrated in Supporting Information Fig. S4, the peak areas of the four PAHs increased with increased extraction time before 30 min and after that there were no significant changes in the peak areas. Therefore, the optimum extraction time for the four PAHs was 30 min. 3.2.5 Sample agitation rate Fast agitation of the sample solution could continuously update the sample solution outside of the fiber and increase the contacting frequencies between the analytes and the fiber, C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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thus reducing the extraction equilibrium time. At the same time, a high stirring rate could help the dispersion of the OMC during the extraction. So stirring intensity is one of the important parameters that require to be optimized. In this work, different stirring speeds, from 200 to 1000 rpm were tested. The results in Supporting Information Fig. S5 revealed that the peak areas of the analytes increased with increased stirring speed from 200 to 800 rpm and then remained almost unchanged. Hence, 800 rpm was selected as the optimum agitation rate for the subsequent experiments. 3.2.6 Effect of ionic strength of the samples Generally, the presence of salt in sample solution could affect the solubility of the analytes and therefore the extraction efficiencies would be enhanced due to the salting-out effect, but on the other hand, it could also increase the viscosity of the solution, which would decrease the extraction capability. The influence of NaCl with the concentration ranging from 0–16% w/v was evaluated. The results showed that the addition of NaCl had a negligible effect on the peak areas of the analytes in the concentration range investigated. So no salt was added to the samples in the following experiments. 3.2.7 Effect of sample solution pH It is well known that the pH of sample solution plays a crucial factor in the extraction performance because the existing forms of the analytes in the sample solution can be affected by the pH value. Therefore, the influence of solution pH on the extraction efficiency of the four PAHs was investigated within the pH range of 2.0–12.0 by adding 1.0 mol/L sodium hydroxide or 1.0 mol/L hydrochloric acid to the sample solutions. The experimental results showed that no obvious peak area changes were observed for the PAHs when the pH of the sample solution was changed, which could be attributed to the fact that the PAHs exist as neutral compounds under ordinary conditions and the PAHs existing forms are unlikely influenced by the change of the sample solution pH. So, the pH of the sample solution was not adjusted.
3.3 Desorption solvent and desorption time After extraction, the hollow fiber was transferred to a centrifuge tube for desorption. In this study, three organic solvents including methanol, acetone, and acetonitrile were evaluated as the desorption solvent for the desorption of the analytes. Under the same extraction and elution conditions, the results showed that the three organic solvents provided similar results, but a better chromatographic peak shape was obtained when methanol was used. Thus, methanol was selected as the desorption solvent. The effect of desorption time and the methanol volume was also investigated. The desorption time was optimized by vortexing from 30 s to 4 min. As a result, four PAHs peak areas reached the highest values at desorption time of 1.0 min, and no obvious increase in peak www.jss-journal.com
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PAHs
LRa) (ng/L)
r
LOD(ng/L)
Table 2. Determination of polycyclic aromatic hydrocarbons pesticides and recoveries in water samples
Sea water (n = 5)
RSD (%) (n = 5) PAHs
FLU ANT FLT PYR
5.0–1000.0 2.0–1000.0 10.0–1000.0 10.0–1000.0
0.9906 0.9913 0.9986 0.9954
1.0 0.4 4.0 4.0
5.3 4.8 4.2 5.9
FLU
ANT a) LR, linear range.
FLT
areas was observed when longer desorption time was applied. Therefore, 1 min desorption time was chosen. The volume of methanol was investigated in the range from 20 to 150 L, and as a result, 100 L methanol yielded the best desorption result.
