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A Porous Aromatic Framework 48/Gel Hybrid Material-Coated Solid-Phase Microextraction Fiber for the Determination of the Migration of Styrene from Polystyrene Food Contact Materials Yuanyuan Jin, Zhongyue Li, Lei Yang, Jun Xu, Le Zhao, Zhonghao Li, and Jiajia Niu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04083 • Publication Date (Web): 21 Dec 2016 Downloaded from http://pubs.acs.org on December 24, 2016

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Analytical Chemistry

A Porous Aromatic Framework 48/Gel Hybrid Material-Coated Solid-Phase Microextraction Fiber for the Determination of the Migration of Styrene from Polystyrene Food Contact Materials Yuanyuan Jina, c, Zhongyue Lia*, Lei Yangc, Jun Xuc, Le Zhaob, Zhonghao Lib, Jiajia Niub* a

School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China Zhengzhou Tobacco Research Institute of CNTC, Zhengzhou, Henan 450001, China c School of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China b

ABSTRACT: A novel solid-phase microextraction (SPME) fiber was fabricated by a porous aromatic framework 48 (PAF-48)/Gel hybrid material through a sol-gel process. PAF-48 is a porous organic framework (POF) material that was polymerized from 1,3,5triphenylbenzene. The uniform pore structure, high surface area, continuous conjugate network and hydrophobicity make PAF-48 expected to have special abilities to absorb and extract styrene as well as some other harmful volatile aromatic compounds (VACs). The PAF-48/Gel-coated fiber was explored for the extraction of styrene and six VACs (benzene, toluene, ethylbenzene and xylenes) from aqueous food simulants followed by gas chromatography (GC) separation. The fiber was found to be very sensitive for the determination of the target molecules with wide linear ranges (0.1–200 or 500 µg⋅kg-1), low limits of detection (LODs, 0.003–0.060 µg⋅kg-1), acceptable precisions (intraday RSD < 5.9%, interday RSD < 7.3%) and long lifetime (> 200 times). Particularly for styrene, the PAF-48/Gel-coated fiber exhibited a much lower LOD (0.006 µg⋅kg−1) compared with most of the reported fibers. Moreover, the PAF-48/Gel-coated fiber had a high extraction selectivity for styrene and VACs over alcohols, phenols, aromatic amines and alkanes and show a molecular sieving effect for the different molecule sizes. Finally, the PAF48/Gel-coated SPME fiber was successfully applied in GC for the determination of the specific migrations of styrene and VACs from polystyrene (PS) plastic food contact materials (FCMs).

1. Introduction Plastics are widely used as packaging materials for foods, pharmaceuticals, detergents, etc. Because they are lightweight, strong, easily processed easily stored, and economical.1 For example, polystyrene (PS) is used in the manufacturing of rigid or foam disposable food contact materials (FCMs).2 However, during the polymerization production process, residual low molecular mass styrene monomer would be found in PS products and has the risk of migrating out of the packaging into food.3 Styrene has a toxic effect on the liver as well as the central nervous system.4 In 2011, styrene was listed as “reasonably anticipated to be a human carcinogen” by the US National Toxicology Panel.5 Therefore, the accurate determination of the specific migration of styrene from PS food packaging is very important for the risk assessment of styrene in FCMs. Presently, gas chromatography (GC) is one of the most regular detection methods for styrene.6 However, the traditional approach is cumbersome, inefficient, and usually needs complex preprocessing that includes solvent extraction as well as concentration before GC analysis. Instead, solid-phase microextraction (SPME) is widely used for the sample pretreatment and enrichment in the field of environmental engineering, biology, clinical medicine and food industry, etc, because it is highly effective and solventfree.7 The application of SPME technology to the determination of the specific migration of styrene will greatly improve the efficiency and sensitivity of the analysis.8,9

