Accepted Manuscript Title: Roles of inorganic oxide nanoparticles on extraction efficiency of electrospun polyethylene terephthalate nanocomposite as an unbreakable fiber coating Author: Habib Bagheri Ali Roostaie PII: DOI: Reference:
S0021-9673(14)01839-1 http://dx.doi.org/doi:10.1016/j.chroma.2014.11.059 CHROMA 356045
To appear in:
Journal of Chromatography A
Received date: Revised date: Accepted date:
17-5-2014 9-11-2014 22-11-2014
Please cite this article as: H. Bagheri, A. Roostaie, Roles of inorganic oxide nanoparticles on extraction efficiency of electrospun polyethylene terephthalate nanocomposite as an unbreakable fiber coating, Journal of Chromatography A (2014), http://dx.doi.org/10.1016/j.chroma.2014.11.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Roles of inorganic oxide nanoparticles on extraction efficiency of electrospun polyethylene
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terephthalate nanocomposite as an unbreakable fiber coating
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Habib Bagheri1, Ali Roostaie
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Environmental and Bio-Analytical Laboratories, Department of Chemistry,
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Sharif University of Technology, P.O. Box 11365-9516, Tehran-Iran
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Abstract
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In the present work, the roles of inorganic oxide nanoparticles on the extraction efficiency of
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polyethylene terephthalate-based nanocomposites were extensively studied. Four fiber coatings
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based on polyethylene terephthalate nanocomposites containing different types of nanoparticles
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along with a pristine polyethylene terephthalate polymer were conveniently electrospun on
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stainless steel wires. The applicability of new fiber coatings were examined by headspace-solid
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phase microextraction of some environmentally important volatile organic compound such as
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benzene, toluene, ethylbenzene and xylene (BTEX), as model compounds, from aqueous
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samples. Subsequently, the extracted analytes were transferred into a gas chromatography by
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thermal desorption. Parameters affecting the morphology and capability of the prepared
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nanocomposites including the type of nanoparticles and their doping levels along with the
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coating time were optimized. Four types of nanoparticles including Fe3O4, SiO2, CoO and NiO
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were examined as the doping agents and among them the presence of SiO2 in the prepared
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nanocomposite was prominent. The homogeneity and the porous surface structure of the SiO2-
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polyethylene terephthalate nanocomposite was confirmed by scanning electron microscopy
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indicating that the nanofibers diameters were lower than 300 nm.
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parameters influencing the extraction and desorption process such as temperature and extraction
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time, ionic strength and desorption conditions were optimized. Eventually, the developed method
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was validated by gas chromatography-mass spectrometry. Under optimized conditions, the
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relative standard deviation values for a double distilled water spiked with the selected volatile
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organic compounds at 50 ng L-1 were 2–7% (n = 3) while the limits of detection were between
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In addition, important
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Corresponding author. Tel.: +98-21-66005718; Fax: +98-21-66012983 E-mail address: [email protected] (H. Bagheri)
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0.7 and 0.9 ng L-1. The method was linear in the concentration range of 10 to 1000 ng L−1 (R2>
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0.9992). Finally, the developed method was applied to the analysis of Kalan dam and tap water
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samples and the relative recovery values were found to be in the range of 86 to 102%.
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Keywords
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Polyethylene
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spectrometry; BTEX; Water analysis
nanocomposites;
Nanoparticles;
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1. Introduction
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Solid phase microextraction (SPME), nowadays, is regarded as a well-established and solvent-
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free extraction technique and has been applied to the determination of a variety of chemicals in
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different samples. In this technique the extraction process is predominantly performed on fragile
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fused silica-based fibers [1, 2] . Moreover, the fiber coatings might be damaged when they are
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exposed to high temperatures of gas chromatography (GC) injectors and the desorbing organic
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solvents of high performance liquid chromatography (HPLC). To have access to appropriate
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fiber coatings is therefore one of the most critical steps in SPME. One important strategy to
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improve the sensitivity of SPME relies on the usage of nanostructured materials in which the
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interactions between the desired
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developments of new fibers are mostly focused on improving the thermal, mechanical and
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chemical stabilities, imparting diverse functionalities and polarities, and enhancing their
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extraction capacity [3–5]. The use of nanocomposites-based coatings [6-8] and replacing the
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fragile fused silica substrates by metallic wires [9,10] are among the recent approaches toward
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achieving an overall stability for the prepared fibers.
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Polyethylene terephthalate (PET) is a semicrystalline polymer and its composites are widely used
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in many versatile applications. Considerable efforts have been devoted to improve various
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physical, and mechanical properties of PET through mixing it with nanoclays to produce layered
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clay-incorporated PET composites. Incorporation of spherical inorganic nanoparticles, i.e. nano-
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SiO2, into the PET matrix is another approach to improve such properties [11–14].
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Nanocomposites are multiphase solid materials where one of the phases has nano-scale
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dimensions, or structures having nanometer-scale repeat distances between the different phases
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[15]. In overall, these materials possess different physical properties in compared with the initial
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chemicals and the fiber coating are enhanced. The
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components. The surface area/volume ratio of the reinforcing materials, with morphological
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characteristics, is a key issue for their spread applications [16]. The nanostructured materials
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with unique properties have become highly popular as it would be possible to design multi-
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functional structures at the nanometer scale. The selection of nanoparticles dopants depends on
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the desired thermal, mechanical, and electrical properties of the nanocomposites [17-19]. Among
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numerous nanomaterials, cobalt (Co)-based nanomaterials has specially attracted many attention
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due to its good electroactivity and low cost [20,21]. For instance, the composite of Co
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hexacyanoferrate nanoparticles (NPs)-carbon nanotube-chitosan has been used to develop
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glucose sensor [22]. Pt–Co NPs supported on single-walled carbon nanotubes (SWCNTs) and Co
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NPs integrated with SWCNTs have been applied for methanol [23] and nitrite detection [24].
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Currently, nano-magnetic particles are attracting more attentions due to their distinguished
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properties, such as surface effect, special magnetic target and good biocompatibility properties
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[25, 26]. Among magnetic nanoparticles, Fe3O4 nanoparticles are particularly attractive because
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of their biocompatibility, low toxicity and ease of preparation [27, 28]. Their unique properties
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such as large surface area, excellent conformation stability and better contact between biocatalyst
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and its substrate have made them quite suitable to be used in electrochemical biosensors [29].
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Silica nanoparticles, due to their chemical inertness, nontoxicity, optical transparency, and
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excellent thermal stability [30-32], have gained significant attentions. They can be widely used
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in catalysis [33], chemical process industry [34], removal of metal ions [35], and metal ion
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preconcentration [36] through polymer coatings or other functional groups. Large specific
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surface area and high thermal stability of nanoporous silica materials (e.g., SBA-15) have made
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them suitable alternative for SPME coatings. The chemical functionalization of SBA-15 by
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incorporating organic groups and its capability as a support for the construction of conductive
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polymer/SBA-15 nanocomposites extends the range of materials applied as SPME coatings [37,
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38]. The recent studies on polypyrrole/SBA-15 [38], polyaniline/silica nanocomposite [15],
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modified silica-polyamide nanocomposite [16] and ZnO/graphene nanocomposite [39] have
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proven that these composites can be incorporated as appropriate coatings for hydrophobic
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compounds.
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One successful approach to prepare such nanocomposites is the electrodeposition of inorganic
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oxides in organic matrices via electrospinning. The electrospinning process has already been
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explained previously [ 40, 41]. The entrapment of the target analytes by the nanostructured
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materials is facilitated due to its enhanced surface area and this justifies the use of minimum
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amount of nanofibers for the extraction which subsequently reduces the volume of desorption
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solvent [ 42].
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In this work, electrospun PET-based nanocomposites were synthesized as new SPME coatings to
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obtain a nanofibers network embedded with highly dispersed inorganic oxides nanoparticles.
