Multiwalled carbon nanotubes coated fibers for solid-phase microextraction of polybrominated diphenyl ethers in water and milk samples before gas chromatography with electron-capture detection

Journal of Chromatography A, 1137 (2006) 8–14

Multiwalled carbon nanotubes coated fibers for solid-phase microextraction of polybrominated diphenyl ethers in water and milk samples before gas chromatography with electron-capture detection Jun-Xia Wang, Dong-Qing Jiang, Zhi-Yuan Gu, Xiu-Ping Yan ∗ Key Laboratory of Functional Polymer Materials, Ministry of Education, Research Center for Analytical Sciences, College of Chemistry, Nankai University, Tianjin 300071, China Received 13 September 2006; received in revised form 2 October 2006; accepted 2 October 2006 Available online 20 October 2006

Abstract Determination of polybrominated diphenyl ethers (PBDEs) in environmental samples has raised great concerns due to the widespread use of PBDEs and their potential risk to humans. Solid-phase microextraction (SPME) is a fast, simple, cost-effective, and green sample preparation technique and is widely used for environmental analysis, but reports on the application of SPME for determination of PBDEs are very limited, and only a few publications dealing with commercial SPME fibers are available for extraction of PBDEs. Herein, we report a novel SPME method using multiwalled carbon nanotubes (MWCNTs) as the SPME fiber coating for gas chromatography with electron-capture detection (GC-ECD) of PBDEs in environmental samples. The MWCNTs coating gave much higher enhancement factors (616–1756) than poly (5% dibenzene–95% dimethylsiloxane) coating (139–384) and activated carbon coating (193–423). Thirty-minute extraction of 10 mL of sample solution using the MWCNTs coated fiber for GC-ECD determination yielded the limits of detection of 3.6–8.6 ng L−1 and exhibited good linearity of the calibration functions (r2 > 0.995). The precision (RSD%, n = 4) for peak area and retention time at the 500 ng L−1 level was 6.9–8.8% and 0.6–0.9%, respectively. The developed method was successfully applied for the analysis of real samples including local river water, wastewater, and milk samples. The recovery of the PBDEs at 500 ng L−1 spiked in these samples ranged from 90 to 119%. No PBDEs were detected in the river water and skimmed milk samples, whereas in the wastewater sample, 134–215 ng L−1 of PBDEs were found. The PBDEs were detected in all whole fat milk samples, ranging from 13 to 484 ng L−1 . In a semiskimmed milk sample, only BDE-47 was found at 21 ng L−1 . © 2006 Elsevier B.V. All rights reserved. Keywords: Solid-phase microextraction; Carbon nanotubes; Polybrominated diphenyl ethers; Gas chromatography; Milk; Water

1. Introduction Since solid-phase microextraction (SPME) was first introduced by Arthur and Pawliszyn [1], the technique has gained a widespread acceptance in many areas. SPME can integrate sampling, extraction, and sample introduction into a single step and become an attractive alternative to most of the conventional sampling techniques. Usually, a section of fiber coated with a thin coating is used to extract the analytes and then the analytes sorbed on the fiber are desorbed and analyzed. The

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0021-9673/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2006.10.003

extraction efficiency of SPME is determined by the distribution of analytes between the matrix and the coating. As the fiber coating plays a key role in SPME, development of fiber coating for highly efficient extraction of the analytes of interest is an important research direction in SPME. Up to now, a number of novel coatings have been developed for the extraction of different kinds of compounds in addition to the commercially available SPME fibers [2] such as polymer materials [3–5], calix [4] open-chain crown ether [6], HPLC chemically bonded silica stationary phases [7], solid sorbents [8,9]. Various approaches including vapor deposition [10], sol–gel technology [11], electrochemical procedures [12], physical deposition [13], and direct use of uncoated fiber, etc. [14] have also been proposed for the production of SPME fibers [2].

