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Determination of benzene, toluene, ethylbenzene and xylenes in soils by multiple headspace solid-phase microextraction

Determination of benzene, toluene, ethylbenzene and xylenes in soils by multiple headspace solid-phase microextraction
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  Introduction Volatile organic compounds (VOC), e.g ., benzene, toluene,ethylbenzene, xylene and cumene (BTEXC), are importantenvironmental contaminants because of their high toxicity andwidespread occurrence. They are present in aviation fuel(gasoline) and are widely used as industrial solvents and rawmaterials for the production of different commodities. 1 Benzene, toluene and ethylbenzene are among compoundsdesignated as “priority pollutants” by the US EPA, and theaction and risk levels of benzene, toluene, ethylbenzene, andxylene are described in the Dutch Government QualityStandards for the Assessment of Soil and WaterContamination. 2 The determination of BTEXC inenvironmental matrices is difficult because of their trace-levelpresence and losses incurred during sample handling, extraction etc . Recently, there has been considerable interest in thedevelopment of SPE columns for the clean-up, extraction, andpre-concentration of liquid samples. 3 SPE applications arefound in different environmental areas, such as soils andsediments, 4,5 industrial effluents 6 and water samples. 7 – 9 SPE hasbeen used for the extraction of BTEXC 10 and pesticides 11 fromsoils and sediments. A few SPE applications have alsoappeared for VOC components in the BTEXC analysis of watersamples; a cartridge containing C 18 adsorbent was used toextract BTEX from industrial effluents 6 and benzene andtoluene from seawater. 7 Although solid-phase micro-extraction(SPME) was also applied to isolate BTEX from water sampleseither by direct adsorption from the liquid 8 or via headspacesampling, 9 the limitation of SPME for the quantitation of sulfur-based VOC has been reported. 12 The VOCs analyzed from airand water samples by open-tubular, wall-coated columns, 13 solvent trapping 14 and semi-VOC from air by atmospheric-pressure chemical ionization mass spectrometry 15 were alsodescribed.Buriganga river water is important because it flows throughthe capital of Bangladesh and has an enormous impact on thesocio-economic development of the country, especiallyindustrial and shipping-marine activities. Tanker-washingsewage, shipping scrap particles and oil spillage are commonfeatures on the river at different ghats (marine terminals). As aresult, the water is continually polluted by various organiccompounds, especially hydrocarbons. In addition, aromaticsolvents are increasingly used in industry, 1 and the wastes aresometimes disposed of in the aquatic environment, whichbecomes increasingly contaminated. To monitor contaminants,we recently determined the concentrations of various normalsaturated hydrocarbons in Buriganga river water 16 and pesticidesin soil 17 by GC, using liquid-liquid extraction and SPE methods,respectively. We also reported on the development of achromatography method for the determination andcharacterization of anionic detergents in river water. 18 Thepresent paper describes the concentration levels of benzene,toluene, ethylbenzene, xylene and cumene in water samplescollected at two depths from the Sadarghat, one of the biggest 1365ANALYTICAL SCIENCES OCTOBER 2003, VOL. 192003 ©The Japan Society for Analytical Chemistry Determination of Benzene, Toluene, Ethylbenzene and Xylene in River Water by Solid-Phase Extraction and Gas Chromatography Mohammad A. M OTTALEB , * † Mohammad Z. A BEDIN , ** and Mohammad S. I SLAM ** *  Department of Chemistry, University of Rajshahi, Rajshahi 6205, Bangladesh **  Department of Chemical Technology and Polymer Sciences, Shahjalal University of Science and Technology, Sylhet 3114, Bangladesh A rapid and reproducible method is described that employs solid-phase extraction (SPE) using dichloromethane, followedby gas chromatography (GC) with flame