Hydrofluoric Acid Etched Stainless Steel Wire for Solid-Phase

Anal. Chem. 2009, 81, 4971–4977

Hydrofluoric Acid Etched Stainless Steel Wire for Solid-Phase Microextraction Hua-Ling Xu, Yan Li, Dong-Qing Jiang, and Xiu-Ping Yan* Research Center for Analytical Sciences, College of Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, China Stainless steel wire has been widely used as the substrate of solid-phase microextraction (SPME) fibers to overcome the shortcomings of conventional silica fibers such as fragility, by many researchers. However, in previous reports various sorbent coatings are always required in conjunction with the stainless steel wire for SPME. In this work, we report the bare stainless steel wire for SPME without the need for any additional coatings taking advantage of its high mechanical and thermal stability. To evaluate the performance of stainless steel wire for SPME, polycyclic aromatic hydrocarbons (PAHs), benzene, toluene, ethylbenzene, chlorobenzene, n-propylbenzene, aniline, phenol, n-hexane, n-octane, n-decane, n-undecane, n-dodecane, chloroform, trichloroethylene, n-octanol, and butanol were tested as analytes. Although the stainless steel wire had almost no extraction capability toward the tested analytes before etching, it did exhibit high affinity to the tested PAHs after etching with hydrofluoric acid. The etched stainless steel wire gave a much bigger enhancement factor (2541-3981) for the PAHs than the other analytes studied (e515). Etching with hydrofluoric acid produced a porous and flower-like structure with Fe2O3, FeF3, Cr2O3, and CrF2 on the surface of the stainless steel wire, giving high affinity to the PAHs due to cation-π interaction. On the basis of the high selectivity of the etched stainless steel wire for PAHs, a new SPME method was developed for gas chromatography with flame ionization detection to determine PAHs with the detection limits of 0.24-0.63 µg L-1. The precision for six replicate extractions using one SPME fiber ranged from 2.9% to 5.3%. The fiberto-fiber reproducibility for three parallel prepared fibers was 4.3-8.8%. One etched stainless steel wire can stand over 250 cycles of SPME without significant loss of extraction efficiency. The developed etched stainless steel wire is very stable, highly selective, and reproducible for the SPME of PAHs. Since the technique was introduced in the early 1990s by Arthur and Pawliszyn,1 solid-phase microextraction (SPME) has attracted increasing attention due to its advantages of simplicity of operation, solventless nature, analyte/matrix separation, and * Corresponding author. Fax: (86)22-23506075. E-mail: [email protected]. (1) Arthur, C. L.; Pawliszyn, J. Anal. Chem. 1990, 62, 2145–2148. 10.1021/ac900743s CCC: $40.75  2009 American Chemical Society Published on Web 05/15/2009

preconcentration.2,3 The technique is based on the establishment of an equilibrium between the analyte and a fiber coated with a stationary phase. The analyte is then desorbed from the fiber to a suitable separation and detection system.2,3 The sorbent coated on the fiber, used to adsorb the analytes from samples, is a key part for the extraction ability of SPME. Widely used commercial coating materials for SPME include poly(dimethylsiloxane) (PDMS),4,5,8 polyacrylate (PA),6-8 poly(dimethylsiloxane)/divinylbenzene (PDMS/DVB),8,9 poly(ethylene glycol)/divinylbenzene (PEG/DVB),10 carboxen/poly(dimethylsiloxane) (CAR/ PDMS),11 carbowax/divinylbenzene (CW/DVB),12 and polyacrylonitrile (PAN).13,14 With different polymer coatings, SPME has been successfully applied for many organic compounds including the groups of benzene, toluene, ethylbenzene, and xylenes (BTEX),15-18 polycyclic aromatic hydrocarbons (PAHs),19-21 pesticides,22 and