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Adaption of this strategy led to the production of a nanocomposite with higher reinforcement and
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extraction efficiency. Effects of different nanoparticles such as Fe3O4, SiO2, CoO and NiO on the
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extraction efficiency were also assayed. The SiO2-PET nanocomposite was found to be superior
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and used as an unbreakable fiber coating for the determination of some selected organic
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compounds in aqueous samples.
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2. Experimental
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2.1. Reagents
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Benzene, toluene, ethyl benzene, o-xylene and m-xylene (BTEX) were purchased from Merck
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(Darmstadt, Germany). Standard solution (1000 mg L−1) of each analyte was prepared in HPLC5
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grade methanol (Merck) and stored in the refrigerator. The working standard solutions were
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prepared weekly by diluting the standard solution with methanol, and more diluted working
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solutions were prepared daily by diluting this solution with double distilled water (DDW).
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Sodium chloride, FeCl3·6H2O, FeCl2·4H2O, HCl and NaOH were purchased from Merck
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(Darmstadt, Germany). PET was purchased from Kolon Industries Inc. (Seoul, Korea) and
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triflouroacetic acid was obtained from Riedel-de Haën (Seelze-Hannover, Germany). All
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solvents used in this study were of analytical reagent grade or HPLC grade. The different
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nanoparticles such as SiO2, CoO and NiO with a diameters range of 12-50 nm were obtained
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from US Research Nanomaterials, Inc (Houston, TX 77084, USA). Nitrogen was used for
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providing the inert atmosphere necessary for synthesis of Fe3O4 NPs.
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2.2 Apparatus
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A GC model Agilent 6820, with a split/splitless injection port, and a flame ionization detection
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system that the carrier gas was Nitrogen (99.999%) at a flow rate of 3 mL min-1, was used for the
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optimization process. For quantitative determination, a Hewlett-Packard (HP, Palo Alto, USA)
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HP 6890 series GC equipped with a split/splitless injector and a HP 5973 mass-selective detector
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system that the MS was operated in the EI mode (70 eV) and Helium (99.999%) was employed
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as carrier gas and its flow-rate was adjusted to 1 mL min-1 were used. A homemade glass inlet
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liner with a total length of 7.7 cm, 6 mm o.d. and 1 mm i.d. was deactivated by
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trimethylchlorosilane used. The separation of analytes was carried out using a capillary column
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HP-5 MS (60 m, 0.25 mm i.d.) with 0.25 µm film thickness (Hewlett-Packard, Palo Alto, CA,
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USA). The column was held at 70 °C for 3 min, increased to 110 °C at a rate of 10 °C min-1 and
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raised to 210 °C at 30 °C min-1 and kept at this temperature for 3 min. The injector temperature
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was set at 240 °C and all injections were carried out on the splitless mode for 3 min. To obtain
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the highest possible sensitivity, the MS detection was operated using time-scheduled SIM based
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on the selection of some mass peaks of the highest intensity for each compound. In the SIM
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mode, the following important ions are used for BTEX: m/z 71 for benzene, m/z 91 and 92 for
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toluene, m/z 91, 106 for ethyl benzene, o-xylene and m-xylene.
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The SPME syringe was made in our laboratories consisting of two spinal needles with gauge
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numbers of 22 and 26. All samples were extracted from a 10 mL glass vials with a PTFE-faced
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septum and aluminum cap. Samples were heated in a homemade glass water bath connected to a
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refrigerated circulating water bath (Neslab, USA) and stirred using a MR Hei-Mix S magnetic
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stirrer (Heidolph, Germany). The electrospinning experiments were performed using a regulated
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power supply (Brandenburg, England). A KDS100 syringe pump (Kd. Scientific Co., U.S.A.)
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was used for the polymer solution delivery in the electrospinning process.The Scanning electron
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microscopy (SEM) images were obtained by a TESCAN VEGA II XMU (Brno, Czech Republic)
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and Fourier transform infrared spectroscopy (FTIR) spectra were recorded by a ABB Bomem
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MB100 (Quebec, Canada).
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2.3 Preparation of magnetic nanoparticles
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Magnetic nanoparticles were synthesized according to the procedure described previously [ 43].
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Briefly, 5.2 g of FeCl3·6H2O and 2 g FeCl2·4H2O and 0.85 mL concentrated HCl were dissolved
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in 25 mL water under the N2 gas. This solution was added drop wise into 250 mL of sodium
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hydroxide solution (1.5M) under the N2 atmosphere while it was vigorously stirred for 30 min
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(1000 rpm). The resulted black precipitates were separated using a 1.4 T magnet and washed
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several times with degassed water. Finally MNPs were dried in vacuum oven at 60 ºC for 5
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hours.
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2.4 Preparation of PET-base nanocomposite
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First, 0.10 g of PET (10 % w/v) was dissolved in 1 mL of TFA and stirred for 20 min to obtain a
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homogenous solution. After complete dissolving of polymer and obtaining a homogeneous
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solution, different types of nanoparticles were added into the polymer solution and it was
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vigorously stirred for 20 min (1000 rpm). Then 0.5 mL of this solution was withdrawn into a 1
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mL syringe which was eventually located in a syringe pump. A homemade SPME device was
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attached to a small electrical motor in a way that the plunger wire could be rotated while
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electrospinning was performed. Under this condition, the external needle and the polymer
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containing syringe needle were connected to the high voltage power supply terminals. A length
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of 1.5 cm from the end part of plunger wire was used for collecting the nanofibers. The other part
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of the SPME assembly was protected from the nanofibers flying towards the collector by a
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packed paper insulator. The distance between the needle and the collector was set at 10 cm. The
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SPME assembly was in perpendicular position in respect to the syringe. A voltage of 16 kV was
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applied for the nanofibers production while a flow rate of 0.5 mL h−1 was set for the polymer
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solution delivery using a syringe pump. All the electrospinning processes were performed under
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the ventilation (Fig. 1a). All fibers were electrospun for 2 min. Finally, the SPME fibers were
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inserted into the GC injector port for conditioning at 240 ◦C for 3 h.
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2.5 The SPME process
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A home-made SPME device [7,16,40] containing the synthesized PET-based nanocomposite
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fiber coating (~ 60 µm film thickness) was employed for the headspace SPME process. During
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the extraction, the aqueous sample was stirred by a magnetic stir bar and a water bath was used
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to control the temperature. The thermal desorption was carried out at 240 °C while the split valve
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of the GC injector was kept closed for 3 min. For extraction purposes, 4 mL of double distilled
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water or real water samples were spiked with the selected analytes in a 7-mL vial containing 0.2
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g of sodium chloride under the maximum stirring rate (1000 rpm). The vials were then sealed
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with a PTFE-faced septum and a cap. The extraction was performed by exposing the PET-based
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nanocomposite fiber coating to the sample headspace for 10 min at 50 °C. Then, the SPME fiber
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was withdrawn from the vial and transferred to the GC injection port for thermal desorption of
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the model analytes.
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3. Results and discussion
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The chemical structure of PET (Fig. 1b) along with its physical properties data encouraged us to
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employ it as a major constituent for preparation of a rather stable nanocomposite for extraction
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purposes. . The influential parameters on the morphology of nanocomposite were examined by
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comparing the extraction efficiency of the model analytes from aqueous samples. Also, the
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effects of type of nanoparticles, doping levels of nanoparticles, polymer concentration and
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coating time on the extraction efficiency of the synthesized nanocomposites were investigated.
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After achieving the most appropriate nanocomposite, the effects of extraction/desorption
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parameters were also optimized.
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3.1. PET-based nanocomposites and influences of nanoparticles
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According to the structure of PET, a strong π–π interaction for this polymer towards organic
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analytes could be anticipated. Unlike conductive polymers, PET can be dissolved in a solvent
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such as TFA for convenient electrospinning. The preliminary results revealed that this polymer
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with good flexibility and mechanical stability could be used as an efficient SPME fiber coating
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without being peeled off from the needle in whole extraction procedure. The effect of PET
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concentration on the extraction efficiency was investigated considering a concentration range of
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4–22% (w/v). According to Fig. 2a , by increasing the polymer concentration to 18%, the
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extraction efficiency was enhanced and then started to decline. Apparently as the polymer
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concentration rises, the sorbent capacity is enhanced, but by further concentration increase a
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thicker fiber coating could be formed which leads to lower surface area and subsequent lower
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extraction efficiency.
