Determination Of Fuel Dialkyl Ethers And Btex In Water

Journal of Chromatography A, 1033 (2004) 193–203

Determination of fuel dialkyl ethers and BTEX in water using headspace solid-phase microextraction and gas chromatography–flame ionization detection Idoia Arambarri, Maitena Lasa, Rosa Garcia, Esmeralda Millán∗ Departamento de Qu´ımica Aplicada, Facultad de Qu´ımica, Universidad del Pa´ıs Vasco, Apdo. 1072, 20080-San Sebastián, Spain Received 25 July 2003; received in revised form 18 January 2004; accepted 26 January 2004

Abstract A simple procedure for the determination of methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), ethyl butyl ether (EBE), tert-amyl methyl ether (TAME), benzene, toluene, ethylbenzene, and xylenes (BTEX) in water using headspace (HS) solid-phase microextraction (HS-SPME) was developed. The analysis was carried out by gas chromatography (GC) equipped with flame ionization detector (FID) and 100% dimethylpolysiloxane fused capillary column. A 27–4 Plackett–Burman design for screening and a central composite design (CCD) for optimizing the significant variables were applied. Fiber type, extraction temperature, sodium chloride concentration, and headspace volume were the significant variables. A 65 ␮m poly(dimethylsiloxane)-divinylbenzene (PDMS-DVB) SPME fiber, 10 ◦ C, 300 g/l, and 20 ml of headspace (in 40 ml vial) were respectively chosen for the best extraction response. An extraction time of 10 min was enough to extract the ethers and BTEX. The relative standard deviation (R.S.D.) for the procedure varied from 2.6 (benzene) to 8.5% (ethylbenzene). The method detection limits (MDLs) found were from 0.02 (toluene, ethylbenzene, and xylenes) to 1.1 ␮g/l (MTBE). The optimized method was applied to the analysis of the rivers, marinas and fishing harbors surface waters from Gipuzkoa (North Spain). Three sampling were done in 1 year from June 2002 to June 2003. Toluene was the most detected analyte (in 90% of the samples analyzed), with an average concentration of 0.56 ␮g/l. MTBE was the only dialkyl ether detected (in 15% of the samples) showing two high levels over 400 ␮g/l that were related to accidental fuel spill. © 2004 Elsevier B.V. All rights reserved. Keywords: Water analysis; Headspace analysis; Solid-phase microextraction; Methyl tert-butyl ether; Dialkyl ethers; BTEX

1. Introduction Because of the extensive use of fuels, their components are ubiquitous environmental contaminants. Of primary concern are the volatile aromatics, benzene, toluene, ethylbenzene, and the three xylene isomers (BTEX). In addition to these compounds the dialkyl ethers, as oxygenates and octane enhancers, are present in gasoline. Methyl tert-butyl ether (MTBE) is the most common oxygenate added to gasoline. Smaller amounts of other ethers have been used for gasoline oxygenation, including ethyl tert-butyl ether (ETBE), and tert-amyl methyl ether (TAME). European directive 98/70/EC allows concentration of oxygenate ethers to 15% (v/v) and limits the aromatics (42%, v/v) and benzene

∗

Corresponding author. Tel.: +34-943015419; fax: +34-943212236. E-mail address: [email protected] (E. Mill´an).

0021-9673/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2004.01.046

(1%, v/v). Although these ethers concentrations are similar to that used in reformulated gasoline in USA, European fuel blends mostly contain concentrations lower than 5%. An increase of this percentage is expected in 2005 due to more restrictive legislation on the aromatic content of gasoline [1]. The presence of these compounds in both ground- and surface water are related to fuel spills, leaking underground storage tanks, and the release of unburned fuel directly into the atmosphere and surface waters [2–4]. However, non-point sources like vehicle exhausts or evaporation of fuel compounds at gasoline stations may be responsible for low surface and rain water concentrations of MTBE [5]. The occurrence of BTEX in the environment is a great concern because of their toxicity. Among them, benzene is the most dangerous; since is recognized as carcinogenic agent. A maximum level of 1 ␮g/l is fixed by European legislation in drinking water [6], and a maximum contaminant level (MCL) of 5 ␮g/l is established by US Environmental

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Protection Agency (USEPA) [3]. Among dialkyl ethers, MTBE is the most widespread used and studied. It is a persistent water contaminant due to its high water solubility and slow rate of degradation [2]. Although MTBE is a suspected human carcinogen, MCL has not been done for it. However, the USEPA has set an advisory level for taste and odor at 20–40 ␮g/l [3]. Analytical methods used to alkyl ethers and BTEX determination include purge and trap (PT), headspace (HS) or direct aqueous injection (DAI) onto gas chromatography (GC) [3,4,7–10]. Solid-phase microextraction (SPME) has been used as an alternative. This type of extraction does not require solvents and it can be carried out directly from the liquid phase or from headspace over the liquid samples (HS). The HS sampling is more advisable when matrix could affect the determination of target analyte. On the other hand, HS versus immersion extraction often shows an important reduction of extraction time [11]. Most of the SPME applications within the fuel compounds and oxygenates have been devoted to MTBE determination. Some of them used direct immersion coupled with GC and MS detector [12–14], the others used headspace followed by GC–MS [15–17] or GC with flame ionization detector (FID) [18]. Both extraction modes have been used in the determination of BTEX [19]. The simultaneous determination of MTBE, ethyl butyl ether (EBE), and BTEX using headspace solid-phase microextraction (HS-SPME) has been presented using two-dimensional gas chromatography [20]. There are several experimental variables affecting the HS-SPME procedure such as type of fiber, stirring rate, temperature, extraction time, and addition of salt. The study considering one by one the variables to get the best possible conditions have been showed in several works [16,20]. However, this procedure requires a high number of runs and also is time consuming. An experimental design, that could take into account simultaneously several variables, seems the most convenient approach searching for the optimal operational conditions in a reasonable number of runs [21,22]. This kind of methodology has been used for optimization of PT–GC–MS conditions to determine MTBE, tert-butyl alcohol (TBA), and BTEX in water [23]. Also, has been applied to obtain the best conditions of HS-SPME–GC–FID for MTBE determination [18]. Since the environmental implications of fuel oxygenates and BTEX, the determination of both types of analytes is now considered and will be more often required. Screening methods that allow simultaneously monitoring both contaminants would be a helpful way to identify the contaminated samples. This work mainly focused on obtaining a screening method for simultaneous determination of alkyl ethers and BTEX using HS-SPME–GC–FID. The experimental design was applied to know the significant variables and to optimize the HS-SPME extraction of analytes from water. The influence of high amounts of BTEX in adsorption process with mixed coating fibers was also checked. Finally, the