(2) Dietz, C.; Sanz, J.; Ca´mara, C. J. Chromatogr., A 2006, 1103, 183–192. (3) Jiang, G. B.; Huang, M. J.; Cai, Y. Q.; Lv, J. X.; Zhao, Z. S. J. Chromatogr. Sci. 2006, 44, 324–332. (4) Koziel, J. A.; Odziemkowski, M.; Pawliszyn, J. Anal. Chem. 2001, 73, 47– 54. (5) Bruheim, I.; Liu, X. C.; Pawliszyn, J. Anal. Chem. 2003, 75, 1002–1010. (6) Pino, V.; Ayala, J. H.; Gonza´lez, V.; Afonso, A. M. Anal. Chem. 2004, 76, 4572–4578. (7) Zuazagoitia, D.; Millań, E.; Garcia, R. Chromatographia 2007, 66, 773– 777. (8) Martin, D.; Ruiz, J. Talanta 2007, 71, 751–757. (9) Miller, M. E.; Stuart, J. D. Anal. Chem. 1999, 71, 23–27. (10) Flamini, R.; Dalla Vedova, A.; Panighel, A.; Perchiazzi, N.; Ongarato, S. J. Mass Spectrom. 2005, 40, 1558–1564. (11) Polo, M.; Llompart, M.; Garcia-Jares, C.; Gomez-Noya, G.; Bollain, M. H.; Cela, R. J. Chromatogr., A 2006, 1124, 11–21. (12) Bagheri, H.; Babanezhad, E.; Es-Haghi, A. J. Chromatogr., A 2007, 1152, 168–174. (13) Musteata, M. L.; Musteata, F. M.; Pawliszyn, J. Anal. Chem. 2007, 79, 6903–6911. (14) Yang, J.; Tsui, C.-P. Anal. Chim. Acta 2001, 442, 267–275. (15) Gaujac, A.; Emı´dio, E. S.; Navickiene, S.; Ferreira, S. L. C.; Do´rea, H. S. J. Chromatogr., A 2008, 1203, 99–104. (16) Legind, C. N.; Karlson, U.; Burken, J. G.; Reichenberg, F.; Mayer, P. Anal. Chem. 2007, 79, 2869–2876. (17) Wittkamp, B. L.; Hawthorne, S. B. Anal. Chem. 1997, 69, 1197–1203. (18) Budziak, D.; Martendal, E.; Carasek, E. J. Chromatogr., A 2008, 1187, 34– 39. (19) Hou, J.-G.; Ma, Q.; Du, X.-Z.; Deng, H.-L.; Gao, J.-Z. Talanta 2004, 62, 241–246. (20) Fernańdez-Gonza´lez, V.; Concha-Gran ã, E.; Muniategui-Lorenzo, S.; Lo´pezMahıá, P.; Prada-Rodrı´guez, D. J. Chromatogr., A 2008, 1196-1197, 65– 72. (21) Wauters, E.; Van Caeter, P.; Desmet, G.; David, F.; Devos, C.; Sandra, P. J. Chromatogr., A 2008, 1190, 286–293. (22) Basheer, C.; Narasimhan, K.; Yin, M. H.; Zhao, C. Q.; Choolani, M.; Lee, H. K. J. Chromatogr., A 2008, 1186, 358–364.

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drugs in biological samples.23,24 However, commercial fibers are expensive and commercial silica-based fibers also show drawbacks such as breakage of the fibers, thermal instability (usually 240-280 °C), bending of the needle, and the stripping of coatings. As conventional silica fibers are fragile and require the users to take great care to use them, other materials have been explored to replace silica rods as firm SPME substrates, such as stainless steel wire,13,19,25-28 platinum wire,12,29-31 anodized aluminum wire,32 gold wire,33,34 copper wire,35 titanium wire,36,44 and NiTi alloy.18,45-49 The coatings are always immobilized on the substrates through physical and chemical forces. But many of these coatings are thermally unstable. To overcome this problem, sol-gel coating technology has been applied to enhance thermal stability and to remove the problem of the stripping of coatings from the substrate.37 Furthermore, electrochemical techniques have been used to produce firm coatings on metal wires for SPME.12,29,31,32,35 Aluminum wires were anodized by direct current in sulfuric acid at room temperature to obtain aluminum oxide on aluminum wire for the extraction of some aliphatic alcohols, BTEX, and petroleum products from gaseous samples with a relative humidity of less than 25%.32 The polyaniline coating was electroplated on a stainless steel wire for SPME of amines in water.26 Polyaniline12,29 or dodecyl sulfate-doped polypyrrole30 film was electrodeposited on platinum wire in sulfuric acid solution using cyclic voltammetry technique for the extraction of phenols29 and PAHs12,30 from water samples. Copper(I) chloride was electrolytically produced on copper wire as an excellent sorbent for some amines from gaseous samples.35 PAHs are a widely distributed group of organic pollutants and are known to be strongly mutagenic and/or carcinogenic, especially for the PAHs containing four or more aromatic rings.38 Recently, some PAHs are suspected to be endocrine disrupters, and much attention has been focused on their possible biological (23) Kumazawa, T.; Lee, X.-P.; Sato, K.; Suzuki, O. Anal. Chim. Acta 2003, 492, 49–67. (24) Kataoka, H. Trends Anal.Chem. 2003, 22, 232–244. (25) Liu, Y.; Shen, Y. F.; Lee, M. L. Anal. Chem. 1997, 69, 190–195. (26) Huang, M. J.; Tai, C.; Zhou, Q. F.; Jiang, G. B. J. Chromatogr., A 2004, 1048, 257–262. (27) Zeng, J. B.; Chen, J. M.; Wang, Y. R.; Chen, W. F.; Chen, X.; Wang, X. R. J. Chromatogr., A 2008, 1208, 34–41. (28) Panavaite˙, D.; Padarauskas, A.; Vicˇkacˇkaite˙, V. Anal. Chim. Acta 2006, 571, 45–50. (29) Bagheri, H.; Mir, A.; Babanezhad, E. Anal. Chim. Acta 2005, 532, 89–95. (30) Mohammadi, A.; Yamini, Y.; Alizadeh, N. J. Chromatogr., A 2005, 1063, 1–8. (31) Wu, J. C.; Mullett, W. M.; Pawliszyn, J. Anal. Chem. 2002, 74, 4855–4859. (32) Djozan, D.; Assadi, Y.; Haddadi, S. H. Anal. Chem. 2001, 73, 4054–4058. (33) Djozan, D.; Bahar, S. Chromatographia 2004, 59, 95–99. (34) Djozan, D.; Bahar, S. Chromatographia 2003, 58, 637–642. (35) Farajzadeh, M. A.; Rahmani, N. A. Talanta 2005, 65, 700–704. (36) Azenha, M. A.; Nogueira, P. J.; Silva, A. F. Anal. Chem. 2006, 78, 2071– 2074. (37) Chong, S. L.; Wang, D. X.; Hayes, J. D.; Wilhite, B. W.; Malik, A. Anal. Chem. 1997, 69, 3889–3898. (38) Mersch-Sundermann, V.; Mochayedi, S.; Kevekordes, S.; Kern, S.; Wintermann, F. Anticancer Res. 1993, 13, 2037–2044. (39) Tran, D. Q.; Ide, C. F.; McLachlan, J. A.; Arnold, S. F. Biochem. Biophys. Res. Commun. 1996, 229, 102–108. (40) King, A. J.; Readman, J. W.; Zhou, J. L. Anal. Chim. Acta 2004, 523, 259– 267. (41) Doong, R.-A.; Chang, S.-M.; Sun, Y.-C. J. Chromatogr., A 2000, 879, 177– 188. (42) Pawliszyn, J. Anal. Chem. 2003, 75, 2543–2558. (43) Meharg, A. A.; Wright, J.; Dyke, H.; Osborn, D. Environ. Pollut. 1998, 99, 29–36.

