Nano-structured lead dioxide as a novel stationary phase for solid-phase microextraction

Journal of Chromatography A, 1134 (2006) 24–31

Nano-structured lead dioxide as a novel stationary phase for solid-phase microextraction Ali Mehdinia a , Mir Fazllolah Mousavi a,∗ , Mojtaba Shamsipur b a

Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran b Department of Chemistry, Razi University, Kermanshah, Iran

Received 27 June 2006; received in revised form 24 August 2006; accepted 29 August 2006 Available online 25 September 2006

Abstract The first study on the high efficiency of nano-structured lead dioxide as a new fiber for solid-phase microextraction (SPME) purposes has been reported. The size of the PbO2 particles was in the range of 34–136 nm. Lead dioxide-based fibers were prepared via electrochemical deposition on a platinum wire. The extraction properties of the fiber to benzene, toluene, ethylbenzene, and xylenes (BTEX) were examined using headspace solid-phase microextraction (HS-SPME) mode coupled to gas chromatography-flame ionization detection (GC-FID). The results obtained proved the suitability of proposed fibers for the sampling of organic compounds from water. The extraction procedure was optimized by selecting the appropriate extraction parameters, including preparation conditions of coating, salt concentration, time and temperature of adsorption and desorption and stirring rate. The calibration graphs were linear in a concentration range of 0.1–100 ␮g l−1 (R2 > 0.994) with detection limits below 0.012 ␮g l−1 level. Single fiber repeatability and fiber-to-fiber reproducibility were less than 10.0 and 12.5%, respectively. The PbO2 coating was proved to be very stable at relatively high temperatures (up to 300 ◦ C) with a high extraction capacity and long lifespan (more than 50 times). Higher chemical resistance and lower cost are among the advantages of PbO2 fibers over commercially available SPME fibers. Good recoveries (81–108%) were obtained when environmental samples were analyzed. © 2006 Elsevier B.V. All rights reserved. Keywords: Nano-structured lead dioxide; BTEX; Headspace-solid phase microextraction; HS-SPME

1. Introduction The greatest advances in solid-phase microextraction (SPME) have taken place over the past decade. The technique has been validated for numerous applications and has shown surprising versatility for a wide variety of analytical problems in the analysis of pharmaceutical, food, and natural products [1,2]. A common belief is that the development and further extension of SPME application greatly depend on new breakthroughs in coating technology [2]. SPME is predominantly performed on commercially available SPME fibers coated with different sorbents having various polarities. The recommended operating temperatures for these fibers are generally within the range of 200–270 ◦ C, which is not suitable enough in many cases. Moreover, these fibers are often prepared by physical deposition of

∗

Corresponding author. Tel.: +98 21 88011001x3474; fax: +98 21 88006544. E-mail address: [email protected] (M.F. Mousavi).

0021-9673/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2006.08.087

a polymer coating on the surface of fused silica fibers, which is the most likely reason for their low thermal and chemical stability [3]. Meantime, silica fibers are expensive, fragile and therefore must be handled with great care. Hence, it is highly desired to have more robust SPME fibers with long lifetimes and relatively low cost. In order to overcome these problems, a large number of home-made fibers were reported in the literature; for example, the efficiency of uncoated fibers, like pencil lead [4–6] and glassy carbon [7,8], have been investigated. Moreover, several modified metal wires, including anodized aluminum [9], copper coated with copper(I) chloride [10] and copper sulfide [11] and anodized zinc [12] have been developed and used as SPME fibers. These metallic fibers were found to be selective for polar and semi-polar organic compounds and showed improved features such as low cost, durability, sensitivity and a vast range of applications. Recently, the use of niobium(V) oxide coated on glass-ceramic rods [13] and ␥-Al2 O3 coated on silica fiber [14] in SPME has been reported in the literatures. A great deal of the past research efforts towards robust SPME

