Determination Of Benzene, Toluene, Ethyl Benzene, Xylenes In Water

ARTICLE IN PRESS

Chemosphere xxx (2007) xxx–xxx www.elsevier.com/locate/chemosphere

Determination of benzene, toluene, ethylbenzene, xylenes in water at sub-ng l 1 levels by solid-phase microextraction coupled to cryo-trap gas chromatography–mass spectrometry Maw-Rong Lee b

a,*

, Chia-Min Chang a, Jianpeng Dou

a,b

a Department of Chemistry, National Chung Hsing University, Taichung 40227, Taiwan, ROC College of Biological and Agricultural Engineering, Jilin University, Changchun 130022, PR China

Received 26 December 2006; received in revised form 1 May 2007; accepted 3 May 2007

Abstract A trace analytical method of benzene, toluene, ethylbenzene and xylenes (BTEX) in water has been developed by using headspace solid-phase microextraction (HS-SPME) coupled to cryo-trap gas chromatography–mass spectrometry (GC–MS). The chromatographic peak shape for BTEX was improved by using cryo-trap equipment. The HS-SPME experimental procedures to extract BTEX from water were optimized with a 75 lm carboxen/polydimethylsiloxane (CAR/PDMS)-coated fiber at a sodium chloride concentration of 267 g l 1, extraction for 15 min at 25 C and desorption at 290 C for 2 min. Good linearity was verified in a range of 0.0001–50 lg l 1 for each analyte (r2 = 0.996–0.999). The limits of detection (LODs) of BTEX in water reached at sub-ng l 1 levels. LODs of benzene, toluene, ethylbenzene, m/p-xylene and o-xylene were 0.04, 0.02, 0.05, 0.01 and 0.02 ng l 1, respectively. The proposed analytical method was successfully used for the quantification of trace BTEX in ground water. The results indicate that HS-SPME coupled to cryo-trap GC–MS is an effective tool for analysis of BTEX in water samples at the sub-ng l 1 level.  2007 Elsevier Ltd. All rights reserved. Keywords: Solid-phase microextraction; Benzene; Toluene; Ethylbenzene; Xylene; Cryo-trap

1. Introduction Benzene, toluene, ethylbenzene, m/p-xylene and o-xylene (BTEX) are ubiquitously environmental contaminants in air, water and soil and are widely used in industries, such as printing, paint, synthetic resin, and synthetic rubber (Holcomb and Seabrook, 1995; Nollet, 2001). BTEX are also abundant in petroleum products, such as fuel oil and gasoline. But BTEX has an effect on human health including neurological diseases or cancer (Chiou et al., 1982). The US Environmental Protection Agency (EPA) establishes the maximum contaminant level (MCL) for benzene 5 lg l 1, for toluene 1000 lg l 1, for ethylbenzene 700 lg l 1 and for xylenes 10 000 lg l 1 in drinking water

*

Corresponding author. Tel.: +886 4 2285 1716. E-mail address: [email protected] (M.-R. Lee).

(USEPA, 2004). The maximum contaminant level of 1 lg l 1 for benzene in drinking water is established by the European Union (EU, 1998). Therefore, in order to protect people’s health, it is necessary to establish an effective and convenient quantification method for monitoring trace BTEX in water. BTEX in water were analyzed by direct aqueous injection (DAI) and gas chromatography combined with flame ionization detector (GC–FID), and limits of detection (LOD) ranged from 0.6 lg l 1 for benzene to 1.1 lg l 1 for o-xylene (Kubinec et al., 2005). BTEX in water samples were also analyzed by headspace solid-phase microextraction (SPME) and GC–FID (Flo´rez Mene´ndez et al., 2000; Almeida and Boas, 2004; Ezquerro et al., 2004), and LOD obtained with PDMS/DVB/CAR fibre ranged from 15 ng l 1 (benzene) to 160 ng l 1 (toluene) (Almeida and Boas, 2004). Substituted benzene was analyzed by liquid-phase microextraction (LPME) and GC–FID, and