The sorbent coating on the fiber plays a key role in SPME, including extraction efficiency and selectivity primarily. Therefore, multifarious materials have been reported as the fiber coatings, such as organic polymers,10 ionic liquids,11 carbon nanomaterials,12 molecular imprinted polymers13 and mesoporous materials.14 Since the in situ growth of a MOF199 film on stainless steel fiber for SPME was reported by Yan’s group in 2009,15 a type of inorganic-organic hybrid microporous materials termed metal organic frameworks (MOFs) have been considered to be an appropriate material for SPME fiber coatings because of their special properties such as programmable porous structures, large surface area and uniform pores.16-19 Recently, as the successors of MOFs, porous organic frameworks (POFs), which are generated by the linkage of organic polymerizable monomer building blocks or sometimes post-polymerization hyper-cross-linking, have been explored.20 Various POFs, such as covalent organic frameworks (COFs),21 polymers of intrinsic microporosity (PIMs),22 conjugated microporous polymers (CMPs),23 hypercrosslinked polymers (HCPs),24 triazine-based organic frameworks (CTFs),25 and porous aromatic frameworks (PAFs),26 have been successfully designed and synthesized in the past decade. Compared with MOFs, POFs have not only programmable porous structures, aromatic frameworks, large surface areas and narrow pore distributions but also better water, acid and alkali tolerances.21-26 POFs have great potential for extraction on the basis of molecular adsorption and sieving. Moreover, they can be used under harsher conditions.

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In 2015, one of the POFs SNW-1 was reported as a SPME fiber coating by Prof. Li’s group, and the prepared fiber exhibited superior enrichment performance of polycyclic aromatic hydrocarbons and volatile fatty acids.27 PAFs are a series of POFs that are polymerized from aromatic monomers. Compared with other POFs, more aromatic PAFs have much stronger enrichment abilities for benzene homologues (styrene, benzene, toluene, ethylbenzene, xylene, etc.) due to the π-π affinity between the sorbents and the target molecules. Therefore, PAFs are appropriate candidates for SPME fiber coatings, especially for the extraction of styrene and other VACs. However, how to prepare a uniform, compact and firm PAF coating on a SPME fiber is a big challenge, because PAFs are difficult to grow in situ on the fiber substrate like MOFs. Due to the high thermal stability, good film-forming property and covalent bonding on the surface of quartz fiber, sol-gel is an ideal process to help PAFs form a film on quartz fibers. In this paper, a SPME fiber with a novel hybrid coating material PAF-48/Gel was prepared by a sol-gel process (Figure 1), which was applied for the determination of the migrations of styrene and some harmful VACs (benzene, toluene, ethylbenzene and xylene) from PS plastic FCMs. PAF-48 is a PAF material that was achieved from 1,3,5triphenylbenzene by an AlCl3-induced Scholl polymerization reaction.28 Sol-gel preparation technology was used to help PAF-48 particles form a uniform, compact and firm coating on the quartz fiber. The high aromaticity, narrow pore distribution, high surface area, and good water, acid and alkali tolerances make the functional component PAF-48 likely to afford special extraction abilities of styrene and VACs. The PAF-48/gel-coated fiber was explored for the extraction of styrene and VACs from aqueous food simulants followed by GC separation with flame ionization detection (FID). Experiment conditions, including the extraction time, extraction temperature, salt concentration, and agitation speed were optimized for the PAF-48/Gel-coated fiber. The extraction performances of the prepared fiber were evaluated. The molecular sieve effect of the PAF-48/Gel coating was discussed systematically, and the extraction selectivity of the PAF-48/Gel coating for styrene and VACs towards alcohols, phenols, aromatic amines and alkanes was studied. Moreover, the PAF-48/Gel was applied for the determination of the specific migrations of styrene and VACs from PS disposable cups to food simulants. 2. Experimental 2.1. Reagents and materials 1,3,5-Triphenylbenzene (99%), methyltrimethoxysilane (MTMOS, 98%), silanol terminated polydimethylsiloxane (OH-PDMS), trifluoroacetic acid (TFA, 99%) and polymethylhydrosiloxane (PMHS, 98%) were bought from the Beijing J&K Technology Co., Ltd (Beijing, China). Anhydrous aluminum chloride (99%) was bought from the Aladdin Chemistry Co., Ltd (Shanghai, China). Methanol, ethanol, acetone, trichloromethane and tetrahydrofuran were bought from the Tianjin Yongda Chemical Reagent Co., Ltd (Tianjin, China). Benzene (99.5%), o-xylene (96%), m-xylene (99%) and p-xylene (99%) were bought from Alfa Aesar (Heysham, U.K.). Toluene (99.8%) was bought from the Tianjin Kemiou Chemical Reagent Co., Ltd (Tianjin, China). Ethylbenzene (99%), styrene (99%), n-hexane (99.5%), nheptane (99%), diphenylmethane (99%), triphenylmethane

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Figure 1. Schematic diagrams of the preparation process of the PAF-48/Gel-coated fiber.

