Isomeric Analysis of BTEXs in the Atmosphere

Chemosphere 63 (2006) 502–508

www.elsevier.com/locate/chemosphere

Isomeric analysis of BTEXs in the atmosphere usingb-cyclodextrin capillary chromatography coupledwith thermal desorption and mass spectrometry

Noureddine Yassaa a,*, Enzo Brancaleoni b,Massimiliano Frattoni b, Paolo Ciccioli b

a Faculty of Chemistry, University of Science and Technology Houari Boumediene, USTHB, BP 32, El-Alia,

Bab-Ezzouar, 16111 Algiers, Algeriab Istituto di Metodologie Chimiche—C.N.R., Area della Ricerca di Roma, Via Salaria Km 29.3, C.P 10, 00016,

Monterotondo Scalo RM, Italy

Received 22 April 2005; received in revised form 25 July 2005; accepted 8 August 2005Available online 20 December 2005

Abstract

An analytical method capable of determining trace levels of BTEX-aromatics (benzene, toluene, ethylbenzene, m-, p-and o-xylenes) in the atmosphere with as high resolution as possible has been developed. The method is based on thepreconcentration of air samples using a multibed tube (Carbopack C, Carbograph 1) at ambient temperature, followedby thermal desorption, and analysis of aromatic species by a b-cyclodextrin capillary chromatography coupled withmass spectrometry. The resolution achieved was sufficient for individual separation of BTEXs as well as m- and p-xyl-enes. The BTEX-ratios have been determined in an air tunnel and in on-road, suburban and rural forest atmosphere.The ethylbenzene/m-xylene ratios could provide a deep insight into anthropogenic related NMHC patterns at differentlocations and under different meteorological conditions and may reflect photochemical processes in the best way.� 2005 Elsevier Ltd. All rights reserved.

Keywords: VOCs; BTEXs; Air tunnel; b-cyclodextrin capillary chromatography; Graphitic carbon adsorbents; Thermal desorption;GC/MS

1. Introduction

Today aromatic hydrocarbons represent about 30%of all non methane hydrocarbons (NMHCs) in urbanair (Becker, 1994). The major source of monocyclic aro-matic hydrocarbons (MAHs) in these areas is anthropo-genic production (Dearth et al., 1992). In urban regions

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* Corresponding author. Fax: +213 212 473 11.E-mail address: [email protected] (N. Yassaa).

automobiles are the dominant source of benzene, tolu-ene, ethylbenzene and the isomers of xylene, commonlycalled BTEXs (Edgerton et al., 1989). These compoundsare added to fuels to increase the octane number and areemitted to the urban atmosphere as a component ofautomobile exhaust and by gasoline evaporation andspillage (Clark et al., 1984). Since in recent years higheramounts of these aromatics are added to substitute forlead, the urban atmospheric concentration of these com-pounds has increased considerably (Perry and Gee,1994). Additional sources of these MAHs are emission

ed.

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N. Yassaa et al. / Chemosphere 63 (2006) 502–508 503

from the chemical industry, waste incinerator and com-posting facilities (Eitzer, 1995) and household chemicals(Sack et al., 1992).

Apart from contributing to urban pollution them-selves by their carcinogenic and mutagenic effects on liv-ing organisms and human health (Shepson et al., 1985;Dumdei et al., 1988), BTEXs take part in reactions pro-moting photochemical smog. Owing to their high reac-tivity towards the OH-radical (Table 1) they cancontribute significantly to chemical processes leadingto ozone formation in the troposphere (Atkinson,1989, 1990, 1994, 2000). As shown in Table 1 the reac-tion rates of BTEX-compounds with OH cover a relativewide range from low-reactive benzene to m-xylene thatexhibits reaction rates comparable to those of alkenes.

In or near an urban environment where severalsources of BTEX-emitters are present, the concentra-tions of alkylated benzenes in the atmosphere do oftenexceed those of benzene and do resemble the overallemissions of the different sources (Field et al., 1992).At rural sites the benzene concentrations are higher rela-tive to those of toluene or other alkylated benzenes.These findings have been explained by several authorsto be the result of an accelerated atmospheric degrada-tion of alkylated benzenes relative to benzene (Atkinson,1990; Clarkson et al., 1996). However, to attribute theinfluence of individual emitters like the road traffic,not only should car fleet be known, but also temporaland spatial variation of the BTEX ratios.

The measurement of toxic organic chemicals, such asbenzene, toluene, xylenes, styrene, and 1,3-butadiene, inambient air is currently of great practical interest. Eval-uation of emission control efforts, determination ofhuman health risks, and confirmation of required reduc-tions in those risks, all require ambient measurements ofthese and related volatile organic compounds (VOCs).