J.-X. Wang et al. / J. Chromatogr. A 1137 (2006) 8–14

Carbon materials have long been used as adsorbents for trapping or separation of organic compounds. In SPME, carbon materials such as polycrystalline graphite [15], low-temperature glassy carbon [16], and activated carbon [8] have been successfully used as the coating or fiber material. Carbon nanotubes (CNTs) are a kind of novel and interesting carbon material first found in 1991 by Iijima [17]. CNTs have curved surface, thus are expected to show a stronger binding affinity for hydrophobic molecules compared with a plannar carbon surface [18]. Furthermore, the internal pores of the CNTs are large enough to allow molecules to penetrate. Large sorption surface is also available on the outside and in the interstitial spaces within the nanotube bundles [18]. All these indicate that CNTs have strong physical adsorption ability to hydrophobic organic pollutants. Multiwalled carbon nanotubes (MWCNTs) have been successfully used as sorbent for solid phase extraction of metal ions [19], bisphenol A, 4-n-nonylphenol and 4-tertoctylphenol [20], chlorobenzenes [21], phthalate esters [22], atrazine and simazine [23], and chlorophenols [24]. Even so, no reports, to the best of our knowledge, have been published on the application of CNTs as the fiber coating for SPME until now. Polybrominated diphenyl ethers (PBDEs) are widely used as brominated flame retardants (BFRs) for many kinds of materials [25,26]. Due to the widespread use of PBDEs since 1970s, they are ubiquitous in the environment, and rapidly increasing levels of PBDEs have been detected in the global environment, human, and other biota. Several epidemiological studies have shown PBDEs to pose health risks [27–31] such as endocrine disruption, adverse neurobehavioral effects, to act as reproductive toxicants, and probable carcinogens. Therefore, the pentaand octa-brominated mixes are now banned in most parts of Europe, and phasing out their use has recently begun in North America [32]. Thus, the determination of PBDEs in environmental samples has raised great concerns recently. Although SPME is a fast, simple, cost-effective, and green sample preparation technique and is widely used for environmental analysis, reports on the application of SPME for determination of PBDEs are quite limited, and only a few publications dealing with commercial SPME fibers are available for the extraction of PBDEs [33–37]. The purpose of the present work was to develop a novel SPME technique using MWCNTs as fiber coating for extraction of PBDEs in environmental samples before gas chromatographic determination with electron-capture detection (GCECD). To this end, five target PBDEs were selected from the 209 possible congeners based on their relative abundance in environmental samples, i.e. 2,2 ,4,4 -tetraBDE (BDE-47), 2,2 ,4,4 ,5-pentaBDE (BDE-99), 2,2 ,4,4 ,6-pentaBDE (BDE100), 2,2 ,4,4 ,5,5 -hexaBDE (BDE-153), and 2,2 ,4,4 ,5,6 hexaBDE (BDE-154). Potential factors affecting the SPME of PBDEs were optimized, and the analytical performance of the developed MWCNTs coated fibers were compared with that of poly(5%dibenzene–95%dimethylsiloxane) and activated carbon coated fibers for SPME of PBDEs. The developed method was applied for the determination of trace PBDEs in river water, wastewater, and milk samples.