0045-6535/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.05.004

Please cite this article in press as: Lee, M.-R. et al., Determination of benzene, toluene, ethylbenzene, xylenes in water ..., Chemosphere (2007), doi:10.1016/j.chemosphere.2007.05.004

ARTICLE IN PRESS 2

M.-R. Lee et al. / Chemosphere xxx (2007) xxx–xxx

LODs of BTEX ranged from 0.3 lg l 1 for o-xylene to 1.6 lg l 1 for toluene in a water matrix (Wang et al., 2006). At present, gas chromatography–mass spectrometry (GC–MS) is widely used to determine BTEX in different water samples. Generally, sample preparation of BTEX from real samples is an important procedure before analysis. Liquid–liquid extraction, solid-phase extraction and purge-and-trap techniques were used for determination of BTEX in the past (Stan and Kirsch, 1995; Beketov et al., 1996; Miermans et al., 2000; Bianchi et al., 2002; Rosell et al., 2003). BTEX in water were analyzed by purge-andtrap and GC–MS, and LOD ranged from 2.8 ng l 1 for o-xylene to 22.6 ng l 1 for ethylbenzene (Bianchi et al., 2002). However, solid-phase microextraction (SPME), as rapid, selective and solvent-free techniques, are more and more widely used for analysis of trace BTEX (Potter and Pawliszyn, 1992; Elke et al., 1998; Fuoco et al., 1999; Koziel et al., 2000; Li et al., 2001; Matisova´ et al., 2002; Almeida and Boas, 2004; Arambarri et al., 2004). BTEX were analyzed by SPME–GC–MS, and LODs ranged from 15 ng l 1 for benzene to 50 ng l 1 for o-xylene in a water matrix (Potter and Pawliszyn, 1992). The aim of this study was to develop a higher sensitivity method for determination of trace BTEX in water samples. In this work, BTEX were adsorbed on the fiber coating of the SPME. A headspace (HS) SPME method was used to extract trace BTEX in water. The cryo-trap could curtail chromatographic peak breadth of volatile compounds and improve chromatographic shape. Therefore, BTEX extract was analyzed by cryo-trap-GC–MS. The conditions for extracting BTEX from water samples are optimized. The optimized HS-SPME–GC–MS method was used to determine BTEX in ground water. 2. Materials and methods 2.1. Chemicals and materials Benzene (99.99%), toluene (99.5%), ethylbenzene (99.97%), o-xylene (99.3%), m-xylene (99.8%), and p-xylene (99.9%) were purchased from TEDIA Company (Fairfield, OH, USA). Sodium chloride (99.8%) was obtained from Riedel-deHae¨n Company (Seelze, Germany). One standard mixture of BTEX were prepared at individual concentration of 100 mg l 1 in acetone and diluted with water to yield the required concentration. All solutions were stored at 4 C in a refrigerator. All chemicals and reagents used in

this work were analytical grade without further purification. The purified water was obtained by an SG Ultra clear water purification system (SG Water Company, Barsbu¨ttel, Germany). All glassware was silanized before it was used by soaking the glassware overnight in toluene solution with 10% dichlorodimethylsilane. The glassware was rinsed with toluene and methanol and then thoroughly dried for 4 h. 2.2. GC–MS GC–MS and some parameters set were in accordance with the description of a literature (Lee et al., 2000). The GC oven temperature program was as follows: 40 C held 2 min, rate 25 C min 1 to 70 C, rate 2 C min 1 to 80 C, rate 25 C min 1 to 250 C, held for 1 min. Electron impact ionization (EI) was used as ionization mode for BTEX analysis. An injector temperature was maintained at 290 C. The mass spectra were obtained at a mass-tocharge ratio (m/z) scan range from 45 to 300 u. Cryo-trap was produced by Scientific Instrument Services Inc. (Mode 961, Ringoes, NJ, USA). BTEX are trapped in the cryotrap accumulation cell, which allows all of BTEX were quantitatively transferred from the cryo-trap cell to the chromatographic column by the helium flow. The temperature of cryo-trap was initially set at 100 C (held 2 min), and then reached sharply to 250 C. To increase sensitivity, the selected ion monitoring (SIM) mode of EI was also applied in quantitative analysis. The most abundant ion was generally monitored as well as quantified and the specific ion was used as the confirmed ion. The compounds of m-xylene and p-xylene were combined and analyzed as sum of the corresponding peak areas because they could not be separated during the GC analysis and have the same fragmentation pathways. In Table 1, the most abundant ion of benzene is m/z 78 ([M]+) ion and the most abundant ion of the other BTEX is m/z 91 ([M H]+ or [M CH3]+) ion. Hence, the fragment ions m/z 78 and 91 were used for quantification of benzene and other BTEX, respectively. 2.3. Sample preparation The SPME fibers were purchased from Supelco Company (Bellefonte, PA, USA). SPME sampling was similar to the description of a literature (Lee et al., 2007). The ground water samples (nearby gas station, paint factory, river and university in Taichung, Taiwan) were stored at

