In Situ Hydrothermal Growth of Metal−Organic Framework 199 Films

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In Situ Hydrothermal Growth of Metal-Organic Framework 199 Films on Stainless Steel Fibers for Solid-Phase Microextraction of Gaseous Benzene Homologues Xiao-Yan Cui, Zhi-Yuan Gu, Dong-Qing Jiang, Yan Li, He-Fang Wang, and Xiu-Ping Yan* Research Center for Analytical Sciences, College of Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, China Metal-organic frameworks (MOFs) have received great attention due to their fascinating structures and intriguing potential applications in various fields. Herein, we report the first example of the utilization of MOFs for solid-phase microextraction (SPME). MOF-199 with unique pores and open metal sites (Lewis acid sites) was employed as the coating for SPME fiber to extract volatile and harmful benzene homologues. The SPME fiber was fabricated by in situ hydrothermal growth of thin MOF-199 films on etched stainless steel wire. The MOF-199-coated fiber not only offered large enhancement factors from 19 613 (benzene) to 110 860 (p-xylene), but also exhibited wide linearity with 3 orders of magnitude for the tested benzene homologues. The limits of detection for the benzene homologues were 8.3-23.3 ng L-1. The relative standard deviation (RSD) for six replicate extractions using one SPME fiber ranged from 2.0% to 7.7%. The fiberto-fiber reproducibility for three parallel prepared fibers was 3.5%-9.4% (RSD). Indoor air samples were analyzed for the benzene homologues using the SPME with the MOF-199-coated fiber in combination with gas chromatography-flame ionization detection. The recoveries for the spiked benzene homologues in the collected indoor air samples were in the range of 87%-106%. The high affinity of the MOF-199-coated fiber to benzene homologues resulted from the combined effects of the large surface area and the unique porous structure of the MOF-199, the π-π interactions of the aromatic rings of the analytes with the framework 1,3,5-benzenetricarboxylic acid molecules, and the π-complexation of the electron-rich analytes to the Lewis acid sites in the pores of MOF-199. Metal-organic frameworks (MOFs) have received an increasing interest due to their fascinating structures1-3 and intriguing * Corresponding author. Fax: (86)22-23506075. E-mail: [email protected]. (1) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705–714. (2) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, 2334– 2375. (3) Fe´rey, G. Chem. Soc. Rev. 2008, 37, 191–214. 10.1021/ac901663x CCC: $40.75  2009 American Chemical Society Published on Web 11/06/2009

potential applications in hydrogen storage,4,5 carbon dioxide capture,6 gas separation,7,8 catalysis,9,10 chiral separation,11,12 sensing,13,14 and imaging.15 At variance with conventionally used microporous inorganic materials such as zeolites, these tailored nanoporous robust MOFs with extremely high surface area (up to more than 5000 m2 g-1)16 are formed by self-assembly, using metal ions as coordination centers, linked together with a variety of rigid-rod-like organic bridging ligands. This structure also provides MOFs with tunable options, organic functionality, high thermal and mechanical stability,1,7 and open metal sites in the skeleton.17,18 Recently, many researches have focused on the rational design and synthesis of MOFs to perform highly selective gas adsorption through controlling the structure and functions of the pores.1,19,20 Moreover, MOFs with open metal sites exhibit high activity and selectivity in adsorption and catalysis21,22 possibly due to their interaction with the targets, (4) Kaye, S. S.; Dailly, A.; Yaghi, O. M.; Long, J. R. J. Am. Chem. Soc. 2007, 129, 14176–14177. (5) Vitillo, J. G.; Regli, L.; Chavan, S.; Ricchiardi, G.; Spoto, G.; Dietzel, P. D. C.; Bordiga, S.; Zecchina, A. J. Am. Chem. Soc. 2008, 130, 8386–8396. (6) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. Science 2008, 319, 939–943. (7) Mueller, U.; Schubert, M.; Teich, F.; Puetter, H.; Schierle-Arndt, K.; Pastre´, J. J. Mater. Chem. 2006, 16, 626–636. (8) Bastin, L.; Ba´rcia, P. S.; Hurtado, E. J.; Silva, J. A. C.; Rodrigues, A. E.; Chen, B. J. Phys. Chem. C 2008, 112, 1575–1581. (9) Alaerts, L.; Se´guin, E.; Poelman, H.; Thibault-Starzyk, F.; Jacobs, P. A.; De Vos, D. E. Chem.sEur. J. 2006, 12, 7353–7363. (10) Alkordi, M. H.; Liu, Y.; Larsen, R. W.; Eubank, J. F.; Eddaoudi, M. J. Am. Chem. Soc. 2008, 130, 12639–12641. (11) Kesanli, B.; Lin, W. B. Coord. Chem. Rev. 2003, 246, 305–326. (12) Li, G.; Yu, W. B.; Cui, Y. J. Am. Chem. Soc. 2008, 130, 4582–4583. (13) Chen, B. L.; Yang, Y.; Zapata, F.; Lin, G. N.; Qian, G. D.; Lobkovsky, E. B. Adv. Mater. 2007, 19, 1693–1696. (14) Chen, B.; Wang, L.; Zapata, F.; Qian, G.; Lobkovsky, E. B. J. Am. Chem. Soc. 2008, 130, 6718–6719. (15) Taylor, K. M. L.; Rieter, W. J.; Lin, W. J. Am. Chem. Soc. 2008, 130, 14358– 14359. (16) Koh, K.; Wong-Foy, A. G.; Matzger, A. J. J. Am. Chem. Soc. 2009, 131, 4184–4185. (17) Chui, S. S. Y.; Lo, S. M. F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D. Science 1999, 283, 1148–1150. (18) Yang, Q. Y.; Zhong, C. L. J. Phys. Chem. B 2006, 110, 655–658. (19) Chae, H. K.; Siberio-Pe´rez, D. Y.; Kim, J.; Go, Y.; Eddaoudi, M.; Matzger, A. J.; O’Keeffe, M.; Yaghi, O. M. Nature 2004, 427, 523–527. (20) Li, H.; Eddaoudi, M.; O’Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276– 279. (21) Britt, D.; Tranchemontagne, D.; Yaghi, O. M. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 11623–11627.