procedure was applied to the analysis of marine and river surface water samples from Gipuzkoa (North Spain).

2. Experimental 2.1. Chemicals and materials Methyl tert-butyl ether (99.8%), ethyl butyl ether (99%), ethyl tert-butyl ether (99%), and tert-amyl methyl ether (97%) were purchased from Aldrich (Sigma-Aldrich Chemie GmbH, Steinheim, Germany). Benzene (>99.5%), ethylbenzene (>99%), o-xylene, m-xylene, p-xylene (>99%) were from Fluka (Sigma-Aldrich Chemie GmbH, Steinheim, Germany). Toluene (99.5%) and methanol (>99.8%) were from Panreac (Panreac Qu´ımica S.A., Barcelona, Spain). Sodium chloride (99.5%) was purchased from Merck (Merck, Damstadt, Germany). SPME holders and fibers [100 ␮m thickness poly(dimethylsiloxane) (PDMS) and 65 ␮m poly(dimethylsiloxane)-divinylbenzene (PDMSDVB)], sample vials (40 ml, amber glass), and PTFE-silicone septa were obtained from Supelco (Bellefonte, PA, USA). Single standard solutions (1000 mg/l) of ethers and BTEX compounds were prepared by spiking each compound in methanol. Then, 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 and mixing them with 20 ml of mineral organic free water previously put in 40 ml vials. The spring mineral water was chosen after its comparison with deionized and bidistilled water regularly used in the laboratory. In the latter there were other organics besides the target analytes that complicate the blank signal. This could be due to the disinfection byproducts present in the drinking water used as source for distilled and deionized water. 2.2. Equipment A HP 6890 gas chromatograph equipped with split– splitless injector and FID detector (Agilent Technologies, Wilmington, DW, USA) was used in all measurements. The injection port fitted with a 0.75 mm i.d. injection liner (Supelco) was operated in the splitless mode, with the split– splitless purge valve opened at 1 min after injection. The injection port temperature was 250 ◦ C and helium served as carrier gas with a flow-rate of 2.2 ml/min. Chromatographic separation was accomplished with a fused-silica capillary column (HP-1, 30 m × 0.32 mm × 0.25 ␮m film thickness, Agilent Technologies). The temperature program used was: 40 ◦ C for 1 min, 20 ◦ C/min to 100 ◦ C, hold 1 min, then 50 ◦ C/min to 200 ◦ C, and hold 2 min. The detector temperature was set at 280 ◦ C. A PC interfaced to the GC using Chemstation software (Agilent Technologies) was used for data acquisition and processing. A Heidolph MR 3003 magnetic stirrer (Heidolph Elektro GmbH & Co KG, Kelheim, Germany) was used. PTFE

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coated stir bars of 25 mm were put in the 40 ml vials just before runs. The experimental matrix designs were performed and evaluated using the STATISTICA software package (StatSoft, Tulsa, USA). 2.3. Water sampling In total 18 sampling sites were chosen in the province of Gipuzkoa (North Spain). Sixteen of the sampling locations were selected based in their proximity to gasoline service stations where fuel oxygenates and BTEX compounds expected to be occurred. Nine of these sites were located in marinas or fishing harbors, and other seven sites were in river waters. Another two sampling points chosen as control sites were in pristine rivers. Three sampling were done in 1 year (June 2002, February 2003, and June 2003). Surface water samples were collected in 100 ml glass vials that were completely filled with no headspace, covered with aluminum foil, closed with a metallic cap, and then transported to the laboratory in a cooler with ice. All the samples maintained in the cooler were analyzed within 48 h of sampling. In order to check a potential contamination from containers, shipping, storage, sample preparation, and measurements a trip blank was included and maintained as the same conditions of the collected water samples. 2.4. Analytical procedure Adequate standards volumes were added immediately before use to the vials containing the salty water (6g/20 ml) to achieve the corresponding micrograms per liter concentration. The vials with salty water had been previously put in water bath to acquire 10 ◦ C. Sampling extraction was performed by HS mode exposing the PDMS-DVB fiber over stirred samples. The extraction was done for 10 min at 10 ◦ C (using a constant-temperature water bath). For all the runs a stirring rate of 1000 rpm was used. After sampling the fiber was withdrawn into the needle of the holder and was immediately placed in the GC injector. The desorption temperature was 250 ◦ C and 1 min was the desorption time for all the runs. No carryover was observed after this desorption time. River water samples were prepared in a similar manner. For practical reasons in the marine waters the amount of sodium chloride added was as much as the saturation level. A great variability of the salinity in these samples, checked by the conductivity measured in situ, was found. Therefore, the addition of salt to get 300 g/l in each of the sea samples was considered an impracticable task for a desirable simple procedure. In order to quantify the ethers and BTEX compounds in the real samples the standard addition procedure was used. Three mixed standards with closer concentrations to levels of the analytes present in the sample were used for quantification.