Table 1 Analytical conditions of BTEX by GC–MS with SIM mode Compound

Molecular weight

Retention time (min)

Quantification ions (m/z)

Confirmed ions (m/z)

Benzene Toluene Ethylbenzene m/p-Xylene o-Xylene

78 92 106 106 106

4.95 5.99 7.78 7.98 8.45

78 91 91 91 91

77, 52, 51 92, 65, 51 106, 77, 65 106, 77, 65 106, 77, 65

Please cite this article in press as: Lee, M.-R. et al., Determination of benzene, toluene, ethylbenzene, xylenes in water ..., Chemosphere (2007), doi:10.1016/j.chemosphere.2007.05.004

ARTICLE IN PRESS M.-R. Lee et al. / Chemosphere xxx (2007) xxx–xxx

4 C in a refrigerator and analyzed within 48 h to avoid storage losses. A 15 ml of sample solution was undertaken in a 40 ml sample vials and closed with a PFTE-coated septum. A 75 lm CAR/PDMS fiber was used to extract BTEX from water samples. Triplicate analyses were performed. Extraction temperature was at 25 C. During extraction, the sample solution was continuously agitated at a constant velocity of 1000 rpm with a Teflon-coated stir bar (0.8 cm · 2.0 cm) on a magnetic stirrer. 3. Results and discussion 3.1. Optimization of SPME conditions The extraction efficiency of the SPME experiment could be affected by type of fiber, desorption temperature, desorption time, absorption time, sample matrix and various other factors (Lagalante and Felter, 2004). Different types of coatings provide different absorption properties for different kinds of analytes. The choice of an appropriate coating is crucial for the SPME method. Various coatings were tested under the conditions: standard solution concentration of BTEX, 40 lg l 1; extraction temperature, 25 C; extraction time, 15 min. Triplicate analyses were performed, relative standard deviations of all fibers were less than 10%. The result in Fig. 1 reveals that the 75 lm CAR/PDMS fiber is the best for simultaneous extraction of BTEX. CAR/PDMS coating (Elke et al., 1998; Koziel et al., 2000; Li et al., 2001; Arambarri et al., 2004) was suitable for analysis of small molecular and nonpolar compounds. Although Cho et al. proposed that CAR/PDMS should be carefully used for determination of BTEX in groundwater samples, there is hardly interference among BTEX components when the concentration of the analyte is at a low level (Cho et al., 2003). Therefore, 75 lm CAR/PDMS was used for the analysis of low-concentration BTEX in this study. The principle of SPME is based on partition equilibrium between the concentration of analytes in a sample and that

