CHEM. RES. CHINESE UNIVERSITIES 2011, 27(2),

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1 CHEM. RES. CHINESE UNIVERSITIES 2011, 27(2), Solid-phase Microextraction with Benzoxy-calix[6]arene Fiber Coupled to Gas Chromatography for the Analysis of Polycyclic Aromatic Hydrocarbons in Water YE Chang-wen, ZHANG Xue-na, HUANG Jiang-yan, LI Xiu-juan * and PAN Si-yi College of Food Science & Technology, Huazhong Agricultural University, Wuhan , P. R. China Abstract Headspace solid-phase microextraction(hs-spme) with sol-gel calix[6]arene-containing fiber followed by gas chromatography with a flame ionization detector was used to examine the composition and distribution of seven polycyclic aromatic hydrocarbons(pahs) in water. The novel SPME fiber exhibited higher extraction efficiency to PAHs compared with poly(dimethylsiloxane) and other calixarene-containing fibers. Extraction/retention mechanism based on the interactions between calixarenes and PAHs was discussed. Owing to the good selectivity and high extraction capability of this calixarene fiber, low detection limits were obtained in a range of ng/l and the relative standard deviation values were 12.3% for all of the analytes. The linear ranges of the proposed method were five orders of magnitude for the tested compounds, with linear correlation coefficients(r) greater than The method was applied to the determination of polycyclic aromatic hydrocarbons in nine water sources in Wuhan City, China. Standard addition method was selected for the quantification and the recovery values were in a satisfactory range. Total PAHs concentrations in the nine surface water samples were found to vary between undetectable and μg/l with two- and three-ring PAHs predominating. Keywords Polycyclic aromatic hydrocarbon; Solid-phase microextraction; Calixarene; Gas chromatography; Water Article ID (2011) Introduction Calixarenes are a class of cyclic oligmers prepared from formaldehyde and para-substituted phenols via cyclic condensation under alkaline condition. They provide well-defined cavities, which simultaneously offer polar(lower-rim) and nonpolar(upper-rim) features. Thus, calixarenes can recognize and separate a wide range of guest species, depending on the bound groups at each rim and the number of repeat units in the macrocycle [1]. Three types of calixarenes bearing different substituted groups, namely, 5,11,17,23-tetra-tert-butyl-25,27-diethoxy-26,28-dihydroxycalix[4]arene(C[4]), 25,27-dihydroxy- 26,28-oxy(2,7 -dioxo-3,6 -diazaoctyl)oxy-p-tert-butylcalix[4] arene(amide bridged-c[4]), and 5,11,17,23-tetra-tert-butyl-25, 27-dihydroxy-26,28-diglycidyloxycalix[4]arene(diglycidyloxy- C[4]), were synthesized and used for the preparation of solidphase microextraction(spme) fibers by sol-gel coating technology in our previous studies [2 4]. The characteristics of these fibers were investigated in detail. Because of the well-defined cavities, good thermal and chemical stabilities of calixarenes, all of these sol-gel fibers have high operational stability and exhibit good selectivity and sensitivity to aromatic compounds, such as benzene substitutes, aromatic amines, chlorophenols, chlorobenzenes, phthalate acid esters and propranolol. Application was carried out accordingly in the fields of environment [4 6], food [7] and biomedicine [8,9] with satisfactory results. Polycyclic aromatic hydrocarbons(pahs), an important group of organic pollutants, with two or more fused aromatic rings have received considerable attention because of their documented carcinogenicity in experimental animals of several species [10]. PAHs can be introduced into the aquatic environment through various routes, such as wastewater discharge, dry and wet deposition(e.g., rain droplets, snow and hail), surface runoff and oil leaking [11]. The analysis of PAHs in environmental water has become an important topic in analytical chemistry, especially after the event of Gulf of Mexico oil spill [12]. Therefore, the development of effective extraction and enrichment techniques for PAHs prior to an analytical procedure is necessary. SPME with the calixarene-containing fiber would be a suitable tool. In contrast to the aromatic compounds mentioned above, PAHs with three or more fused aromatic rings may be too large to form good complexes with calix[4]arenes. If this is true, better inclusions into larger calixarenes should be possible. As a result, an SPME fiber coated with 5,11,17,23,29,35-hexa-tertbutyl-37, 39, 41-trihydroxy-38, 40, 42-trimethoxy calix[6]arene (C[6])/hydroxyl-terminated silicone oil(c[6]/oh-tso) was prepared with the similar procedure to that of calix[4]arenes *Corresponding author. lixiujuan@mail.hzau.edu.cn Received July 13, 2010; accepted September 6, Supported by the National Natural Science Foundation of China(No ) and the Fund of the Ministry of Education of China(No ).

