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Available online at www.sciencedirect.com

Journal of Chromatography A, 1171 (2007) 1–7

Liquid–liquid extraction/headspace/gas chromatographic/massspectrometric determination of benzene, toluene,ethylbenzene, (o-, m- and p-)xylene and styrene

in olive oil using surfactant-coated carbonnanotubes as extractant

Carolina Carrillo-Carrion, Rafael Lucena,Soledad Cardenas, Miguel Valcarcel ∗

Department of Analytical Chemistry, Marie Curie Building (Annex), Campus de Rabanales, University of Cordoba,E-14071 Cordoba, Spain

Received 2 May 2007; received in revised form 13 August 2007; accepted 11 September 2007Available online 21 September 2007

bstract

BTEX-S compounds are widely distributed in the environment and can be present in different foodstuffs, including olive oil. Taking into accounthe risks of the exposure to these compounds, analytical methods for their determination in different matrices are mandatory. In this paper, the usef surfactant-coated multiwalled carbon nanotubes as additive in liquid–liquid extraction is applied for the determination of single-ring aromaticompounds in olive oil samples. After sample treatment, the aqueous extracts are subsequently analyzed by headspace/gas chromatography/masspectrometry allowing the determination of BTEX-S within ca. 15 min. Each stage of the proposed LLE/HS/GC/MS configuration involves a

electivity enhancement avoiding the interference of other compounds of the sample matrix. Limits of detection were in the range 0.25 ng mL−1

obtained for ethylbenzene) and 0.43 ng mL−1 (for benzene). The repeatability of the proposed method expressed as RSD varied between 1.9%styrene) and 3.3% (benzene) (n = 11). 2007 Elsevier B.V. All rights reserved.

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eywords: Carbon nanotubes; Pseudophase; Liquid–liquid extraction; Gas chr

. Introduction

The acronym BTEX-S defines the mixture of benzene,oluene, ethylbenzene, the three xylenes isomers (ortho, metand para) and styrene; all being harmful volatile organic com-ounds (VOCs). BTEX-S are emitted to the environment fromn extensive variety of sources including combustion products ofood and fuels, industrial paints, adhesives, degreasing agents

nd aerosols [1]. Their widespread presence in samples of envi-

onmental concern (air, water and soil) and their lipophilic natureeads to their accumulation in different foodstuffs, mainly edibleils and fat [2]. Therefore the human exposure to these aromatic

∗ Corresponding author. Tel.: +34 957 218 616; fax: +34 957 218 616.E-mail address: qa1meobj@uco.es (M. Valcarcel).

Ab[lso(

021-9673/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.chroma.2007.09.039

graphy; BTEXS; Olive oil

ydrocarbons can occur by ingestion (consuming contaminatedater or food), inhalation or absorption through the skin. The

ffects of exposure to these substances comprise changes in theiver and harmful effects on the kidneys, heart, lungs, and ner-ous system [3]. In order to reduce the human intake of theseazardous substances a chemical control (and consequentlyethods of analysis) is desirable.Gas chromatography (GC) is the main alternative of choice

or the deterination of BTEX-S in environmental samples.lthough the direct analysis of the samples’ headspace haseen traditionally employed for the determination of BTEX-S4,5], new extraction strategies including the so-called solvent-

ess sample preparation techniques [6,7] are gaining importance,ince they improve the selectivity and sensitivity of the devel-ped methodologies. In this sense, solid phase microextractionSPME) [8–10] and single drop microextraction (SDME) [11]

2 Chromatogr. A 1171 (2007) 1–7

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Fig. 1. Schematic diagram of the headspace/gas chromatography/mass spec-teg

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C. Carrillo-Carrion et al. / J.

ave been proposed as useful alternatives to the conventionalpproach.

On the other hand, only a few methods face up the determina-ion of these compounds in olive oil due to the complex nature ofhis matrix. HS has been succesfully applied on the developmentf screening [12] and confirmation [13] methodologies achiev-ng limits of detection in the range of 2–10 �g kg−1. Lowerimits of detection have been achieved by means of a HS-SPMExtraction step [14], allowing the determination of volatile andemivolatile aromatic hydrocarbons in virgin olive oil. Purge-nd-trap technique has been also proposed for the determinationf styrene in olive oil sample at the ng kg−1 level [15].