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although the mechanism remains to be confirmed.23 These properties make MOFs ideal choices for gas-selective adsorption, especially in sample pretreatments. Solid-phase microextraction (SPME), first introduced by Pawliszyn and co-workers in 1990,24 integrates sampling, extraction, and sample introduction into one step with less solvent consumption and is considered one of the most promising methods in sample preparation. The technique is based on the adsorption of analytes from the sample to the SPME fiber which is coated with sorbent. The analytes are then desorbed into suitable separation and detection devices.25,26 Generally, the sorbent coated onto the fiber is the most important part of SPME. Although a number of SPME coatings are commercially available, such as poly(dimethylsiloxane) (PDMS),27,28 polyacrylate (PA),29 poly(dimethylsiloxane)/divinylbenzene (PDMS/DVB),30 polyethylene glycol/divinylbenzene (PEG/DVB),31 carboxen/poly(dimethylsiloxane) (CAR/ PDMS),32 and carbowax/divinylbenzene (CW/DVB),33 their performance is not always satisfactory for the extraction of large numbers of varied analytes due to their instability at high temperature or in some organic solvents and limited selectivity and coatings. To overcome these problems, myriad coatings for the SPME fiber have been introduced, including calix[4] openchain crown ether,34 a chemically bonded silica stationary phase for HPLC,35 polycrystalline graphites,36 low-temperature glassy carbon,37 active carbon,38 carbon nanotubes,39,40 metal oxides,41-44 and recently developed biocompatible coatings.45,46 The modular construction of MOFs allows their pore structures to be systematically tuned to yield the desired shape, size, and (22) Horike, S.; Dinca˘, M.; Tamaki, K.; Long, J. R. J. Am. Chem. Soc. 2008, 130, 5854–5855. (23) Lee, Y. G.; Moon, H. R.; Cheon, Y. E.; Suh, M. P. Angew. Chem., Int. Ed. 2008, 47, 7741–7745. (24) Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990, 62, 2145–2148. (25) Vas, G.; Ve´key, K. J. Mass Spectrom. 2004, 39, 233–254. (26) Aulakh, J. S.; Malik, A. K.; Kaur, V.; Schmitt-Kopplin, P. Crit. Rev. Anal. Chem. 2005, 35, 71–85. (27) Arthur, C. L.; Killam, L. M.; Motlagh, S.; Lim, M.; Potter, D. W.; Pawliszyn, J. Environ. Sci. Technol. 1992, 26, 979–983. (28) Chong, S. L.; Wang, D.; Hayes, J. D.; Wilhite, B. W.; Malik, A. Anal. Chem. 1997, 69, 3889–3898. (29) Magdic, S.; Boyd-Boland, A.; Jinno, K.; Pawliszyn, J. J. Chromatogr., A 1996, 736, 219–228. (30) Miller, M. E.; Stuart, J. D. Anal. Chem. 1999, 71, 23–27. (31) Flamini, R.; Dalla Vedova, A. D.; Panighel, A.; Perchiazzi, N.; Ongarato, S. J. Mass Spectrom. 2005, 40, 1558–1564. ´ .; Ortiz, G.; Pons, B.; Tena, M. T. J. Chromatogr., A 2004, (32) Ezquerro, O 1035, 17–22. (33) van Doorn, H.; Grabanski, C. B.; Miller, D. J.; Hawthorne, S. B. J. Chromatogr., A 1998, 829, 223–233. (34) Liu, M.-M.; Zeng, Z.; Lei, Y.; Li, H.-B. J. Sep. Sci. 2005, 28, 2306–2318. (35) Liu, Y.; Lee, M. L.; Hageman, K. J.; Yang, Y.; Hawthorne, S. B. Anal. Chem. 1997, 69, 5001–5005. (36) Aranda, R.; Kruus, P.; Burk, R. C. J. Chromatogr., A 2000, 888, 35–41. (37) Giardina, M.; Olesik, S. V. Anal. Chem. 2003, 75, 1604–1614. (38) Martos, P. A.; Pawliszyn, J. Anal. Chem. 1999, 71, 1513–1520. (39) Wang, J.-X.; Jiang, D.-Q.; Gu, Z.-Y.; Yan, X.-P. J. Chromatogr., A 2006, 1137, 8–14. (40) Zhang, W.; Sun, Y.; Wu, C.; Xing, J.; Li, J. Anal. Chem. 2009, 81, 2912– 2920. (41) Djozan, D.; Assadi, Y.; Haddadi, S. H. Anal. Chem. 2001, 73, 4054–4058. (42) Budziak, D.; Martendal, E.; Carasek, E. Anal. Chim. Acta 2007, 598, 254– 260. (43) de Oliveira, A. F.; da Silveira, C. B.; de Campos, S. D.; de Campos, E. A.; Carasek, E. Talanta 2005, 66, 74–79. (44) Xu, H.-L.; Li, Y.; Jiang, D.-Q.; Yan, X.-P. Anal. Chem. 2009, 81, 4971–4977. (45) Musteata, M. L.; Musteata, F. M.; Pawliszyn, J. Anal. Chem. 2007, 79, 6903–6911. (46) Mullett, W. M.; Pawliszyn, J. J. Sep. Sci. 2003, 26, 251–260.