2 No.2 YE Chang-wen et al. 199 and applied to the analysis of PAHs in soil samples with the aid of gas chromatography(gc) [13]. The fiber showed higher extraction efficiency towards PAHs in comparison with C[4]/OH-TSO and commercial polydimethylsiolxane(pdms) fiber. In this paper, we prepared another new calix[6]arene fiber with benzoxy substituents on its lower-rim [14], and simultaneously introduced OH-TSO, C[4]/OH-TSO and C[6]/OH-TSO fibers to investigate the host-guest inclusion interactions with PAHs as probes. One-way analysis of variance(anova) and Duncan s multiple-range tests were employed to explore the significant differences in extraction efficiency among these fibers. And then, a robust method was developed for determining PAHs in water based on headspace SPME-GC(HS- SPME-GC) using this new fiber. At last the method was applied to investigate the composition and distribution of seven PAHs in the urban lakes and rivers in Wuhan City, China. 2 Experimental 2.1 Chemicals, Reagents and Materials Indene, naphthalene, biphenyl, fluorene, phenanthrene, anthracene, fluoranthene and pyrene were obtained from Merck (Darmstadt, Germany). Methanol of analytical grade was purchased from Shanghai Chemical Factory, China. The stock solution of these PAHs was prepared in methanol at a concentration of each PAH 1 mg/ml. Working solutions at different concentrations were prepared by diluting the stock solution in an appropriate volume of methanol. A total of nine sampling sites in Wuhan City were selected, including seven urban lakes(ul1 7), the giant Yangtze River and the Han River. The water samples were collected on April 2, 2008, in 2 L pre-cleaned glass bottles with a frame that allows the bottle to be opened underwater to avoid the collection of the surface microlayer. To eliminate randomicity, each sample consisting of 5 subsamples was collected within a surface area of 100 m 100 m, about 20 m far from the shore, and was well mixed. After sampling, the water samples were transported to the laboratory immediately and stored in a refrigerator until April 3 when they were analyzed. All the solutions and samples were stored at 4 C in the dark in sealed glass containers completely filled(without headspace) to avoid analytes losses and to ensure reproducibility. 2.2 Instruments and Analytical Conditions Quantification of PAHs was performed on an SE-54 capillary column(30 m 0.32 mm 0.25 μm) in an SP-6890A capillary GC system(shandong Lunan Ruihong Chemical Instrument Corporation, Tengzhou, China) equipped with a capillary split/splitless injector system and a flame ionization detector(fid). Online data collection and processing were done on Chromatopac model N2000(Hangzhou Mingtong Technology Co., Ltd., Hangzhou, China). The column temperature was programmed from 90 C(hold for 1 min) at 8 C/min to 260 C(hold for 5 min). The temperatures were 300 C for the injector and 320 C for the FID. Nitrogen was used as the carrier gas at a linear velocity of cm/s in the splitless mode for all the analyses. GC peak identification was conducted by comparing the gas chromatographic retention time with that of an authentic standard. Meanwhile, the Agilent 6890N gas chromatograph coupled to a 5975B mass-selective detector (MSD, Agilent Technologies, Palo Alto, CA, USA) was also used for the identification of the analytes. A magnetic stirrer DF-101S(Zhengzhou Greatwall Scientific Industrial and Trading Co., Ltd., Zhengzhou, China) with a maximum stirring speed of 1250 r/min was employed for stirring the sample during extraction. Laboratory-made SPME syringes with sol-gel coated fibers were used to transfer the extracted sample to the GC injector for analysis. The preparation of sol-gel coated OH-TSO(70 μm), C[4]/OH-TSO(72 μm), C[6]/OH-TSO(70 μm) and benzoxy-c[6]/oh-tso(70 μm) fibers conformed to the methodologies reported in the literature [2,13,14]. The commercially available PDMS fiber(100 μm) was obtained from Supelco(Bellefonte, PA, USA). 