In the present paper, a new alternative for BTEX-S determi-ation in olive oil samples is proposed. The method makes use ofhe exceptional adsorption properties of multiwall carbon nan-tubes (MWCNTs). BTEX-S are liquid–liquid extracted fromhe olive oil samples using a surfactant aqueous dispersion of

WCNTs as extracting medium, this aqueous phase being fur-her analyzed by HS/GC/MS. The presence of the dispersed car-on nanotubes in the extractant enhances not only the selectivityut also the sensitivity of the determination achieving detectionimits at least 10 times lower than the direct HS method.

. Experimental

.1. Reagents and samples

All reagents were of analytical grade. The analytes: benzene,oluene, ethylbenzene, m-, p- and o-xylene, and styrene wereurchased from Sigma–Aldrich (Madrid, Spain). Stock standardolutions of each analyte were prepared in methanol, purchasedrom Panreac (Barcelona, Spain), at a concentration of 1 g L−1

nd stored in glass-stoppered bottles in the dark at 4 ◦C. Work-ng solutions were prepared by dilution of the stocks in theppropriate solvent.

MWCNTs, purity of 95%, were obtained fromigma–Aldrich. The diameters and lengths are in the range of0–50 nm and 5–20 �m, respectively. The solubilization, byispersion, of MWCNTs in an aqueous medium were achievedollowing a previously optimized procedure [16]: an accuratelyeighted amount of 5.0 mg of the nanotube was mixed with30 mg of solid sodium dodecyl sulphate (SDS). The solidixture was placed in a 50 mL volumetric flask, filled with

istilled water and sonicated in an ultrasonic bath (50 W,0 Hz) during 20 min. The final dispersion contained SDSnd MWCNTs at concentrations of 9 mM and 0.1 mg mL−1,espectively.

Olive oil samples were purchased at a local supermarket. Inrder to increase the samples variability, olive oil from differentears and storing conditions were employed. Once in the lab,hey were stored in amber glass bottles without headspace atoom temperature until the analysis.

A blank refined olive oil sample, previously analyzed by

S/GC/MS, was employed for calibration and comparison stud-

es. The blank sample was also subjected to a stirring process inn opened glass bottle during 5 h, in order to guarantee the totalbsence of the target analytes.

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rometry system employed for the determination of BTEX-S. LLE, liquid–liquidxtraction; MWCNTs, multiwall carbon nanotubes; HS, headspace module, GC,as chromatograph and MS, mass spectrometer.

.2. Apparatus

The instrumental set-up used, depicted in Fig. 1, consistsf three modules: a MPS2 32-space headspace autosamplerGerstel, Mulhein an der Ruhr, Germany) including a roboticrm and an oven for sample heating/headspace generation, aewlett-Packard HP6890 gas chromatograph (Palo Alto, CA,SA) equipped with a HP-5MS fused silica capillary col-mn (30 m × 0.32 mm i.d., and 0.25 �m of thickness) and aP5973 mass spectrometer based on a quadrupole analyser andphotomultiplier detector. For individual analyte identification

nd quantification, the program temperature was as follows:min at 40 ◦C, raised up to 80 ◦C at 5 ◦C min−1, ramped at0 ◦C min−1 up to 200 ◦C and kept finally at 200 ◦C for 3 min.n automated injector fitted with a 2.5 mL gastight HS-syringeas used for the introduction of 2.5 mL of the homogenizedeadspace from the vial into the gas chromatograph. Helium5.0 grade purity, Air Liquide, Seville, Spain), regulated by aigital pressure and flow controller, was used as carried gas1.4 mL min−1) for driving the analytes to the detector. Theass spectrometer detector was operated in full scan mode,

y scanning a mass range from m/z 40 to 180 at a rate of.36 scan s−1; electron impact ionization (70 eV) was used fornalyte fragmentation. The MS source and quadrupole temper-tures were maintained at 230 and 150 ◦C, respectively. Totalon current chromatograms were acquired and processed using1701BA Standalone Data Analysis software. The peak areas

alculated from the total ion current chromatogram were usedor quantification of benzene, toluene, ethylbenzene and (m-,

-)xylene. Taking into account that o-xylene and styrene over-apped, quantification and identification of both compoundsere achieved by extracting a specific m/z fragment (91 for-xylene and 104 for styrene). The resulting chromatograms

. Chromatogr. A 1171 (2007) 1–7 3

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Fig. 2. Chromatogram obtained for a blank refined olive oil sample spiked with2C(

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C. Carrillo-Carrion et al. / J

ere integrated, the peak areas being used for quantificationurposes.