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surface characteristics by the judicious choice of metal-containing secondary building units (SBUs) and/or bridging linkers,47,48 making them ideal candidates for the SPME coatings for a variety of analytes. However, to the best of our knowledge, no effort has been devoted to the utilization of MOFs as novel coatings for SPME so far. Herein, we report the first example of the utilization of MOFs for SPME. MOF-199 (also called HKUST-1 or Cu-BTC)17 with unique pores and open metal sites (Lewis acid sites) was employed as the coating for the SPME of volatile and harmful benzene homologues. The SPME fiber was fabricated by in situ hydrothermal growth of thin MOF-199 films on etched stainless steel wire. Potential factors and mechanisms for the SPME of benzene homologues were investigated and discussed. The MOF-199coated fiber exhibited high affinity, wide linearity, and excellent reproducibility for the SPME of benzene homologues. EXPERIMENTAL SECTION Instrumentation. A Shimadzu GC-9A system equipped with a flame ionization detector (FID) was employed for all experiments. The GC capillary column (SE-54, 30 m length × 0.53 mm i.d. × 1.0 µm) was purchased from the Lanzhou Institute of Chemical Physics (Lanzhou, China). The column temperature was maintained at 40 °C for 5 min and then programmed at 5 °C min-1 to the final temperature of 90 °C for 1 min. The injector and detector temperatures were both set at 275 °C. The high-purity nitrogen (99.99%, BOC Gases Co. Ltd., Tianjin, China) was used as the carrier gas at a flow rate of 40 mL min-1. Hydrogen and air were maintained at flow rates of 55 and 470 mL min-1, respectively. Splitless injections were used throughout. A commercial SPME manual holder and the fibers coated with 30 µm of PDMS and 65 µm of PDMS/DVB (Supelco, Bellefonte, PA) were used for comparison. The fibers were conditioned in the GC inject port according to the manufacturer. The sampling chambers used in this study were 120 mL glass vials sealed by an aluminum cap with a rubber septum. The scanning electron microscopy (SEM) micrographs were recorded on a Shimadzu SS-550 scanning electron microscope at 15.0 kV. The X-ray diffraction (XRD) experiments were performed on a D/max-2500 diffractometer (Rigaku, Japan) using Cu KR radiation (λ ) 1.5418 Å). Thermogravimetric analysis (TGA) was carried out on a thermal gravimetric analyzer (Rigaku, Japan) from roomtemperatureto600°CunderN2.TheBrunauer-Emmett-Teller (BET) surface area of the evacuated MOF-199 was measured on a TriStar 3000 sorptometer (Micromeritics) using nitrogen adsorption at 77 K. Chemicals and Reagents. All chemicals used were at least of analytical grade. Hydrochloric acid and nitric acid were purchased from the Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). Hydrofluoric acid (40.0%) was purchased from Tianjin Standard Science and Technology Co., Ltd. (Tianjin, China). Ultrapure water (18.2 MΩ cm) was obtained from a WaterPro Water Purification System (Labconco Corp., Kansas City, MO). Cu(NO3)2 · 3H2O was purchased from the Tianjin Guangfu Fine Chemical Research Institute. 1,3,5-Benzenetri(47) Eddaoudi, M.; Moler, D. B.; Li, H. L.; Chen, B. L.; Reineke, T. M.; O’Keeffe, M.; Yaghi, O. M. Acc. Chem. Res. 2001, 34, 319–330. (48) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469–472.