(99%), propylbenzene (98%), butylbenzene (99%), 1phenylpentane (98%), trimethylbenzene (97%), triethylbenzene (97%), triisopropylbenzene (95%), 2chlorophenol (98%), 2,4-dimethylphenol (99%), 3methylphenol (98%), 2,4-dichlorophenol (99%), 4chlorophenol (99%), n-butanol (99.5%), n-amyl alcohol (99.5%), n-hexyl alcohol (98%), n-octyl alcohol (99%), noctane (99.5%), n-nonane (99%), n-decane (99.5%), aniline (99%), o-toluidine (99.5%), 4-chloroaniline (99%) and 2,3diaminotoluene (97%) were bought from the Beijing J&K Technology Co., Ltd (Beijing, China). Phenol and cresols (99.5%) were brought from Dr. Ehrenstorfer GmbH (Augsburg, Germany). 2-Nitrophenol (≥99%) and n-propanol (≥99.5%) were bought from the Sigma-Aldrich Co. LLC (St. Louis, USA). The fused-silica fibers were taken from Fiber Home Telecommunication Technologies (Wuhan, China). 5 µL syringes were bought from the Shanghai Gaoge Industrial and Trading Co., Ltd. (Shanghai, China). Headspace vial (20 mL) was purchased from Agilent Technologies (California, USA). The commercially available fibers for comparison including polydimethylsiloxane (PDMS, 100 µm), carboxen/polydimethylsiloxane (CAR/PDMS, 75 µm) and polydimethylsiloxane/divinylbenzene (PDMS/DVB, 65 µm) were purchased from Supelco (Bellefonte, USA). 2.2. Instrumentation Powder X-ray diffraction (PXRD) measurements were performed on a D8 Advance X-ray diffractometer (Bruker, Karlsruhe, Germany) using Cu-Kα radiation, 40 kV and 40 mA with the scan speed of 6°⋅min-1. Nitrogen adsorption and desorption were performed on an Autosorb-1C (Quantachrome, Florida, USA) at 77 K. The scanning electron microscope (SEM) images were obtained on a JSM-6010LA (JEOL, Tokyo, Japan). Thermogravimetric analyses (TGA) were performed using a Discovery TGA (TA, New Castle County, USA) at the heating rate of 5°C⋅min-1 in a dried air atmosphere. The Fourier transform infrared (FTIR) spectra were recorded (4000−400 cm−1 region) on an Optics VERTEX 70 Fourier transform infrared spectrometer (Bruker, Karlsruhe, Germany) using a KBr pellet. The size distribution data were collected on Zetasizer Nano ZS laser particle analyzer (Malvern, Malvern, UK). An American-made Agilent 6890N equipped with a split/splitless injector and a FID was used for determination of


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all the analytes in this study. Helium (99.999%) and nitrogen (99.999%) were employed as carrier gas and makeup gas, whose flow rates were adjusted to 1.0 mL⋅min-1 and 40 mL⋅min-1, respectively. The flow rates of synthesized air (volume ratio of nitrogen to oxygen was 79:21) and hydrogen (generated by Parker H2PD-300-220 hydrogen generator) were adjusted to 40 mL⋅min-1 and 400 mL⋅min-1, respectively. All chromatographic analyses were carried out with an HPINNOWAX capillary column (60 m × 0.25 mm i.d. × 0.25 µm) (Agilent, California, USA), 230°C injector temperature, 250°C detector temperature and splitless mode for 2 min. The initial column temperature was set at 40°C (held for 3 min) and increased to 130°C at a rate of 10°C ⋅min-1 (held for 3 min), and then raised to 200°C at 20°C⋅min-1. 2.3. Preparation 2.3.1. Synthesis of PAF-48 PAF-48 was prepared referring to the reported method.28 Anhydrous aluminum chloride (3.75 mmol) was added into 40 mL of anhydrous trichloromethane under nitrogen gas. The mixture was kept at 60°C for 3 h with stirring (denoted as 1). A 0.85 mmol aliquot of 1,3,5-triphenylbenzene was dissolved in 20 mL of anhydrous trichloromethane under a nitrogen atmosphere (denoted as 2). Solution 2 was added into 1 slowly, and then, the mixture was stirred at 60°C for 24 h. After the reaction finished, the system was cooled down to room temperature, filtered and washed with hydrochloric acid solution (1 M), methanol, and acetone successively to remove the unreacted monomers and catalyst residues. The product was further purified by Soxhlet extraction with ethanol, tetrahydrofuran and trichloromethane for 48 h in turn. Finally, the product was dried in vacuum for 8 h at 80°C. 2.3.2. Pretreatment of the silica fiber After the protecting polyimide layer of the silica fiber was removed with acetone, one tip (approximately 3 cm in length) of the fiber was immersed into a NaOH solution (1 M) for exposing the maximum number of Si-OH groups, and then, it was treated with HCl solution (1 M) to neutralize the redundant -OH ions. Finally, the fiber was washed with deionized water and dried in air. 2.3.3. Preparation of the PAF-48/Gel-coated fiber The sol-gel approach was employed for fabricating the PAF48/Gel-coated fiber. 100 mg of the synthesized PAF-48, 300 µL of MTMOS, 180 mg of OH-PDMS and 30 mg of PMHS were added into a 0.50 mL centrifuge tube and mixed thoroughly by vortex mixing for 5 min. Then, 150 µL of TFA (95%) was added into the mixture and vortex mixed for 5 min to obtain PAF-48/Sol. The pretreated silica fiber was dipped into PAF-48/Sol to a depth of approximately 3 cm and kept there for 5 min. PAF-48/Sol was coated onto the surface of the silica fiber after the fiber was pulled out. The PAF-48/Solcoated fiber was dried at 60°C for 2 min. This process was repeated several times until a desired coating thickness of approximately 60 µm was obtained. Finally, the PAF-48/Gelcoated fiber was got by aging at 240°C for 1 h. 2.4. Enrichment procedure The stock solution for each compound was prepared to 1000 mg⋅L-1 methanol solution and stored in a refrigerator. The working standard solutions were diluted with deionized water according to the experiment need. 5 g of solution, including working standard solutions and real sample, was introduced