The most common method for monitoring BTEXs inair at the low microgram per cubic meter concentrationlevel involves collecting samples with containers (e.g.,plastic bags, glass or metal canisters) or solid traps (e.g.,

Table 1Rate constants KOH for the gas-phase reactions of OH withBTEX-compounds according to Atkinson (1990) and typicallife times si for BTEX-compounds, respectively

BTEX-compound 10�12 · KOH

(cm3 molecule�1 s�1)si (h)

Benzene 1.23 56.5Toluene 5.96 11.7Ethylbenzene 7.1 9.8o-Xylene 13.0 5.1p-Xylene 14.3 4.9m-Xylene 23.6 2.9

Life time si was calculated using data from Simpson (1995).According to Simpson average European summer time OH-concentrations at noon are about 4 · 106 (molecule cm�3).

Tenax, XAD, silica gel, or carbon) followed by the com-bination of gas chromatography (GC) with either a flameionization detector (FID) or mass spectrometry (MS).Recently novel commercial techniques, typically theDifferential Optical Absorption Spectroscopy (DOAS)(Platt, 1994) and on-line BTEX-measurements, have beendeveloped for routine BTEX-monitoring in the atmo-sphere.

In general, the requirements of sampling and analy-sing devices can be summarized as follows: (i) regardlessof the method employed, the sampling must be represen-tative and quantitative, (ii) the sampling procedureshould be so simple that it can be performed by unskilledpersons and under difficult circumstances, e.g., in regionswhere no electric power supply is available, (iii) no dete-rioration or losses of the sample between the samplingand the analysis most occur and (iv) the chromato-graphic columns must separate as many components aspossible.

Sorbent traps are well suited for field samplingbecause they are easy to handle and use simple equip-ment and adsorbed compounds are later recovered bysolvent elution or thermal desorption. Regardless thefact that DOAS and on-line BTEX measurements areexpensive instruments and they require electric powersupply, their utilization is not straightforward and ofteninvolves drastic calibration conditions.

As shown in Table 1 (Atkinson, 1990), BTEXs exhi-bit different reactivity towards the OH-radical andozone-forming potential in the troposphere, the deter-mination of individual aromatic hydrocarbons is ofparticular interest. Various authors have used specificBTEX-ratios for either discriminating emission sourcesor photochemical processes (Nelson and Quigley, 1983;Rappengluck and Fabian, 1998). Since m-xylene is themost reactive BTEX-compound, to obtain a realisticpicture of photochemical activity it is imperative todetermine its single concentration independently to thatof p-xylene.

Because they are not separated with ordinary capil-lary columns the aerial concentrations of m- and p-xyl-enes are given together in the most of NMHCmeasurements in air and the BTEXs-ratios are oftendetermined considering the total amounts of the bothisomers. Although xylene isomers are now well sepa-rated with specific stationary phases such as liquid crys-tals or cyclodextrins, their qualitative evaluation in airsamples were attempted only recently in Algiers urbanatmosphere using charcoal sorbents followed by solventelution and b-cyclodextrin capillary chromatographyequipped with a flame ionization detector (Yassaaet al., 1999). Due to their low thermal stability, charcoalsorbents require liquid elution as a method for com-pound extractions. However, large air volumes (ca.100 l) are required to meet the sensitivity of the detectionsystem and hence the sampling times are too long (8 h)

504 N. Yassaa et al. / Chemosphere 63 (2006) 502–508

to give useful information in situation in which theconcentration of a chemical changes rapidly due to thevariation of emission strength or atmospheric photo-chemical activity. These limitations have provided usthe impetus for developing simple, inexpensive, nottime-consuming and sensitive method based on b-cyclo-dextrin capillary chromatography/mass spectrometrycombined with a preconcentration device using carbontraps and a thermal desorption apparatus. This methodhas been successfully applied in the evaluation of enan-tiomeric monoterpenes discrimination in terrestrial plantemission and in the atmosphere (Yassaa et al., 2001).

In this study, a quantitative evaluation of BTEX-aro-matics was accomplished in tunnel air and in ambientatmosphere of urban, suburban, and rural forest areasby combined use of carbon traps with multi-layer bedsfor air samples collection and thermal desorption systeminstalled on GC–MS equipped with a b-cyclodextrincapillary column. The BTEX-ratios were evaluated toassess the relative reactivity of individual species in theatmosphere characterized by different typology.

2. Experimental

2.1. Site description

Air samples were collected in: (1) the biggest tunnelof Rome located in the heart of the city, (2) on-roadat Viale di Liegi characterized by rush traffic and locatedin the Rome city center, (3) a big green park at Vila-Adasituated at about 15 km far from Rome city center and(4) Montelibretti located at about 30 km from Romeand characterized by very weak traffic.