Fig. 1. Extraction efficiencies of 40 lg l

1

3

in the solid-phase fiber coating (Arthur and Pawliszyn, 1990). Although a stir bar could accelerate the mass transfer of analytes through the aqueous matrix, the time of extraction, the temperature and headspace of sample vial have an effect on the partition equilibrium. Extractions were performed from 5 to 30 min to determine the effect of extraction time (Fig. 2). Fresh samples were used for each extraction time studied. BTEX have all reached partition equilibrium in 15 min. The extraction temperature determines the mass transfer rate of BTEX from water into fiber. Extraction temperatures from 25 to 75 C were investigated. The maximum amount extracted was at 25 C, and then decreased gradually as the temperature increased further. The reason probably is the partition coefficient of BTEX on fiber coating decreased as the temperature increased. The more BTEX volatilized from water matrix to gas phase as extraction temperature rose, the more analytes were adsorbed on the fiber, but at the same time the much more analytes were desorbed from fiber for higher extraction temperature, therefore the total abundance of BTEX gradually decreased as the extraction temperature increased. The headspace of vial would also affect extraction efficiencies for sample by SPME. The absolute amount of BTEX on headspace of sample vial was relevant to headspace and solution volume of vial. A solution of 1, 2, 5, 10, 15 and 20 ml was added in 40 ml of vial, respectively. The results in Fig. 3 show the peak areas of BTEX increased with the volume of solution to a maximum at 15 ml except o-xylene. Therefore, 15 ml of BTEX water solution in 40 ml vial are extracted for 15 min by HS-SPME at 25 C. The desorption temperature and desorption time determine the amount of analytes desorbed from the fiber coating, as determined by the SPME method. The desorption temperature was investigated with a range of 260–300 C. The results indicate that the peak areas of BTEX increased with the desorption temperature, reaching a maximum desorption amount at 290 C. Desorption time was investigated within a range of 0.5–4 min, by leaving the fiber in the injector for an increasing period of time and maintain-

BTEX in water with various fiber coatings adsorption for 15 min at 25 C (n = 3).

Please cite this article in press as: Lee, M.-R. et al., Determination of benzene, toluene, ethylbenzene, xylenes in water ..., Chemosphere (2007), doi:10.1016/j.chemosphere.2007.05.004

ARTICLE IN PRESS 4

M.-R. Lee et al. / Chemosphere xxx (2007) xxx–xxx

Fig. 2. Effect of extraction time on peak areas of 40 lg l

1

Fig. 3. Effect of solution volume in 40 ml vial on peak areas of 40 lg l

ing the temperature of the injector at 290 C. The amount of BTEX desorption increased with desorption time and reached a maximum after 2 min. Therefore, a 290 C desorption temperature and 2 min desorption time were used in the experiment. The SPME fiber can be continuously used during the experiment without any carry over after desorbed 290 C for 2 min. Changes in the sample matrix had a significant effect on the signal intensities of analytes obtained by SPME (Lee et al., 2000, 2007). The analyte of neutral molecular form is extracted easily by SPME. In order to investigate the effect of salting out on the signal intensities of analytes, sodium chloride was added to the water samples to yield final concentrations of sodium chloride of 0.067, 0.133, 0.200, 0.267 and 0.333 g ml 1. A blank solution (40 lg l 1 of BTEX standard solution) with no added sodium chloride was also tested. Sodium chloride is able to increase

BTEX in water produced by HS-SPME at 25 C.

1

BTEX in water produced by HS-SPME for 15 min at 25 C.

the ionic strength and improve the amount of analytes extracted by the SPME fiber. The results indicate that the peak areas of BTEX increase with the amount of NaCl to a maximum at 0.267 g ml 1. Therefore, the extraction is pursued at additional sodium chloride concentration of 0.267 g ml 1. 3.2. Method validation The linearity, limits of detection and precision were calculated when the optimum conditions for the HS-SPME– cryo-trap-GC–MS procedure were established. The linearity of the HS-SPME method was examined by extracting standard solutions spiked 0.0001, 0.001, 0.01, 1, 10 and 50 lg l 1 BTEX, respectively. Triplicate injections were performed. The r2 were above 0.996. Lack-of-fit and Mandel’s fitting test were performed to check the goodness of fit

Please cite this article in press as: Lee, M.-R. et al., Determination of benzene, toluene, ethylbenzene, xylenes in water ..., Chemosphere (2007), doi:10.1016/j.chemosphere.2007.05.004

ARTICLE IN PRESS M.-R. Lee et al. / Chemosphere xxx (2007) xxx–xxx

and linearity (Bianchi et al., 2002; Prˇikryl et al., 2006). Lack-of-fit tests demonstrated that the linear models were adequate because the whole p values were more than 0.05 at significance level of 95%. Mandel’s fitting tests were also performed for the mathematical verification of linearity. F values calculated lower than the tabulated F-value at the confidence level of 95% indicating that a quadratic regression would not provide a significantly better fit than a linear one (Table 2). The linear range experiments provided