into a 20 mL headspace vial. The extraction conditions (time, temperature, salt concentration, agitation speed, etc.) were adjusted according to what was required by the experiment. The self-made fiber was fixed on the reformed a 5 µL syringe and a headspace mode was employed. 2.5. Optimization of the HS-SPME extraction procedure The influences of the experiment conditions including extraction time, extraction temperature, salt concentration, and agitation speed on the extraction efficiency were investigated sequentially. In these processes, the desorption conditions were set at 230°C for 0.5 min, and sample concentration was 100 µg⋅kg-1. 2.6. Evaluation of the extraction performance 2.6.1 Evaluation of the extraction efficiency The extraction experiments of PAF-48/Gel-coated fiber and the commercial CAR/PDMS (75 µm), PDMS/DVB (65 µm) and PDMS (100 µm) fibers for styrene and VACs were carried out. The experimental conditions were as follows: sample concentration, 100 µg⋅kg-1; extraction time, 30 min; extraction temperature, 40°C; no salt added; agitation speed, 400 rpm; desorption temperature, 230°C; and desorption time, 0.5 min. 2.6.2. Selectivity and evaluation of the molecular sieve effect The extraction experiments of PAF-48/Gel-coated fiber for (a) benzene, toluene, ethylbenzene, p-xylene, m-xylene, o-xylene, styrene, n-propanol, n-butanol, n-amyl alcohol, n-hexyl alcohol, and n-octyl alcohol; (b) benzene, toluene, ethylbenzene, p-xylene, m-xylene, o-xylene, styrene, 2chlorophenol, o-cresol, phenol, 2,4-dimethylphenol, 3methylphenol, 2,4-dichlorophenol, 2-nitrophenol, and 4chlorophenol; (c) benzene, toluene, ethylbenzene, p-xylene, m-xylene, o-xylene, styrene, aniline, o-toluidine, 4chloroaniline, 2,3-diaminotoluene; (d) n-hexane, n-heptane, noctane, n-nonane, n-decane, benzene, toluene, ethylbenzene, p-xylene, m-xylene, o-xylene, and styrene; (e) benzene, diphenylmethane, and triphenylmethane; (f) benzene, trimethylbenzene, triethylbenzene, and triisopropylbenzene; and (g) benzene, toluene, ethylbenzene, propylbenzene, butylbenzene, 1-phenylpentane, and 1-bromo-4-nhexylbenzene were carried out. The extraction conditions for (a) and (d)−(g) were as follows: sample concentration, 100 µg⋅kg-1; extraction time, 30 min; extraction temperature, 40°C; salt concentration, 30%; agitation speed, 400 rpm; desorption temperature, 230°C; and desorption time, 0.5 min. The extraction temperature for (b) and (c) was 50°C, and the other conditions were the same as for (a) and (d)−(g). 2.6.3. Evaluation of the analytical parameters The extraction conditions were as described in 2.6.1. 2.7. Real sample test The disposable cups were purchase locally. Three liquid food simulants: hot water (approximately 95°C), water at room temperature (approximately 25°C) and a 10% (v/v) ethanol aqueous solution at room temperature (approximately 25°C) were poured into the cups for 2 h, 24 h and 24 h, respectively, before analysis. 3. Results and discussion 3.1. Characterization of PAF-48 and PAF-48/Gel coating First, the FTIR spectra were measured to verify the preparation process (Figure S-1 in the Supporting Information). PAF-48 and its monomer 1,3,5-triphenybenzene