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fibers employed sol–gel technology, which provides efficient incorporation of organic components into inorganic polymeric structures in solution under extraordinarily mild thermal conditions [15]. Recently sol–gel/titanium wire fibers were used for the analysis of BTEX [16]. There are two distinct types of SPME coatings available commercially. Poly(dimethylsiloxane) (PDMS) and poly(acrylate) (PA), which are the most widely used, extract analytes via absorption. The remaining coatings, PDMS-DVB (divinylbenzene), Carbowax-DVB, Carbowax-TR (template resin) and Carboxen, extract analytes via adsorption. Extraction of analytes by many new porous coatings is based on adsorption rather than absorption [17]. Meanwhile, the materials used as working electrodes in electrochemistry are potential candidates for adsorption of organic and inorganic compounds. For instance, glassy carbon has been used as a stationary phase for HPLC [18], SFC [19,20] and SPME [4] because of its mechanical and chemical stability, as well as its high surface area. Lead dioxide coated onto inert substrate has become popular electrode due to its good electrical conductivity, favorable overpotential for oxygen evolution and high chemical inertness [21]. It possesses wide electrochemical and industrial applications due to its distinctive properties such as low cost, good conductivity, high stability in acid media and adsorption properties [22]. A variety of other applications including its use as a positive active material in lead acid batteries [23], waste water treatment [24] and being entrapped into polymeric materials in solid-phase reactors as an oxidative agent [25] have been reported in the literature. Due to its suitable adsorption properties, lead dioxide has been employed for the adsorption of several metals including bismuth [26], monovalent and bivalent metal nitrates [27] and several organic compounds [28,29]. In this work, we examined the application of nano-structured PbO2 as a SPME fiber coating, because of its high adsorption capacity. A group of compounds including benzene, toluene, ethylbenzene, o-, m- and p-xylenes (BTEX) was used to study the suitability of the new coating for SPME purposes. It should be noted that these compounds have already been studied and well characterized as PDMS fibers [30]. The results revealed the great potential of nano-structured of PbO2 for HS-SPMEGC analysis. 2. Experimental 2.1. Reagents and solutions Analytical grade benzene, toluene, ethylbenzene, xylenes (o, m- and p-) and methyl-isobutyl ketone (MIBK) were obtained from Aldrich. Single standard solutions (1000 mg l−1 ) of BTEX compounds were prepared by spiking of each compound in methanol (HPLC grade, Merck) and were stored at 4 ◦ C and used within 4 weeks. Working aqueous solutions, prepared just before use, were made from the stock methanolic solutions by spiking appropriate amounts and dilution to the mark with deionized water in 25-ml volumetric flasks. The appropriate volume of aqueous samples was then extracted using the HSSPME technique in 20-ml vials. For the preparation of fibers,