5

the necessary information to estimate LODs, based on the lowest detectable peak with a signal-to-noise ratio of three. LODs of SPME used to determine BTEX in water relies on the amount of analytes adsorbed by coating on the fiber and the sensitivity of the GC–MS. Under the experimental conditions, LODs were 0.01–0.05 ng l 1 in water. The precision of the HS-SPME method was evaluated by analyzing BTEX at two concentration levels for each analyte. The results in Table 2 showed that the RSDs

Table 2 Linear range, limit of detection (LOD) and precision of BTEX in water by SPME–cryo-trap-GC–MS Compound

Linear range (lg l 1)

r2

LOD (ng l 1)

Lack-of-fit, pa

Mandel’s fitting test, Fa

RSD (%, n = 9) 0.1 lg l

Benzene Toluene Ethylbenzene m/p-Xyleneb o-Xylene a b

0.0001–50 0.0001–50 0.0001–50 0.0001–50 0.0001–50

0.998 0.998 0.996 0.999 0.998

0.04 0.02 0.05 0.01 0.02

1.000 0.862 0.999 0.606 0.661

0.01 2.27 0.18 4.16 4.17

11.2 8.9 11.6 8.4 7.8

1

40 lg l

1

5.2 4.5 6.8 3.1 4.8

Confidence interval, 95%. The value of m/p-xylene expressed the sum of m-xylene and p-xylene.

Table 3 Concentration (lg l 1) of BTEX in water samples by SPME–cryo-trap-GC–MS Real water sample S1 S2 S3 S4 S5 S6 S7

b

Benzene

Toluene

Ethylbenzene

m/p-Xylenea

o-Xylene

5.9 ± 0.5 0.72 ± 0.08
6.2 ± 0.8 0.95 ± 0.04 < 36 ± 4.20 24.2 ± 2.2 35.2 ± 6.0 1.21 ± 0.09

0.62 ± 0.05 0.15 ± 0.02 < ND 0.28 ± 0.06 7.4 ± 0.9 <

0.85 ± 0.04 0.10 ± 0.03 < 1.60 ± 0.05 0.52 ± 0.02 12.1 ± 0.4 <

0.35 ± 0.05 0.12 ± 0.02 < 0.56 ± 0.04 0.98 ± 0.05 4.0 ± 0.4 <

a

The value of m/p-xylene expressed the sum of m-xylene and p-xylene. S1–S3: ground water nearby different gas station in Taichung; S4–S5: ground water nearby different paint factory in Taichung; S6: ground water nearby some university in Taichung; S7: river water in Taichung. c < :
Fig. 4. Mass ion chromatogram of ground water S6 analyzed by HS-SPME–cryo-trap-GC–MS.

Please cite this article in press as: Lee, M.-R. et al., Determination of benzene, toluene, ethylbenzene, xylenes in water ..., Chemosphere (2007), doi:10.1016/j.chemosphere.2007.05.004