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exhibited similar absorption peaks as found in the literature.28 After the polymerization reaction, the absorption peaks in the 1510−1425 cm−1 region and the 790−570 cm−1 region that belong to the C−H and C−C vibrations of the benzene ring obviously weaken. It is obvious that the intensity of the infrared absorption of C−C becomes weaker, which could be attributed to the formation of polymeric networks that restrict the stretching and deformation vibrations of molecules. The hybrid material PAF-48/Gel exhibits both the specific absorption peaks of PAF-48 and gel. The PXRD spectrum (Figure S-2 in the Supporting Information) of the synthesized PAF-48 sample indicates that it has an amorphous texture, which was consistent with the literature.28 PAF-48 and PAF-48/Gel express good thermal stabilities according to the results of TGA. As shown in Figure S-3 of the Supporting Information, the decomposition temperature of the monomer of PAF-48 (1,3,5-triphenybenzene) is approximately 350°C. PAF-48 loses less weight before 420°C, and then the skeleton of PAF-48 decomposes as the temperature increases. The hybrid material PAF-48/Gel shows better thermal stability compared with PAF-48. The specific surface area of PAF-48 calculated using the BET model is 1308 m2⋅g-1. Additionally, the pore size distribution of PAF-48 is calculated according to Density Functional Theory (DFT), which is a mainstream method for the pore size analysis of PAFs.26, 28 It contains three types of pores with widths of approximately 0.54, 0.80 and 1.17 nm, respectively (Figure S-4 in the Supporting Information). Approximately 85% of the cumulative pore volume is concentrated below the pore width of 2 nm, and 64% of the cumulative pore volume is concentrated below the pore width of 1 nm (Figure S-5 in the Supporting Information), indicating that the pore distribution of PAF-48 is relatively narrow and most pores are micropores. The size distribution report demonstrates that PAF-48 particles exhibit a narrow size distribution and have a diameter of approximately 990 nm (Figure S-6 in the Supporting Information). Furthermore, the SEM images (Figure 2) of the PAF-48/Gel-coated fiber show that the PAF-48 particles on the surface of the silica fiber are uniform and well-knit. It could be attributed the good film forming properties of gel as well as the strong connection between gel and quartz fiber through Si-OH. During the gel formation process, PAF particles are twined and fixed by the net of gel; simultaneously, the gel is linked on the surface of the quartz fiber through SiOH. Additionally, there are many channels formed in the course of the formation of the gel,29 and as a result, there are enough passageways for the target molecules to enter into the hybrid coating and contact the active ingredient PAF-48.

Figure 2. SEM images of the PAF-48/Gel-coated fiber.