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Pb(NO3 )2 (Sigma–Aldrich, 99.99%), HClO4 (35%, Merck), NaF (Merck), Nafion 117 (5% in a mixture of lower aliphatic alcohols and water, Fluka) and CH3 OH (HPLC grade, Merck) were employed. 2.2. Apparatus GC analysis was performed on a Hewlett-Packard 6890N GC (Agilent Technologies, DE, USA) equipped with FID and split/splitless injector. Chromatographic separations were carried out using a HP-5 fused silica column (30 m length, 0.32 mm I.D. and 0.25 ␮m film thickness; HP, DE, USA). The oven temperature was set at 45 ◦ C for 3 min, before temperature programming to 220 ◦ C at 10 ◦ C min−1 and then holds for 5 min at 220 ◦ C. The FID temperature was 300 ◦ C and the injection port was held at 250 ◦ C. Nitrogen (99.999%) at a flow rate of 1.3 ml min−1 was used as the carrier gas. Under these GC conditions, the three xylenes isomers were separated into two peaks (one for the mixture of m-, p-xylene and another for o-xylene). Thus, in quantification, we integrated the area of m-, p-xylene peaks together and calculated the whole concentration of these two xylenes. The samples were injected in splitless mode. Deionized water was prepared using a Milli-Q (Millipore, Bedford, MA, USA) purification system. A RH basic hot plate-stirrer (IKA-Werke, Staufen, Germany) was used to heat and stir the sample during the extraction procedure. PTFE coated stir bars of 8 mm (O.D. 3 mm) were put in the 20 ml vials just before runs. Surface characteristic studies of the prepared lead dioxide fibers were performed by scanning electron microscopy using a Philips XL30 instrument. A SPME fiber holder was purchased from Azar Electrode Co. (Urmia, Iran). A potentiostat & galvanostat Autolab system with PGSTAT30 and GPES 4.9 software (Eco Chemie, Utrecht, The Netherlands), was used for the electrochemical preparation of PbO2 . 2.3. SPME fiber preparation Prior to the electrodeposition, the Pt wires were subjected to acetone cleaning in an ultrasonic bath to remove the organic contaminants on the surface before rising in ultrapure water. A platinum plate was used as a counter electrode and an Ag|AgCl|KCl(sat) was employed as the reference electrode. PbO2 was prepared according to a procedure reported elsewhere with some modification [31]. An aqueous solution of 0.1 M Pb(NO3 )2 /0.2 M HClO4 /0.01 M NaF containing 30 v/v% methanol and 1 v/v% of 5% nafion were prepared for depositing PbO2 on Pt substrate (250 ␮m I.D., 2 cm long section of platinum wire was covered by the PbO2 film). A constant current method was used to anodically deposit PbO2 . Constant currents ranged from 20 ␮A cm−2 to 10 mA cm−2 were applied and the best condition was chosen based on morphology studies and extraction efficiency. At lower currents (less than 20 ␮A cm−2 ) the production of fiber was disturbed and longer time required for its production. On the other hand, electroplating at higher currents (higher than 10 mA cm−2 ) produced fibers, which were not stable enough mechanically. All exper-

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iments were performed at room temperature (25 ± 1 ◦ C). The PbO2 film formed on the surface was subsequently washed with deionized water, then with methanol or acetone for 3 min and dried under nitrogen protection at room temperature. Finally, it was conditioned at 250 ◦ C for 30 min in the GC injector before use. A commercial SPME device assembly (Azar Electrode, Urmia, Iran) was used as a SPME device. In this assembly a platinum wire (250 ␮m O.D.) coated with PbO2 was mounted inside the septum-piercing needle of the syringe and attached to the plunger. 2.4. SPME sampling conditions Deionized water was pipetted into a 20-ml vial containing a magnetic stirrer bar. After addition of an appropriate volume of stock solution, the vial was sealed with a silicon-rubber septum and an aluminum cap. The fiber was exposed to the headspace of solution by piercing the septum with the SPME needle assembly and then depressing the plunger. After extraction, the fiber was withdrawn into the needle and removed from sample vial. The analytes were then thermally desorbed in the GC injection port at 250 ◦ C for 3 min. The identification of compounds was made by comparison of the retention time of GC peaks with those of the standard compounds. 3. Results and discussion 3.1. Optimum fiber preparation The morphology of deposited PbO2 strongly depends on the experimental conditions used. PbO2 can exist in different phases; two distinct phases are ␣- and ␤-phases. It is well known that the crystal structure of PbO2 deposits depends on the pH of the deposition solution: ␣-PbO2 is obtained from bases, ␤-PbO2 from acids [32]. It is also known that ␤-PbO2 has a porous structure but ␣-PbO2 has a compact structure [33]. Therefore, the porous structure of ␤-PbO2 provides an enhanced surface area for SPME. Consequently, acid medium was chosen for the deposition of PbO2 . Based on our preliminary experiments, PbO2 produced from Pb(NO3 )2 solution, without any additive, has neither proper adhesion on Pt substrate no enough porous structure; hence, some additives were used to improve its mechanical stability and structural porosity. It has been shown that fluoride ions can increase the PbO2 deposition rate and significantly improved its adhesion to the substrate [34]. Since the deposition of PbO2 accompanied by oxygen evolution, the addition of alcohol and nafion solution may increase the overpotential for oxygen evolution and subsequently influence the surface morphology of the deposited PbO2 and its adhesion on the substrate [31]. Moreover, the addition of nafion found to increase the amount of coating the PbO2 fibers, which is necessary for enhance extraction efficiency of the SPME fiber. Four types of PbO2 fibers were prepared at four different current densities from solutions of the same composition and used for the extraction of BTEX compound, and the results for toluene as a model compound were compared in Fig. 1. As seen,