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The enhancement of surface area and porosity surly has a direct effect on the extraction
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efficiency. The presence of different types of inorganic oxides NPs in polymer structure is an
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efficient way to fulfill this purpose. For investigating the effect of NPs type on the extraction
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capability of PET nanocomposite, fourNPs including Fe3O4, SiO2, CoO and NiO at equal weight
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percent were embedded in the electrospun PET network. Fig. 2b shows that PET-based
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nanocomposites containing inorganic NPs exhibit higher extraction efficiencies for the model
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organic analytes Our results revealed that the silica-PET nanocomposite
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extraction efficiencies in compared with those containing CoO, NiO and Fe3O4 NPs (SiO2-PET
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> CoO-PET > NiO-PET > Fe3O4-PET > PET nanofibers). This trend might be due to the
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effective dispersion of silica NPs throughout the polymer network compared with the others. The
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addition of silica nanoparticles was found to be the most appropriate nanofiller. For investigating
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could yield higher
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the effect of silica NPs doping level on extraction capability, five silica-PET nanocomposites
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were doped with 0.0018, 0.010, 0.020, 0.030 and 0.040 g of silica NPs (1- 22%, w/v). As shown
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in Fig. 2c , the extraction capability of the silica-PET nanocomposite was enhanced at the
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expense of more silica NPs . Apparently for most analytes, increasing the silica doping level up
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to 0.040 g led to an improvement in extraction efficiency, while at higher values , the
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electrospining process became rather difficult. According to these results a doping level of 0.040
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g of the silica NPs was chosen as the optimum value. The SPME coating thickness can be
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controlled by the electrospinning time. As the electrospinning time was increased, the fiber
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coating became thicker, providing a higher extraction capacity. According to Fig. 2d , the time of
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electrospinning was selected at 3 min which led to the formation of a SPME fiber coating with a
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desirable thickness.
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3.2. Characterization of the SiO2-PET nanocomposite
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The FTIR spectra obtained for NiO, SiO2, Fe3O4 and CoO NPs are show in Fig. 3. The
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absorption peaks appeared at 466, 1100 and 576 cm-1 could be assigned to Ni-O, Si-O and Fe-O
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stretching vibrations, respectively. The spectrum of CoO NPs exhibited two absorption peaks at
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525 and 646 cm-1 corresponding to stretching vibration mode of Co-O and the bridging vibration
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of O-Co-O bond, respectively. The presence of these NPs in electrospun PET nanofibers was
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monitored by FTIR (Fig. 4). The FTIR spectra of PET-based nanocomposites and PET
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nanofibers clearly show the absorption peaks at 683 and 725 cm-1 corresponding to bending
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motion of phenyl ring and coupled vibration of carbonyl and deformation of phenyl group,
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respectively. Also, the absorption peaks appeared at 1268 and 1503 cm-1 correspond to C-O-C
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in-plane ring deformation and aromatic ring vibration of C-C group, respectively. The absorption
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peak at 1720 cm-1 correspond to C=O stretching vibration of PET. Moreover, the peaks at 466,
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1100, 576, 525 and 646 cm-1 in the spectra of PET-based nanocomposites clearly confirms the
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presence of four nanoparticles, individually doped in the nanocomposite.
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The SEM image of the dispersed iron oxide nanoparticles in water shows that the synthesized
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MNPs have rather high surface area (Fig. 5a). The morphology of SiO2-PET nanocomposite and
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PET nanofibers was compared using SEM (Figs. 5b,5c). The SEM images revealed that the
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SiO2-PET nanocomposite coating film is porous and homogeneous along the stainless steel wire
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while the nanocomposite dimensions are below 300 nm (Fig 5 b). Also, interestingly the
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produced nanofibers in SiO2-PET nanocomposite in compared with PET nanofibres are thinner
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(Figs 5 b,5c). Certainly the higher porosity of SiO2-PET nanocomposite could lead to the
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increased surface area, adsorption sites availability and higher mass transfer for analytes during
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the sorption/desorption steps. The presence and dispersion of SiO2 nanoparticles throughout the
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coating increases the spaces among the nanofibers in which much faster analytes diffusion in the
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PET network becomes more likely.
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3.3. Optimization of HS- SPME procedure
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For the adapted HS-SPME procedure, the effects of extraction temperature/time, desorption
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temperature/time and salt concentration on the extraction of the model compounds were
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optimized.
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The addition of NaCl to the sample caused an increase in the efficiency of the extraction for all
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the tested analytes (Fig. 6a). At salt concentrations above 25%, the responses for all analytes
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remained almost constant and this value was used as the optimum quantity.
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The extraction temperature is a pronounced parameter in headspace-SPME mode because it
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could have considerable effect on the equilibrium status of extraction. The extraction
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temperature has diverse effects on the extraction efficiency. At high temperatures the analytes in
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the sample solution can escape from their matrix and enter the headspace, causing the increase of
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analytes concentration in the headspace and subsequent enhancement of higher extraction rates .
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Nevertheless, the sorbent/headspace distribution coefficient also decreases at higher
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temperatures, resulting in a decrease in the equilibrium amount of the extracted analyte. A
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temperature range of 25–70 ◦C was considered and according to Fig. 6b, the amounts of extracted
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analytes were enhanced as temperature raised to 50 ◦C and then started to decline. At higher
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temperatures, the heat transfer to the metallic needle possibly leads to thermal desorption of the
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previously adsorbed analytes. Considering these results, a temperature of 50 ˚C was selected for
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the further experiments.
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Both desorption time and temperature are influential in the recovery process and they must be
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optimized. The desorption time should be kept as minimum as possible in order to prevent any
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carryover effect. The maximum desorption temperature is limited by the thermal stability of the
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sorbent. To prevent the coating from the harsh desorption conditions and increase its life time,
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the lowest possible temperature should be applied for desorption. The desorption temperature
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was varied from 100 to 280 ◦C. Although, the rise of desorption temperature increases the
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desorption efficiency, for the sake of the coating life time, a temperature of 240 ◦C was chosen
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for further experiments. While the desorption temperature was set at 240 ◦C, the desorption time
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was varied within a range of 1–5 min, by leaving the SPME device in the injection port of GC.
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Clearly, desorption was increased by time and the maximum value of 3 min was selected.
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An extraction time interval, ranging from 5 to 25 min, was also studied. As shown in Fig. 6c, the
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extraction profiles for all analytes increased up to 10 min and after that they reached to the
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equilibrium. In overall, an extraction time of 10 min was finally chosen.3.3.1 Durability and
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stability of PET -SiO2 nanocomposite
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The coating lifetime is a very important issue as far as the practical consideration is concerned.
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Also, durability together with chemical and thermal stability of the fiber coating are also
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important issues in SPME. They correspond to the extraction capability of the fiber coating after
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frequent usage and exposing to different solvents and high temperatures.
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In order to achieve a complete desorption with no carry-over effects, the fiber coating thermal
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resistance is a very important parameter for SPME applications. The PET -SiO2 nanocomposite
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fiber coating was conditioned for 1 h at different temperatures (100-300 C) and then used for
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headspace sampling of BTEX. The ratios (r) of BTEX were obtained by comparison of the peak
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area at each temperature versus the value achieved at 240 C (Table 1). The extraction ability of
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the fiber coating was barely affected by the temperature ranging from 240 to 300 C. However,
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the temperature of 240 C was selected to extend the life time of the fiber coating.