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3. Results and discussion The different properties of the ethers (MTBE, TAME, EBE, and ETBE) and BTEX aromatic hydrocarbons affect the HS-SPME extraction process. Both ethers and BTEX are volatile as indicated by the moderate to high vapor pressure (from 6.6 to 251 mm Hg, at 25 ◦ C). However, the oxygenates do not rapidly partition from water to air because of their high aqueous solubility (for MTBE is 50,000 mg/l). Therefore, they present moderately low Henry’s Law constant (H). This constant gives the ratio of the partial pressure of the analyte in the gas phase to the concentration in the water that is in equilibrium with the partial pressure (expressed in atmosphere cubic meters per mol). When H is divided by the product of the gas constant (R, in atm m3 /mol ◦ K) and the temperature (T, in ◦ K), the resulting H/RT is referred to as the dimensionless Henry’s Law constant. A compound with an H/RT value of 0.05 or larger is volatile from water; and with a low value, tends to remain in water phase [2]. At 25 ◦ C, H/RT for MTBE is 0.02 and for TAME is 0.05; in contrast, H/RT for benzene is 0.22 and for ethylbenzene is 0.35 [2,3]. Taking into consideration these different properties, in all the runs in the screening and optimization designs the following concentrations were used. For ethers the aqueous solution contained 500 ␮g/l of each MTBE, EBE, ETBE, and TAME. For benzene the concentration was 100 ␮g/l, and for the rest of aromatics (toluene, ethylbenzene, and xylenes) the concentration was 20 ␮g/l. 3.1. Screening design Screening is the first step in the efficient assessment of the factors involved in the studied analytical system. If a large number of factors are involved, reduced factorial designs are employed. A particular type of those designs is the Plackett–Burman design, which assumes that the interactions can be completely ignored and so the main effects are calculated with a reduced number the experiments. With this design k factors are studied in k + 1 runs, where the total number or runs must be a multiple of 4. On the basis of the literature and the experience of the laboratory [15,16,18–20] seven variables were selected to define the experimental field. The variables considered were: type of fiber, pH value, concentration of sodium chloride in water, headspace volume, extraction temperature, extraction time, and desorption time. Six variables were continuous and one (fiber type) was qualitative or categorical. In the latter, either (1) PDMS fiber or (2) PDMS-DVB fiber option was used. The two coatings were chosen in order to contrast the PDMS general phase with the PDMS-DVB mixed phase that is a semipolar coating and presents complementary properties. The pH value was between 3 and 9. The concentration of salt ranged from 0 to 300 g/l. The HS volume varied from 10 (1/4 of total volume in 40 ml vial) to 30 ml (3/4 of total volume in 40 ml vial). The temperature, maintained by

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water bath, was from 10 to 40 ◦ C. The extraction time was from 2 to 20 min. And, the desorption time ranged from 0.5 to 3 min. The variables and their code, and the low and high levels considered are shown in Table 1. A 27–4 Plackett–Burman design was applied to evaluate the main effects. Three replicates per run were carried out, to obtain a value for experimental error. In total, the design matrix had 24 runs randomly carried out trying to nullify the effect of extraneous or nuisance variables. The design matrix and the response for the studied analytes (peak area in arbitrary units) are also given in Table 1. The data obtained were evaluated by ANOVA test (not included here) and visualized by using main effects Pareto charts (Fig. 1). The Pareto charts showed in Fig. 1 belong to three of the ethers (MTBE, ETBE, and TAME) and three of the aromatics (toluene, ethylbenzene and o-xylene). In the charts, the bar lengths are proportional to the absolute value of the estimated main effects. These charts also include a vertical line corresponding to 95% confidence interval. An effect, which exceeds this reference line, may be considered significant as regard to the response. The sign of the main effects showed that the response would be improved or not on passing a given factor from the lower to the high level. In this study, the fiber type was the most important variable, with a positive effect for the PDMS-DVB coating. The next most influential factor was sodium chloride concentration with a positive effect. Headspace volume (HSvol ) and extraction temperature (Textr ) were other two significant effects in the ethers results. The better effectiveness of mixed coatings (carboxenPDMS, DVB-carboxen-PDMS) compared to PDMS for these type of analytes using HS-SPME have been also showed in several works [15,20,24]. The positive effect of the addition of sodium chloride have been remarked by other authors [15,16,19,20]; varying the amount of added salt, 10% (w/w), 5 g/20 ml or even saturation. Headspace volume is often a variable not considered in the optimization procedure, and usually 0.5 volume ratio (ml of solution/ml of vial) is used. The latter is the headspace volume ratio chosen for single determination of MTBE [15,18]; for BTEX [19,24], and for MTBE, EBE and BTEX [20]. Therefore, the extraction temperature presents a wider range. For MTBE determination goes from room temperature to 35 ◦ C [15,16,18], and for BTEX varies from low to room temperature [19,24]. In the MTBE, EBE, and BTEX simultaneous determination a 40 ◦ C temperature was chosen [20]. The results of this screening design led to use the PDMSDVB fiber and to eliminate three variables for the following optimization step: pH, extraction time, and desorption time. Hence, the actual pH value of the salty aqueous dilution (around 5.8), 10 min for extraction time, and 1 min for desorption time was chosen for the following step. The time of 10 min for extraction, in headspace mode, was also used for determination of MTBE [15], and MTBE, EBE, and BTEX [20]. In other works the extraction time