Fig. 1. The comparison of extraction efficiency between the PbO2 fibers coated at various current density. Deionized water (7 ml) spiked with 10 ␮g l−1 toluene, 5% (w/v) NaCl, HS-SPME at 80 ◦ C for 40 min.

the extraction efficiency on the fiber prepared at 10 mA cm−2 current density was higher than that of the other types of fibers. These results can be explained according to the SEM graphs shown in Fig. 2. Fig. 2 shows the longitudinal SEM image of PbO2 -coated Pt wire at 10 mA cm−2 (a) and SEM photographs of PbO2 deposited onto Pt substrate by applying constant currents of 20 ␮A cm−2 for 60 min (b), 200 ␮A cm−2 for 30 min (c), 2 mA cm−2 for 15 min (d) and 10 mA cm−2 for 10 min (e, f). It should be noted that, as expected, longer times were needed at lower current densities. The estimated thicknesses of all prepared coatings were between 70 and 75 ␮m; hence, the differences among extraction efficiencies cannot be related to this parameter. The PbO2 deposited at low current densities gave uniform globular grains (Fig. 2b and c). The deposit was flat and dense. At higher current densities, the morphology of deposited PbO2 was characterized by rice-shaped surfaces as shown in Fig. 2d. Further increase the current density shows two different structures on Pt substrate, flower-like and cornflower-like structures with highly porous surfaces (Fig. 2e and f). Fig. 2g and h shows flower-like and cornflower-like structures at higher magnification. Nanostructures of PbO2 with the diameter in rang of 34–136 nm can be observed in these figures. PbO2 with nanostructures greatly increases its effective area, which is a favorable feature in view of a possible application as extracting phase of SPME. Such a porous nanostructure should significantly increase the available surface area of the fiber, and therefore, increase its extraction capacity. The density of nucleation is extremely sensitive to the current density [35]. At low current densities, mass transport process is fast compared with electron transfer and the deposit structure can be well formed. While at higher current densities, concentration polarization is severe and adatoms no longer reach their equilibrium position on the electrode surface. In addition, at the increased overpotential, nucleation of additional growth centers remains a more frequent event, so that the deposit formed will be less ordered and outward growth of the layer becomes of increasing importance. On the other hand, the deposition of PbO2 is at the potential where oxygen evolving occurred. At low current densities, most of the current is used for the formation of PbO2 and the deposit will grow three-dimensionally and uniformly. At increased current densities, the charge used for oxygen evolving increases, which in turn, results in an uneven surface. Since

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Fig. 2. (a) Longitudinal SEM image of PbO2 -coated Pt wire at 10 mA cm−2 and SEM photographs of PbO2 deposited onto Pt substrate by applying constant current densities of (b) 20 ␮A cm−2 for 60 min, (c) 200 ␮A cm−2 for 30 min, (d) 2 mA cm−2 for 15 min, (e, f) 10 mA cm−2 for 10 min, (g, h) higher magnification of PbO2 nanostructure produced at 10 mA cm−2 for 10 min.

PbO2 is a highly conducting oxide, the potential will decrease substantially once nucleation has occurred; subsequently, oxygen evolution may become easier on the PbO2 surface, resulting in a rice-like structure. With further increase in current density, oxygen evolution dominates the electrode process. Here, the isolated nuclei can hardly overlay to form a uniform layer and the growth becomes favorable in the vertical direction [31].