687
PYR
Spiked (ng/L) 0.0 40.0 200.0 0.0 16.0 80.0 0.0 80.0 400.0 0.0 80.0 400.0
Found (ng/L) ndb) 35.56 197.8 ndb) 14.51 73.3 ndb) 80.03 400.6 ndb) 78.93 399.9
Lake water (n = 5)
Ra) (%)
RSD (%)
88.89 98.92
6.2 5.1
90.61 91.63
5.6 4.7
100.0 100.1
5.3 6.8
98.63 99.95
5.2 4.5
Found (ng/L) 6.25 41.03 200.1 2.81 18.50 82.01 ndb) 79.97 400.1 ndb) 79.30 399.7
Ra) (%)
RSD (%)
86.96 96.96
6.2 7.1
98.06 99.00
7.4 5.1
99.96 100.0
4.6 5.9
99.13 99.94
4.1 6.3
a) R: recovery of the method. b) nd, not detected.
3.4 Linearity, repeatability, and limits of detection The performance parameters of the OMC-HF-LPME technique, such as repeatability, linearity, and LOD were investigated to estimate the efficiency and the feasibility for its application to the analysis of environmental water samples. A series of PAH-free double-distilled water samples spiked with each of the PAHs at different concentration levels (0.5, 1.0, 2.0, 5.0, 10.0, 20.0, 50.0, 100.0, 1000.0 ng/L) were prepared for the establishment of the calibration curve. Each concentration level was carried out in triplicate, and results were presented in Table 1. Based on the S/N of 3, the LOD was 1.0 ng/L for FLU, 0.4 ng/mL for ANT and 4 ng/mL for both FLT and PYR. The linear concentration ranges of the method were from 5 to 1000.0 ng/L for FLU, from 2.0 to 1000.0 ng/L for ANT and from 2.0 to 1000.0 ng/L for both FLT and PYR, with the correlation coefficients of 0.9954– 0.9986. The characteristic calibration data obtained are listed in Table 1.
The repeatability study was carried out by performing five parallel analyses at two concentration levels (40.0 and 200.0 ng/L for FLU, 16.0 and 80.0 ng/L for ANT, 80.0 and 400.0 ng/L for FLT and PYR). The RSDs varied from 4.2–5.9% (Table 1). These results indicated that the developed method of OMC-HF-LPME had good repeatability, high sensitivity, and wide linear range.
3.5 Environmental water samples analysis The different environmental water samples were applied to validate the applicability of the method. The results shown in Table 2 indicate that no residues of the PAHs were detected in sea water samples; only a low concentration of FLU (6.25 ng/L) and ANT (2.81 ng/L) was found in lake water sample. To assess the accuracy of the method, water samples were spiked at two concentration levels (40.0 and
Figure 2. The typical chromatograms for the extracted polycyclic aromatic hydrocarbons from lake water (A), and the blank lake water sample spiked with 50 ng/L FLU, 10 ng/L ANT, and 100 ng/L FLT and PYR (B).
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200.0 ng/L for FLU, 16 and 80 ng/L for ANT, 80 and 400 ng/L for FLT and PYR) and the data showed that satisfactory recoveries for the FLU, ANT, FLT, and PYR were between 88.96 and 100.17% with RSDs between 4.2 and 5.9%, indicating a good accuracy of the method for the determination of the four PAHs in environmental water samples. Figure 2 shows typical chromatograms of the blank and spiked lake water sample.
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4 Conclusions In this study, OMC-HF-LPME combined with HPLC–FLD was developed for the first time and successfully applied for the determination of four polycyclic aromatic hydrocarbons in environmental water samples. After the optimization of the extraction conditions, an effective enrichment procedure with high sensitivity, good linearity, and good precision was obtained. The results proved that developed OMC-HFLPME method has excellent preconcentration and cleanup efficiency, and can be a good alternative approach for the determination of other environmental pollutants. Financial support from the National Natural Science Foundation of China (No. 31171698, 31471643), the Innovation Research Program of Department of Education of Hebei for Hebei Provincial Universities (LJRC009), the Natural Science Foundations of Hebei (B2012204028), the Scientific and Technological Research Foundation of Department of Education of Hebei Province (ZD20131033) and the Natural Science Foundation of Agricultural University of Hebei (ZD201405) are gratefully acknowledged.
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