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effects on human health.39 As the concentrations of PAHs are very low in the environment (undetectable to several hundreds of nanograms per liter in surface waters),50 a proper method is desired to preconcentrate them prior to detection. Different fiber coatings have been applied for SPME of PAHs. PDMS- and PAcoated fibers were applied for the simultaneous separation and determination of 16 PAHs with gas chromatography/mass spectrometry (GC/MS).40,41 Aniline-based coating on platinum wire was employed for SPME of PAHs with GC/MS from water samples.12 Herein, we report a simple and rapid method to prepare an etched stainless steel wire fiber for SPME with high stability, durability, and repeatability. The performance of the prepared fiber was demonstrated by extracting PAHs. The etched stainless steel wire fiber was characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), energy-dispersive spectrometry (EDS), and the adsorption mechanism was discussed. The etched stainless steel wire fiber offered good selectivity for the SPME of PAHs. On the basis of the excellent affinity of the prepared fiber to PAHs, a novel SPME method was developed for preconcentration and separation of PAHs in spiked river water and wastewater samples before gas chromatography with flame ionization detection (GC-FID). EXPERIMENTAL SECTION Materials and Chemicals. Stainless steel wire (304, 300 µm) was obtained from Shanghai Gaoge Industrial and Trade Co., Ltd. (Shanghai, China). Hydrofluoric acid (40.0%) and NaCl were from Tianjin Standard Science and Technology Co., Ltd. (Tianjin, China). Acenaphthene (96%), fluorene (97%), phenanthrene (98%), fluoranthene (98%), and pyrene (98%) were purchased from Tianchang Chemical Co., Ltd. (Anshan, Liaoning, China). Chloroform (>99.0%) was purchased from Tianjin Chemical Reagent No. 3 Plant (Tianjin, China). Butanol (>98.0%), n-decane, and chlorobenzene (>99.0%) were purchased from Tianjin Chemical Reagent No. 2 Plant (Tianjin, China). Phenol (>99.0%) was obtained from Tianjin Chemical Reagent No. 1 Plant (Tianjin, China). Benzene (g99.5%), n-hexane (>99.0%), n-octane (g99.5%), biphenyl (>99.0%), n-undecane (g99.5%), and n-dodecane (g99.5%) were from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). n-Octanol was purchased from Tianjin Bodi Chemical Co., Ltd. (Tianjin, China). Toluene (g99.5%) was obtained from Tianjin Standard Science and Technology Co., Ltd. (Tianjin, China). Doubly deionized water (DDW, 18.2 MΩ cm) was obtained from a WaterPro water purification system (Labconco Corporation, Kansas City, MO). All chemicals used were of analytical grade and used as received. Stock solutions of naphthalene (99%, The second Chemical Factory of Tianjin, Tianjin, China), acenaphthene, fluorene, phenanthrene, fluoranthene, and pyrene of 5000 mg L-1 were prepared using ethanol as solvent. All the stock solutions were stored at 4 °C in darkness. A mixture of these PAHs was prepared by diluting (44) Cao, D. D.; Lu, J. X.; Liu, J. F.; Jiang, G. B. Anal. Chim. Acta 2008, 611, 56–61. (45) Budziak, D.; Martendal, E.; Carasek, E. J. Chromatogr., A 2007, 1164, 18– 24. (46) Budziak, D.; Martendal, E.; Carasek, E. J. Chromatogr., A 2008, 11981199, 54–58. (47) Budziak, D.; Martendal, E.; Carasek, E. Microchim. Acta 2009, 164, 197– 202.