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3.2.1. Extraction time HS-SPME is a dynamic distribution procedure between aqueous phase, gas phase and coating materials, therefore the amount of the adsorbates would reach equilibrium in a certain time generally.30 In the experiment, the extraction time was investigated from 10 to 40 min. As shown in Figure 3a, styrene and six VACs achieved the highest point within 30 min and then slightly decreased in 40 min. As a result, 30 min was regarded as the optimized extraction condition for all the seven analytes. 3.2.2. Extraction temperature Raising temperature can promote the volatilization of the analytes from the aqueous phase into the gas phase, which is advantageous for extraction. On the other side, a high temperature will reduce the partition coefficient between coating materials and gas phase, which would reduce the adsorbed amount of analytes.31 In this study, the extraction temperature was examined from 30 to 50°C. As shown in Figure 3b, styrene, m-xylene, ethylbenzene, p-xylene could nearly reach equilibrium in 40°C, and the profiles of toluene, o-xylene, benzene increased first and then decreased. Considering the overall extraction efficiencies, 40°C was chosen as the suitable extraction temperature. 3.2.3. Salt concentration When the organics in the aqueous solution are extracted, salt can affect the extraction amount of the fiber coating.31 Here, different amount of NaCl (0−30%, w/v) was added into the sample solution. The extraction efficiencies of benzene appear fall-rise tendency, and the profiles of the other analytes decline with the increase of NaCl. Overall, no NaCl addition was the highest point. The reducing tendency could be ascribed that the Na+ and Cl- may affect equilibrium relationship of the analytes in the fiber coating, gas phase and aqueous phase by changing the ionic strength, viscosity and density of the sample solution.31 The influence is not all favorable for SPME.32 Therefore, no salt was added for the further experiment. 3.2.4. Agitation speed Opportune agitation can improve the rate of mass transfer from the sample solution to headspace.33 As shown in Figure 3d, the extraction efficiencies obviously increase from 300 to 400 rpm, while lightly decline from 400 to 500 rpm. Thus, 400 rpm was the most suitable condition for PAF-48/Gel to extract styrene and VACs. 3.3. Extraction performance of the PAF-48/Gel-coated fiber 3.3.1. Comparison of the extraction efficiency between the PAF-48/Gel-coated fiber and other SPME fibers The extraction efficiency of the PAF-48/Gel-coated fiber was compared with those of the commercial 75 µm of CAR/PDMS, 65 µm of PDMS/DVB and 100 µm of PDMS fibers (Figure 4). The PAF-48/Gel-coated fiber performed with obvious superiority for the extraction of styrene and VACs over the three commercial coatings. It can be attributed to the following three points: (1) The uniform pore size and high surface area of microporous PAF-48 affording enough molecular absorbing abilities or possibility. (2) The distinct continuous conjugate network of PAF-48 arising from the pure benzene frameworks structure could offer a powerful drive to attract the aromatic

3.2. Optimization of the HS-SPME extraction procedure


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Figure 3. Factors affecting the extraction efficiency for styrene and VACs: (a) extraction time; (b) extraction temperature; (c) salt concentration; and (d) agitation speed. The error bar shows the standard deviation for triplicate extractions.

Figure 4. Extraction efficiency comparison of the PAF-48/Gelcoated fiber with the commercial CAR/PDMS, PDMS/DVB and PDMS SPME fibers for styrene and VACs. The error bar shows the standard deviation for triplicate extractions.

molecules by π-π affinity. (3) Another extraction impetus is the “hydrophobic-hydrophobic” interaction between PAF-48 and the hydrophobic styrene and VACs. To demonstrate the functionality of PAF-48, the extraction efficiency of the PAF-48/Gel-coated fiber was compared with the simple gel-coated fiber. As shown in Figure S-7 in the

Supporting Information, the extraction efficiency of the Gelcoated fiber was much lower than that of PAF-48/Gel-coated fiber. It indicated that the gel in the PAF-48/Gel coating did not affect the extraction performance, and PAF-48 was the functional component for the extraction of styrene and VACs. 3.3.2. Analytical parameters The linear ranges, correlation coefficients (R), limits of detection (LODs, S/N = 3), limits of quantification (LOQs, S/N = 10) and method precision of the PAF-48/Gel-coated fiber for the HS-SPME of styrene and VACs analyzed on GC are listed in Table 1. The linearity was measured by standard solutions with five or six different concentrations; the PAF48/Gel-coated fiber exhibited good linearities in the wide range of 0.1−200 or 500 µg⋅kg-1 with the value of R between 0.9991 and 1.0000. The LODs and the LOQs are in the ranges of 0.003−0.060 µg⋅kg-1 and 0.011−0.202 µg⋅kg-1, respectively. The intraday and interday RSDs of six replicated extractions were less than 5.9% and 7.3 %, respectively. The lifetime of the PAF-48/Gel-coated fiber was more than 200 times, because the extraction performance did not degrade significantly after 200 injection tests. It can be seen that the PAF-48/Gel-coated fiber is very sensitive for the determination of styrene and VACs. Especially for styrene, the LOD of the fiber was low at 0.006 µg⋅kg−1, which is lower than those of most of the reported fibers (Table 2). Thus, the PAF-48/Gel-coated fiber is a good choice for the detection of styrene.