Figure 1. Sketch view of the vertical section of the developed SPME device: 1, handle grip; 2, push-pull rod (stainless steel tube, 100 mm length × 1.1 mm o.d. × 0.8 mm i.d.); 3, stainless steel capillary (150 mm length × 0.30 mm o.d. × 0.10 mm i.d.); 4, main body of the microsyringe (calibrated glass tube); 5, uncoated part of the SPME fiber (stainless steel wire, 3 cm length × 0.09 mm diameter); 6, needle of the microsyringe (50 mm length × 0.6 mm o.d. × 0.4 mm i.d.); 7, MOF-199-coated part of the SPME fiber (stainless steel wire, 3 cm length × 0.15 mm diameter).

carboxylic acid (H3BTC) was purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Benzene, toluene, chlorobenzene, ethylbenzene, trimethylbenzene, and p-xylene were purchased from the Beijing Chemical Reagent No. 1 Plant (Beijing, China). Styrene was purchased from the Tianjin Chemical Reagent No. 1 Plant (Tianjin, China). Stock solutions of the analytes (1000 mg L-1) were prepared using acetone as the solvent. All the stock solutions were stored at 4 °C in darkness. Working solutions were prepared by step-by-step dilution with acetone just before use. A gas mixture of the benzene homologues was prepared by direct vaporization of 5 µL of a working solution in a sealed vial for 5 min at 50 °C just before use. Caution! Take care to avoid direct contact with benzene homologues and hydrofluoric acid because of their high toxicity, and prepare all the solutions in a well-ventilated hood. Fabrication of the SPME Fiber. Stainless steel wires with a length of 6.0 cm were used to fabricate the SPME fibers. Half (3.0 cm in length) of the stainless steel wire was etched by hydrofluoric acid44 to generate a rough surface with a diameter of 0.15 mm, washed gently with ultrapure water, and dried in air (part 7 of Figure 1). For in situ hydrothermal growth of the MOF199 films onto the hydrofluoric acid-etched part of the stainless steel wire, 0.437 g of Cu(NO3)2 · 3H2O and 0.640 g of H3BTC were dissolved in 6 mL of ultrapure water and 18 mL of ethanol, respectively. The two solutions were mixed in a 35 mL Teflon liner. The hydrofluoric acid-etched stainless steel wire was carefully immersed into the solution in the Teflon liner. Then the Teflon liner was sealed and placed into the steel autoclave at 120 °C for 8 h. After cooling to room temperature, the MOF199-deposited fiber was taken out of the Teflon liner with great care, washed gently with ultrapure water and ethanol sequentially, and dried at 120 °C for 30 min. Thus, a new MOF-199coated fiber with a length of 3.0 cm was obtained. The thickness of the coated MOF-199 films was measured to be