System control was achieved with an HP1701CA MS Chem-tation (Agilent Technologies) on a Pentium 4 computer whichlso controlled the whole system.

.3. Reference method

The reference method, described elsewhere [13], consists ofhe direct analysis of the samples’ headspace by GC/MS. For thisurpose, 10 mL of sample were placed in a 20 mL headspacelass vial. The vial was sealed with PTFE/silicone septa and00 �L of ethyl acetate was added, by means of an appropriateyringe, as chemical modifier. The vial was heated in the oven at5 ◦C during 30 min with mechanical stirring, to ensure the equi-ibration of the BTEXS residues between the gaseous phase andhe liquid sample. Finally, 2.5 mL of the resulting headspace wasnjected in the chromatograph. The chromatographic conditionsere identical than those described for the proposed method.

.4. Analytical procedure

Before the analysis, real olive oil samples were liquid–liquidxtracted using the surfactant-coated aqueous dispersion ofWCNTs as extractant. In a 20 mL vial, 9 mL of sample andmL of the extractant were placed, and 100 �L of methanolas added for extraction enhancing. After mixing, the vial was

losed and manually shaken during 30 s in order to facilitate thelose contact between phases. Finally, the phases were separatednd 6 mL of the aqueous extracts were placed into 10 mL glassials, hermetically sealed with PTFE/silicone septa and placedn the autosampler. The robotic arm took each vial from theray and placed it into the oven where extracts were heated at0 ◦C for 15 min under mechanical stirring to ensure the equili-ration of the BTEX-S between the two phases (the headspacend the aqueous dispersion of MWCNTs). Then, 2.5 mL of theaseous phase was sampled by inserting the needle of the gas-ight syringe (heated at 80 ◦C) through the septum cap and driveno the column where the separation took place. The BTEX-Sere identified by comparison of the retention times and mass

pectra of the chromatographic peaks with those obtained fortandards under the same chromatographic and mass spectro-etric conditions. A representative chromatogram is shown inig. 2. In order to quantify o-xylene and styrene, which presentsimilar retention time, specific m/z ratios (91 and 104, respec-

ively) were extracted from the total ion chromatogram and theesulting peaks were integrated.

.5. Carbon nanotubes recovery

Dispersed MWCNTs were mixed with methanol and waternder continuous magnetic stirring. Under these conditions the

ispersion breaks, liberating the solid nanotubes which are col-ected by filtration and sequentially washed with fresh portionsf n-hexane, pure water and methanol. Finally, MWCNTs areried in an oven at 50 ◦C.

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0 ng mL−1 of each analyte. The chromatographic peaks of interest are indicated.hromatographic peaks of interest: (1) benzene, (2) toluene, (3) ethylbenzene,

4) m-xylene, (5) p-xylene, (6) o-xylene and (7) styrene. SD, solvent delay.

. Results and discussion

Carbon nanotubes (CNTs) have strong adsorption affinity towide variety of organic compounds and are also characterizedy their high sorption surface. These interesting properties haveeen exploited in some analytical methodologies. In this way,NTs have been used as sorbent material in solid phase extrac-

ion (SPE) [17–19], as fibre material coating in SPME [20],s stationary phase in gas chromatography [21–23] and also asseudostationary phase in capillary electrophoresis [24].

The main limitation when CNTs are used as sorbent materials their aggregation tendency, which reduces the active surfacerea of this material and therefore their effectiveness. Differ-nt methodologies have been proposed in order to overcomehis problem, the most used being chemical derivatization andispersion in a surfactant medium. In the second approach,he apolar hydrocarbon chain of the surfactant interacts withhe nanotubes allowing their homogeneous distribution in thequeous medium. Surfactant-coated MWCNTs aqueous solu-ion enhances the adsorption properties of MWCNTs, reducinghe aggregation, and can be used as pseudophase or additive iniquid–liquid extraction [16].