the stock solutions with ethanol, and working standard solutions were prepared by diluting the standard solutions with DDW just before use. Note: take care to avoid direct contact of all the analytes studied, especially for PAHs, benzene, toluene, ethylbenzene, chlorobenzene, n-propylbenzene, trichloroethylene, and chloroform, and prepare all the solutions in a well-ventilated hood because of their high toxicity. Instrumentation. A Shimadzu GC-9A system equipped with an FID was employed for all experiments. The GC capillary column (SE-54, 30 m long × 0.53 mm i.d. × 1.0 µm) was purchased from Lanzhou Institute of Chemical Physics (Lanzhou, China). The column temperature was maintained at 180 °C for 1.5 min and then programmed at 4 °C min-1 to the last temperature of 220 °C for 5 min. The injector and detector temperatures were both set at 320 °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 SPME holder was purchased from Shanghai Gaoge Industrial and Trade Co., Ltd. (Shanghai, China). A model 85-1 stir plate (Jintan Instruments Co. Ltd., Jintan, Jiangsu) and a Teflon-coated stir bar (9.9 mm × 5.9 mm × 5 mm) were used for agitation. The 20 and 10 mL glass vials with the same inner diameter fitted with crimped aluminum caps lined with PTFE septa were obtained from Agilent Technologies (Wilmington, DE). The SEM micrographs of the stainless steel wire before and after etching were recorded on a Shimadzu SS-550 scanning electron microscope at 15.0 kV. The chemical composition of the surfaces of the etched stainless steel wire was investigated using XPS (XPSKratos Axis Ultra DLD spectrometer), employing a monochromatic Al KR X-ray source (hν ) 1486.6 eV), hybrid (magnetic/ electrostatic) optics, and a multichannel plate and delay line detector (DLD). All XPS spectra were recorded using an aperture slot of 300 µm × 700 µm; survey spectra were recorded with a pass energy of 160 eV, and high-resolution spectra with a pass energy of 40 eV. Commercial PDMS (30 µm) and PDMS/DVB (65 µm) coated silica SPME fibers (Supelco, Bellefonte, PA) were also used for comparative purpose. The fibers were conditioned as recommended by the manufacturer before use. Preparation of SPME Fiber. A stainless steel wire was immersed in hydrofluoric acid for 15 min at 40 °C. The surface of the wire gradually turned black while small gas bubbles were released during etching. The etched part of the stainless steel wire was taken out of hydrofluoric acid and washed gently by DDW. Then the etched stainless steel wire was assembled to a 5 µL microsyringe (Shanghai Gaoge Industrial and Trade Co., Ltd. Shanghai, China) and conditioned at 300 °C for 4 h under nitrogen in the GC injector before use. Samples. Local river water and wastewater samples were collected for determination of PAHs. The samples were centrifuged at 2000 rpm for 2 min, and the supernatants were collected in precleaned glass bottles, which were thoroughly washed with detergents, water, methanol, and DDW and dried before use, and analyzed immediately after sampling. Determination of Enhancement Factors. Enhancement factor (EF) was defined as the ratio of the chromatographic peak

area response per microgram per liter of the analyte with the etched stainless steel wire for the SPME of 10 mL of standard solution containing 10 µg L-1 of each individual PAH and 50 µg L-1 of the other individual analytes to that with direct injection of 1 µL of standard solution containing 10 mg L-1 of each individual analyte. The EF values were determined for two SPME modes. In mode 1, the stainless steel wire was immersed into 10 mL of sample solution in a 20 mL vial closed with an airtight cap with 10 mL of headspace, whereas in mode 2 the stainless steel wire as immersed into 10 mL of sample solution in a 10 mL vial closed with an airtight cap without headspace. Analytical Procedures. To carry out the extraction, 10 mL of aqueous standard solution (2.5-50 µg L-1 of each individual PAH) or sample solution was placed in a 20 mL glass vial. The vial was immediately closed airtight with a Teflon-lined cap after introducing the magnetic stir bar. The needle of the SPME device passed through the septum, and the etched stainless steel wire used as the SPME fiber was pushed to immerse it into the standard or sample solution. After the extraction under stirring at the rate of 800 rpm for 30 min, the needle was removed from the vial and immediately transferred to the GC injection port for thermal desorption at 320 °C for 4 min and subsequent analysis. Between two extractions, the fiber was conditioned at 320 °C for 10 min. The internal standard calibration method was employed for quantification with biphenyl (10 µg L-1) as the internal standard. RESULTS AND DISCUSSION Consideration of Etched Stainless Steel Wire for SPME of PAHs. It is well-known that SPME fiber mainly consists of two parts: the sorbent and the substrate. The sorbents of SPME, through which analytes are enriched, are usually porous polymer materials. Stainless steel wire has been used for the substrate of SPME to overcome the shortcomings of conventional silica fibers, such as fragility, by many researchers.13,19,25–28 The stainless steel wire was always coated by various sorbents for SPME, such as biocompatible sorbents,13 mesoporous C16-MCM-41,19 methacrylic acid-trimethylolpropanetrimethacrylate copolymers,27 hightemperature silicone glue,28 and porous bonded silica particles.25 In this work, we used the bare stainless steel wire for SPME without the need for any additional coatings taking advantage of its high mechanical and thermal stability. It was observed that the stainless steel wire had almost no extraction capability toward the tested PAHs before etching but exhibited high affinity to the tested PAHs after etching with hydrofluoric acid (Figure 1). To find out the reasons for the high affinity of the etched stainless steel wire to PAHs, the surface morphology and chemical composition of the stainless steel wire before and after etching were analyzed. Comparison of the SEM images of the stainless steel wire before and after etching (Figure 2, parts a and b vs parts c and d) shows that the surface of the stainless steel wire was smooth before etching but became rough and porous with a fine flower-like structure after etching. Such rough and porous flower-like structure of the etched stainless steel wire should significantly increase the surface area of the fiber and ensure the sample capacity of the fiber.37 XPS experiments revealed that the etching process produced new compounds (mainly Fe2O3, FeF3, Cr2O3, and CrF2) on the surface of the stainless steel wire Analytical Chemistry, Vol. 81, No. 12, June 15, 2009