ionization detection for the determination of benzene, toluene, ethylbenzene,xylene and cumene (BTEXC) from Buriganga River water of Bangladesh. The method was applied to detect BTEXC ina sample collected from the surface, or 5 cm depth of water. Two-hundred milliliters of n -hexane-pretreated and filteredwater samples were applied directly to a C 18 SPE column. BTEXC were extracted with dichloromethane and the BTEXconcentrations were obtained to be 0.1 to 0.37  g ml –1 . The highest concentration of benzene was found as 0.37  g ml –1 with a relative standard deviation (RSD) of 6.2%; cumene was not detected. The factors influencing SPE e.g ., adsorbenttypes, sample load volume, eluting solvent, headspace and temperatures, were investigated. A cartridge containing a C 18 adsorbent and using dichloromethane gave a better performance for the extraction of BTEXC from water. Averagerecoveries exceeding 90% could be achieved for cumene at 4˚C with a 2.7% RSD. (Received October 1, 2002; Accepted July 17, 2003) † To whom correspondence should be addressed.E-mail: Mottaleb.Mohammad@epamail.epa.govM. A. M. present address: EPA/NRC Postdoctoral ResearchAssociate, Environmental Chemistry Branch, EnvironmentalSciences Division, National Exposure Research Laboratory,U.S. Environmental Protection Agency, P.O. Box 93478, LasVegas, NV 89193-3478, USA.  marine terminals, on the Buriganga river. Also discussed arethe recoveries of BTEXC using different adsorbents for SPEcolumns and factors influencing SPE adsorption, such as thesample load volume, eluting solvent, headspace andtemperatures. Experimental  Apparatus and reagents A Varian gas chromatograph (Model 3300) equipped with aflame ionization detector (FID) was used in this study. A DB-1fused-silica mega-bore analytical column (15 m × 0.53 mm i.d.,1.5  m thickness) and a phenyl-methyl deactivated guardcolumn were used. An integrator (Varian Model 4290) gave thepeak area and retention time of the peaks separated by GC-FID.Methanol (HPLC grade), hexane, dichloromethane andchloroform were obtained from Merck Ltd. (Germany). VOCsand internal standards were purchased from Sigma ChemicalsLtd., USA. The C 18 , C 8 and phenyl (PH) (500 mg, 3 ml) SPEcartridges were obtained from the Supelco Ltd. Stock and working standard solutionsStock solution . An aliquot (10  l) of each of the BTEXCconstituents and internal standard dichlorobenzene (DCB) weredissolved in 100 ml of dichloromethane. The concentrations of the BTEXC components corresponded to 87.7, 86.7, 86.7, 86.1,86.1  g ml –1 , respectively and the concentration of DCBcorresponded to 130.5  g ml –1 . Working standard solutions . Four standard solutions weremade for measuring the linearity of the GC response. Theconcentrations of the standard solutions are given in Table 1. Collection of river water samples Contaminated water samples were collected in 1-l dark glassbottles on September 21, 1999 from the Sadarghat. A map of the Buriganga river and the location of the Sadarghat area havebeen reported. 16 Cleaned bottles were rinsed with sample waterprior to sample collection. Ten-liter samples were collected atthe surface and ten samples were also collected at a depth of 5cm. The distance of the sample collection point from the riverbank was about 200 m. Method development work was carriedout using double-distilled water containing BTEXC at the sameconcentrations as those found in the river water.  Extraction of VOC from synthetic and river water samples A 10 ml volume of a synthetic sample was extracted with 10ml of hexane in a 30 ml vial; the layers were allowed toseparate. Prior to SPE work, 2 to 4 ml of the organic layer wasremoved and stored in a sealed glass vial at 4˚C. The extractionof a river-water sample was carried out essentially according toa reported method. 