.1. Optimization/selection of variables

Liquid–liquid extraction is a key step of the whole analyticalrocess as it is the source for sensitivity and also selectivitynhancement. The effectiveness of MWCNTs dispersed in aurfactant medium as analyte extractant has been proven in aecent publication [16]. The referenced study demonstrated that

WCNTs act as the real “driving force” in the extraction, withegligible effect of the surfactant medium.

In the present paper, different factors that can affect the effi-iency of the extraction, namely: phases’ ratio, the addition ofhemical modifiers and extraction time, were studied. Aqueousarbon nanotubes dispersion, containing SDS and MWCNTs atnal concentrations of 9 mM and 0.1 mg mL−1, was employeds extracting medium. A real olive oil sample, spiked with

TEX-S at individual concentrations of 50 ng mL−1, was used

n the variables’ selection process. The ratio between samplend extractant volumes was studied within the interval 7:1 to:3, obtaining the best results when a 1:1 ratio is used. Finally,

4 C. Carrillo-Carrion et al. / J. Chromatogr. A 1171 (2007) 1–7

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Fig. 3. Influence of the headspace temperature (A

he addition of chemical modifiers (organic solvents) was stud-ed, resulting in 100 �l of methanol being found to be the bestlternative. This small amount of methanol facilitates the extrac-ion of the analytes and does not affect the pseudophase stability.he extraction time was studied within the interval 0.5–10 min.espite the concentration of extracted analyte slightly increas-

ng with the extraction time, 0.5 min was selected in order toncrease the sample throughput of the method. In the same way,he possible losses of target analytes in the transference of thextracts from the extraction vial to the headspace vial were takennto account. No evidence of volatile losses was found using aipette as transference tool between vials and working at roomemperature.

Headspace is the extraction technique used in the proposedethod. Different variables, including chemical and instru-ental ones, were considered with the aim of enhancing the

ensitivity of the determination. For this purpose, a control oliveil sample was spiked with 50 ng mL−1 of toluene, which waselected as model analyte, and analyzed using the proposedethod. The samples were liquid–liquid extracted following the

bove-mentioned procedure and the extracts were analyzed byhe HS/GC/MS method. First, the influence of the volume ofhe extract (viz. surfactant-coated carbon nanotubes containinghe BTEX-S) on the analyte transfer to the headspace phase wastudied within the interval 3.0–7.0 mL using 10 mL glass vials.

volume of 6.0 mL was fixed as optimum because this ratio ofxtract/headspace volumes provided the highest MS response.hen, different organic solvents, namely: methanol, isopropanolnd ethyl acetate, were assayed as chemical modifiers. It is wellnown that the release of volatiles from the sample matrix isavored by the presence of these modifiers; however, in thisase negligible effect on the analytical signal was observed and

herefore no chemical modifier was added to the samples.

The instrumental variables that markedly affected theeadspace enrichment were also optimized. Oven temperatureas studied in the range 40–90 ◦C and equilibration time was

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able 1igures of merit of the proposed method

Linear range (ng mL−1) LOD (ng mL−1)

enzene 1.5–200 0.43oluene 1.5–200 0.42thylbenzene 1–200 0.25

m + p)-Xylenea 1–200 0.26-Xylene 1–200 0.32tyrene 1–200 0.38

a The linear range corresponds to the mixture of both isomers.

equilibration time (B) on the toluene’ peak area.

ptimized in the interval 5–30 min. Fig. 3 shows the variationf the chromatographic peak areas with the temperature (A) andime (B) for toluene. As can be seen, the highest signals werebtained for 80 ◦C and 15 min, so these values were fixed asptimal.

.2. Analytical features of the LLE/HS/GC/MS system

Analytical curves were obtained using a completely blankefined olive oil spiked with the analytes at different con-entrations (between 1 and 200 ng mL−1). Calibration modelsere constructed by plotting the peak area against the ana-

yte concentration for each BTEX-S. By using the proposedhromatographic conditions, the peaks for o-xylene and styreneverlapped (this problem can be overcome using a 45 m lengtholumn). In this case, individual quantification and identificationere achieved by extracting the m/z fragment 91 for o-xylene

nd 104 for styrene and the chromatographic peaks obtainedere integrated. Taking into account the fragmentation patternf the analytes and the relative intensity of the fragment ionselected, negligible influence on the sensitivity was observed inomparison with that obtained for the rest of analytes. The fig-res of merit of the calibration graphs are summarized in Table 1.he detection limits were calculated as the concentration provid-

ng an analytical signal three times higher than the backgroundoise. The precision of the method expressed as relative stan-ard deviation was estimated at two concentration levels (20 and0 ng mL−1) by analyzing 11 independent aliquots of refinedlive oil spiked with the seven analytes studied. The values arelso listed in Table 1.