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Figure 1. Chromatograms of PAHs using stainless steel wire before etching (a) and after etching (b) as SPME fiber. Concentration of each PAH, 20 µg L-1; NaCl added, 10% w/v; extraction time, 30 min; agitation speed, 800 rpm; injection temperature, 320 °C.

Figure 3. XPS spectra of Fe 2p and Cr 2p for the stainless steel wire before etching (a) and after etching (b).

Figure 2. Scanning electron micrographs of the surface of the stainless steel wire before (a and b) and after (c and d) etching. The images a and c are at a magnification of 200; images b and d are at a magnification of 10 000.

(Figure 3). EDS analysis showed that the weight percentages of Fe, Cr, and Ni on the surface of the untreated stainless steel wire were 69.3%, 19.6%, and 7.8%, respectively. The weight percentages of Fe and Cr on the surface of the etched stainless steel wire decreased to 43.2% and 17.0%, respectively, whereas that of O (oxygen) increased from 2.4% to 25.4% due to the formation of their oxides. Etching with hydrofluoric acid produced a porous and flower-like structure with Fe2O3, FeF3, Cr2O3, and CrF2 on the surface of stainless steel wire, giving high affinity to the PAHs. Optimization of SPME Conditions for the Determination of PAHs. Owing to the high affinity of the etched stainless steel wire to PAHs, potential factors affecting the SPME of PAHs, such as extraction time, concentration of NaCl, agitation speed, desorption temperature, and desorption time, were optimized for GCFID determination. Extraction time is a key factor affecting extraction efficiency. Generally, extraction efficiency increases with the increase of extraction time before equilibrium. Figure 4 shows the plots of the peak area data (PA) as a function of extraction time (t) 4974


Figure 4. Effect of extraction time on extraction efficiency. Concentration of each analyte (except naphthalene, 10 µg L-1), 20 µg L-1; other conditions as in Figure 1.

according to PA(t) ) a(1 - exp(bt)); thus, the time to reach 95% of steady state for naphthalene, acenaphthene, fluorene, phenanthrene, fluoranthene, and pyrene was calculated to be 31, 36, 46, 50, 52, and 52 min, respectively. We can see that the increasing order for the time to reach 95% of steady state for the PAHs (naphthalene < acenaphthene < fluorene < phenanthrene < fluoranthene ) pyrene) is in good agreement with the decreasing order of the diffusion coefficients of the PAHs in water (naphthalene > acenaphthene > fluorene > phenanthrene > pyrene > fluoranthene). The above results indicate that the diffusion of the analytes from water to the SPME fiber is probably the limiting

factor for the adsorption. As SPME is a nonexhaustive approach, it can be designed on the basis of principles of equilibrium, preequilibrium, and permeation.42 If the achieved analytical sensitivity is sufficient for quantitative analysis, it is not necessary to reach equilibrium. Meanwhile, considering the flux of the samples, 30 min was selected as the extraction time for the subsequent experiments. Addition of salt into aqueous solution, usually NaCl, always decreases the solubility of nonpolar organic compounds in water, which not only increases the distribution coefficient of solutes to the SPME fiber but also enhances the transfer of analytes from the water phase to air phase, especially for more volatile compounds. The former effect is favorable, whereas the latter effect is unfavorable for the direct SPME in this work. Also, addition of the salt increases the viscosity of the solution, and decreases the diffusion rate of the analytes, which is unfavorable to the transfer of the analytes into the SPME fiber in direct SPME.19 To evaluate the effect of NaCl concentration on the present direct SPME, extraction was performed with 10 mL of sample solution containing various concentrations of NaCl (0%, 5%, 10%, 15%, and 20%). The results show that the peak area of acenaphthene and fluorene increased with NaCl concentration from 0% to 20% w/v, whereas the peak area of the other PAHs increased with NaCl concentration only to 5% w/v, then decreased with further increase of NaCl concentration up to 20% w/v due likely to increased viscosity of the solution and decreased diffusion rate of the analytes and enhanced mass transfer of analytes from the water phase to air phase. Therefore, 10% w/v NaCl was used as a compromise in further extraction to enhance the sensitivity of the most analytes. In principle, extraction efficiency of analytes can also be improved with increasing agitation speed because it can accelerate mass transfer of the analytes between the aqueous sample and the SPME fiber. Also, agitation increases the mass transfer of the analytes from water to air, which is unfavorable to the direct SPME mode used in the present work, especially for more volatile compounds. In addition, too fast agitation may impair the reproducibility. Study on the effect of agitation speed on extraction efficiency shows a positive effect on the peak area of fluorene, phenanthrene, fluoranthene, and pyrene but no significant effect on that of the other analytes when agitation speed was less than 800 rpm (Figure 5). Further increase of agitation speed led to a slight decrease in that of fluorene, acenaphthene, and naphthalene due likely to the increased mass transfer rate of the analyte from aqueous phase to air. Extraction efficiency was higher at 800 rpm for all analytes. So an 800 rpm setting was adopted. Desorption temperature and time must be sufficient to release all the analytes. Three temperatures, i.e., 280, 300, and 320 °C, were examined to rate the effect of desorption temperature. The results show that a temperature of 320 °C was sufficient for complete desorption of all these six PAHs in 4 min. (48) Budziak, D.; Martendal, E.; Carasek, E. Anal. Chim. Acta 2008, 629, 92– 97. (49) Budziak, D.; Martendal, E.; Carasek, E. Anal. Chim. Acta 2007, 598, 254– 260. (50) Manoli, E.; Samara, C. Trends Anal. Chem. 1999, 18, 417–428.