used was from 5 to 30 min for MTBE [16,18], and from 4 to 30 min for BTEX [19,24]. 3.2. Optimization design The second step is concerned with improving the output in an analytical system, as a function of several experimental factors. Many designs for modeling are based on the central composite design (CCD, sometimes called a response surface design). It is assumed that the central point for each factor is 0, and the design is symmetric around this. A CCD is constructed as several superimposed designs. In this study the three variables and its low, central, and high levels were the followings: extraction temperature (Text , 15–22–30 ◦ C), sodium chloride concentration (NaCl, 120–185–250 g/l), and headspace volume (HSvol , 15–20–25 ml). Those values are showed in Table 2. CCD consisted of the points of factorial design (2N ) augmented with (2N + 1) star points. In this work, 23 augmented with (2 × 3 + 1). The star points were located at +α and −α from the center of the experimental domain. An axial distance α was selected with a value of 1.6818 in order to establish the rotatability condition. With the inclusion of this condition, the design generates information equally in all directions; i.e., a rotation of design about the origin does not alter the variance contours. The runs at the center of the experimental field were performed three times more. Therefore, in total the matrix of CCD design involved 18 experiments. The values corresponding to every variable in each experiment are shown in Table 2. The experiments were randomly carried out, and each run was done with three independent samples. The average values of the three data (in arbitrary units of peak area) are shown in Table 2. Taking into consideration the above results, the following step was to find the select conditions of the independent variables (Text , NaCl, and HSvol ) that maximize the desirability of the responses on the dependent variables (the analytes MTBE, ETBE, EBE, TAME, and BTEX). This could be done by response–desirability profiling from central composite designs in STATISTICA program. Firstly, adequate models (i.e., prediction equations) have to be found to predict responses of the dependent variables based on the levels of the independent variables; and second, the levels of the independent variables that simultaneously product most desirable predicted responses on the dependent variables have to be determined. The model used in STATISTICA program for standard central composite design is a second degree polynomial, response surface model. The regression coefficients obtained are used in computing predicted values for the dependent variables at different combinations of the independent variables levels. Then, the desirability function for each dependent variable has to be fixed. This was done by assigning desirability values of 0.0 (for undesirable, lowest result in this work), 0.5 (medium), and 1.0 (for very desirable, highest result in this work). After doing the desirability

Table 1 Experimental variables, levels, 27–4 Plackett–Burman design matrix, and results (in peak area arbitrary units) for MTBE, ETBE, EBE, TAME, and BTEX determination with HS-SPME Variable

Coded

Level Low

High

Fiber pH NaCl HSvol Textr textr tdesor

PDMS (1) 3 0 10 10 2 0.5

PDMS/DVB (2) 9 300 30 40 20 3

Run

Fiber

pH

NaCl

HSvol

Textr

textr

tdesor

MTBE

ETBE

EBE

TAME

Benzene

Toluene

Ethyl benzene

m,p-Xylene

o-Xylene

1 14 6 20 15 12 9 23 21 18 4 22 16 10 7 2 3 11 5 8 24 17 13 19

1 2 2 2 1 2 1 1 1 2 2 2 2 2 1 2 1 1 1 2 2 1 1 1

3 3 3 9 9 9 3 9 3 3 9 3 9 3 9 3 9 9 3 9 9 3 3 9

0 300 300 0 300 0 0 300 300 0 0 300 300 0 300 0 0 0 300 300 300 0 300 0

30 10 10 30 10 30 30 10 30 10 30 10 30 10 10 10 10 10 30 30 30 30 30 10

40 40 40 10 10 10 40 10 10 10 10 40 40 10 10 10 40 40 10 40 40 40 10 40

20 2 2 2 20 2 20 20 2 20 2 2 20 20 20 20 2 2 2 20 20 20 2 2

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 3.0 3.0 0.5 0.5 3.0 3.0 0.5 3.0 3.0 3.0 3.0 3.0 3.0 0.5 3.0 3.0

20 226 204 167 39 142 32 68 144 64 160 235 319 68 54 63 8 12 116 329 420 24 152 6

75 1233 1005 558 142 606 95 270 440 300 551 1173 920 326 203 284 25 34 377 928 1210 66 477 16

225 6913 4388 2451 620 3235 296 1131 1559 2061 2366 5372 3555 2528 823 2082 90 111 1492 3460 4639 177 1843 55

126 1892 1592 785 246 927 162 429 740 576 780 1784 1901 630 325 558 44 63 607 1921 2456 118 787 30

42 797 481 1230 75 1030 55 146 199 332 1049 603 399 512 102 348 17 19 195 402 559 29 231 10

33 1007 564 747 94 872 46 172 181 496 677 741 477 788 117 555 16 18 191 480 670 25 227 10

66 1734 871 818 235 1069 95 391 357 999 735 1413 945 1361 278 1020 42 42 386 1000 1321 56 458 23

144 3964 1995 1668 534 2224 205 870 817 2176 1504 3291 2273 2898 625 2210 86 86 891 2439 3237 115 1060 49

73 1868 944 609 248 853 101 397 437 913 559 1554 1209 1155 283 915 37 40 457 1277 1720 61 557 21