about 40 µm by SEM, and the amount of the coated MOF-199 films was estimated to be about 2 mg by weighing the scraped films from the prepared SPME fibers (average data from five SPME fibers prepared in parallel). Assembly of the SPME Device. The sketch view of the vertical section of the developed SPME device is shown in Figure 1. A 5 µL GC microsyringe (Shanghai Gaoge Industrial and Trade Co. Ltd., Shanghai, China) was used to prepare the SPME device. The hand grip (part 1 of Figure 1) was connected to the push-pull rod of the microsyringe (stainless steel tube, 100 mm length × 1.1 mm o.d. × 0.8 mm i.d.) (part 2 of Figure 1) and the stainless steel capillary (150 mm length × 0.30 mm o.d. × 0.10 mm i.d.) (part 3 of Figure 1) with a piece of adhesive. The main body of the microsyringe is a calibrated glass tube (part 4 of Figure 1). The needle of the microsyringe (part 6 of Figure 1) is a stainless tube with a bevel (50 mm length × 0.6 mm o.d. × 0.4 mm i.d.) which ensures the passage of the MOF-199-coated fiber. The MOF-199-coated SPME fiber (part 7 of Figure 1) was firmly fixed by inserting the uncoated part (3 cm length × 0.09 mm diameter) of the stainless steel wire (part 5 of Figure 1) into the stainless steel capillary (part 3 of Figure 1), which was assembled into a 5 µL GC microsyringe by replacing the plunger of the microsyringe. The SPME fiber was detachable from the stainless steel capillary (part 3 of Figure 1) when necessary. Before use, the fiber was conditioned in the GC injector at 280 °C for 30 min under nitrogen. Collection of Indoor Air Samples. Before collection, 120 mL glass vials were evacuated down to 1.33 kPa and sealed. The indoor air samples were collected from three locations (a furniture market, a newly decorated bedroom, and an office) by opening and placing the pre-evacuated vials for 8 h to equilibrate. All collections were performed at room temperature with a relative humidity of ∼20%. The chromatographic areas of the analytes with an external standard calibration were used for quantification. Analytical Procedures. A 5.0 µL aliquot of standard solution was introduced to a 120 mL gastight sealed glass vial and then gasified and homogenized in the vial at 50 °C for 5 min. To evaluate the effect of the relative humidity, 0-4.0 µL of ultrapure water was further introduced into the sample vial which was filled with the standard, the resulting solution was well mixed, and the vapor headspace was allowed to re-equilibrate at 50 °C for 15 min. The needle of the SPME device was passed through the septum, and the MOF-199-coated wire used as the SPME fiber was pushed to be exposed to the standard vapor or sample gas. After extraction for 20 min, the needle was removed from the vial and immediately transferred to the GC injection port for thermal desorption at 275 °C for 1 min and subsequent analysis. Between two extractions, the fiber was conditioned at 275 °C for 5 min.

RESULTS AND DISCUSSION Characterization of the MOF-199-Coated Fibers. In the present work, the MOF-199 films were in situ grown on the surface of the stainless steel fiber hydrothermally, as described in the Experimental Section. The powder scraped from the fiber was confirmed as MOF-199 by comparing the XRD patterns of Analytical Chemistry, Vol. 81, No. 23, December 1, 2009

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Figure 2. (A) XRD patterns of simulated MOF-199 diffraction patterns and the MOF-199 scraped from the prepared fibers. (B) TG curve of the MOF-199 scraped from the prepared fibers.

the powder and the simulated diffraction patterns (Figure 2A). The well-tallied patterns indicate that MOF-199 with high quality was successfully coated on the fiber.17,49 The thermogravimetric (TG) analysis shows that the MOF199 is stable at temperatures below 289 °C (Figure 2B). This facecentered-cubic structure contains three types of pores, of which two larger square-shaped pores (12 Å in diameter) are reported to penetrate the basic structure in all three dimensions and are connected with pore windows about 8.0 Å in diameter.50,51 Such a porous structure provides a potential approach to selective extraction of analytes based on size selectivity. Meanwhile, MOF199 possesses a Lewis acid coordination site on the interior of the pore walls, which greatly enhances the extraction ability based on electron selectivity due to the electronic charge transfer from the electron-rich analytes to the Lewis acid sites and makes MOF199 more selective for more electron rich analytes.21,22 The SEM images for the surface of the MOF-199-coated fiber are shown in Figure 3. Typical octahedrally shaped crystals of MOF-199 uniformly precipitated on the fiber with relatively uniform size and structure. Under our synthesis conditions, the thickness of the coating is about 40 µm with a mass of about 2 mg (average data of five fibers prepared in parallel). The MOF199 scraped from the fibers was found to have an amazingly high surface area of 1458 m2 g-1, ensuring the excellent adsorption ability and analyte capacity of the fibers. (49) Schlichte, K.; Kratzke, T.; Kaskel, S. Microporous Mesoporous Mater. 2004, 73, 81–88. (50) Yang, L.; Naruke, H.; Yamase, T. Inorg. Chem. Commun. 2003, 6, 1020– 1024. (51) Schlichte, K.; Kratzke, T.; Kaskel, S. Microporous Mesoporous Mater. 2004, 73, 81–88.