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The PET -SiO2 nanocomposite fiber coating was regularly checked and no considerable changes
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in its extraction ability could be observed after 125 runs, while for each run, the fiber kept at 240
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C at least for 3 min. Moreover, the chromatograms obtained from the blank fiber led to
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insignificant changes, indicating the fiber coating sufficient stability.
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Solvent stability is very important for SPME-LC analysis and SPME derivatization technique,
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since both of them require the fiber to be directly immersed into the appropriate solvent for a
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certain time. Solvent stability was also assessed since the coating must have no bleeding during 14
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its contact with the organic solvent. The coating was therefore immersed in several solvents with
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different polarities including water, methanol, dichloromethane and acetonitrile. Then, it was
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exposed to high temperatures (up to 240 C) for 30-min intervals and no loss of performance
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could be observed.
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3.4 Method validation
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Using the optimized conditions, the developed method was validated for determination of BTEX
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in the spiked distilled water sample. The method detection limit and other analytical features for
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the selected analytes are tabulated in Table 1. The correlation coefficients for all analytes were
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satisfactory (r2>0.9992) and limits of detection (LOD), based on a signal-to-noise ratio of 3:1,
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were between 0.7 and 0.9 ng L-1. The intraday precision was evaluated by determining the peak
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area of each BTEX in three spiked replicates at two different concentration levels. The intraday
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precision showed RSD% values ranging from 6 to 9% at 0.05 ng mL-1 and 2 to 6% at 0.15 ng
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mL-1 for the selected volatile compounds (Table 2 ). The interday precision ranged from 5 to
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11% at 0.05 ng mL-1. To evaluate the applicability of the current method, extraction and analysis
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were performed on water samples obtained from Kalan dam and drinking sources. Real water
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samples were spiked at a concentration level of 50 ng L−1 and the analysis was carried out under
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the optimized SPME conditions . Relative recoveries in the range of 86–102% for the selected
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analytes were achieved.
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Table 3 compares some statistical data obtained from this work with other relevant reports [41-
334
42]. Clearly the synthesized nanocomposite fiber coating (~ 60 µm film thickness) exhibits
335
comparable analytical performance to other SPME fibres. In addition, the synthesis of the
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nanocomposite coating can be carried out easily while it is rather low cost, convenient, rapid and
337
durable. Additionally, the long life time of this novel fiber coating was validated by no tangible
338
change in extraction efficiency of the selected BTEX after 125 times.
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4. Conclusion
341
In this study, a SPME technique coupled with GC-MS based on the use of SiO2–PET
342
nanocomposite fiber coating was developed for trace determination of some organic model
343
compounds, in water samples. Relative recoveries along with other analytical data approved the
344
candidacy of the synthesized nanocomposite as an appropriate SPME fiber coating for extracting
345
aromatic compounds. The electrospun SiO2–PET nanocomposite mat, due to non-smooth and
346
porous structure, provides high specific surface area with increased activated sites. The
347
developed method based on the use of SiO2–PET nanocomposite coating, is facile, simple, rapid
348
and inexpensive and can be employed for the determination of BTEX with sufficient sensitivity
349
and reproducibility. Also chemical structure of nanocomposite film contributes to hydrophobic
350
and π-π interaction between analytes and the PET polymer, making the SPME fiber coating a
351
sensitive and unbreakable probe for extracting organic compounds.
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Acknowledgements
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The Research Council and Graduates School of Sharif University of Technology (SUT) are
355
thanked for supporting the project.
356 357
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490 491
Figure captions
492
Fig. 1: Schematic diagram of (a) the synthesizing setup for PET-based nanocomposites and (b) structure of PET.
ip t
493 494 495
Fig. 2: Effects of (a) concentration of polymer, (b) different types of nanoparticles, (c) amount of silica nanoparticles and (d) the coating time on the extraction efficiency.
cr
496 497
Fig. 3: FTIR spectra of different types of NPs.
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499
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498
500
505 506 507 508 509
M
503 504
Fig. 5: SEM images of (a) magnetic nanoparticles ( b) SiO2-PET nanocomposite and ( c) PET nanofibers.
d
502
Fig. 4: FTIR spectra of PET-based nanocomposites.
Ac ce pt e
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Fig. 6: Effect of (a) salt concentration, (b) extraction temperature and (c) extraction time on the extraction efficiency.
24
Page 23 of 33
509
Fig. 1
511 512
513 514
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cr
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510
(a)
(b)
515 516 25
Page 24 of 33
517 518
Fig. 2
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519
(b)
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(a)
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cr
520
(d)
(c)
521 522 523 524 525 526
26
Page 25 of 33
527
Fig. 3
528
ip t
529
531 532 533 534 535
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d
M
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cr
530
536 537 27
Page 26 of 33
538
Fig. 4
542 543 544 545
d
541
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540
M
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us
cr
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539
546 547 548 28
Page 27 of 33
Fig. 5
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cr
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549
550 551 552
Fig. 6 29
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553 554
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cr
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555
an
(b)
556 557 558 559 560
Ac ce pt e
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(a)
(c)
561 562 563
Table1- Thermal stability of PET-SiO2coated SPME fibera 30
Page 29 of 33
◦
160
200
240
260
280
300
Benzene
0.21
0.30
0.45
0.85
1.00
1.02
1.00
0.99
Toluene
0.21
0.29
0.43
0.84
1.00
1.04
1.04
1.01
Ethyl benzene
0.20
0.31
0.46
0.76
1.00
1.05
1.06
M-xylene
0.21
0.29
0.43
0.84
1.00
1.04
O-xylene
0.21
0.30
0.45
0.85
1.00
1.03
ip t
140
1.03
1.04
1.05
1.02
us
1.04
a
M
an
The ratio (r) is defined by the peak areas at each temperature with respect to peak are obtained at ◦ 240 C.
d
565
100
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564
Compound
cr
Condition temperature ( C)
31
Page 30 of 33
566 567
Table 2- Some analytical data obtained from HS-SPME of BTEX using the SiO2-PETnanocomposite fiber coating and GC-MS
568 LDR1 ( ng L-1)
Compound
(R2)
LOD2 (ng L−1)
intra-day RSD3a% (n=3)
inter-day RSDb% (n=3)
intra-day Relative recovery%
fiber-tofibreRSDc % (n=3)
0.90
9
6
Toluene
5-1000
0.9999
0.90
7
5
Ethyl benzene Ortho xylene Metha xylene
5-1000
0.9997
0.70
6
2
5-1000
0.9998
0.90
8
5
5-1000
0.9992
0.85
6
4
Linear dynamic range Limit of detection 3 Relative standard deviation a Spiked at 0.05 ng L-1 b Spiked at 0.15 ng L-1 c Spiked at 0.05 ng L-1
86
95
8
95
96
5
99
102
8
102
99
7
100
98
an
1
11
cr
0.9995
us
5-1000
Tap water
d
M
2
Ac ce pt e
569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597
Benzene
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Kalan dam water
32
Page 31 of 33
Table 3- Comparison of analytical characteristics for proposed fiber with other fibers in determination of BTEX compounds LDRa
LODb
RSDc%
Polydimethylsiloxane /Divinylbenzene
10–2000 ng
0.75-1.4
5.4–8.3
10–800
5.4–14.8
3.9-8.2
Polydimethylsiloxane
10–25,000
0.75–10
Cobalt oxide nanoparticles
10–300,000
1–11
Sol–gel-carbon aerogels
1–1500
Graphite
0.6–11090
SiO2-PETnanocomposite
10-1000
µg L-1 µg L-1 µg L-1 ng L-1
cr
Water
[ 45]
S0021-9673(14)01839-1 http://dx.doi.org/doi:10.1016/j.chroma.2014.11.059 CHROMA 356045
To appear in:
Journal of Chromatography A
Received date: Revised date: Accepted date:
17-5-2014 9-11-2014 22-11-2014
Please cite this article as: H. Bagheri, A. Roostaie, Roles of inorganic oxide nanoparticles on extraction efficiency of electrospun polyethylene terephthalate nanocomposite as an unbreakable fiber coating, Journal of Chromatography A (2014), http://dx.doi.org/10.1016/j.chroma.2014.11.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Roles of inorganic oxide nanoparticles on extraction efficiency of electrospun polyethylene
2
terephthalate nanocomposite as an unbreakable fiber coating
3
Habib Bagheri1, Ali Roostaie
4
Environmental and Bio-Analytical Laboratories, Department of Chemistry,
5
Sharif University of Technology, P.O. Box 11365-9516, Tehran-Iran
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6
Abstract
8
In the present work, the roles of inorganic oxide nanoparticles on the extraction efficiency of
9
polyethylene terephthalate-based nanocomposites were extensively studied. Four fiber coatings
10
based on polyethylene terephthalate nanocomposites containing different types of nanoparticles
11
along with a pristine polyethylene terephthalate polymer were conveniently electrospun on
12
stainless steel wires. The applicability of new fiber coatings were examined by headspace-solid
13
phase microextraction of some environmentally important volatile organic compound such as
14
benzene, toluene, ethylbenzene and xylene (BTEX), as model compounds, from aqueous
15
samples. Subsequently, the extracted analytes were transferred into a gas chromatography by
16
thermal desorption. Parameters affecting the morphology and capability of the prepared
17
nanocomposites including the type of nanoparticles and their doping levels along with the
18
coating time were optimized. Four types of nanoparticles including Fe3O4, SiO2, CoO and NiO
19
were examined as the doping agents and among them the presence of SiO2 in the prepared
20
nanocomposite was prominent. The homogeneity and the porous surface structure of the SiO2-
21
polyethylene terephthalate nanocomposite was confirmed by scanning electron microscopy
22
indicating that the nanofibers diameters were lower than 300 nm.