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Fiber type pH value NaCl-Salt concentration (g/l) Headspace volume (ml) Extraction temperature (◦ C) Extraction time (min) Desortion time (min)

197

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Fig. 1. Pareto charts of the main effects obtained from 27–4 Plackett–Burman design for MTBE, TAME, ETBE, toluene, ethylbenzene, and o-xylene.

specifications, for each dependent variable is possibly to obtain predicting responses at several levels of single independent variable holding the remainder variables constant at their current setting. There is also the option that allows obtaining the profile of overall response desirability at each level of the independent variable, holding the levels of all other independent variables constant at their current values. These can be visualized in prediction profiles and desirability graphs for each analyte, and overall response desirability graphs for each independent variable (graphs not showed here). The overall desirability was computed obtaining a value of 0.9115; with the settings of Text , HSvol , and NaCl

at 9.9 ◦ C, 20 ml, and 294.3 g/l, respectively. The comparison of those conditions with the obtained in the single determination of MTBE [18] showed the similar values for salt concentration and headspace volume. However, a higher extraction temperature value (20 ◦ C) was best for individual MTBE determination. In Fig. 2 is presented the desirability surface response developed by the model in 3D plots. Those graphs are useful for interpreting graphically the effect on overall response desirability of each pair of independent variables. In Fig. 2a the variables were NaCl concentration and headspace volume, and in Fig. 2b salt concentration and extraction

I. Arambarri et al. / J. Chromatogr. A 1033 (2004) 193–203

199

Table 2 Experimental variables, levels, matrix of central composite design (CCD), and results (in peak area arbitrary units) for MTBE, ETBE, EBE, TAME, and BTEX determination with HS-SPME Variable

Coded

(◦ C)

Level Low

Centre

High

Extraction temperature NaCl- Salt concentration (g/l) Headspace volume (ml)

Textr NaCl Hsvol

15 120 15

22 185 20

30 250 25

Run

Textr

NaCl

HSvol

MTBE

ETBE

EBE

TAME

Benzene

Toluene

Ethyl benzene

m,p-Xylene

o-Xylene

1 11 5 16 12 9 7 17 15 13 3 18 10 6 4 2 8 14

15 22 30 22 22 10 30 22 22 22 15 22 35 30 15 15 30 22

120 185 120 185 185 185 120 185 185 76 120 185 185 250 250 250 250 294

15 12 15 20 28 20 25 20 20 20 25 20 20 15 25 15 25 20

241 268 251 313 359 278 247 291 317 191 242 330 341 390 362 354 436 524

1012 1178 1087 1300 1313 1199 920 1183 1331 797 1022 1343 1318 1586 1522 1527 1471 1955

5989 6738 5507 7274 6024 7821 4168 6717 7240 4400 6015 6794 5871 7294 8760 9038 6097 8733

1554 1783 1680 1972 2107 1835 1442 1814 2009 1230 1585 2056 2042 2416 2316 2417 2480 3210

1097 1090 858 1198 793 1486 557 917 112 727 1038 985 778 952 1311 1336 750 1093

993 1054 707 1068 692 1490 491 872 1048 733 1010 935 721 841 1206 1228 718 901

1635 2312 1704 2226 1440 2505 1198 1833 2210 1620 1875 2099 1751 1981 2225 2499 1555 2043

4007 5890 4279 5600 3731 6247 3123 4713 5688 4063 4689 5472 4650 5236 5396 6755 4170 5386

1701 2499 1954 2356 1748 2571 1399 2063 2483 1642 1944 2419 2150 2439 2388 2840 1995 2561

(C)

(C) (C)

(C)

C, central point.

temperature. The best global response was reached when the headspace volume was around 20 ml (in vial of 40 ml), salt concentration was close to 300 g/l, and the extraction temperature near to 10 ◦ C. For practical reasons, the two latter were the values chosen instead of the previously indicated (294.3 g/l and 9.9 ◦ C). Resulting from this study, the working conditions to obtain the best simultaneous response for the studied analytes were: PDMS-DVB coating fiber, 300 g/l sodium chloride concentration, 10 ◦ C extraction temperature, and 20 ml (in a 40 ml glass vial) HS volume. The other fixed conditions

for no significant variables were: pH of the solution, 10 min extraction time, and 1 min desorption time. 3.3. Method performance and BTEX influence Procedure repeatability, expressed as relative standard deviation (R.S.D.), was obtained from the results of five independent samples in the same day. The concentrations of the analytes were the same as previously used in the screening and optimization runs: 500 ␮g/l for the ethers, 100 ␮g/l for benzene, and 20 ␮g/l for toluene, ethylbenzene

Fig. 2. Response surfaces for global desirability estimated from the central composite design. (a) NaCl concentration and headspace volume; and (b) NaCl concentration and extraction temperature.