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2. Experimental 2.1. Chemicals and materials The MWCNTs used as the SPME fiber coating are 20–40 nm in diameter and 5–15 ␮m in length (L-MWNT2040, Shenzhen Nanotech Port, Shenzhen, China). The specific surface area and thermal conductivity of the MWCNTs are 40–300 m2 g−1 and ∼2000 W m−1 k−1 , respectively. The SPME fiber used was a fused-silica fiber (125 ␮m in diameter, Yongnian Optical Fiber, Hebei, China). Activated carbon (Tianjin Binhai Chemicals, Tianjin, China) and poly (95%dimethylsiloxane–5%dibenzene) (PDMS-DB, Supelco, Bellefonte, PA, USA) were also used as fiber coating for a comparative purpose. The five PBDEs standards (50 mg L−1 for each) were purchased from AccuStandard (New Haven, CT, USA) and stored in brown bottles in the refrigerator. Doubly deionized water (DDW, 18.2 M cm−1 ) was obtained from a WaterPro water purification system (Labconco, Kansas City, MO, USA). A mixture of these PBDEs was prepared by diluting the standard solution with methanol and then further diluting the methanol solution with DDW. All solvents used were of analytical grade from Tianjin Taixing (Tianjin, China). 2.2. Instrumentation A Shimadzu (Tokyo, Japan) GC-9A system equipped with electron-capture detector was used for all analyses. The GC was fitted with SE-54 column (20 m, 0.53-mm I.D., 1.0 ␮m) from Lanzhou Institute of Chemical Physics (Lanzhou, China). 99.999% nitrogen (BOC Gases, Tianjin, China) was used as carrier gas at a flow rate of 70 mL min−1 . The injector temperature was set at 295 ◦ C for sample injection. The detector temperature was set at 295 ◦ C, and the column temperature was set at 245 ◦ C. A Model 85-1 stir plate (Jintan Instruments, Jintan, China) and a polytetrafluoroethylene (PTFE)-coated stir bar (9.9 mm × 5.9 mm × 5 mm) were used for agitation. The 20mL sample vials were purchased from Agilent Technologies (Palo Alto, CA, USA). The SEM micrographs of the MWCNTs before and after coating on the surface of the SPME fiber were obtained on a Shimadzu SS-550 scanning electron microscope at 15.0 kV.

63 Ni

2.3. Preparation of MWCNTs coated fiber for SPME Prior to coating, the end of the fused-silica fiber (2 cm) was dipped in acetone for 3 h to remove the protective polyimide layer, cleaned with water, and air dried at room temperature. The MWCNTs powder was dispersed in dimethylformamide (DMF) under sonication to prepare a 20 mg mL−1 of MWCNTs suspension. The fiber coating was prepared by depositing an aliquot of the suspension onto the pretreated fiber section, and then the fiber was subsequently heated at ∼160 ◦ C to remove the solvent. This procedure was repeated until the coating was of a required thickness (∼40 ␮m). The coated fiber was preheated at 80 ◦ C for 30 min, and then aged at 280 ◦ C for 4 h

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in a GC injection port under helium protection before SPME experiments. A modified syringe assembly was used as an SPME device. The fiber coated with MWCNTs was protected by a section of quartz capillary and inserted into the needle of a 5-␮L syringe to replace the plunger for SPME experiments. 2.4. Preparation of activated carbon and PDMS-DB coated fibers for comparison The activated carbon coated fiber was prepared in the same way as the MWCNTs coated fiber. For the preparation of the PDMS-DB coated fiber, 20 mg of PDMS-DB was dissolved in 1 mL of chloroform under sonication. Then, the pretreated section of the fiber was dipped into the polymer solution and followed by drying to remove the chloroform. The thickness of the film was controlled by the laboratory-made SPME device. The activated carbon coated fiber and PDMS-DB coated fiber were preheated and aged as the MWCNTs coated fiber.

3. Results and discussion 3.1. Preparation of the MWCNTs coated SPME fiber The method for preparing the CNTs-modified electrodes was used to prepare the CNTs coating [38,39]. Briefly, CNTs suspension was deposited on the fiber and followed by drying. MWCNTs suspension with a high concentration of 20 mg mL−1 in DMF was obtained by sonication for 10 min. The suspension with lower concentration was difficult to deposit on the fiber. The coating deposited with more concentrated suspension was not uniform. The thickness of the film was controlled with a homemade SPME device. The stainless steel plunger of commercial 5-␮L GC syringe was replaced by a section of hollow quartz capillary to protect the coated fiber. The coated fiber should move through the protected quartz capillary. The thickness of the coating was about 40 ␮m, which was just the space between the fiber and the protected quartz capillary. Fig. 1 shows the SEM images of the MWCNTs before coating and after coating on the fiber surface, demonstrating the presence of MWCNTs as a homogenous coating on the surface of the fiber.