ARTICLE IN PRESS 6

M.-R. Lee et al. / Chemosphere xxx (2007) xxx–xxx

were lower than 12% in aqueous matrix. Therefore, BTEX in sorption on a CAR/PDMS-coated fiber in HS-SPME were deemed acceptable for determining at sub-ng l 1 levels in water solution. 3.3. HS-SPME–cryo-trap-GC–MS of real samples The proposed method was used to quantify BTEX in ground water. The HS-SPME was operated under the optimum conditions. Triplicate analyses were performed. The results in Table 3 shows that BTEX was present in ground water. Fig. 4 shows the mass ion chromatogram of S6, which the contents of BTEX were 4.0 lg l 1 (o-xylene) to 35.2 lg l 1 (toluene). Recoveries in the range 102–106% were obtained for all samples. The results indicate the suitability of the HS-SPME–cryo-trap-GC–MS method for analyzing trace BTEX in water. 4. Conclusions Results from this study indicate that SPME coupled to GC–MS is a precise method for reproducibly analyzing trace BTEX in water. Better sensitivity were obtained by headspace SPME combined with GC–MS, and the chromatographic shape is improved further by cryo-trap. Detection limits of BTEX in water at sub-ng l 1 concentration levels were achieved and linear ranges were over five orders of magnitude for all the analytes. Earlier studies on determination of BTEX were at from lg l 1 to ng l 1 level and linear ranges were less than four orders of magnitude. BTEX were indeed present in ground water samples probably contaminated by gasoline, paint and thinners, and chemical solvents. Acknowledgements The authors thank the National Science Council of the Republic of China for financially supporting this research under contract No. NSC92-2113-M-005-024. References Almeida, C.M.M., Boas, L.V., 2004. Analysis of BTEX and other substituted benzenes in water using headspace SPME-GC–FID: method validation. J. Environ. Monitor. 6, 80–88. Arambarri, I., Lasa, M., Garcia, R., Milla´n, E., 2004. Determination of fuel dialkyl ethers and BTEX in water using headspace solid-phase microextraction and gas chromatography–flame ionization detection. J. Chromatogr. A 1033, 193–203. Arthur, C.L., Pawliszyn, J., 1990. Solid phase microextraction with thermal desorption using fused silica optical fibers. Anal. Chem. 62, 2145–2148. Beketov, V.I., Parchinski, V.Z., Zorov, N.B., 1996. Effects of highfrequency electromagnetic treatment on the solid-phase extraction of aqueous benzene, naphthalene and phenol. J. Chromatogr. A 731, 65– 73. Bianchi, F., Careri, M., Marengo, E., Musci, M., 2002. Use of experimental design for the purge-and-trap-gas chromatography–mass spectrometry determination of methyl tert.-butyl ether, tert.-butyl

alcohol and BTEX in groundwater at trace level. J. Chromatogr. A 975, 113–121. Chiou, C.T., Schmedding, D.W., Manes, M., 1982. Partition of organic compounds in octanol–water systems. Environ. Sci. Technol. 16, 4–10. Cho, H.-J., Baek, K., Lee, H.-H., Lee, S.-H., Yang, J.-W., 2003. Competitive extraction of multi-component contaminants in water by carboxen–polydimethylsiloxane fiber during solid-phase microextraction. J. Chromatogr. A 988, 177–184. Elke, K., Jermann, E., Begerow, J., Dunemann, L., 1998. Determination of benzene, toluene, ethylbenzene and xylenes in indoor air at environmental levels using diffusive samplers in combination with headspace solid-phase microextraction and high-resolution gas chromatography–flame ionization detection. J. Chromatogr. A 826, 191– 200. EU, 1998. Council Directive 98/83/EC of 3 November 1998 on quality of water intended for human consumption, Official Journal of the European Communities, L 330, 5/12/1998, 0032–0054. ´ ., Ortiz, G., Pons, B., Tena, M.T., 2004. Determination of Ezquerro, O benzene, toluene, ethylbenzene and xylenes in soils by multiple headspace solid-phase microextraction. J. Chromatogr. A 1035, 17–22. Flo´rez Mene´ndez, J.C., Ferna´ndez Sa´nchez, M.L., Sa´nchez Urı´a, J.E., Ferna´ndez Martı´nez, E., Sanz-Medel, A., 2000. Static headspace, solid-phase microextraction and headspace solid-phase microextraction for BTEX determination in aqueous samples by gas chromatography. Anal. Chim. Acta 415, 9–20. Fuoco, R., Ceccarini, A., Onor, M., Marrara, L., 1999. Analysis of priority pollutants in environmental samples by on-line supercritical fluid chromatography cleanup–cryo-trap-gas chromatography–mass spectrometry. J. Chromatogr. A 846, 387–393. Holcomb, L.C., Seabrook, B.S., 1995. Indoor concentrations of volatile organic compounds: implications for comfort, health and regulation. Indoor Environ. 4, 7–26. Koziel, J., Jia, M.Y., Pawliszyn, J., 2000. Air sampling with porous solidphase microextraction fibers. Anal. Chem. 72, 5178–5186. Kubinec, R., Adamusˇcˇin, J., Jurda´kova´, H., Foltin, M., Ostrovsky´, I., Kraus, A., Soja´k, L., 2005. Gas chromatographic determination of benzene, toluene, ethylbenzene and xylenes using flame ionization detector in water samples with direct aqueous injection up to 250 ll. J. Chromatogr. A 1084, 90–94. Lagalante, A.F., Felter, M.A., 2004. Silylation of acrylamide for analysis by solid-phase microextraction/gas chromatography/ion-trap mass spectrometry. J. Agric. Food Chem. 52, 3744–3748. Lee, M.R., Song, Y.S., Hwang, B.H., Chou, C.C., 2000. Determination of amphetamine and methamphetamine in serum via headspace derivatization solid-phase microextraction–gas chromatography–mass spectrometry. J. Chromatogr. A 896, 265–273. Lee, M.R., Chang, L.Y., Dou, J., 2007. Determination of acrylamide in food by solid-phase microextraction coupled to gas chromatography– positive chemical ionization tandem mass spectrometry. Anal. Chim. Acta 582, 19–23. Li, K., Santilli, A., Goldthorp, M., Whiticar, S., Lambert, P., Fingas, M., 2001. Solvent vapour monitoring in work space by solid phase micro extraction. J. Hazard. Mater. 83, 83–91. Matisova´, E., Medved’ova´, M., Vraniakova´, J., Sˇimon, P., 2002. Optimisation of solid-phase microextraction of volatiles. J. Chromatogr. A 960, 159–164. Miermans, C.J.H., van der Velde, L.E., Frintrop, P.C.M., 2000. Analysis of volatile organic compounds, using the purge and trap injector coupled to a gas chromatograph/ion-trap mass spectrometer: review of the results in Dutch surface water of the Rhine, Meuse, Northern Delta Area and Westerscheldt, over the period 1992–1997. Chemosphere 40, 39–48. Nollet, L.M.L. (Ed.), 2001. Handbook of Water Analysis. Marcel Dekker, New York (Chapter 33). Potter, D.W., Pawliszyn, J., 1992. Detection of substituted benzenes in water at the pg/ml level using solid-phase microextraction and gas chromatography–ion trap mass spectrometry. J. Chromatogr. A 625, 247–255.