.3. Analysis of samples

The proposed method was used for the determination ofTEX-S in commercial olive oil samples. Ten real sam-les, coming from different places and packed under different

R2 RSD (%) (at 20 ng mL−1) RSD (%) (at 50 ng mL−1)

0.998 4.0 3.30.997 3.6 2.80.998 2.4 1.80.998 2.2 1.50.999 2.5 1.80.999 2.6 1.9

C. Carrillo-Carrion et al. / J. Chromatogr. A 1171 (2007) 1–7 5

Table 2Analysis of real olive oil samples using the proposed method

Typeb Packagec Concentration (ng mL−1) ± SDa

Benzene Toluene Ethylbenzene (m + p)-Xylene o-Xylene Styrene

Sample 1 EVOO G 173.7 ± 5.0 115.5 ± 3.5 n.dd n.d n.d n.dSample 2 EVOO G 20.2 ± 0.6 28.4 ± 0.7 n.d n.d n.d 2.29 + 0.1Sample 3 EVOO P(2004) 75.1 ± 2.8 50.9 ± 0.9 n.d n.d n.d 102.8 ± 1.0Sample 4 VOO G 23.2 ± 0.7 235.6 ± 3.8 17.6 ± 0.3 17.0 ± 0.1 16.1 ± 0.2 8.8 ± 0.1Sample 5 VOO P(2004) 16.5 ± 0.6 15.7 ± 0.5 14.8 ± 0.3 14.5 ± 0.1 14.0 ± 0.2 101.9 ± 2.5Sample 6 OO P(2006) 6.6 ± 0.2 8.9 ± 0.3 8.3 ± 0.1 8.4 ± 0.1 8.0 ± 0.2 7.8 ± 0.2Sample 7 OO P(2005) 36.0 ± 0.9 31.5 ± 1.2 34.3 ± 0.4 25.0 ± 0.1 19.8 ± 0.2 32.2 ± 0.1Sample 8 ROO G 44.2 ± 1.4 64.1 ± 2.1 4.8 ± 0.1 4.5 ± 0.1 4.6 ± 0.1 4.7 ± 0.1Sample 9 ROO M de 4.0 ± 0.1 3.1 ± 0.1 2.9 ± 0.1 2.8 ± 0.1 3.5 ± 0.1Sample 10 ROO P(2006) 46.5 ± 1.3 80.2 ± 2.5 5.6 ± 0.1 5.3 ± 0.1 5.3 ± 0.1 5.1 ± 0.1

a SD; standard deviation.b EVOO, extra virgin olive oil; VOO, virgin olive oil; OO, olive oil and ROO, refined olive oil.

ar of packing is indicated into brackets.

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ples were spiked with the BTEX-S at five concentrations levels(25, 40, 50, 80 and 100 ng mL−1). The first set of samples wasanalyzed using the reference method, while the second set was

Table 3Comparison of the results obtained with the proposed method and those obtainedby direct HS analysis

SEFa SSEFb R2

Benzene 20.2 0.5 0.9990Toluene 39.3 0.8 0.9998Ethylbenzene 45.1 0.6 0.9997(m + p)-Xylene 44.3 0.3 0.9998

c G, glass bottle; P, plastic bottle and M, metal can. For plastic bottles, the yed n.d, non detected. Concentration below the detection limit.e d, detected at a concentration lower than the quantification limit.

onditions, were analyzed in triplicate. The results are presentedn Table 2. In this table the type and package conditions of thenalyzed samples are also indicated.