16 Each (500 ml) water sample was shakenvigorously for 30 min with 50 ml of hexane at 4˚C. Theaqueous layer was separated and extracted again with 25 ml of hexane. The combined extracts were then stored at 4˚C forSPE. Prior to direct use of the water sample in SPE, the sample(200 ml) was filtered by a 0.45  m nylon membrane. Solid-phase extraction The column was activated with 3 ml 50% methanol and pre-equilibrated with 3 ml 1% methanol. The river-water (100 ml)sample was loaded on the column at 3 ml min –1 . Elution wascarried out with 2 portions of 2 ml aqueous 1% methanol.Finally, solutes were eluted with two aliquots of 2 ml of dichloromethane. Similar elution profiles were obtained forrecovery experiments. DCB (200  l or 5%) was added as aninternal standard prior to a GC analysis. Passing samplesthrough a dryer containing sodium sulfate only eliminated tracesof water. Calculation of response factor and concentration of components The relative response factor of a component (  R F ) to theinternal standard of DCB is given by  R F = × ,where C  DCB and C  c represent the concentrations of DCB and thecomponent analyte, respectively, in terms of  g/ml. The terms  A DCB and  A c indicate the peak-area counts from the integrator forDCB and the component analyte, respectively. The responsefactors for all components were calculated as mentioned above,and the concentration of each component ( C  c ) was calculated asfollows: C  c = × . C  DCB ———  R F  A c ———  A DCB  A c —— C  c C  DCB ———  A DCB 1366ANALYTICAL SCIENCES OCTOBER 2003, VOL. 19 Standard 18.778.678.678.618.61Standard 217.1417.3417.3517.2217.22Standard 326.3126.0126.0125.8325.83Standard 435.0834.6834.6834.4434.44SolutionBenzeneTolueneEthylbenzeneXyleneCumeneConcentration of BTEXC standard solution/  µ g ml –1 Table1Concentration of a BTEXC standard solution used for GC optimizationFig.1GC-FID detection chromatogram for a standard solution of BTEXC, and internal standard (dichlorobenzene). Conditions: gasflow rates: N 2 (carrier gas), 4 ml min –1 ; H 2 , 33 ml min –1 ; air, 330 mlmin –1 ; detector temperature, 180˚C; injector temperature, 180˚C;mega-bore, DB-1 fused-silica analytical column; oven temperatureprogram: 35˚C for 5 min, to 70˚C at 5˚C min –1 , to 180˚C at 15˚Cmin –1 and hold 10 min at 180˚C; and sample injected volume, 1  l.  Results and Discussion GC optimization The GC-FID system used was optimized before the VOCmeasurement. Separations were achieved with differenttemperature programs. A good separation of individual BTEXCconstituents, including DCB, was obtained under the followingconditions: injector temperature, 180˚C; column oventemperature, 35˚C for 5 min, to 70˚C at 5˚C min –1 , to 180˚C at15˚C min –1 and hold at 180˚C for 10 min. The detectortemperature was 180˚C and the flow rate of the nitrogen carriergas was 4 ml min –1 . Figure 1 is a GC-FID chromatogram of astandard solution of the BTEXC components, showing that thecomponents and DCB were well-resolved. The linearity of thedetector response was also demonstrated by injecting theworking standard solutions into the GC-FID instrument. Figure2 depicts the calibration graphs of the peak area versus theconcentration. The FID gave good linearity of the response forthe detection of each of the BTEXC constituents. Hence, it wasdecided that the above conditions could be used for thedetermination of BTEXC from the river-water samples. Presence of VOC in water samples Table 2 summarizes the concentration of BTEXC in the river-water samples analyzed by SPE-GC-FID. The presence of ethylbenzene, xylene and cumene was not detected using theexperimental conditions described previously, although tracelevels of benzene and toluene were found. To detect the otherconstituents of the BTEXC family, an increased volume of 200ml of water sample was directly applied to the SPE at 4˚C. Anappreciable amount of benzene and toluene, including tracelevels of ethylbenzene and xylene, were obtained in the river-water sample. However, cumene was never found. Figure 3shows a representative GC-FID chromatogram of the river-water samples. The chromatograms in Figs. 3(A) and 3(B)correspond to the surface and 5 cm depth of water, collectedfrom the Buriganga river. These were obtained when 2  l SPEeluted samples were injected into the GC and showed a