2.3 SPME Procedure Before analysis, the fibers were conditioned in the injection port of GC to ensure no residual contamination occurred. For each SPME analysis, 5 ml of water spiked with PAHs standards was placed into a 10 ml glass vial containing a little magnetic spin bar(ptfe) and a certain amount of NaCl (between 0 and 1.8 g) depending on the experiments. Then, the vial was tightly capped with a PTFE stopper and an aluminum cap. During the headspace extraction, the aqueous sample was continuously stirred with a magnetic stir bar(800 r/min). After extraction, the SPME fiber containing analytes was withdrawn from the vial and inserted into the GC injection port for thermal desorption. 3 Results and Discussion 3.1 Coating Evaluation ANOVA and Duncan s multiple-range tests were employed to explore the significant differences in extraction capability among PDMS, OH-TSO, C[4]/OH-TSO, C[6]/OH-TSO and benzoxy-c[6]/oh-tso fibers for eight PAHs(Fig.1). Three replicates were performed for each kind of fiber and the average values were reported. One-way ANOVA indicated that the extraction efficiency to PAHs was significantly affected(p<0.05) by the type of fiber coating except for fluoranthene and pyrene. Duncan s multiple-range tests showed that the means with the same letter were not significantly different for these coatings at 5% level. As shown in Fig.1, the extraction efficiencies of the three kinds of calixarene-containing fibers were higher than those of PDMS and OH-TSO fibers. It is obvious that calixarenes play a vital role in the extraction process. C[6]/OH-TSO fiber had higher means of extraction capability than that of C[4]/OH-TSO fiber. Statistically significant differences were found between the two fibers for the low molecular weight(lmw) PAHs, namely, indene, naphthalene, biphenyl and fluorene; but no significant differences were found for phenanthrene, anthracene, fluoranthene and pyrene.

3 200 CHEM. RES. CHINESE UNIVERSITIES Vol.27 Especially for fluoranthene and pyrene, the two fibers showed similar extraction capability. The coefficient of variation(cv) of extraction efficiencies of C[6]/OH-TSO and C[4]/OH-TSO fibers(given in the parentheses) also decreased with the increase of the ring number of PAHs. That is to say, compared with C[4]/OH-TSO fiber, C[6]/OH-TSO fiber has a higher affinity for PAHs with two aromatic rings and a similar affinity for PAHs with three and four aromatic rings. and C[4]/OH-TSO fibers should be attributed to the sizes of the cavities. The suitable size matching between calix[6]arene cavity and the LMW PAHs leads to stronger host-guest inclusion interactions. However, neither calix[4]arene nor calix[6]arene can form good complexes with PAHs with three or four aromatic rings. The extraction capability of benzoxy-c[6]/oh-tso fiber is superior to those of other four fibers for all the eight PAHs. Statistically significant differences in extraction efficiency were found between the two calix[6]arene-containing fibers for indene, naphthalene, biphenyl, fluorene and phenanthrene. As shown in Fig.2, the difference between the two calix[6]arenes originates from the substituents at the lower-rims. The higher affinity of benzoxy-c[6]/oh-tso fiber for PAHs should be explained by the substituent of benzoxy, which increases the π-π interactions. 3.2 Method Validation Fig.1 Comparison of extraction capability to PAHs by laboratory-made OH-TSO, C[4]/OH-TSO, C[6]/ OH-TSO, benzoxy-c[6]/oh-tso sol-gel fibers and commercial PDMS fiber via ANOVA and Duncan s multiple-range tests * P<0.05; ** P<0.01. a. Indene; b. naphthalene; c. biphenyl; d. fluorene; e. phenanthrene; f. anthracene; g. fluoranthene; h. pyrene. Spiking level: 100 μg/l. SPME conditions: extraction temperature: 60 C; extraction time: 30 min; c(nacl)=300 g/l. Data in parentheses are the coefficients of variation of extraction efficiency between C[6]/OH-TSO and C[4]/ OH-TSO. PDMS(100 μm); OH-TSO(70 μm); C[4]/OH-TSO (72 μm); C[6]/OH-TSO(70 μm); benzoxy-c[6]/oh-tso(70 μm). As reported elsewhere [2 6], the high extraction capability of calixarene-containing sol-gel SPME coatings to PAHs is due to the π-π interactions, hydrophobic interactions and the potential host-guest inclusion