Some conclusions can be inferred from these results. Inhe majority of samples analyzed, the total concentration ofTEX-S is lower than the maximum recommended value of000 ng mL−1. The overall BTEX-S concentration resulted toe significantly higher in extra virgin olive oil (EVOO) and vir-in olive oil (VOO) than in refined olive oil (ROO) because aain part of the volatile fraction is lost during the refining pro-

ess. In addition, refined olive oil (labelled as samples 8, 9 and0) presented a low concentration of xylenes, which are unde-ectable in extra virgin olive oil samples (samples 1, 2 and 3).

oreover, while benzene and toluene were detected in all sam-les, the presence of styrene is directly related to the packagingaterial, the concentration of styrene being higher when the

amples are stored in plastic bottles.Finally, a relationship between styrene concentration and the

torage time was also observed. Samples bottled in 2006 (sam-les 6 and 10) contained a lower concentration of styrene thathose with a longer packing time (samples 3, 5 and 7).

The typical chromatograms obtained for real samples areepicted in Fig. 4.

.4. LLE/HS versus HS direct analysis

Headspace generation can be considered as a simple sam-ling technique which allows the determination of volatileompounds, at the same time reducing the effect of potentialnterferences presented in the sample matrix as long as these areess volatile than the analyte. HS analysis has been frequentlysed for the direct analysis of olive oil, not only for the individ-al quantification of a broad variety of compounds but also forample characterization and qualitative analysis. However, in

he proposed method a new approach is presented, using a prioriquid–liquid extraction of the samples using a surfactant aque-us dispersion of MWCNTs as extracting medium. This LLEtep was envisaged to enhance both the sensitivity and selectiv-

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live oil sample, (B) mixture of refined and virgin olive oils and (C) virgin oliveil sample. The retention time of each analyte is also indicated.

ty of the determination. In order to confirm the usefulness ofhis proposal a direct comparison of the new methodology withreference method was performed.

For this purpose, two series of blanks refined olive oil sam-

-Xylene 45.4 0.9 0.9992tyrene 39.0 1.1 0.9976

a Sensitivity enhancement factor.b Standard error of SEF.

6 C. Carrillo-Carrion et al. / J. Chromatogr. A 1171 (2007) 1–7

Table 4Effect of the pseudophase’ recovery on the analytical signal

Peak area (fresh pseudophase) Peak area (recovered pseudophase) Variation (%) RSDa (%)

Benzene 4,592,807 4,385,476 4.5 4.0Toluene 5,147,460 4,934,847 4.1 3.6Ethylbenzene 3,991,271 3,882,121 2.7 2.4(m + p)-Xylene 6,806,315 6,642,351 2.4 2.2o 9,290S 9,928

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a Relative standard deviation of the method at the same concentration level (s

iquid–liquid extracted before the analysis. Finally, the resultsere expressed for each analyte in the form:

p = KSc + b

here Sp and Sc are the signals provided by the proposed and theeference method, respectively. K, the slope of the linear regres-ion models, can be considered as the sensitivity enhancementactor (SEF) obtained with the new method for the correspond-ng analyte. Table 3 summarizes the SEF values as well as thetandard error associated obtained for each analyte. The linearttings obtained were acceptable in all instances (R2 > 0.997)nd the intercept (b) of the models (data not given) were neg-igible compared with the slopes (in the range 0.01–0.5%). Asan be seen, a clear sensitivity enhancement is obtained in allases, the lower value being for benzene (20) and the highest for-xylene (45.4). With the prior liquid–liquid extraction step annhancement of sensitivity is obtained despite the distributionoefficient of BTEX-S between a hydrophobic organic phase andhe dispersed MWCNTs phase being lower than 1. This detailan be explained taking into account two facts: (a) Headspaceeneration depends directly on the donor liquid phase natureaqueous or organic) and for BTEX-S, the release to the gashase is favored if an aqueous phase, instead of an organic one, ismployed; (b) At 80 ◦C, the headspace temperature used for thispplication, the MWCNTs dispersion becomes unstable whichavors the transfer of the analytes to the gaseous phase.

The average of the mentioned factors establishes the enhance-ent sensitivity factor for each analyte.

.5. Pseudophase regeneration

The question of recovery of MWCNTs after use is interestingo reduce the cost of the methodology. In order to evaluate theotential reusing of MWCNTs a simple study was developed. Alank refined olive oil sample spiked with 20 ng mL−1 of all thenalytes was analyzed following the proposed method. Aliquotsf the spiked sample were extracted using fresh and regener-ted MWCNTs pseudophase and the chromatograms obtainedere compared in terms of peak area (see Table 4). As can be

een, negligible differences were observed between the resultsif these differences are compared with the RSD of the method)nd the variability can be attributed to the inherent imprecisionf the method.