2.5. Solid-phase microextraction

3.2. Optimization of SPME

A 20-mL glass vial was used as a sample container, and 10 mL of sample solution was placed into the sample vial with a stir bar. The syringe was fixed at a suitable height above the sample vial so that the section of coated fiber was completely immersed into the sample solution for 30-min SPME at ambient temperature. After extraction, the fiber was removed from the sample vial and immediately inserted into the heated GC injector at 295 ◦ C for 2-min desorption. Before sampling, the fiber was preheated in the injector port to ensure no carryover of analytes from previous extractions.

Various parameters that affect the efficiency for the SPME of PBDEs were studied and subsequently optimized, including extraction time, magnetic stirring rate, ionic strength, desorption temperature, and desorption time. The extraction time is an important parameter for the extraction performance. The extraction time profile of the five PBDEs is shown in Fig. 2. The amount of analytes extracted (corresponding to the resulting peak areas) greatly increased as the extraction time increased from 0 to 35 min. A 30-min extraction time was sufficient to achieve high extraction efficiency, although the equilibrium was not reached. Because SPME is a nonexhaustive approach, the method can be designed on the basis of principles of equilibrium, preequilibrium, and permeation [40]. Considering the total throughout time, the extraction time was set at 30 min. Magnetic stirring is most commonly used in manual SPME experiments to accelerate the extraction. Study on the effect of the stirring rate revealed that the chromatographic peak areas of the analytes increased as the stirring rate increased from 0 to 1200 rpm, which was the maximal allowable stirring rate of the stir plate. For further experiments, a stirring rate of 1200 rpm was chosen. Experiments were conducted to determine the salt effect on the extraction efficiencies at various concentrations of NaCl (0, 1, 2, 3, and 4%, w/v). It was found that the ionic strength of sample solution had negative effect on the extraction performance. The chromatographic peak areas of the analytes decreased when the concentration of NaCl increased from 0 to 4% (w/v). So, in the following experiments, no salt was added to the samples. The desorption temperature is an important factor for the desorption process. With the desorption temperature increasing from 280 to 295 ◦ C, the signal intensity increased. Since further increase in the injector temperature would damage the GC

2.6. Sample preparation One river water sample was collected from a local river. An industrial effluent water sample was sampled from a plastic factory (Xinxiang, China). Milk samples including three whole-fat milk samples (fat >3.2%), one semiskimmed milk sample (fat <1.5%), and one skimmed milk sample (fat <0.5%) were randomly bought from local supermarkets. The water samples were directly extracted by the MWCNTs coated fiber. For milk samples, the saponification was used to remove the matrix interferences and made the combined PBDEs free. 15 mL of milk sample was added to 15 mL of 50% NaOH solution and 2 mL of ethanol. Then, the mixed solution was saponified for 30 min in the 70 ◦ C water bath. After saponification, the sample was extracted with 50 mL petroleum ether to remove the added NaOH because the presence of NaOH could decrease the efficiency of subsequent SPME. The extract was vaporized to dryness with a gentle nitrogen flow. The residue was dissolved in 10 mL doubly deionized water, and the resultant aqueous solution was extracted by the MWCNTs coated fiber for GC-ECD analysis.

J.-X. Wang et al. / J. Chromatogr. A 1137 (2006) 8–14

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Fig. 2. Influence of extraction time on the chromatographic peak areas of the PBDEs at 1000 ng L−1 . Sample volume, 10 mL; stirring rate, 1200 rpm; desorption temperature, 295 ◦ C; desorption time, 2 min.

out effect. Thus, in the subsequent experiments, desorption time was set at 2 min. 3.3. Comparison of the SPME efficiency of the MWCNTs coated fiber with activated carbon, and PDMS-DB coated fibers

Fig. 1. SEM images of the MWCNTs: (A) before coating at magnifications of 10,000, (B) after coating on the fiber surface at magnifications of 10,000, and (C) after coating on the fiber surface at magnifications of 200.