Figure 5. Effect of agitation speed on extraction efficiency of PAHs. Other conditions as in Figure 1. Table 1. Enhancement Factors (EFs) for the Analytes Obtained with the Etched Stainless Steel Wire for the SPMEa EF (mean ± σ, n ) 3) analyte

mode 1b

mode 2c

naphthalene acenaphthene fluorene phenanthrene fluoranthene pyrene benzene toluene ethylbenzene chlorobenzene n-propylbenzene n-hexane n-octane n-decane n-undecane n-dodecane chloroform trichloroethylene n-octanol phenol butanol aniline

2714 ± 113 2853 ± 102 3532 ± 267 3981 ± 118 2820 ± 95 2541 ± 80 26 ± 1 290 ± 18 515 ± 23 489 ± 27 474 ± 32 95 ± 4 260 ± 14 414 ± 4 398 ± 23 261 ± 23 25 ± 1 153 ± 17 220 ± 3 not extracted not extracted not extracted

2906 ± 150 2888 ± 68 3677 ± 60 3930 ± 41 2653 ± 22 2560 ± 99 29 ± 1 322 ± 12 573 ± 7 578 ± 6 554 ± 24 132 ± 5 238 ± 4 440 ± 2 457 ± 11 287 ± 21 31 ± 0 175 ± 1 251 ± 20 not extracted not extracted not extracted

log KAWd log KOWe -1.76 -2.01 -2.46 -2.79 -3.43 -3.32 -0.65 -0.57 -0.49 -0.85 -0.55 1.84 2.08 2.45 2.87 2.48 -0.81 -0.49 -2.99 -4.87 -3.47 -4.08

3.29 3.98 4.18 4.45 4.9 4.88 1.97 2.84 3.15 2.96 3.69 4.05 5.18 6.69 6.8 6.54 2.01 2.42 3.15 1.63 0.88 0.92

a Conditions: extraction time, 30 min; added NaCl, 10% w/v; agitation speed, 800 rpm. b The etched stainless steel wire was immersed into 10 mL of sample solution in a 20 mL vial closed with an airtight cap for direct SPME with 10 mL of headspace. c The etched stainless steel wire was immersed into 10 mL of sample solution in a 10 mL vial closed with an airtight cap for direct SPME without headspace. d Air-to-water partitioning coefficient. Data for fluoranthene, n-octanol, phenol, and butanol were obtained from R. Sander: Henry’s law constants (http:// www.mpch-mainz.mpg.de/∼sander/res/henry.html). The other data were obtained from ref 51. e The octanol-water partitioning coefficients (KOW) were from ref 43 and a databank at http://logkow.cisti.nrc.ca/ logkow/search.html.

Selectivity of the Etched Stainless Steel Wire for SPME. To evaluate the selectivity of the etched stainless steel wire for SPME, different types of organic compounds were selected as analytes. These analytes include PAHs, toluene, ethylbenzene, chlorobenzene, n-propylbenzene, n-hexane, n-octane, n-decane, n-undecane, n-dodecane, chloroform, trichloroethylene, n-octanol, aniline, phenol, and butanol. The EF values obtained for two SPME modes (with and without headspace) as described in the ExperiAnalytical Chemistry, Vol. 81, No. 12, June 15, 2009

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Figure 6. Comparison of enhancement factors (EFs) for the PAHs obtained by the etched stainless steel fiber to commercial PDMS (30 µm) and PDMS/DVB (65 µm) SPME fibers.