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. Conclusions

In the present work, the potential of the use of multiwall car-on nanotubes as additives in liquid–liquid extraction has beenemonstrated. The extraction step enhances the sensitivity of theetermination reducing the limit of detection at least 10 timesn comparison with the direct headspace analysis method. Thextraction process reduces the amount of matrix componentshat must be injected into the chromatographic system, whichlearly improves the selectivity of the determination. It alsoncreases the lifetime of the column and other chromatographicomponents.

Compared with the HS-SPME approach, the proposedethod is a valid alternative in terms of sensitivity, precision and

ime of analysis. Although the LLE-procedure is more manualnd requires higher sample’ volume, it is cheaper taking intoccount the price of the SPME fibres and their stability. Carry-ver problems, which sometimes appear in SPME applications,re here avoided since every sample is extracted with a freshseudophase.

cknowledgement

Financial support from the Spanish DGICyT (GrantTQ2004-01220) is gratefully acknowledged.

eferences

[1] R.M. Alberici, G.C. Zampronio, R.J. Poppi, M.N. Eberlin, Analyst 127(2002) 303.

[2] S. Vichi, L. Pizzale, L.S. Conte, S. Buxaderas, E. Lopez-Tamames, FoodControl 18 (2007) 1204.

[3] R.J. Irwin, M. VanMouwerik, L. Stevens, M.D. Seese, W. Basham, Envi-ronmental Contaminants Encyclopedia, Fort Collins, Colorado, USA,1997.

[4] J.L. Perez-Pavon, A. Guerrero-Pena, C. Garcıa Pinto, B. Moreno Cordero,J. Chromatogr. A 1047 (2004) 101.

[5] A. Serrano, M. Gallego, J. Chromatgr. A 1045 (2004) 181.[6] W. Wardencki, J. Curylo, J. Namiesnik, J. Biochem. Biophys. Methods 70

(2007) 275.[7] D.A. Lambropoulou, I.K. Konstantinou, T.A. Albanis, J. Chromatogr. A

1152 (2007) 70.[8] O. Ezquerro, G. Ortiz, B. Pons, M.T. Tena, J. Chromatogr. A 1035 (2004)

17.[9] I. Arambarri, M. Lasa, R. Garcia, E. Millan, J. Chromatogr. A 1033 (2004)

193.10] B. Tang, U. Isacsson, J. Chromatogr. A 1137 (2006) 15.11] A. Przyjazny, J.M. Kokosa, J Chromatogr. A 977 (2002) 143.

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[21] M. Stadermann, A.D. McBrady, B. Dick, V.R. Reid, A. Noy, R.E. Synovec,

C. Carrillo-Carrion et al. / J

12] F. Pena, S. Cardenas, M. Gallego, M. Valcarcel, Anal. Chim. Acta 526(2004) 77.

13] F. Pena, S. Cardenas, M. Gallego, M. Valcarcel, J. Chromatogr. A 1052(2004) 137.

14] S. Vichi, L. Pizzale, L.S. Conte, S. Buxaderas, E. Lopez-Tamames, J.Chromatogr. A 1090 (2005) 146.

15] C. Nerin, C. Rubio, J. Cacho, J. Salafranca, Chromatographia 41 (1995)216.

16] C. Carrillo-Carrion, R. Lucena, S. Cardenas, M. Valcarcel, Analyst 132(2007) 551.

17] G.Z. Fang, J.X. He, S. Wang, J. Chromatogr. A 1127 (2006) 12.

[[[

omatogr. A 1171 (2007) 1–7 7

18] Q. Zhou, J. Xiao, W. Wang, G. Liu, Q. Shi, J. Wang, Talanta 68 (2006)1309.

19] Y. Cai, Y. Cai, S. Mou, Y. Lu, J. Chromatogr. A 1081 (2005) 245.20] J.X. Wang, D.Q. Jiang, Z.Y. Gu, X.P. Yan, J. Chromatogr. A 1137 (2006)

8.

O. Bakajin, Anal. Chem. 78 (2006) 5639.22] M. Karwa, S. Mitra, Anal. Chem. 78 (2006) 2064.23] C. Saridara, S. Mitra, Anal. Chem. 77 (2005) 7094.24] Y. Xu, S.F.Y. Li, Electrophoresis 27 (2006) 4025.