23
parameters influencing the extraction and desorption process such as temperature and extraction
24
time, ionic strength and desorption conditions were optimized. Eventually, the developed method
25
was validated by gas chromatography-mass spectrometry. Under optimized conditions, the
26
relative standard deviation values for a double distilled water spiked with the selected volatile
27
organic compounds at 50 ng L-1 were 2–7% (n = 3) while the limits of detection were between
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In addition, important
1
Corresponding author. Tel.: +98-21-66005718; Fax: +98-21-66012983 E-mail address: [email protected] (H. Bagheri)
1
Page 1 of 33
28
0.7 and 0.9 ng L-1. The method was linear in the concentration range of 10 to 1000 ng L−1 (R2>
29
0.9992). Finally, the developed method was applied to the analysis of Kalan dam and tap water
30
samples and the relative recovery values were found to be in the range of 86 to 102%.
31
Keywords
32
Polyethylene
33
spectrometry; BTEX; Water analysis
nanocomposites;
Nanoparticles;
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41 42 43 44
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chromatography-mass
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38
Gas
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1. Introduction
45
Solid phase microextraction (SPME), nowadays, is regarded as a well-established and solvent-
46
free extraction technique and has been applied to the determination of a variety of chemicals in
47
different samples. In this technique the extraction process is predominantly performed on fragile
48
fused silica-based fibers [1, 2] . Moreover, the fiber coatings might be damaged when they are
49
exposed to high temperatures of gas chromatography (GC) injectors and the desorbing organic
50
solvents of high performance liquid chromatography (HPLC). To have access to appropriate
51
fiber coatings is therefore one of the most critical steps in SPME. One important strategy to
52
improve the sensitivity of SPME relies on the usage of nanostructured materials in which the
53
interactions between the desired
54
developments of new fibers are mostly focused on improving the thermal, mechanical and
55
chemical stabilities, imparting diverse functionalities and polarities, and enhancing their
56
extraction capacity [3–5]. The use of nanocomposites-based coatings [6-8] and replacing the
57
fragile fused silica substrates by metallic wires [9,10] are among the recent approaches toward
58
achieving an overall stability for the prepared fibers.
59
Polyethylene terephthalate (PET) is a semicrystalline polymer and its composites are widely used
60
in many versatile applications. Considerable efforts have been devoted to improve various
61
physical, and mechanical properties of PET through mixing it with nanoclays to produce layered
62
clay-incorporated PET composites. Incorporation of spherical inorganic nanoparticles, i.e. nano-
63
SiO2, into the PET matrix is another approach to improve such properties [11–14].
64
Nanocomposites are multiphase solid materials where one of the phases has nano-scale
65
dimensions, or structures having nanometer-scale repeat distances between the different phases
66
[15]. In overall, these materials possess different physical properties in compared with the initial
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44
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chemicals and the fiber coating are enhanced. The
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Page 3 of 33
components. The surface area/volume ratio of the reinforcing materials, with morphological
68
characteristics, is a key issue for their spread applications [16]. The nanostructured materials
69
with unique properties have become highly popular as it would be possible to design multi-
70
functional structures at the nanometer scale. The selection of nanoparticles dopants depends on
71
the desired thermal, mechanical, and electrical properties of the nanocomposites [17-19]. Among
72
numerous nanomaterials, cobalt (Co)-based nanomaterials has specially attracted many attention
73
due to its good electroactivity and low cost [20,21]. For instance, the composite of Co
74
hexacyanoferrate nanoparticles (NPs)-carbon nanotube-chitosan has been used to develop
75
glucose sensor [22]. Pt–Co NPs supported on single-walled carbon nanotubes (SWCNTs) and Co
76
NPs integrated with SWCNTs have been applied for methanol [23] and nitrite detection [24].
77
Currently, nano-magnetic particles are attracting more attentions due to their distinguished
78
properties, such as surface effect, special magnetic target and good biocompatibility properties
79
[25, 26]. Among magnetic nanoparticles, Fe3O4 nanoparticles are particularly attractive because
80
of their biocompatibility, low toxicity and ease of preparation [27, 28]. Their unique properties
81
such as large surface area, excellent conformation stability and better contact between biocatalyst
82
and its substrate have made them quite suitable to be used in electrochemical biosensors [29].
83
Silica nanoparticles, due to their chemical inertness, nontoxicity, optical transparency, and
84
excellent thermal stability [30-32], have gained significant attentions. They can be widely used
85
in catalysis [33], chemical process industry [34], removal of metal ions [35], and metal ion
86
preconcentration [36] through polymer coatings or other functional groups. Large specific
87
surface area and high thermal stability of nanoporous silica materials (e.g., SBA-15) have made
88
them suitable alternative for SPME coatings. The chemical functionalization of SBA-15 by
89
incorporating organic groups and its capability as a support for the construction of conductive
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polymer/SBA-15 nanocomposites extends the range of materials applied as SPME coatings [37,
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38]. The recent studies on polypyrrole/SBA-15 [38], polyaniline/silica nanocomposite [15],
92
modified silica-polyamide nanocomposite [16] and ZnO/graphene nanocomposite [39] have
93
proven that these composites can be incorporated as appropriate coatings for hydrophobic
94
compounds.
95
One successful approach to prepare such nanocomposites is the electrodeposition of inorganic
96
oxides in organic matrices via electrospinning. The electrospinning process has already been
97
explained previously [ 40, 41]. The entrapment of the target analytes by the nanostructured
98
materials is facilitated due to its enhanced surface area and this justifies the use of minimum
99
amount of nanofibers for the extraction which subsequently reduces the volume of desorption
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solvent [ 42].
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In this work, electrospun PET-based nanocomposites were synthesized as new SPME coatings to
102
obtain a nanofibers network embedded with highly dispersed inorganic oxides nanoparticles.
103
Adaption of this strategy led to the production of a nanocomposite with higher reinforcement and
104
extraction efficiency. Effects of different nanoparticles such as Fe3O4, SiO2, CoO and NiO on the
105
extraction efficiency were also assayed. The SiO2-PET nanocomposite was found to be superior
106
and used as an unbreakable fiber coating for the determination of some selected organic
107
compounds in aqueous samples.