3.2. The lifetime of coating According to the literature data [36], the thermogravimetric analysis (TGA) of PbO2 exhibited a significant weight loss of about 3.5% at about 350 ◦ C, which is associated with the release of water. On the other hand, differential scanning calorimetry (DSC) of the electrochemically prepared ␤-PbO2 is reported to give a small endothermic peak at 350 ◦ C, which reflects the

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of benzene on the extraction of MIBK was less pronounced as compared to the effect of MIBK on the extraction of benzene. This can be explained taking into account that MIBK revealed higher affinity to the fibers examined than benzene did. As a result, MIBK effect on extraction benzene was more significant. 3.4. Optimization of SPME procedure Experimental conditions, like adsorption and desorption times and temperatures, sample volume, ionic strength and stirring rate were optimized before validating the analytical method. To find the best adsorption time, the fiber was exposed to the headspace of the spiked samples containing 10 ␮g l−1 of BTEX for 5, 10, 20, 30, 40 and 60 min. The profile for ethylbenzene is shown in Fig. 4A and other profiles were the same for subsequent compounds. As can be seen in Fig. 4A, the responses of the target compound increased with increasing extraction time until 40 min and then remained more or less constant. Since at a 40 min time the responses of BTEX compounds reach more than 90% of maximum peak area, it was selected as the extraction time. The optimization of desorption time results in achieving complete desorption of the adsorbed analyte, improved sensitivity, and avoid carry-over effects. After extraction of spiked aqueous samples for 30 min, the fiber was subjected to desorption at 250 ◦ C in the GC injection port for 1, 3, 5 and 10 min. The peak areas obtained for each desorption time of ethylbenzene are compared in Fig. 4A. As is obvious, a 3 min desorption time yielded the highest peak areas, and, thus, it was selected as optimum desorption time for further experiments. To test for carry-over effects of the system, samples containing large amounts of analytes (10 mg l−1 ) were extracted for 60 min, and the fiber was subjected to the desorption process in the hot GC injector for 3 min. After the chromatographic run, a second desorption of the fiber was performed at the same temperature to examine the possible remaining analytes, that could contaminate further analyses. Fortunately, no peaks were observed after the second desorption process, further supporting the use of a 3 min desorption time for the SPME procedure. In order to study the effect of temperature on the extraction process, the procedure was followed at varying temperatures in the range 25–90 ◦ C for 40 min. It should be noted that a continuous stirring at 500 rpm was applied to make sure that the extracted solutions are perfectly agitated and to reach a faster equilibration. After the adsorption process, the analytes were thermally desorbed into the injection port of a gas chromatograph at 250 ◦ C for 3 min. Fig. 4B illustrates that an increase in

Fig. 3. The desorption baseline of SPME under 350 ◦ C (A), 300 ◦ C (B) and column blank at the same condition (C). Chromatographic conditions: column 80 ◦ C at a ramp of 15 ◦ C min−1 to 290 ◦ C, hold for 15 min.

dehydration of PbO(OH)2 or H2 PbO3 , and a second wider peak at 450 ◦ C, which results from the decomposition of the PbO2 to non-stoichiometric PbOn compounds (1 < n < 2) [37]. The experimental results shown in Fig. 3 clearly revealed that the coating phase of the fiber can withstand desorption temperature up to 300 ◦ C, which indicates that the SPME with this coating has the potential to analyze volatile and most of semivolatile compounds. Fig. 3 shows that the fiber coating has no bleeding up to 300 ◦ C. However, when it was subjected to 350 ◦ C, some extraneous peaks were observed. This is probably due to the release of some PbO2 particles from the substrate. The fiber can be used over 50 times without any measurable decrease in coating thickness (examined under microscope) and extraction performance. 3.3. Matrix competition effect While absorption is a non-competitive process, adsorption is by definition competitive, and a molecule with higher affinity for the surface can replace a molecule with lower affinity. Thus, the amount of the analyte extracted by the fiber from a sample can be significantly affected by sample matrix composition [17]. To further study the properties of the new porous solid coating, a water sample spiked with two concentrations of MIBK was also studied and the extraction efficiency was investigated using headspace extraction. Table 1 illustrates the effect of interfering compound on the amount of the analyte extracted from sample for the PbO2 fibers. In this experiment, the concentration of the interfering compound was kept constant for both of the analyte concentration levels. It can be observed from this table that the presence of the interfering compound caused a reduction in the amount of the analyte extracted by the fibers. However, the effect