interactions. In the meantime, high surface area of the sol-gel coating also provides enhanced extraction efficiency in SPME. As for C[4]/OH-TSO and C[6]/OH-TSO fibers, the only difference comes from the structures of the two types of calixarenes. Fig.2 shows the simplified structures of calix[4]arene and the two types of calix[6]arenes. The calix[4]arene contains ethoxy groups and the calix[6]arene contains methoxy groups at the lower-rims, which has little effect on the extraction of PAHs. Meanwhile, the C[6]/OH-TSO fiber shows no additional π-π interactions compared with the C[4]/OH-TSO fiber since they show similar extraction capability to fluoranthene and pyrene. Therefore, the main difference in the extraction efficiency of C[6]/OH-TSO Fig.2 Structures of calix[4]arene and two types of calix[6]arenes chemically bonded in the sol-gel coatings In this work, headspace extraction of PAHs from water was done with the benzoxy-c[6]/oh-tso fiber. Four parameters affecting extraction efficiency were investigated and the optimal operating conditions obtained were 300 g/l NaCl, an agitation of 800 r/min, and an extraction time of 30 min at 60 C. The SPME-GC-FID method was validated by thoroughly examining the linear range, repeatability, limits of detection(lods) and recovery. This validation test was carried out via spiked ultrapure water. The results are presented in Table 1. The linear range was from 0.1 μg/l to 1000 μg/l for naphthalene, biphenyl, fluorene, phenanthrene and anthracene, 0.01 μg/l to 100 μg/l for fluoranthene and pyrene. Correlation coefficients between and were obtained, depending on the compounds, showing a high degree of correlation between concentration and peak area. The repeatability was investigated by performing eight extractions under the identical conditions. The repeatability, expressed as relative standard deviation (RSD), gave a satisfactory outcome, which ranged from 7.3% to 12.3%. The LODs were calculated based on three times the average background noise divided by the detection sensitivity and ranged from 0.34 ng/l to 6.50 ng/l, which were lower than those obtained by HS-SPME-GC-MS method in aqueous samples with PDMS fiber except for naphthalene [15]. Table 1 Relative standard deviations(rsds), limits of detections(lods), linearities, and correlation coefficients of proposed method PAH RSD a (%) (n=8) Linear range/(μg L 1 ) LOD b / (ng L 1 ) Correlation coefficient Naphthalene Biphenyl Fluorene Phenanthrene Anthracene Fluoranthene Pyrene a. Spiking level: 100 ng/l; b. detection limits were estimated on the basis of a signal to noise ratio of 3:1 in which the concentration of each analyte in the sample solution agreed with the lowest one in the linearity.

4 No.2 YE Chang-wen et al Evaluation of Interference from Water Matrices Table 2 shows the interference from different water matrices. These results were gained by comparing the peak areas in the chromatograms given by HS-SPME of different real water with the corresponding peak areas obtained in ultrapure water, with ultrapure water as references(100%). Table 2 also provides the results of ANOVA on FID responses in terms of peak areas for the seven analytes. Statistically significant differences(p<0.05) in the peak areas observed between different water matrices can be explained by matrix effects(ph, total organic carbon, conductivity and so on). Therefore, quantitative analysis of real samples based on calibration in model water will not be feasible. The standard addition calibration could be a more effective method in order to avoid systematic errors caused by matrix effect. Table 2 Evaluation of interference from water matrices on PAHs * Matrix Mean relative response to ultrapure water(%) Ultrapure water UL UL UL UL UL UL UL Yangtze River Han River F P < < * Spiking level: 100 μg/l. 3.4 Recovery Study and Real Samples To confirm the applicability of the proposed method to extract PAHs from real water samples, recovery tests were performed based on the addition of known amounts of PAHs, as shown in Table 3. Although the matrices in water samples Table 3 