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and xylenes. R.S.D. was from 2.6 (benzene) to 8.5% (ethylbenzene). The values are similar to those found (<8.5%) working with PT–GC–MS [4]. And also, very close to the values obtained for the BTEX compounds (3–7%) when HS-SPME–GC–FID, with PDMS fiber, was used [19]. The method detection limit (MDL) was considered as the lowest concentration of the analyte that in the whole process can differentiate from the background levels. It should be measured from a real sample that has been treated as stated in the method procedure [25]. Some works used LOD calculated by a signal-to-noise ratio of 3 [3]. Other works obtained it from the calibration plot [23]. In this study, the MDL of each compound was calculated by the concentration corresponding to the intercept in the calibration plot of the y-average blank plus three times the standard deviation from the five independent blank runs. Five point calibration curves were used with a range of 2.0–20 ␮g/l for ethers and 0.25–2.5 ␮g/l for BTEX. The calibration equations obtained and used for MDL calculations were the followings: MTBE (y = 10.144x + 1.867, R2 = 0.998); ETBE (y = 53.748x + 39.431, R2 = 0.992); TAME (y = 61.007x + 46.549, R2 = 0.994); EBE (y = 101.986x + 180.225, R2 = 0.996); benzene (y = 224.286x + 3.500, R2 = 0.9996); toluene (y = 561.029x + 30.800, R2 = 0.9991); ethylbenzene (y = 359.314x + 6.800, R2 = 0.9994); m,p-xylene (y = 729.486x + 16.600, R2 = 0.9999); o-xylene (y = 361.143x + 8.000, R2 = 0.9997). From those calibration equations the MDLs obtained were: 0.2 (EBE), 0.3 (ETBE), 0.5 (TAME), and 1.1 ␮g/l (MTBE). For the aromatics those were the MDL values: 0.02 (toluene, ethylbenzene, and xylenes), and 0.07 ␮g/l (benzene). The comparison with other works that determined several analytes in the same matrix showed that the MDLs obtained were similar or slightly better than others found using the same technique (HS-SPME–GC–FID). The differences with those works were mainly in the type of fiber (PDMS or PDMS-carboxen) and in the extraction temperature (room temperature or 40 ◦ C) [19,20]. The results were similar to those indicated using ASTM Method D4815 (flame ionization detection) [7]. Also, they were better than the obtained (≤2 ␮g/l for most of the analytes) working with DAI–GC–MS technique [10]. However, the values were higher than the showed levels (ng/l magnitude order) by the researches where SPME or PT techniques were followed by GC and the more specific MS detector [3,13,23,26]. The comparison with works devoted only to determination of MTBE, showed very close values of MDL using static headspace GC–MS [27], and PT–GC–PID [28] (1.2–2.0 ␮g/l and 1.0 ␮g/l, respectively), and slightly higher working with PT–GC–FID (0.2 ␮g/l) [29]. The use of selective MS detector provided lower MDLs than FID did. The level of 10 ng/l was obtained working with SPME–GC–MS and HS-SPME–GC–MS [12,16], and 50 ng/l using PT–GC–MS [9]. In a previous work, using HS-SPME–GC–FID tech-

nique, with the single MTBE determination as goal, the MDL value obtained was 0.45 ␮g/l [18]. The increase of the MDL level is consequence of the cross competition with the other analytes (BTEX and ethers) present in the sample matrix that affects the adsorption process in this type of fiber. Adsorption plays an important role in extraction mechanism of a mixed coating with porous solid-phase like PDMS-DVB, while absorption occurs in PDMS. Composition of matrix can greatly affect the amount of analyte extracted when compounds with higher affinity for the PDMS-DVB phase replace compounds with less affinity [30]. Competitive extraction during solid-phase microextraction among BTEX [24], and the influence of BTEX on MTBE and tert-butyl alcohol have been remarked [8]. In the latter two works the mixed carboxen-PDMS fiber was used. Working with BTEX have been reported that the relatively affinity of each component differs according to the total amount of analytes, and the general affinity to the fiber followed the order xylene > ethylbenzene > toluene > benzene [24]. The next experiments were planned in order to known the influence of the BTEX concentration not only on the MTBE but also on the rest of the studied ethers (ETBE, EBE, TAME) working with PDMS-DVB fiber in HS-SPME mode. In all the runs for the extraction process the optimized conditions were used. The concentrations of each of the ethers were fixed at 500 ␮g/l. The BTEX concentrations added were: 0, 20 (benzene, 10 ␮g/l; toluene, etylbenzene and each of xylenes, 2 ␮g/l), 100 (benzene, 50 ␮g/l; toluene, etylbenzene and each of xylenes, 10 ␮g/l), 200 (benzene, 100 ␮g/l; toluene, etylbenzene and each of xylenes, 20 ␮g/l), 400 (benzene, 200 ␮g/l; toluene, etylbenzene and each of xylenes, 40 ␮g/l), and 2000 ␮g/l (benzene, 1000 ␮g/l; toluene, etylbenzene and each of xylenes, 200 ␮g/l). From the data, the two general affinities for the fiber could be indicated. For the ethers followed the order EBE > TAME > ETBE > MTBE; and for the aromatics the affinity order was m,p-xylene > o-xylene > ethylbenzene > toluene > benzene. The latter was similar to the order showed using carboxen-PDMS fiber [24]. The normalized peak areas considering as reference the system without BTEX were calculated. In Fig. 3 are represented the normalized values for MTBE, ETBE, EBE, and TAME versus the BTEX concentration. It can be seen that the higher the concentration of BTEX the lower the ether peak areas were, and hence the normalized values represented. At 20 ␮g/l of BTEX, the areas decrease around 25%, and at 400 ␮g/l, the peak areas decrease more than 60% compared with of the highest peak areas. This decrease in response working with the mixing coating carboxen-PDMS has been showed [8]. The authors justified the decrease in MTBE signal by the replacement of less polar aromatic compounds, especially when total aromatics amount is above 1 mg/l. In those cases they recommended dilution to minimize the adverse matrix effect.

I. Arambarri et al. / J. Chromatogr. A 1033 (2004) 193–203

201

Normalized peak area

1. 2 1. 0 0. 8 0. 6 0. 4 0. 2 0. 0

0

500

1000

1500

Total BTEX concentration

2000

2500

g/l)

Fig. 3. Normalized peak area of 500 ␮g/l of each ether as a function of the total concentration of BTEX using 65 ␮m PDMS/DVB fiber with headspace sampling. (䉬), MTBE; (䊏), ETBE; (×), EBE; (䉱), TAME.