mental Section are compared in Table 1. The EF values for direct immersion of the SPME fiber without headspace are presented for two purposes. One is to examine whether the air-to-water partitioning coefficient (KAW) of the analyte controls the EF value for the SPME with headspace. The other is to use EFs as the primary estimation of selectivity. For the SPME with 10 mL of headspace (mode 1), the etched stainless steel wire gave much bigger EFs (2541-3981) for PAHs than the other analytes studied (e515). To evaluate whether the air-to-water partitioning coefficient (KAW) of the analyte controls the EF value for the SPME with 10 mL of headspace (mode 1), the SPME experiments were also carried out without headspace (mode 2) for comparison. For mode 2, the KAW of the analyte should not affect the EF value of the analyte due to no headspace air. In comparison with mode 1 where volatile analytes have a chance to be lost in headspace air, mode 2 is expected to give bigger EFs, especially for the analytes with larger KAW. The results in Table 1 show without headspace only about 10% increase of EFs for most analytes, even for highly volatile compounds, n-hexane, n-octane, n-decane, n-undecane, and n-dodecane. Meanwhile, even without headspace the etched stainless steel wire also gave much bigger EF values for PAHs than the other analytes studied regardless of their KAW. These results suggest that KAW is not a dominating factor for the determined EFs for the SPME with 10 mL of headspace, even for highly volatile analytes studied such as n-hexane, n-octane, n-decane, n-undecane, and n-dodecane. Thus, the EF values for the SPME with 10 mL of headspace were suggested to be controlled mainly by the affinity of the etched stainless steel

wire to the analytes. The etched stainless steel wire gave much higher affinity to PAHs than the other analytes studied, resulting in high EF values for PAHs. The affinity of the etched stainless steel wire to the analytes depends not only on the surface characteristic of the etched stainless steel wire but the nature of the analytes as well. The hydrophobicity of the analytes was evaluated as one of the potential factors. The hydrophobicity (log KOW) of the analytes is also shown in Table 1. Hydrophilic phenol, butanol, and aniline cannot be extracted with the etched stainless steel wire (Table 1). Except n-octane, n-decane, n-undecane, n-dodecane, and n-hexane, the EFs positively correlate with the hydrophobicity of the analytes to a certain degree. The presence of a delocalized π system in the target molecules plays an important role in the selectivity of the etched stainless steel wire for SPME. For example, the hydrophobicities of n-octane, n-decane, n-undecane, and n-dodecane are higher than those of the PAHs (fluoranthene, pyrene, phenanthrene, fluorene, acenaphthene), but the PAHs exhibit much bigger EFs. Although the hydrophobicity of n-octanol and ethylbenzene is similar, ethylbenzene shows much bigger EF than n-octanol. Even the hydrophobicity of n-decane is much higher than that of ethylbenzene, their EFs are similar. The above results show that the target molecules with a delocalized π system have higher affinity to the etched stainless steel fiber. Recent molecular modeling and spectroscopic studies have suggested that relatively strong interactions can occur between aromatic π donors and metal cations in aqueous solutions and/or on mineral surfaces, i.e., cation-π interactions.52-56 A charge-induced dipole-dipole interaction of the electron-deficient iron with the electron-rich aromatic ring structure of the PAHs was proposed by Mader et al. as the reason for greater affinity of PAHs to iron oxide surfaces.56 Also, a cation-π interaction was supposed by Mu¨ller et al. for the crucial noncovalent binding for the specific sorption of pyrene to goethite-coated quartz.55 In this work, we also attribute the high affinity of the PAHs to the etched stainless steel fiber to the cation-π interaction between the PAHs and the metal oxides on the surface of the etched stainless steel fiber. From the above discussion, the selectivity of the etched stainless steel wire depends not only on the hydrophobicity of the analytes but the presence of delocalized π-bonds in the analytes as well. The big EFs for the PAHs results from the presence of the delocalized π system and high hydrophobicity of the PAHs, through which high physicochemical affinity was produced between the PAHs and the porous structure and metal compounds on the surface of the etched stainless steel wire. Comparison of the Etched Stainless Steel Wire Fiber with Commercial SPME Fibers. The etched stainless steel wire fiber was compared with two commercial SPME fibers (30 µm PDMS

Table 2. Analytical Figures of Merit for SPME Using Etched Stainless Steel Wire for GC-FID Determination of PAHs analyte

DLs/µg L-1

linear range/µg L-1

linearity/r2

precision for one fiber (RSD, n ) 6)/%

fiber-to-fiber reproducibility (RSD, n ) 3)/%

naphthalene acenaphthene fluorene phenanthrene fluoranthene pyrene

0.24 0.26 0.30 0.42 0.58 0.63

2.5-20 2.5-30 2.5-50 2.5-50 2.5-50 2.5-50

0.9893 0.9935 0.9943 0.9939 0.9988 0.9983

3.7 2.9 2.9 4.4 5.3 5.1

4.3 8.8 7.8 8.0 4.5 7.3

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Table 3. Analytical Results (µg L-1, Mean ( s, n ) 3) for the Determination of PAHs in Water Samples river water 1 analyte naphthalene acenaphthene fluorene phenanthrene fluoranthene pyrene a

no spiking a

nd nd nd nd nd nd

river water 2

wastewater

spiked with 10 µg L-1

no spiking


no spiking


9.5 ± 0.0 9.3 ± 0.4 9.7 ± 0.3 9.1 ± 0.3 8.8 ± 0.4 8.8 ± 0.3

nd nd nd nd nd nd

9.9 ± 0.1 10.2 ± 0.3 9.7 ± 0.2 10.0 ± 0.6 9.0 ± 0.3 8.5 ± 0.3

nd nd nd nd nd nd

9.7 ± 0.3 9.9 ± 0.2 9.9 ± 0.3 10.3 ± 0.3 9.3 ± 0.5 9.0 ± 0.6

Not detected.