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Figure 3. Scanning electron micrographs of the MOF-199-coated fiber at a magnification of (A) 100× and (B) 500×.

Figure 4. Effect of the extraction time on the extracted amounts of the benzene homologues (sample volume, 120 mL; extraction temperature, 40 °C; desorption time, 1.0 min; desorption temperature, 275 °C; concentration of the analytes, 3.6 µg L-1 for benzene, toluene, ethylbenzene, p-xylene, styrene, and trimethylbenzene and 4.6 µg L-1 for chlorobenzene). The error bar shows the standard deviation of triplicate extractions.

Effects of the Experimental Conditions on the SPME of Benzene Homologues. Potential factors affecting the extraction efficiency of the SPME of benzene homologues with the MOF199-coated fiber were optimized. These factors include the extraction time and temperature and desorption time and temperature. The effect of the extraction time on the extraction efficiency was examined from 5 to 30 min (Figure 4). The adsorption of benzene reached equilibrium within 3 min, while the extracted amounts of the other analytes remarkably increased with the

Figure 5. Effect of the extraction temperature on the extracted amounts of the benzene homologues (sample volume, 120 mL; extraction time, 20 min; desorption time, 1.0 min; desorption temperature, 275 °C; concentration of the analytes, 3.6 µg L-1 for benzene, toluene, ethylbenzene, p-xylene, styrene, and trimethylbenzene and 4.6 µg L-1 for chlorobenzene). The error bar shows the standard deviation for triplicate extractions.

extraction time in the first 10 min and then quickly leveled off. These results indicate the fast adsorption kinetics of the analytes on MOF-199.21 Benzene exhibited the fastest kinetics for the adsorption on MOF-199 due to its smallest molecular size among the tested analytes. To ensure high efficiency and reproducibility for the studied compounds, a 20 min extraction was selected for further experiments. Studies on the effect of the extraction temperature from 20 to 95 °C revealed that the extracted amounts of the analytes increased as the extraction temperature increased from 20 to 40 °C and then decreased with a further increase of the extraction temperature (Figure 5). Therefore, 40 °C was the most efficient extraction temperature and was chosen in the further experiments. The desorption time and temperature are both important parameters in the desorption process. The extracted amounts of the analytes gradually increased as the desorption temperature increased from 220 to 275 °C and then remained unchanged with a further increase of the desorption temperature to 280 °C. Since a further increase in the injection temperature would not only damage the injector, but also shorten the lifetime of the fiber, we did not examine the effect of desorption temperatures higher than 280 °C. In further experiments, the desorption temperature was set at 275 °C. A proper desorption time can lead to achievement of complete desorption of the analytes from the MOF-199-coated fiber and ensure accurate quantitative analysis. We increased the desorption time from 0.05 to 2.0 min and found that the extracted amounts of the analytes reached the maximum at 0.3 min and then remained stable with a further increase of the desorption time to 2.0 min. To eliminate the carryover effect, we chose 1.0 min as the desorption time for further experiments. Effect of the Humidity. Previous studies have demonstrated that the open metal sites of MOF-199 can be easily occupied by H2O in air.17 Thus, the existence of water vapor may lead to fiber inactivation due to its competition with the analytes for the open metal sites. Therefore, in this work we tested the effect of the relative humidity on the SPME (Figure 6). No significant decrease in the extracted amounts of the analytes except benzene was observed at a relative humidity of less than 20%. However, the extracted amounts of the analytes more rapidly