Sample PAHs added/ (ng L 1 ) showed significant influence on the extraction efficiency, the recovery values of the method were in a satisfactory range (mostly between 80% and 120%), which demonstrated the suitability of the method for extracting PAHs from real samples. Recoveries at 2 levels of studied PAHs Recovery(%, n=3) UL UL UL UL UL UL UL Yangtze River Han River The method developed for the determination of PAHs was applied to the analysis of the main water sources in Wuhan City, China. The concentrations of individual PAH and total of 7 PAHs in the nine water resources are shown in Table 4. As can be seen, the PAHs with two- and three-ring were the most abundant PAHs, which accounted for 93.1% of the total PAHs. A great variety of PAHs were found in the surface water of Yangtze River(in which four PAHs were detected), which is an important drinking-water source for Wuhan residents. Fluoranthene was not detected in all of the samples. The total concentrations of 7 PAHs ranged from undetectable at UL1 to a maximum of μg/l at UL3, with an average of μg/l in

5 202 CHEM. RES. CHINESE UNIVERSITIES Vol.27 the nine surface water samples. Fig.3 illustrates the water samples obtained from UL6. HS-SPME-GC-FID chromatograms of unspiked and spiked Table 4 PAHs concentrations in water samples * Sample Concentration of PAHs/(μg L 1 ) Total UL1 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. UL2 n.d. n.d. n.d. n.d. n.d. n.d UL n.d n.d. n.d. n.d. n.d UL4 n.d. n.d. n.d n.d. n.d. n.d UL5 n.d n.d. n.d. n.d. n.d UL6 n.d n.d n.d. n.d UL7 n.d. n.d. n.d. n.d n.d. n.d Yangtze River n.d n.d. n.d Han River n.d. n.d n.d. n.d. n.d. n.d Total n.d * n.d.: Concentration was lower than the LODs. level compared with those of other water resources in the world. It could be related to the industrial structure of Wuhan. In consideration of the quality and safety of drinking water, real-time monitoring of PAHs in drinking water resources such as rivers and lakes should be carried out. Experimental results conclusively demonstrate that SPME with the benzoxy-calix[6]arene fiber followed by GC-FID analysis can be used for such a purpose. Fig.3 HS-SPME-GC-FID chromatograms of PAHs in water samples obtained from UL6 (A) Spiking level: 100 μg/l; (B) unspiked water. Peaks: 1. naphthalene; 2. biphenyl; 3. fluorene; 4. phenanthrene; 5. anthracene; 6. fluoranthene; 7. pyrene. Extraction conditions: extraction temperature: 60 C; extraction time: 30 min; c(nacl)=300 g/l. 4 Conclusions An analytical method based on SPME-GC-FID has been developed for the simultaneous extraction and quantitative analysis of seven trace PAHs in water samples. With the help of the sol-gel benzoxy-c[6]/oh-tso coated fiber, the method gives high sensitivity, good repeatability and satisfactory recovery. This study also provides useful data at the levels of PAHs in the surface water in Wuhan City, China. The extent of PAHs pollution in Wuhan is in the rank of moderate pollution References [1] Chen Y. K., Chen Y. Y., Org. Lett., 2000, 2, 743 [2] Li X., Zeng Z., Gao S., Li H., J. Chromatogr. A, 2004, 1023, 15 [3] Li X., Gong S., Zeng Z., Chromatographia, 2005, 62, 519 [4] Wang W., Gong S., Cao Q., Chen Y., Li X., Zeng Z., Chromatographia, 2005, 61, 75 [5] Li X., Zeng Z., Zhou J., Anal. Chim. Acta, 2004, 509, 27 [6] Li X. J., Ye C. W., Huo X. L., Zeng Z. R., Microchim Acta, 2010, 168, 161 [7] Li X., Zeng Z., Zhou J., Gong S., Wang W., Chen Y., J. Chromatogr. A, 2004, 1041, 1 [8] Li X., Zeng Z., Hu M., Mao M., J. Sep. Sci., 2005, 28, 2489 [9] Li X., Zeng Z., Chen Y., Talanta, 2004, 63, 1013 [10] Mumtaz M., George J., Toxicological Profile for Polycyclic Aromatic Hydrocarbons, US Department of Health and Human Services, Atlanta, 1995, 133 [11] Schenkman L., Science, 2010, 328, 1215 [12] Poster D. L., Baker J. E., Environ. Sci. Technol., 1996, 30, 341 [13] Zhang L. F., Li X. J., Liu C. X., Zeng Z. R., Chinese J. Anal. Chem., 2007, 35, 1269 [14] Ye C. W., Gao J., Yang C., Liu X. J., Li X. J., Pan S. Y., Anal. Chim. Acta, 2009, 641, 64 [15] King A. J., Readman J. W., Zhou J. L., Anal. Chim. Acta, 2004, 523, 259