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2. Experimental
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2.1. Reagents
111
Benzene, toluene, ethyl benzene, o-xylene and m-xylene (BTEX) were purchased from Merck
112
(Darmstadt, Germany). Standard solution (1000 mg L−1) of each analyte was prepared in HPLC5
Page 5 of 33
grade methanol (Merck) and stored in the refrigerator. The working standard solutions were
114
prepared weekly by diluting the standard solution with methanol, and more diluted working
115
solutions were prepared daily by diluting this solution with double distilled water (DDW).
116
Sodium chloride, FeCl3·6H2O, FeCl2·4H2O, HCl and NaOH were purchased from Merck
117
(Darmstadt, Germany). PET was purchased from Kolon Industries Inc. (Seoul, Korea) and
118
triflouroacetic acid was obtained from Riedel-de Haën (Seelze-Hannover, Germany). All
119
solvents used in this study were of analytical reagent grade or HPLC grade. The different
120
nanoparticles such as SiO2, CoO and NiO with a diameters range of 12-50 nm were obtained
121
from US Research Nanomaterials, Inc (Houston, TX 77084, USA). Nitrogen was used for
122
providing the inert atmosphere necessary for synthesis of Fe3O4 NPs.
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2.2 Apparatus
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A GC model Agilent 6820, with a split/splitless injection port, and a flame ionization detection
126
system that the carrier gas was Nitrogen (99.999%) at a flow rate of 3 mL min-1, was used for the
127
optimization process. For quantitative determination, a Hewlett-Packard (HP, Palo Alto, USA)
128
HP 6890 series GC equipped with a split/splitless injector and a HP 5973 mass-selective detector
129
system that the MS was operated in the EI mode (70 eV) and Helium (99.999%) was employed
130
as carrier gas and its flow-rate was adjusted to 1 mL min-1 were used. A homemade glass inlet
131
liner with a total length of 7.7 cm, 6 mm o.d. and 1 mm i.d. was deactivated by
132
trimethylchlorosilane used. The separation of analytes was carried out using a capillary column
133
HP-5 MS (60 m, 0.25 mm i.d.) with 0.25 µm film thickness (Hewlett-Packard, Palo Alto, CA,
134
USA). The column was held at 70 °C for 3 min, increased to 110 °C at a rate of 10 °C min-1 and
135
raised to 210 °C at 30 °C min-1 and kept at this temperature for 3 min. The injector temperature
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was set at 240 °C and all injections were carried out on the splitless mode for 3 min. To obtain
137
the highest possible sensitivity, the MS detection was operated using time-scheduled SIM based
138
on the selection of some mass peaks of the highest intensity for each compound. In the SIM
139
mode, the following important ions are used for BTEX: m/z 71 for benzene, m/z 91 and 92 for
140
toluene, m/z 91, 106 for ethyl benzene, o-xylene and m-xylene.
141
The SPME syringe was made in our laboratories consisting of two spinal needles with gauge
142
numbers of 22 and 26. All samples were extracted from a 10 mL glass vials with a PTFE-faced
143
septum and aluminum cap. Samples were heated in a homemade glass water bath connected to a
144
refrigerated circulating water bath (Neslab, USA) and stirred using a MR Hei-Mix S magnetic
145
stirrer (Heidolph, Germany). The electrospinning experiments were performed using a regulated
146
power supply (Brandenburg, England). A KDS100 syringe pump (Kd. Scientific Co., U.S.A.)
147
was used for the polymer solution delivery in the electrospinning process.The Scanning electron
148
microscopy (SEM) images were obtained by a TESCAN VEGA II XMU (Brno, Czech Republic)
149
and Fourier transform infrared spectroscopy (FTIR) spectra were recorded by a ABB Bomem
150
MB100 (Quebec, Canada).
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2.3 Preparation of magnetic nanoparticles
153
Magnetic nanoparticles were synthesized according to the procedure described previously [ 43].
154
Briefly, 5.2 g of FeCl3·6H2O and 2 g FeCl2·4H2O and 0.85 mL concentrated HCl were dissolved
155
in 25 mL water under the N2 gas. This solution was added drop wise into 250 mL of sodium
156
hydroxide solution (1.5M) under the N2 atmosphere while it was vigorously stirred for 30 min
157
(1000 rpm). The resulted black precipitates were separated using a 1.4 T magnet and washed
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several times with degassed water. Finally MNPs were dried in vacuum oven at 60 ºC for 5
159
hours.
160
2.4 Preparation of PET-base nanocomposite
162
First, 0.10 g of PET (10 % w/v) was dissolved in 1 mL of TFA and stirred for 20 min to obtain a
163
homogenous solution. After complete dissolving of polymer and obtaining a homogeneous
164
solution, different types of nanoparticles were added into the polymer solution and it was
165
vigorously stirred for 20 min (1000 rpm). Then 0.5 mL of this solution was withdrawn into a 1
166
mL syringe which was eventually located in a syringe pump. A homemade SPME device was
167
attached to a small electrical motor in a way that the plunger wire could be rotated while
168
electrospinning was performed. Under this condition, the external needle and the polymer
169
containing syringe needle were connected to the high voltage power supply terminals. A length
170
of 1.5 cm from the end part of plunger wire was used for collecting the nanofibers. The other part
171
of the SPME assembly was protected from the nanofibers flying towards the collector by a
172
packed paper insulator. The distance between the needle and the collector was set at 10 cm. The
173
SPME assembly was in perpendicular position in respect to the syringe. A voltage of 16 kV was
174
applied for the nanofibers production while a flow rate of 0.5 mL h−1 was set for the polymer
175
solution delivery using a syringe pump. All the electrospinning processes were performed under
176
the ventilation (Fig. 1a). All fibers were electrospun for 2 min. Finally, the SPME fibers were
177
inserted into the GC injector port for conditioning at 240 ◦C for 3 h.
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2.5 The SPME process
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A home-made SPME device [7,16,40] containing the synthesized PET-based nanocomposite
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fiber coating (~ 60 µm film thickness) was employed for the headspace SPME process. During
182
the extraction, the aqueous sample was stirred by a magnetic stir bar and a water bath was used
183
to control the temperature. The thermal desorption was carried out at 240 °C while the split valve
184
of the GC injector was kept closed for 3 min. For extraction purposes, 4 mL of double distilled
185
water or real water samples were spiked with the selected analytes in a 7-mL vial containing 0.2
186
g of sodium chloride under the maximum stirring rate (1000 rpm). The vials were then sealed
187
with a PTFE-faced septum and a cap. The extraction was performed by exposing the PET-based
188
nanocomposite fiber coating to the sample headspace for 10 min at 50 °C. Then, the SPME fiber
189
was withdrawn from the vial and transferred to the GC injection port for thermal desorption of
190
the model analytes.
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3. Results and discussion
193
The chemical structure of PET (Fig. 1b) along with its physical properties data encouraged us to
194
employ it as a major constituent for preparation of a rather stable nanocomposite for extraction
195
purposes. . The influential parameters on the morphology of nanocomposite were examined by
196
comparing the extraction efficiency of the model analytes from aqueous samples. Also, the
197
effects of type of nanoparticles, doping levels of nanoparticles, polymer concentration and
198
coating time on the extraction efficiency of the synthesized nanocomposites were investigated.
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After achieving the most appropriate nanocomposite, the effects of extraction/desorption
200
parameters were also optimized.
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3.1. PET-based nanocomposites and influences of nanoparticles
203
According to the structure of PET, a strong π–π interaction for this polymer towards organic
204
analytes could be anticipated. Unlike conductive polymers, PET can be dissolved in a solvent
205
such as TFA for convenient electrospinning. The preliminary results revealed that this polymer
206
with good flexibility and mechanical stability could be used as an efficient SPME fiber coating
207
without being peeled off from the needle in whole extraction procedure. The effect of PET
208
concentration on the extraction efficiency was investigated considering a concentration range of
209
4–22% (w/v). According to Fig. 2a , by increasing the polymer concentration to 18%, the
210
extraction efficiency was enhanced and then started to decline. Apparently as the polymer
211
concentration rises, the sorbent capacity is enhanced, but by further concentration increase a
212
thicker fiber coating could be formed which leads to lower surface area and subsequent lower
213
extraction efficiency.