Table 1 The effect of an interfering compound on the amount of analyte extracted by the PbO2 Analyte concentration (␮g l−1 )

Mass of analyte extracted by the fiber (ng) Benzene

5 50

MIBK

No MIBK

1 mg l−1

24 49

21 45

MIBK

Difference (%)

No benzene

1 mg l−1 benzene

Difference (%)

12 8

74 240

67 225

9 6

Extraction of benzene in the presence of MIBK, and extraction of MIBK in the presence of benzene.

A. Mehdinia et al. / J. Chromatogr. A 1134 (2006) 24–31

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obtained at 15% w/v NaCl for the analytes. Thus, this NaCl concentration was selected for subsequent studies. To evaluate the influence of agitation on the extracted amount of BTEX, spiked aqueous samples at 10 ␮g l−1 were analyzed without and with agitation by means of a magnetic stirrer at 500, 750, 1000 and 1250 rpm during the adsorption process. The results shown in Fig. 4C revealed that agitation enhanced the extraction process, so that it reaches a maximum at a stirring rate of 1000 rpm. 3.5. Analytical performance characteristics For the linearity study eleven concentration solutions including: 0.05 (0.06 for toluene), 0.08, 0.1, 0.5, 1.0, 5.0, 10, 50, 100, 500, 1000 ␮g l−1 , were evaluated and seven of them were in the linearity range of 0.1–100 ␮g l−1 . Each solution was submitted to the HS-SPME analysis for five times. The linear regression values, limit of detections (LODs) and relative standard deviations (RSDs) for benzene, toluene, ethylbenzene and xylenes were evaluated (Table 2). Table 2 shows that the linear regression with correlation coefficients greater than 0.994 are obtained for the analytes. The linear dynamic range (LDR) was 0.1–100 ␮g l−1 for all compounds. The RSD values for the analytes (repeatability for one fiber) ranged between 4.7 and 10.0%, while fiber-to-fiber RSD ranged between 9.1 and 12.5%. The LODs for benzene, toluene, ethylbenzene, (m,p)-xylene and o-xylene were: 0.027, 0.054, 0.012, 0.032 and 0.031 ␮g l−1 , respectively, at a signal-to-noise ratio (S/N) of three. A comparison between the LODs obtained in this study with those reported by using a PDMS fiber [30], revealed that although the linearity range of the proposed method is narrower than that based on PDMS fiber (0.1–100 versus 0.8–2000 ␮g l−1 ), its detection limits for all BTEX compounds is improved significantly. 3.6. Application to real samples Fig. 4. Effect of adsorption and desorption times (A), adsorption and desorption temperatures (B) and sample volume and stirring rate (C) on the extraction efficiency of ethylbenzene by the PbO2 SPME fiber.

extraction efficiency was observed when temperature increased from 25 to 80 ◦ C for ethylbenzene. When the extraction temperature exceeded 80 ◦ C, a significant decrease in sensitivity was also observed for the analyte. Thus, a working temperature of 80 ◦ C was selected for further experiments. The desorption temperature was varied from 200 to 270 ◦ C at a desorption time of 3 min and the results are shown in Fig. 4B. It is seen that maximum efficiency is obtained at 250 ◦ C. The effect of variations in sample volume, in the range of 3–10 ml was investigated and the results are shown in Fig. 4C. As seen, a 7-ml sample volume results in the highest extraction efficiency. An electrolyte like NaCl is often added to the sample in order to increase the ionic strength and enhance the amount of analyte extracted by the fiber. Experiments were carried out at different salt concentrations, ranging from 0 to 30% w/v (which is the saturated concentration of NaCl in water). The results shown (data not shown) revealed that the highest peak areas are