The co-occurrence of ethers, mainly MTBE, with high levels of BTEX has been registered in several contaminated samples. For instance, in ground water wells contaminated by underground storage tank leakage or gasoline spillage [4,9]. In those samples the dilution is the advisable option trying to reduce the competition displacement in mixed coating fiber like PDMS-DVB or carboxen-PDMS. However, in most of the surface water samples BTEX are at low concentration or even not detectable. In the results of the study of Dutch surface water in the period 1992–1997 the maximum level was for toluene in the Rhine river with an amount of 1.1 ␮g/l [26]. The analysis of a 249 storm water runoff samples gave median contaminant concentrations (when detected) from 0.07 for ethyl benzene to 0.11 ␮g/l for toluene [3]. In the water analysis in marinas showed that the most common compound of BTEX found with MTBE was toluene where concentrations ranged from 1 to 2.0 ␮g/l [28]. In those samples with low amounts, quantification can be done using the standard addition method. The levels of the standards have to be as close as possible to the actual amounts of the analytes in the sample. With this method the use of internal standard is avoid, since I.S. should be

Fig. 4. (a) Chromatogram of the standard mixture (500 ␮g/l of each ether, 100 ␮g/l of benzene, and 20 ␮g/l of toluene, ethylbenzene and xylenes), (b) chromatogram of a marina water sample. Compound identification number: 1, methanol; 2, MTBE; 3, ETBE; 4, benzene; 5, TAME; 6, EBE; 7, toluene; 8, ethylbenzene; 9, m,p-xylene; and 10, o-xylene.

considered as another interfering analyte that could enter in competition with the target analytes for the fiber. 3.4. Application The optimized method was applied to the analysis of 18 superficial water samples. The sampling was done three times in June 2002, in February, and in June 2003. In all the samples the standard addition method was used for quantification of detected analytes. The ranges of the BTEX compounds are showed in Table 3. Fig. 4b shows

Table 3 Range of BTEX concentrations (␮g/l) in superficial waters of Gipuzkoa (North Spain) Samples

Benzene

Toluene

Ethylbenzene

m,p-Xylene

o-Xylene

June 2002 River waters Marinas/fishing harbors

nd to 8.87 nd to 9.64

nd to 3.38 0.06 to 2.44

nd to 0.36 nd to 0.14

nd to 0.94 0.02 to 0.16

nd to 0.39 nd to 0.18

February 2003 River waters Marinas/fishing harbors

nd to 0.13
nd to 0.53 0.05 to 1.82

nd to 0.07
nd to 0.12 nd to 0.34

nd to 0.06
June 2003 River waters Marinas/fishing harbors

nd to 0.07 nd to 1.21

nd to 0.26 0.03 to 6.41

nd to 0.06 nd to 0.66

nd to 0.05 0.03 to 1.58

nd to 0.05 0.03 to 1.28

nd, not detected;
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the chromatogram from one marina water sample obtained with the proposed analytical procedure. For comparison, a chromatogram of the standard mixture has been included in the Fig. 4a. MTBE was the only alkyl ether present in 15% of the analyzed samples, all of them in June 2002. Two high MTBE levels found (more than 400 ␮g/l) were related to accidental fuel spill, one in a marina recreational area and the other in a fuel service station. In six samples the MTBE obtained was higher than the USEPA advisory level for taste and odor (20–40 ␮g/l). The highest amounts of MTBE often were found concurrently with the highest levels of BTEX. Respect to BTEX, those were in a great number of samples in all the sampling period. Toluene was the analyte most detected (in 90% of the total number of samples analyzed), followed by xylenes and ethylbenzene. Five samples give concentrations of benzene higher than 1 ␮g/l (level of EU drinking water standard). Average concentration of toluene varied from 0.44 (continental waters) to 0.63 ␮g/l (marine waters). More than 80% of the samples contain BTEX amount less than 1 ␮g/l. Only 5% of the samples presented BTEX values higher than 10 ␮g/l, being the maximum BTEX amount found 14 ␮g/l. The comparison of alkyl ethers (mainly MTBE) and BTEX in continental surface water from different areas showed similar values to most of the samples analyzed in this work. Concentrations of MTBE and toluene found in Rhine river respectively were 1.5 and 1.1 ␮g/l [26]. MTBE and BTEX were detected in several samples of stormwater runoff, where elevate contaminant concentrations were related to direct runoff from gas station [3]. In Europe the studies about MTBE in the aquatic environment have been extensively done in Germany [16,17]. The ranges varied depending of the water type; i.e., from river water was from < 10–1674 ng/l, and for industrial waste waters from <10–28422 ng/l [17]. The MTBE has been also determined in marine and recreational areas in California [14,31] and in South Spain coast [32]. In the latter study the MTBE concentration ranged from below LOD to 1842 ␮g/l.