Please cite this article in press as: Lee, M.-R. et al., Determination of benzene, toluene, ethylbenzene, xylenes in water ..., Chemosphere (2007), doi:10.1016/j.chemosphere.2007.05.004

ARTICLE IN PRESS M.-R. Lee et al. / Chemosphere xxx (2007) xxx–xxx Prˇikryl, P., Kubinec, R., Jurda´kova´, H., Sˇevcˇ´ıky, J., Ostrovsk, I., Soja´k, L., Berezkin, V., 2006. Comparison of needle concentrator with SPME for GC determination of benzene, toluene, ethylbenzene, and xylenes in aqueous samples. Chromatographia 64, 65–70. Rosell, M., Lacorte, S., Ginebreda, A., Barcelo´, D., 2003. Simultaneous determination of methyl tert.-butyl ether and its degradation products, other gasoline oxygenates and benzene, toluene, ethylbenzene and xylenes in Catalonian groundwater by purge-and-trap-gas chromatography–mass spectrometry. J. Chromatogr. A 995, 171–184.

7

Stan, H.J., Kirsch, N.H., 1995. GC–FID determination of chloroben-. zene isomers in methanogenic batch-cultures from river sediments. Int. J. Environ. Anal. Chem. 60, 33–40. USEPA, 2004. List of Contaminants & their Maximum Contaminant Level (MCLs) in drinking water. http://www.epa.gov/safewater/ mcl.html#organic. Wang, J.-X., Jiang, D.-Q., Yan, X.-P., 2006. Determination of substituted benzenes in water samples by fiber-in-tube liquid phase microextraction coupled with gas chromatography. Talanta 68, 945–950.

Please cite this article in press as: Lee, M.-R. et al., Determination of benzene, toluene, ethylbenzene, xylenes in water ..., Chemosphere (2007), doi:10.1016/j.chemosphere.2007.05.004