Figure 6. Effect of the relative humidity at 40 °C on the extracted amounts of the analytes from gaseous standards under optimal conditions (concentration of the analytes, 3.6 µg L-1 for benzene, toluene, ethylbenzene, p-xylene, styrene, and trimethylbenzene and 4.6 µg L-1 for chlorobenzene). The error bar shows the standard deviation for triplicate extractions.

decreased as the relative humidity increased to more than 30% due to the competitive occupation of the open metal sites of MOF199 by H2O molecules.17 Analytical Figures of Merit. Table 1 summarizes the analytical figures of merit for the developed SPME method. The enhancement factor (EF) was defined as the ratio of the sensitivity of an analyte after extraction to that before extraction (i.e., by direct injection of 1 µL of standard solution) using the chromatographic peak area for quantification. The MOF-199-coated fiber not only offered large EFs from 19 613 (benzene) to 110 860 (pxylene), but also exhibited wide linearity with 3 orders of magnitude for the tested benzene homologues. The limits of detection (LODs) for the tested benzene homologues ranged from 8.3 (toluene) to 23.3 (styrene) ng L-1, while the limits of quantification (LOQs) ranged from 22.1 (toluene) to 62.1 (styrene) ng L-1. The relative standard deviation (RSD) for six replicate extractions of a standard gas mixture of the tested benzene homologues at 5.0 µg L-1 ranged from 2.0% to 7.7%. The fiber-to-fiber reproducibility (RSD) obtained with three fibers fabricated under the same conditions was less than 9.4%. Comparison of the Developed MOF-199-Coated Fiber with Other SPME Fibers. The developed MOF-199-coated fiber was compared with commercial PDMS (30 µm) and PDMS/DVB (65 µm) fibers and the hydrofluoric acid-etched stainless steel fiber44 for the SPME of the benzene homologues under the same conditions. The results show that the commercial PDMS fiber and homemade hydrofluoric acid-etched stainless steel fiber gave no obvious chromatographic peaks of the benzene homologues under the conditions given in Figure 7. The PDMS/DVB fiber worked better than the PDMS fiber but much worse than the MOF-199-coated fiber for the SPME of the benzene homologues (Figure 7). The excellent performance of the MOF-199-coated fiber likely resulted from the combined effects of the large surface area and unique porous structure of the MOF-199, the π-π interactions of the aromatic rings of the analytes with the framework BTC molecules, and the π-complexation of the electron-rich analytes to the Lewis acid sites in the pores of MOF199. (52) Go´recki, T.; Yu, X. M.; Pawliszyn, J. Analyst 1999, 124, 643–649. (53) Pawliszyn, J. Anal. Chem. 2003, 75, 2543–2558.

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Table 1. Characteristic Data of the Developed SPME-GC-FID Method for the Determination of Benzene Homologues under the Optimized Conditions

analyte benzene toluene chlorobenzene ethylbenzene p-xylene styrene trimethylbenzene

Kes

fiber-to-fiber LOD LOQ precision reproducibility linearity linear range (ng L-1, S/N ) 3) (ng L-1, S/N ) 8) (RSD, n ) 6) (%) (RSD, n ) 3) (%) (r2) (ng L-1)

EF

1.95 × 104 19 613 7.23 × 104 50 753 8.81 × 104 56 233 8.83 × 104 56 301 1.21 × 106 110 860 1.79 × 105 77 031 1.26 × 105 67 040

14.5 8.3 19.7 18.9 12.7 23.3 11.9

38.7 22.1 52.5 50.4 33.9 62.1 31.7

7.7 4.9 5.6 2.6 2.0 3.7 3.6

8.8 3.5 5.9 9.4 7.7 9.2 8.2

0.9956 0.9986 0.9995 0.9969 0.9979 0.9997 0.9975

72-18000 72-18000 92-23000 72-18000 36-18000 72-18000 72-18000

The fiber-sample distribution constant (Kes) is defined as52,53

Kes ) Se/Cs

(1)

where Cs is the concentration of the analyte in the matrix and Se is the solid extraction phase surface concentration of the adsorbed analyte. To facilitate the calculation, we suppose that the concentration of the analyte is uniform in every part of the fiber at equilibrium. The determined values of Kes for the analytes ranged from 1.95 × 104 to 1.21 × 106.

Table 2. Analytical Results for the Determination of Benzene Homologues in Indoor Air Samples sample 1 analyte benzene toluene chlorobenzene ethylbenzene p-xylene styrene trimethylbenzene

-1

concn (ng L ) 151 ± 13 647 ± 32