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Table 1. The linear range, correlation coefficient (R), limit of detection (LOD), limit of quantification (LOQ) and method precision (relative standard deviation, RSD) of the proposed SPME-GC method using PAF-48/Gel-coated fiber. Linear range

Analytes

(µg⋅kg-1)

Method precision (RSD %)

Correlation coefficient (R)

LOD

LOQ

(µg⋅kg−1, S/N = 3)

(µg⋅kg−1, S/N = 10)

Intraday

Interday

0.202

5.7

5.9

Benzene

0.1−200

0.9999

0.060

Toluene

0.1−200

1.0000

0.009

0.031

4.6

5.6

Ethylbenzene

0.1−500

0.9999

0.006

0.02

4.8

7.3

p-xylene

0.1−500

0.9999

0.006

0.02

5.0

6.9

m-xylene

0.1−500

0.9998

0.005

0.016

5.1

6.8

o-xylene

0.1−500

0.9991

0.003

0.011

5.4

5.8

Styrene

0.1−500

1.0000

0.006

0.019

5.9

6.9

Table 2. The comparison of linear range, correlation coefficient (R), limit of detection (LOD) between the developed methods with other works.

SPME-Fiber

PAF-48/Gel

Linear range -1

(µg⋅kg )

LOD

Correlation coefficient (R)

(µg⋅kg−1, S/N = 3)

Volume of headspace bottle (mL)

Sample volume (mL)

Detection method

Ref.

0.1−500

1.0000

0.006

20

5

GC

This Work

a

Ag/Zn/PEG

1−10000

0.999

0.280

10

5

GC

Ref. 34

CAR/PDMS

1−20000

0.999

0.33

10

5

GC

Ref. 34

PDMS

1−15000

0.999

0.47

10

5

GC

Ref. 34

PAb

1−100

0.9999

0.3

22

10

GC-MSc

Ref. 35

a

PEG, polyethylene glycol; PA, polyacrylate; c MS, mass spectrometry. b

3.3.3. Selectivity of the PAF-48/Gel-coated fiber As show in Figures 5a−5c, the enrichment capacities of the PAF-48/Gel coating for alcohols, phenols and aromatic amines were much poorer than those for styrene and VACs. As mentioned in 3.3.1, the high extraction ability of the homemade fiber for styrene and VACs are attributed to the synergism of the microporous structure, the continuous conjugate network and the hydrophobicity of PAF-48. Here, the determinant of selective extraction is the “hydrophobichydrophobic” interactions between PAF-48 and styrene as well as VACs. The hydrophobicity of PAF-48, as the result of the pure benzene structure without any other group, makes PAF-48 easily attract nonpolar styrene and VACs instead of polar alcohols, phenols and aromatic amines. The extraction efficiencies of the PAF-48/Gel coating for styrene, VACs and alkanes are compared in Figure 5d. The enrichment capacities for the five linear alkanes were significantly lower than those of the aromatic styrene and VACs. This is because the continuous conjugate network of PAF-48 can not only utilize the hydrophobicity but also the ππ affinity between the styrene, VACs and PAF-48 framework. Here, the aromatic analytes and the five linear alkanes have

the similar hydrophobic and molecule size, the selectivity depends on the dominant role of the π-π affinity. In summary, the PAF-48/Gel-coated SPME fiber can selectively extract styrene and VACs over alcohols, phenols, aromatic amines and alkanes, which is very advantageous for the analysis of complex real samples. 3.3.4. Molecular sieving effect of the PAF-48/Gel-coated fiber Three group experiments were conducted to discuss the molecular sieving effect of the PAF-48/Gel-coated fiber and the commercial PDMS/DVB fiber. The normalized extraction efficiencies of the analytes are shown in Figure 6. The extraction efficiencies of the PDMS/DVB fiber for these three groups of analytes were not much different. The data of the PAF-48/Gel-coated fiber for the three groups exhibited similar tendencies that the extraction efficiency decreased with increasing molecular size of the analyte, which should be attributed to the specific properties of microporous PAF-48. When molecules with different sizes pass through the surface of PAF-48, it is easier for the molecules with smaller sizes to access to the micropores and difficult for those with larger sizes. Therefore, applying the microporous PAFs materials as


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Figure 5. Extraction efficiency comparison of the PAF-48/Gel-coated fiber for styrene and other VACs with (a) alcohols; (b) phenols; (c) aromatic amines; and (d) alkanes.