214
The enhancement of surface area and porosity surly has a direct effect on the extraction
215
efficiency. The presence of different types of inorganic oxides NPs in polymer structure is an
216
efficient way to fulfill this purpose. For investigating the effect of NPs type on the extraction
217
capability of PET nanocomposite, fourNPs including Fe3O4, SiO2, CoO and NiO at equal weight
218
percent were embedded in the electrospun PET network. Fig. 2b shows that PET-based
219
nanocomposites containing inorganic NPs exhibit higher extraction efficiencies for the model
220
organic analytes Our results revealed that the silica-PET nanocomposite
221
extraction efficiencies in compared with those containing CoO, NiO and Fe3O4 NPs (SiO2-PET
222
> CoO-PET > NiO-PET > Fe3O4-PET > PET nanofibers). This trend might be due to the
223
effective dispersion of silica NPs throughout the polymer network compared with the others. The
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addition of silica nanoparticles was found to be the most appropriate nanofiller. For investigating
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could yield higher
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Page 10 of 33
the effect of silica NPs doping level on extraction capability, five silica-PET nanocomposites
226
were doped with 0.0018, 0.010, 0.020, 0.030 and 0.040 g of silica NPs (1- 22%, w/v). As shown
227
in Fig. 2c , the extraction capability of the silica-PET nanocomposite was enhanced at the
228
expense of more silica NPs . Apparently for most analytes, increasing the silica doping level up
229
to 0.040 g led to an improvement in extraction efficiency, while at higher values , the
230
electrospining process became rather difficult. According to these results a doping level of 0.040
231
g of the silica NPs was chosen as the optimum value. The SPME coating thickness can be
232
controlled by the electrospinning time. As the electrospinning time was increased, the fiber
233
coating became thicker, providing a higher extraction capacity. According to Fig. 2d , the time of
234
electrospinning was selected at 3 min which led to the formation of a SPME fiber coating with a
235
desirable thickness.
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3.2. Characterization of the SiO2-PET nanocomposite
238
The FTIR spectra obtained for NiO, SiO2, Fe3O4 and CoO NPs are show in Fig. 3. The
239
absorption peaks appeared at 466, 1100 and 576 cm-1 could be assigned to Ni-O, Si-O and Fe-O
240
stretching vibrations, respectively. The spectrum of CoO NPs exhibited two absorption peaks at
241
525 and 646 cm-1 corresponding to stretching vibration mode of Co-O and the bridging vibration
242
of O-Co-O bond, respectively. The presence of these NPs in electrospun PET nanofibers was
243
monitored by FTIR (Fig. 4). The FTIR spectra of PET-based nanocomposites and PET
244
nanofibers clearly show the absorption peaks at 683 and 725 cm-1 corresponding to bending
245
motion of phenyl ring and coupled vibration of carbonyl and deformation of phenyl group,
246
respectively. Also, the absorption peaks appeared at 1268 and 1503 cm-1 correspond to C-O-C
247
in-plane ring deformation and aromatic ring vibration of C-C group, respectively. The absorption
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peak at 1720 cm-1 correspond to C=O stretching vibration of PET. Moreover, the peaks at 466,
249
1100, 576, 525 and 646 cm-1 in the spectra of PET-based nanocomposites clearly confirms the
250
presence of four nanoparticles, individually doped in the nanocomposite.
251
The SEM image of the dispersed iron oxide nanoparticles in water shows that the synthesized
252
MNPs have rather high surface area (Fig. 5a). The morphology of SiO2-PET nanocomposite and
253
PET nanofibers was compared using SEM (Figs. 5b,5c). The SEM images revealed that the
254
SiO2-PET nanocomposite coating film is porous and homogeneous along the stainless steel wire
255
while the nanocomposite dimensions are below 300 nm (Fig 5 b). Also, interestingly the
256
produced nanofibers in SiO2-PET nanocomposite in compared with PET nanofibres are thinner
257
(Figs 5 b,5c). Certainly the higher porosity of SiO2-PET nanocomposite could lead to the
258
increased surface area, adsorption sites availability and higher mass transfer for analytes during
259
the sorption/desorption steps. The presence and dispersion of SiO2 nanoparticles throughout the
260
coating increases the spaces among the nanofibers in which much faster analytes diffusion in the
261
PET network becomes more likely.
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3.3. Optimization of HS- SPME procedure
264
For the adapted HS-SPME procedure, the effects of extraction temperature/time, desorption
265
temperature/time and salt concentration on the extraction of the model compounds were
266
optimized.
267
The addition of NaCl to the sample caused an increase in the efficiency of the extraction for all
268
the tested analytes (Fig. 6a). At salt concentrations above 25%, the responses for all analytes
269
remained almost constant and this value was used as the optimum quantity.
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Page 12 of 33
The extraction temperature is a pronounced parameter in headspace-SPME mode because it
271
could have considerable effect on the equilibrium status of extraction. The extraction
272
temperature has diverse effects on the extraction efficiency. At high temperatures the analytes in
273
the sample solution can escape from their matrix and enter the headspace, causing the increase of
274
analytes concentration in the headspace and subsequent enhancement of higher extraction rates .
275
Nevertheless, the sorbent/headspace distribution coefficient also decreases at higher
276
temperatures, resulting in a decrease in the equilibrium amount of the extracted analyte. A
277
temperature range of 25–70 ◦C was considered and according to Fig. 6b, the amounts of extracted
278
analytes were enhanced as temperature raised to 50 ◦C and then started to decline. At higher
279
temperatures, the heat transfer to the metallic needle possibly leads to thermal desorption of the
280
previously adsorbed analytes. Considering these results, a temperature of 50 ˚C was selected for
281
the further experiments.
282
Both desorption time and temperature are influential in the recovery process and they must be
283
optimized. The desorption time should be kept as minimum as possible in order to prevent any
284
carryover effect. The maximum desorption temperature is limited by the thermal stability of the
285
sorbent. To prevent the coating from the harsh desorption conditions and increase its life time,
286
the lowest possible temperature should be applied for desorption. The desorption temperature
287
was varied from 100 to 280 ◦C. Although, the rise of desorption temperature increases the
288
desorption efficiency, for the sake of the coating life time, a temperature of 240 ◦C was chosen
289
for further experiments. While the desorption temperature was set at 240 ◦C, the desorption time
290
was varied within a range of 1–5 min, by leaving the SPME device in the injection port of GC.
291
Clearly, desorption was increased by time and the maximum value of 3 min was selected.
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Page 13 of 33
An extraction time interval, ranging from 5 to 25 min, was also studied. As shown in Fig. 6c, the
293
extraction profiles for all analytes increased up to 10 min and after that they reached to the
294
equilibrium. In overall, an extraction time of 10 min was finally chosen.3.3.1 Durability and
295
stability of PET -SiO2 nanocomposite
296
The coating lifetime is a very important issue as far as the practical consideration is concerned.
297
Also, durability together with chemical and thermal stability of the fiber coating are also
298
important issues in SPME. They correspond to the extraction capability of the fiber coating after
299
frequent usage and exposing to different solvents and high temperatures.
300
In order to achieve a complete desorption with no carry-over effects, the fiber coating thermal
301
resistance is a very important parameter for SPME applications. The PET -SiO2 nanocomposite
302
fiber coating was conditioned for 1 h at different temperatures (100-300 C) and then used for
303
headspace sampling of BTEX. The ratios (r) of BTEX were obtained by comparison of the peak
304
area at each temperature versus the value achieved at 240 C (Table 1). The extraction ability of
305
the fiber coating was barely affected by the temperature ranging from 240 to 300 C. However,
306
the temperature of 240 C was selected to extend the life time of the fiber coating.