similarchromatographic elution pattern with different magnitudes of the BTEX components peak. Blank experiments wereperformed prior to sample injection. Selection of adsorbents and eluting solvent  To select the suitability of adsorbents and eluting solvents, thepercentage recovery of BTEXC constituents was investigatedusing C 18 , C 8 and PH cartridges with CH 2 Cl 2 and CHCl 3 solvents. The recoveries obtained when 2 ml portions of standard VOC solutions were passed through different SPEcolumns and eluted with two portions of 2 ml of dichloromethane or chloroform at 4˚C. The recovery results arepresented in Table 3. The extractions were performedsimultaneously for each solvent. Regardless of the solventsused, higher recoveries were obtained for xylene and cumene.This may have been due to a less evaporative loss of the twocomponents because of their higher boiling point. Moreover, incomparison between CH 2 Cl 2 and CHCl 3 solvents, it wasobserved that slightly better recoveries were obtained whenCH 2 Cl 2 was used as the eluting solvent (Table 3). This isprobably due to a more non-polar interaction between a bonded 1367ANALYTICAL SCIENCES OCTOBER 2003, VOL. 19 Fig.2Calibration curve for optimization of the GC-FID system forthe analysis of BTEXC in river-water samples. Concentration of theinjected BTEXC solution (please see Experimental, preparation of stock and working standard solutions). The operating conditions arethe same as in Fig. 1.Benzene0.350  ±  0.0349.80.372  ±  0.02306.2Toluene0.234  ±  0.03314.30.167  ±  0.028917.4 Ethylbenzene 0.145  ±  0.023015.80.104  ±  0.020920.2Xylene0.124  ±  0.020616.10.144  ±  0.01359.36CumeneAnalyte0 cm depthAverageconc.SD a RSD, b %Averageconc.SD a RSD, b %5 cm depth Concentration of BTEXC component in river water/  µ g ml –1 ——————Table2SPE of river-water samples at 4˚C (C 18  column, sample volume 200 ml)a. SD represents standard deviation, which was calculated from each of three measurements. b. RSD means relative standard deviation.Fig.3GC-FID chromatograms for the river-water samples. For(A), water was collected from the surface, and for (B), water wascollected from 5 cm depth. The operating conditions are the same asin Fig. 1.  phase and the CH 2 Cl 2 system.  Effect of the sample load volume The effect of the sample volume on the SPE recovery is oneof the most important factors, because the SPE performance isaffected by the amount of sample volume loaded on a particularcolumn. 6,10 The break-through volume of a C 18 column (500mg, 3 ml capacity) was determined by passing a number of VOC standard solutions to a volume of up to 500 ml. Knownmasses of the analytes were introduced. There were noappreciable changes in the recovery rates up to a sample volumeof 400 ml. The percentage recovery of benzene and toluenedecreases more rapidly than ethylbenzene, xylene and cumene.This observation confirms the fact that ethylbenzene, xyleneand cumene possess higher breakthrough volumes than benzeneand toluene (Fig. 4). Similar break-through volume curves wereobtained for C 8 and phenyl substituted SPE columns. These arenot shown. Factors affecting the SPE performance investigationsTemperature . To investigate the effect of the temperature onSPE performance, a cartridge containing C 18 material wasemployed with CH 2 Cl 2 solvent at temperatures of 20˚C and 4˚C.An effect of temperature on the SPE recovery of BTEXCconstituents was found to occur (Table 4). It has been observedthat both liquid-liquid and solid-phase extractions providedslightly better recoveries when experiments were carried out ata temperature of 4˚C. The relative standard deviations (RSD)were calculated for each of the BTEXC constituents andtemperatures. At 20˚C, the RSD of recoveries were between 3.3and 4.0%; however, at 4˚C improved recoveries of theconstituents were achieved with RSD values of 2.7 to 3.5%.  