Three well water samples were collected from various regions of Tehran (Iran), including Baghershahr, Tarasht and Ozgol, and filtered before use. By using the linear regressions shown in Table 2, the proposed method was successfully applied to determine BTEX in well waters and the results obtained from five repeated measurements are shown in Table 3. For all three studied well water samples, the benzene and toluene contents were under the detection limits, but low contents of ethylbenzene and xylenes (4.17–4.33 ␮g l−1 ) were detected in Baghershahr well water. It is because of this fact that this region was in the vicinity of the refinery of Tehran. Good spike recoveries were achieved for all these three samples at a spiking level of 5 ␮g l−1 . These results demonstrate that real sample matrices had little effect on the efficiency of the proposed HS-SPME method, which is therefore suitable for the analysis of trace levels of BTEX from real water samples. Fig. 5A and B show a typical chromatogram obtained for the Baghershahr well water non-spiked sample and spiked with 1 ␮g l−1 BTEX, respectively. The large peak just after 2 min came from the methanol added into the sample solution.

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Table 2 Analytical performance of the method for the analysis of BTEX in aqueous samples Analyte

Benzene Toluene Ethylbenzene m,p-Xylenes o-Xylenes

LDR (␮g l−1 )

R2

0.1–100 0.1–100 0.1–100 0.1–100 0.1–100

LOD (␮g l−1 )

RSD %

0.998 0.994 0.999 0.999 0.994

One fiber

Fiber-to-fiber

PbO2

PDMS [30]

7.3 10.0 7.8 4.7 6.8

11.6 12.5 10.5 9.1 11.3

0.027 0.054 0.012 0.032 0.031

0.6 0.2 0.08 0.08 0.08

Table 3 Contents of BTEX (␮g l−1 , mean ± SD, n = 5) in well waters determined by SPME-GC with home-made PbO2 -coated fiber Analyte

Benzene Toluene Ethylbenzene m,p-Xylene o-Xylene a b

Baghershahr

Tarasht

Found

Recoveryb

n.d.a

92 87 95 103 84

n.d. 0.23 ± 0.02 0.46 ± 0.02 1.19 ± 0.10

(%)

Ozgol

Found

Recovery (%)

Found

Recovery (%)

n.d. n.d. n.d. n.d. n.d.

84 94 97 87 90

n.d. n.d. n.d. n.d. n.d.

85 92 108 81 99

Not detected. Recoveries determined at spiked levels of 5 ␮g l−1 of target compounds.

pounds. It was thermally stable, so could be used for more than 50 times. The fiber exhibited fast equilibrium in the extraction and adsorption steps with the porous structure of PbO2 particles. The fiber was applied to the determination of BTEX in aqueous samples and acceptable results were obtained. The main disadvantages of this new fiber might be that it might not be suitable for the analysis of organolead compounds because of releasing of Pb particles, and its ability for oxidizing of the easily oxidized materials. For instance, PbO2 was used for the analysis some compounds such as levodopa [25], ondansetron [28], codeine [29] as an oxidizing agent. The advantages of the PbO2 fiber include the chemical resistance, low cost and high porous structure. An extension towards using of this new fiber for extraction of electrochemically produced compounds is the long-term goal of the project and, on the other hand, porous PbO2 prepared by electrodeposition might be interesting materials for the oxidative degradation of organic pollutants. Acknowledgements This work has been supported by a grant from the Tarbiat Modares University Research Council and the authors gratefully acknowledge the support of this research by the Iranian National Center for Oceanography. Fig. 5. HS-SPME-GC–FID chromatograms of (A) non-spiked Baghershahr well water and (B) spiked well water with 1 ␮g l−1 of BTEX. Peaks: benzene (1), toluene (2), ethylbenzene (3), m- and p-xylene (4), and o-xylene (5).

4. Conclusions A new nano-structured PbO2 -coated SPME fiber was prepared and investigated for the determination of BTEX com-

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