instrument, the desorption temperature was set at 295 ◦ C. To avoid carry-over effect, the desorption time should be sufficient for quantitative desorption of the extracted analytes from the surface of the MWCNTs coated SPME fiber. For this reason, experiments were performed with various depsortion times (1, 2, 3, 4, and 5 min) to test the effect of desorption time on the chromatographic peak areas of the analytes. It was found that the peak areas of the analytes slightly increased as the desorption time increased from 1 to 2 min and then remain unchanged with further increase of desorption time to 5 min. The repeated desorptions with a desorption time of 2 min indicated no carry-

To compare the extraction efficiency of the MWCNTs coating with activated carbon and PDMS-DB coatings, identical procedure as described in Sections 2.3 and 2.4 was applied to prepare three SPME fibers coated with MWCNTs, activated carbon, and PDMS-DB, respectively with the same thickness of coating. Enhancement factor (EF) was defined as the ratio of the peak area after extraction to the peak area of direct injection of 1 ␮L standard solution. The EFs of the PBDEs obtained by using the above three kinds of SPME fibers are summarized in Table 1. The results in Table 1 show that the MWCNTs are quite effective as fiber coating for the SPME of PBDEs, giving much higher enhancement factors (616–1756) than poly(5% dibenzene–95% dimethylsiloxane) coating (139–384) and activated carbon coating (193–423). Comparison of the present MWCNTs coated fiber with commercial SPME fibers was not made as different coating thickness and production methods for commercial fibers, and the present MWCNTs coated fiber can result in unfair conclusions as to the extraction efficiency of the coating materials themselves. 3.4. Analytical figures of merit The characteristic data of the developed SPME method are summarized in Table 2. To investigate the linearity of the developed method, 28–2800 ng L−1 of the PBDEs solutions were analyzed. All the five PBDEs exhibited good linearity (r2 > 0.9954). 10 mL of a mixed standard solution containing 20 ng L−1 for each PBDE was submitted to the SPME for evaluating the limits of detection (LODs) and the limits of quantitation (LOQs). The LODs (S/N = 3) and LOQs (S/N = 8) of the five PBDEs were in the range of 3.6–8.6 ng L−1 and 9.6–22.9 ng L−1 , respec-

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Table 1 Comparison of the enhancement factors (EFs) of PBDEs obtained by SPME with MWCNTs, activiated carbon and PDMS-DB as the fiber coatingsa Compound

Chemical structure

MWCNTs

PDMS-DB

Activated carbon

BDE-47

1756

384

423

BDE-100

1126

285

291

BDE-99

1031

247

271

BDE-154

629

139

193

BDE-153

616

233

196

a Obtained under the following conditions: sample volume, 10 mL; extraction time, 30 min; stirring rate, 1200 rpm; desorption temperature, 295 ◦ C; desorption time, 2 min.

tively. The repeatability study was performed with four replicate extractions and determinations of aqueous sample containing 500 ng L−1 for each compound. The precision (RSD, n = 4) for peak area and retention time was 6.9–8.8% and 0.6–0.9%, respectively. The MWCNTs coated fiber allowed ∼100 replicate extractions without measurable loss of performance. The fiber-to-fiber reproducibility (RSD) evaluated by determining the chromatographic peak areas of the five PBDEs at 500 ng L−1 extracted with three MWCNTs coated fibers prepared in the same way ranged from 8 to 16%. 3.5. Sample analysis The MWCNTs coated fiber was applied to the SPME of the five PBDEs in real water and milk samples for their determina-

tion by GC-ECD. The recovery of the PBDEs at 500 ng L−1 spiked in the water and milk samples ranged from 90 to 119%. The analytical results for the determination of the five PBDEs in the water and milk samples are given in Table 3. No PBDEs were detected in a river water sample, whereas in a wastewater sample, 134–215 ng L−1 of PBDEs were found. The PBDEs were detected in all three samples of whole fat milk, ranging from 13 to 484 ng L−1 . In a semiskimmed milk sample, only BDE-47 was found at 21 ng L−1 , while in a skimmed milk sample, no PBDEs were detected. The chromatograms of industrial wastewater sample and a standard solution obtained by the developed SPME-GC-ECD technique are shown in Fig. 3. The observed broadening of the chromatogram peaks in the Fig. 3 likely resulted from the large inner diameter of capillary column (0.53 mm I.D.) used in this work.