and 65 µm PDMS/DVB) in respect to the EF values for the PAHs studied. The SPME experiments followed the procedures for mode 1 described in Experimental Section to determine the EF values. We selected the two commercial SPME fibers because the six PAHs are nonpolar and semivolatile compounds. PDMS is a nonpolar coating and suitable for analyzing PAHs. PDMS/DVB is a middlepolar coating but has affinity for benzenoid compounds. The results in Figure 6 show that the etched stainless steel fiber gave much bigger EF values for PAHs than the two commercial fibers. Analytical Figures of Merit. The analytical figures of merit for the SPME using etched stainless steel wire for GC-FID determination of PAHs following the analytical procedures described in Experimental Section are summarized in Table 2. The detection limits (DLs) (S/N ) 3) of the developed method were in the range of 0.24-0.63 µg L-1. The chromatographic peak area was a linear function of the concentration of naphthalene from 2.5 to 20 µg L-1, acenaphthene from 2.5 to 30 µg L-1, and others from 2.5 to 50 µg L-1. The EFs for PAHs ranged from 2541 to 3981. The relative standard deviation (RSD) for six replicate extractions of the PAHs at 10 µg L-1 using one SPME fiber was 2.9-5.3%. The fiber-to-fiber reproducibility for three parallel prepared fibers for SPME of the PAHs at 10 µg L-1 was 4.3-8.8% (RSD). The durability experiment for one fiber shows no significant change in the extraction efficiency over 250 cycles of SPME. These results show that the developed stainless steel wire is very stable and reproducible for SPME. Application to Water Samples. In recent years, PAHs contamination in water systems has drawn increasing attention as the PAHs in surface water result mainly from atmospheric deposition, runoff from contaminated soils, and deposition from sewage discharges.57 The developed method using the etched stainless steel wire as the SPME fiber was applied to locally collected river water and wastewater samples for the determination of naphthalene, acenaphthene, fluorene, phenanthrene, fluoranthene, and pyrene by GC-FID. The internal standard calibration method with biphenyl as the internal standard was employed for quantification. The analytical results are given in Table 3. No naphthalene, acenaphthene, fluorene, (51) Mackay, D.; Shiu, W. Y. J. Phys. Chem. Ref. Data 1981, 10, 1175–1199. (52) Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303–1324. (53) Zhu, D. Q.; Herbert, B. E.; Schlautman, M. A.; Carraway, E. R. J. Environ. Qual. 2004, 33, 276–284. (54) Zhu, D. Q.; Herbert, B. E.; Schlautman, M. A.; Carraway, E. R.; Hur, J. J. Environ. Qual. 2004, 33, 1322–1330. (55) Mu ¨ ller, S.; Totsche, K. U.; Ko ¨gel-Knabner, I. Eur. J. Soil Sci. 2007, 58, 918–931. (56) Mader, B. T.; Goss, K. U.; Eisenreich, S. J. Environ. Sci. Technol. 1997, 31, 1079–1086. (57) Zeledoń-Torun õ, Z. C.; Lao-Luque, C.; de las Heras, F. X. C.; Sole-Sardans, M. Chemosphere 2007, 67, 505–512.

Figure 7. Chromatograms of (a) river water 1 with spiking of 10 µg L-1 of biphenyl; (b) river water 1 with spiking of 10 µg L-1 of (1) naphthalene, (2) biphenyl, (3) acenaphthene, (4) fluorene, (5) phenanthrene, (6) fluoranthene, and (7) pyrene; (c) a mixture of standard solution containing 10 µg L-1 of (1) naphthalene, (2) biphenyl, (3) acenaphthene, (4) fluorene, (5) phenanthrene, (6) fluoranthene, and (7) pyrene. Extraction time, 30 min; agitation speed, 800 rpm; injection temperature, 320 °C. Note: Biphenyl used as an internal standard.

phenanthrene, fluoranthene, and pyrene were detected in the water samples studied. The recoveries of the PAHs at 10 µg L-1 spiked in these water samples ranged from 85% to 103%. Chromatograms of a river water sample with and without spiking PAHs and biphenyl are shown in Figure 7. CONCLUSIONS We have demonstrated the preparation and characterization of etched stainless steel wire for SPME. The main advantages of the developed etched stainless steel wire fiber include good selectivity for PAHs, simple preparation, cost-effectiveness, hightemperature resistance, firmness, good reproducibility, and longterm stability. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 20775037, 20705014), the National Basic Research Program of China (2006CB705703), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20070055055), the Fok Ying Tong Education Foundation (No. 114041), and the Tianjin Natural Science Foundation (No. 08JCYBJC00600). Received for review December 29, 2008. Accepted April 30, 2009. AC900743S Analytical Chemistry, Vol. 81, No. 12, June 15, 2009

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Hydrofluoric Acid Etched Stainless Steel Wire for Solid-Phase

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