Figure 6. Normalized extraction efficiencies of the PAF-48/Gelcoated fiber and the commercial PDMS/DVB fiber for three groups of analytes with different molecular sizes.

a SPME fiber coating will provide a selective enrichment capability based on the molecular sieving effect. 3.4. Analysis of real samples The HS-SPME-GC method using the PAF-48/Gel coating was applied to determine the specific migrations of styrene and VACs from PS plastic FCMs to three food simulants. The PS plastic FCM used here was a commercially available PS disposable cup, and the three food simulants were hot water, water at room temperature and a 10% (v/v) ethanol aqueous solution, respectively, according the (EU) No 10/2011 regulation.36 The analytical results are summarized in Table 3, and the chromatograms of the food simulants are included in

Figure 7. Chromatograms of blank and three food simulants: No. 1, hot water; No. 2, water at room temperature; and No. 3, 10% ethanol aqueous solution at room temperature. Peak identity: 1, toluene; 2, ethylbenzene; 3, p-xylene; 4, m-xylene; 5, o-xylene; 6, styrene.

Figure 7. The styrene specific migrations from the PS disposable cup to the three food simulants were detected within the concentration range from 3.22 ± 0.18 to 14.84 ± 0.31 µg⋅kg-1. The recoveries of styrene for spiked samples ranged from 94.9 ± 6.3% to 102.0 ± 3.6%. The specific migrations of other VACs were detected within the concentration range from 0.09 ± 0.01 to 2.15 ± 0.05 µg⋅kg-1, and the recoveries for spiked samples ranged from 92.0 ± 5.6% to 104.7 ± 4.7%.



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Table 3. Analytical results for the determination of migration of styrene and VACs from PS plastic FCM to three food simulants by PAF-48/Gel-coated fiber.

Analytes

No. 1 food simulant hot water

No. 2 food simulant water at room temperature

(about 95°C)

(about 25°C)

Migration

Migration

(µg⋅kg )

Recovery (%, n = 3)

nda

101.7 ± 5.5

-1

Benzene

No. 3 food simulant 10% of ethanol aqueous solution (about 25°C) Migration

(µg⋅kg )

Recovery (%, n = 3)

(µg⋅kg-1)

Recovery (%, n = 3)

nda

95.3 ± 2.5

-

-

-1

a

Toluene

1.73 ± 0.05

96.3 ± 4.5

0.52 ± 0.04

101.0 ± 4.4

nd

Ethylbenzene

2.14 ± 0.06

98.3 ± 5.9

0.49 ± 0.03

96.0 ± 2.6

0.12 ± 0.02

98.3 ± 4.5

102.0 ± 6.1

0.09 ± 0.01

92.0 ± 5.6

p-xylene m-xylene

1.34 ± 0.06 1.59 ± 0.05

101.3 ± 6.4 104.7 ± 4.7

nd

a

nd

a a

nd

a

97.0 ± 4.0

97.3 ± 6.0

nd

a

95.0 ± 2.6

98.8 ± 5.3

3.22 ± 0.18

97.0 ± 4.0

o-xylene

2.15 ± 0.05

94.7 ± 4.5

nd

Styrene

14.84 ± 0.31

94.9 ± 6.3

10.36 ± 0.26

94.0 ± 4.6

102.0 ± 3.6

a

Not detected, Recovery of spiked 5.0 µg⋅kg-1 styrene and 1.0 µg⋅kg-1 VACs.

4. Conclusions A novel PAF-48/Gel hybrid coating on quartz was prepared as a SPME fiber by a sol-gel method, and a highly efficient and sensitive detection method for the specific migrations of styrene and VACs from PS plastic FCMs to food simulants was developed by combining HS-SPME and GC. The functional component of the prepared coating is PAF-48, which is a PAF material with a microporous structure, a continuous conjugate network and a good hydrophobicity. The PAF-48/Gel-coated fiber was explored for the HS-SPME of styrene and six VACs (benzene, toluene, ethylbenzene and xylenes) from aqueous food simulants, and it exhibited wide linearities, Low LODs and LOQs, acceptable precisions and long lifetime. This developed fiber exhibits a highly specific selectivity for styrene and VACs over alcohols, phenols, aromatic amines and alkanes with a high extraction ratio due to the “hydrophobic-hydrophobic” and “π-π affinity” interactions. Moreover, the fiber showed a molecular sieving effect for the molecules with different sizes arising from the regular pores of PAF-48. The PAF-48/Gel-coated fiber was used to determine the specific migrations of styrene and VACs from PS disposable cups to food simulants with a good sensitivity and high recovery.

ASSOCIATED CONTENT Supporting Information Figures S-1−S-7. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (Z. L.)

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant No. 21101059, 21401046) and the PhD Fund of Henan Polytechnic University (B2011-030).

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Table of Contents


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Figure 3 115x89mm (300 x 300 DPI)



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Figure 4 69x56mm (300 x 300 DPI)


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Gel Hybrid Material Coated Solid

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