Table 2 Characteristic data of the developed SPME-GC-ECD method for determination of PBDEs under the same conditions as in Table 1 LODs (ng L−1 , S/N = 3)

LOQs (ng L−1 , S/N = 8)

RSDs (n = 4) (%) Peak

BDE-47 BDE-100 BDE-99 BDE-154 BDE-153 a

3.6 4.9 5.9 7.7 8.6

9.6 13.1 15.7 20.5 22.9

areaa

6.9 8.0 8.8 7.0 7.2

Peak area in ␮V S. The concentration of the standard solution was 500 ng L−1 for each compound.

Linearity (r2 )

Linear range (ng L−1 )

0.9991 0.9956 0.9954 0.9971 0.9967

28-2800 28-2800 28-2800 28-2800 28-2800

Retention time 0.7 0.7 0.7 0.9 0.6

J.-X. Wang et al. / J. Chromatogr. A 1137 (2006) 8–14

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Table 3 Analytical results for the determination of PBDEs in water and milk samples under the same conditions as in Table 1 BDE-47 Concentrationa (ng L−1 ) River water Waste water Milk 1c Milk 2c Milk 3c Milk 4d Milk 5e a b c d e

nd (not detected) 134 ± 19 12.7 ± 2.2 52.2 ± 7.8 32.0 ± 3.2 21.3 ± 3.4 nd

BDE-100 Recb

(%)

94 ± 7 90 100 98 99 105 117

± ± ± ± ± ±

10 7 7 6 9 5

Concentration (ng L−1 ) nd 147 ± 10 nd 20.6 ± 3.5 34.4 ± 5.2 nd nd

BDE-99 Rec (%) 93 ± 9 91 100 95 99 97 115

± ± ± ± ± ±

9 8 6 10 8 8

Concentration (ng L−1 )

BDE-154 Rec (%) 97 ± 9

nd 155 ± 20 nd 19.6 ± 3.9 135 ± 11 nd nd

90 99 93 100 98 105

± ± ± ± ± ±

11 10 11 5 10 6

Concentration (ng L−1 ) nd 215 ± 4 nd nd 225±20 nd nd

BDE-153 Rec (%) 97 ± 8 95 100 102 99 99 119

± ± ± ± ± ±

9 10 8 9 9 11

Concentration (ng L−1 ) nd 212 ± 20 nd nd 56.9 ± 6.3 nd nd

Rec (%) 94 ± 10 98 100 103 99 91 104

± ± ± ± ± ±

8 8 9 6 10 6

Mean ± s. Spiked concentration were 500 ng L−1 of each compound. Whole fat milk. Semiskimmed milk. Skimmed milk.

Fig. 3. Chromatograms obtained by the developed method for (A) standard solution containing 500 ng L−1 of each PBDE, (B) industrial wastewater sample. Sample volume, 10 mL; extraction time, 30 min; stirring rate, 1200 rpm; desorption temperature, 295 ◦ C; desorption time, 2 min.

4. Conclusions We have developed a novel SPME technique with MWCNTs as the fiber coating for GC-ECD determination of trace PBDEs in environmental samples. The results have demonstrated the high efficiency of the MWCNTs coated fiber for the SPME of PBDEs. The developed method is promising for the determination of trace PBDEs in environmental and biological samples. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 20437020) and the National Basic Research Program of China (No. 2003CB415001). References [1] C.L. Arthur, J. Pawliszyn, Anal. Chem. 62 (1990) 2145. [2] C. Dietz, J. Sanz, C. C´amara, J. Chromatogr. A 1103 (2006) 183.

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