307
The PET -SiO2 nanocomposite fiber coating was regularly checked and no considerable changes
308
in its extraction ability could be observed after 125 runs, while for each run, the fiber kept at 240
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C at least for 3 min. Moreover, the chromatograms obtained from the blank fiber led to
310
insignificant changes, indicating the fiber coating sufficient stability.
311
Solvent stability is very important for SPME-LC analysis and SPME derivatization technique,
312
since both of them require the fiber to be directly immersed into the appropriate solvent for a
313
certain time. Solvent stability was also assessed since the coating must have no bleeding during 14
Page 14 of 33
314
its contact with the organic solvent. The coating was therefore immersed in several solvents with
315
different polarities including water, methanol, dichloromethane and acetonitrile. Then, it was
316
exposed to high temperatures (up to 240 C) for 30-min intervals and no loss of performance
317
could be observed.
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3.4 Method validation
320
Using the optimized conditions, the developed method was validated for determination of BTEX
321
in the spiked distilled water sample. The method detection limit and other analytical features for
322
the selected analytes are tabulated in Table 1. The correlation coefficients for all analytes were
323
satisfactory (r2>0.9992) and limits of detection (LOD), based on a signal-to-noise ratio of 3:1,
324
were between 0.7 and 0.9 ng L-1. The intraday precision was evaluated by determining the peak
325
area of each BTEX in three spiked replicates at two different concentration levels. The intraday
326
precision showed RSD% values ranging from 6 to 9% at 0.05 ng mL-1 and 2 to 6% at 0.15 ng
327
mL-1 for the selected volatile compounds (Table 2 ). The interday precision ranged from 5 to
328
11% at 0.05 ng mL-1. To evaluate the applicability of the current method, extraction and analysis
329
were performed on water samples obtained from Kalan dam and drinking sources. Real water
330
samples were spiked at a concentration level of 50 ng L−1 and the analysis was carried out under
331
the optimized SPME conditions . Relative recoveries in the range of 86–102% for the selected
332
analytes were achieved.
333
Table 3 compares some statistical data obtained from this work with other relevant reports [41-
334
42]. Clearly the synthesized nanocomposite fiber coating (~ 60 µm film thickness) exhibits
335
comparable analytical performance to other SPME fibres. In addition, the synthesis of the
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nanocomposite coating can be carried out easily while it is rather low cost, convenient, rapid and
337
durable. Additionally, the long life time of this novel fiber coating was validated by no tangible
338
change in extraction efficiency of the selected BTEX after 125 times.
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4. Conclusion
341
In this study, a SPME technique coupled with GC-MS based on the use of SiO2–PET
342
nanocomposite fiber coating was developed for trace determination of some organic model
343
compounds, in water samples. Relative recoveries along with other analytical data approved the
344
candidacy of the synthesized nanocomposite as an appropriate SPME fiber coating for extracting
345
aromatic compounds. The electrospun SiO2–PET nanocomposite mat, due to non-smooth and
346
porous structure, provides high specific surface area with increased activated sites. The
347
developed method based on the use of SiO2–PET nanocomposite coating, is facile, simple, rapid
348
and inexpensive and can be employed for the determination of BTEX with sufficient sensitivity
349
and reproducibility. Also chemical structure of nanocomposite film contributes to hydrophobic
350
and π-π interaction between analytes and the PET polymer, making the SPME fiber coating a
351
sensitive and unbreakable probe for extracting organic compounds.
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Acknowledgements
354
The Research Council and Graduates School of Sharif University of Technology (SUT) are
355
thanked for supporting the project.
356 357
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490 491
Figure captions
492
Fig. 1: Schematic diagram of (a) the synthesizing setup for PET-based nanocomposites and (b) structure of PET.
ip t
493 494 495
Fig. 2: Effects of (a) concentration of polymer, (b) different types of nanoparticles, (c) amount of silica nanoparticles and (d) the coating time on the extraction efficiency.
cr
496 497
Fig. 3: FTIR spectra of different types of NPs.
an
499
us
498
500
505 506 507 508 509
M
503 504
Fig. 5: SEM images of (a) magnetic nanoparticles ( b) SiO2-PET nanocomposite and ( c) PET nanofibers.
d
502
Fig. 4: FTIR spectra of PET-based nanocomposites.
Ac ce pt e
501
Fig. 6: Effect of (a) salt concentration, (b) extraction temperature and (c) extraction time on the extraction efficiency.
24
Page 23 of 33
509
Fig. 1
511 512
513 514
Ac ce pt e
d
M
an
us
cr
ip t
510
(a)
(b)
515 516 25
Page 24 of 33
517 518
Fig. 2
ip t
519
(b)
Ac ce pt e
d
M
(a)
an
us
cr
520
(d)
(c)
521 522 523 524 525 526
26
Page 25 of 33
527
Fig. 3
528
ip t
529
531 532 533 534 535
Ac ce pt e
d
M
an
us
cr
530
536 537 27
Page 26 of 33
538
Fig. 4
542 543 544 545
d
541
Ac ce pt e
540
M
an
us
cr
ip t
539
546 547 548 28
Page 27 of 33
Fig. 5
Ac ce pt e
d
M
an
us
cr
ip t
549
550 551 552
Fig. 6 29
Page 28 of 33
553 554
us
cr
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555
an
(b)
556 557 558 559 560
Ac ce pt e
d
M
(a)
(c)
561 562 563
Table1- Thermal stability of PET-SiO2coated SPME fibera 30
Page 29 of 33
◦
160
200
240
260
280
300
Benzene
0.21
0.30
0.45
0.85
1.00
1.02
1.00
0.99
Toluene
0.21
0.29
0.43
0.84
1.00
1.04
1.04
1.01
Ethyl benzene
0.20
0.31
0.46
0.76
1.00
1.05
1.06
M-xylene
0.21
0.29
0.43
0.84
1.00
1.04
O-xylene
0.21
0.30
0.45
0.85
1.00
1.03
ip t
140
1.03
1.04
1.05
1.02
us
1.04
a
M
an
The ratio (r) is defined by the peak areas at each temperature with respect to peak are obtained at ◦ 240 C.
d
565
100
Ac ce pt e
564
Compound
cr
Condition temperature ( C)
31
Page 30 of 33
566 567
Table 2- Some analytical data obtained from HS-SPME of BTEX using the SiO2-PETnanocomposite fiber coating and GC-MS
568 LDR1 ( ng L-1)
Compound
(R2)
LOD2 (ng L−1)
intra-day RSD3a% (n=3)
inter-day RSDb% (n=3)
intra-day Relative recovery%
fiber-tofibreRSDc % (n=3)
0.90
9
6
Toluene
5-1000
0.9999
0.90
7
5
Ethyl benzene Ortho xylene Metha xylene
5-1000
0.9997
0.70
6
2
5-1000
0.9998
0.90
8
5
5-1000
0.9992
0.85
6
4
Linear dynamic range Limit of detection 3 Relative standard deviation a Spiked at 0.05 ng L-1 b Spiked at 0.15 ng L-1 c Spiked at 0.05 ng L-1
86
95
8
95
96
5
99
102
8
102
99
7
100
98
an
1
11
cr
0.9995
us
5-1000
Tap water
d
M
2
Ac ce pt e
569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597
Benzene
ip t
Kalan dam water
32
Page 31 of 33
Table 3- Comparison of analytical characteristics for proposed fiber with other fibers in determination of BTEX compounds LDRa
LODb
RSDc%
Polydimethylsiloxane /Divinylbenzene
10–2000 ng
0.75-1.4
5.4–8.3
10–800
5.4–14.8
3.9-8.2
Polydimethylsiloxane
10–25,000
0.75–10
Cobalt oxide nanoparticles
10–300,000
1–11
Sol–gel-carbon aerogels
1–1500
Graphite
0.6–11090
SiO2-PETnanocomposite
10-1000
µg L-1 µg L-1 µg L-1 ng L-1
cr
Water
[ 45]