4. Conclusions A screening HS-SPME method for the simultaneous determination of fuel alkyl ethers and BTEX in water was developed. It was also shown the usefulness of experimental design taken in the optimization of extraction conditions for HS-SPME method. The main advantages of the method were based on the simplicity of the equipment and rapidity. A PDMS-DVB fiber for the extraction and GC–FID for the determination was the equipment required. A time of 10 min was enough for the extraction of the studied analytes. Considering the results, the proposed method could be suitable for screening, and also for determining the content of MTBE and other alkyl ethers simultaneously with BTEX in surface contaminated waters. Because the competency

among the analytes for the adsorption process, the method could apply better in samples with levels of total BTEX below 20 ␮g/l. Also, the addition standard procedure was advisable for quantification. The limiting concentration was for MTBE among the ethers (1.1 ␮g/l) and for benzene among the BTEX (0.07 ␮g/l). However, these levels were well enough to satisfy the European standard for drinking water in the case of benzene (1 ␮g/l), and the USEPA advisory levels for taste an odor in the case of MTBE (20–40 ␮g/l). Samples with low levels could be analyzed by means of more sensitive detectors such as MS coupled to GC. The method was applied to the analysis of the rivers, marinas and fishing harbors surface waters from Gipuzkoa (North Spain). Three sampling were done in one year (June 2002–June 2003). Toluene was the most detected analyte (in 90% of the samples), with an average concentration of 0.56 ␮g/l. MTBE was the only dialkyl ether detected (in 15% of the samples) showing two high levels over 400 ␮g/l that were related to accidental fuel spill.

Acknowledgements The authors acknowledge the financial support from the Diputación Foral de Gipuzkoa.

References [1] Directive 98/70/EC of the European Parliament and of the Council of 13 October 1998 relating to the quality of petrol and diesel fuels and amending Council Directive 93/12/EEC, Official Journal L 350, 28/12/1998, 0058–0068. [2] P.J. Squillace, J.F. Pankow, N.E. Korte, J.S. Zogorski, Environ. Toxicol. Chem. 16 (1997) 1836. [3] R.C. Borden, D.C. Black, K.V. McBlief, Environ. Pollut. 118 (2002) 141. [4] M. Rosell, S. Lacorte, A. Ginebreda, D. Barceló, J. Chromatogr. A 995 (2003) 171. [5] C. Achten, A. Kolb, W. Püttmann, Atmos. Environ. 35/36 (2001) 6337. [6] Council Directive 98/83/EC of 3 November 1998 on quality of water intended for human consumption, Official Journal L 330, 5/12/1998, 0032–0054. [7] R.U. Halden, A.M. Happel, S.R. Schoen, Environ. Sci. Technol. 35 (2001) 1469. [8] L. Black, D. Fine, Environ. Sci. Technol. 35 (2001) 3190. [9] J. Klinger, C. Stieler, F. Sacher, H.-J. Brauch, J. Environ. Monit. 4 (2002) 276. [10] L. Zwank, T.C. Schmidt, S.B. Haderlein, M. Berg, Environ. Sci. Technol. 36 (2002) 2054. [11] J. Pawliszyn, Solid Phase-Microextraction: Theory and Practice, Wiley-VCH, New York, 1997. [12] C. Achten, W. Püttmann, Environ. Sci. Technol. 34 (2000) 1359. [13] D.A. Cassada, Y. Zhang, D.D. Snow, R.F. Spalding, Anal. Chem. 72 (2000) 4654. [14] J.L. Zuccarello, J.A. Ganske, D.B. Green, Chemosphere 51 (2003) 805. [15] F. Piazza, A. Barbieri, F.S. Violante, A. Roda, Chemosphere 44 (2001) 539.

I. Arambarri et al. / J. Chromatogr. A 1033 (2004) 193–203 [16] C. Achten, A. Kolb, W. Püttmann, Fresenius J. Anal. Chem. 371 (2001) 519. [17] C. Achten, A. Kolb, W. Püttmann, P. Seel, R. Gihr, Environ. Sci. Technol. 36 (2002) 3652. [18] J. Dron, R. Garcia, E. Millán, J. Chromatogr. A 963 (2002) 259. [19] J.C. Flórez, M.L. Fernández, J.E. Sánchez, E. Fernández, A. Sanz-Medel, Anal. Chim. Acta 415 (2000) 9. [20] R.B. Gaines, E.B. Ledford, J.D. Stuart, J. Microcolumn Sep. 10 (1998) 597. [21] P.W. Araujo, R.G. Brereton, Trends Anal. Chem. 15 (1996) 26. [22] P.W. Araujo, R.G. Brereton, Trends Anal. Chem. 15 (1996) 63. [23] F. Bianchi, M. Careri, E. Marengo, M. Musci, J. Chromatogr. A 975 (2002) 113. [24] H.-J. Cho, K. Baek, H.-H. Lee, S.-H. Lee, J.-W. Yang, J. Chromatogr. A 988 (2003) 177.

203

[25] P. Bruce, P. Minkkinen, M.-L. Riekkola, Mikrochim. Acta 128 (1998) 93. [26] C.J.H. Miermans, L.E. van der Velde, P.C.M. Frintrop, Chemosphere 40 (2000) 39. [27] D.T. O’Neill, E.A. Rochette, P.J. Ramsey, Anal. Chem. 74 (2002) 5907. [28] Y.-J. An, D.H. Kampbell, G.W. Sewell, Environ. Pollut. 118 (2002) 331. [29] D.P. Lince, L.R. Wilson, G.A. Carlson, A. Bucciferro, Environ. Sci. Technol. 35 (2001) 1050. [30] T. Górecki, X. Yu, J. Pawliszyn, Analyst 124 (1999) 643. [31] J.S. Brown, S.M. Bay, D.J. Greenstein, W.R. Ray, Mar. Pollut. Bull. 42 (2001) 957. [32] M. Mezcua, A. Agüera, M.D. Hernando, L. Piedra, A.R. Fernández-Alba, J. Chromatogr. A 999 (2003) 81.