Headspace . In order to investigate the effect of headspace,recovery experiments for each of the BTEXC components wereperformed using a C 18 column in an environment at atemperature of 4˚C. This temperature provided recoveries of 79to 91% of the constituents (Table 4). Parallel extractions of thecomponents with and without headspace were performed. Theresults are presented Table 5. It can be seen that the incurredlosses of the BTEXC constituents occurred in the range of 9 to12% due to allowing a headspace. Since trace levels of BTEXCcomponents could be found in water, or other real samples, theabove losses (9  – 12%) are quite significant. Thus, it wasdecided that the existence of a headspace increases the 1368ANALYTICAL SCIENCES OCTOBER 2003, VOL. 19 CH 2 Cl 2 C 18 76.0   2.580.5   2.583.7   3.587.1   4.090.3   3.3C 8 70.0   3.568.3   2.074.6   3.583.7   1.579.6   1.4PH73.0   1.577.8   3.570.4   4.076.3   1.582.5   2.3CHCl 3 C 18 74.0   3.078.0   2.580.0   2.084.0   3.087.0   4.0C 8 69.0   3.567.0   2.572.0   3.080.0   2.577.0   3.0PH71.2   3.076.0   3.069.0   4.075.0   3.078.0   2.5SolventBenzeneTolueneEthylbenzeneXyleneCumeneRecoveries (%)   standard deviation (SD) a SPEcolumnTable3Recovery of BTEXC from different SPE columns eluted with CH 2 Cl 2  and CHCl 3  at a temperature of 4˚Ca. SD values were calculated from each of five measurements.Fig.4Effect of the sample load volume on the recoveries of theBTEXC constituents with a C 18 SPE column using dichloromethanesolvent.Benzene76    3.03.979    2.83.5Toluene82    2.93.586    2.52.9Ethylbenzene84    3.33.991    3.23.5Xylene87    3.54.088    2.73.1Cumene89    2.93.391    2.52.7AnalytesAt a temperature of 20˚CAt a temperature of 4˚CRecovery,%SD a RSD b ,%Recovery,%SD a RSD b ,%Table4Effect of the temperature on the recovery of BTEXC with CH 2 Cl 2  using a C 18  cartridgea. SD represents standard deviation, which was calculated from each of five measurements.b. RSD means relative standard deviation.Benzene67.0   2.076.0   3.011.8Toluene72.0   3.581.0   2.511.1Ethlbenzene74.0   3.584.0   2.511.9Xylene79.0   2.787.0   4.09.2Cumene82.0   2.890.0   3.58.9AnalyteRecovery, %SD a Recovery, %SD a HeadspaceNo headspaceRecoverydifference, %Table5Effect of the headspace on the recovery of BTEXC with CH 2 Cl 2  using a C 18  cartridge at 4˚Ca. SD represents the standard deviation, which was calculated from each of five measurements.  evaporative losses, which decreases the percentage recovery.This study was conducted while allowing no headspace in thesampling and at a temperature of 4˚C. Acknowledgements The authors would like to thank the Bangladesh Universitygrants commission for financial support to carry out thisresearch. References 1.J. J. G. Klist, in “ Chemistry and Analysis of VOCs in the Environment  ”, ed. H. J. T. Bloemen and J. Burn, 1993 ,Glasgow.2.Environmental Quality Standards for Soil and Water,Netherlands Ministry of Housing, Physical Planning andEnvironment, 1991 , Leidschendam.3.L. A. Berrueta, B. Gallo, and F. Vicente, Chromatographia , 1995 , 40 , 474.4.P. Loconto,  LC-GC Int  ., 1991 , 4 , 10.5.Z. Zhang and J. Pawliszyn,  Anal. Chem ., 1995 , 67  , 34.6.I. S. Deans, C. M. Davidson, D. Littlejohn, and L. Brown,  Analyst  , 1993 , 118  , 1375.7.W. A. Saner, J. R. Djadamec, R. W. Sager, and T. J. Kleen,  Anal. Chem ., 1979 , 51 , 2180.8.C. L. Auther, J. Pawliszyn, and P. R. Belardi,  J. High. Res.Chromatogr  ., 1992 , 15 , 741.9.Z. Zhang and J. Pawliszyn,  Anal. Chem ., 1993 , 65 , 1843.10.K. M. Meney, C. M. Davidson, and D. Littlejohn,  Analyst  , 1998 , 123 , 195.11.M. J. Redondo, M. J. Ruiz, B. Boluda, and G. Font, Chromatographia , 1993 , 36  , 147.12.R. A. Murray,  Anal. Chem ., 2001 , 73 , 1646.13.B. C. D. Tan, P. J. Marriott, H. K. Lee, and P. D. Morrison,  Analyst  , 2000 , 125 , 469.14.M. A. Stone and L. T. Taylor,  Anal. Chem ., 2000 , 72 , 1268.15.L. Charles, L. S. Riter, and R. G. Cooks,  Anal. Chem ., 2001 , 73 , 5061.16.M. A. Mottaleb, M. Ferdous, M. S. Islam, and M. A.Hossain,  Anal. Sci ., 1999 , 15 , 995.17.M. A. Mottaleb and M. Z. Abedin,  Anal. Sci ., 1999 , 15 ,283.18.M. A. Mottaleb,  Mikrochim. Acta , 1999 , 132 , 31. 1369ANALYTICAL SCIENCES OCTOBER 2003, VOL. 19
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