2.2. Air sample collection and analysis

Air samples were enriched on glass tubes (15 cm ·0.3 cm I.D.) filled with a bed of Carbopack C (0.034 g)and Carbograph 1 (0.17 g), set in series. The graphiticmaterials from 20 to 40 mesh particle size were suppliedby Supelco (Bellefonte, PA, USA) and LARA s.r.l.(Rome, Italy), respectively. The features of these newmaterials and their efficiency were discussed elsewhere(Brancaleoni et al., 1999). Before sample collection, car-tridges were cleaned at 300 �C under a flow-rate ofhelium (100 ml min�1). After 10 min purging, the trapswere closed with tight connectors and stored in large(10 l) sealed glass containers till ambient air samplingwas performed. Air was drawn through the adsorptiontraps by using battery operated samplers (Alpha 1 type)supplied by Ametek (Mansfield & Green Division,Largo, FA, USA). Flow rates ranging from 200 to330 ml min�1 were used for sample collection. Aftersample collection, traps were wrapped in aluminium foiland stored at room temperature until they were sub-

jected to chemical determinations. After removing oxy-gen and the excess of water from the adsorbents by aback-flushing step, traps were thermally desorbed at250 �C and VOCs cryofocused in an empty liner keptat �120 �C. Injection into the capillary column wasachieved by fast heating the liner from �150 to+150 �C in 10 s.

The separation of desorbed VOCs was performed ona b-cylodextrin capillary column (CYCLODEX-B,30 m-long, 0.256 mm ID, 0.25 lm film thickness) sup-plied by J&W Scientific (California, USA). It was con-nected to a HP 5890 gas chromatograph coupled witha HP 5970B mass selective detector (MSD) (HewlettPackard instruments, Palo Alto, CA, USA). Thermaldesorption of sampled tubes was performed by connect-ing a Chrompack (Middleburg, The Netherlands) TCT/PT1 CP4001 desorption unit to the gas chromatograph.The MSD was operated in electron impact mode withthe following conditions: potential ionization 70 eV;source temperature 230 �C; transfer line temperature280 �C and scan range from 20 to 250 m/z. A scan fre-quency of 3 scans s�1 was used for generating the masschromatogram. Selective detection of arene compoundswas achieved by plotting the current profiles generatedby the most specific fragments generated by electronimpacts. Positive identification was obtained by combin-ing the mass spectral information with the elutionsequence obtained through the analysis of pure com-pounds.

3. Results and discussion

Since both the initial temperature and the tempera-ture program are quite critical for the separation of iso-mers on cyclodextrin coated columns, several tests wereperformed to achieve the best resolution not only forgeometric isomers but also for optic isomers (Yassaaet al., 2001). Tests carried out by changing the initialtemperature of the column from 30 �C to 60 �C and tem-perature gradient from 1 to 3 �C min�1 showed that thebest compromise between analysis time, the resolutionof the aromatic species present in the standard mixtureand their separation from all other VOCs was achievedby keeping the column at 40 �C for 5 min and byincreasing the temperature of the oven up to 200 �C ata rate of 1.5 �C min�1.

To simulate the adsorption and desorption processesto which air samples were subjected, liquid standardmixtures containing toluene, m-, p- and o-xylenes andethylbenzene were injected into the capillary columnusing the thermal desorption system. Small aliquots(usually 1–2 ll) of the standard mixture were injectedinto the adsorption traps under a flow of helium of100 ml min�1. The sample was ready for the analysisafter the bulk of methanol was removed by the trap

Table 2Elution order of some VOCs separated with CP-SIL 8 CB andCYCLODEX-B capillary columns

Elution order in CP-SIL 8 CB Elution orderin CYCLODEX-B

Benzene n-Heptanen-Heptane BenzeneToluene n-Octanen-Octane TolueneEthylbenzene n-Nonanep/m-Xylenes p-Xylene

m-Xyleneo-Xylene Ethylbenzenen-Nonane o-Xylenen-Propylbenzene 1-Ethyl-3-methylbenzene1-Ethyl-3-methylbenzenes n-Propylbenzene


and aromatic hydrocarbons distributed through theadsorbing beds. This was obtained by passing 4 l ofhelium through the cartridge. Fig. 1 displays an exampleof extracted ion chromatogram at m/z 91 that wasobtained by submitting to GC/MS analysis of this stan-dard mixture.

The xylene isomers as well as m- and p-xylenes wereresolved at baseline and each compound was eluted farfrom the other. The typical BTEX elution sequencecommonly observed in ordinary stationary phases likeCP SIL 8 CB was somewhat reversed for certaincompounds using cyclodextrins as phases (Table 2). Inparticular, ethylbenzene eluted after p-xylene and m-xylene and before o-xylene. As these compounds presentsubstituents at isomeric positions, this can be attributedto the conic structures of b-cyclodextrin which leads tothe formation of inclusion complexes.

The first experiments were aimed at assessing that thetrapping materials and the thermal desorption processwere not a source of artifacts in the analysis of aromaticcompounds. These tests were performed by comparingthe theoretical values of, p-xylene/m-xylene and ethyl-benzene/m-xylene ratios present in a standard mixturewith those obtained by submitting the mixture to GC–MS analysis after adsorption of the BTEX mixture onsolid sorbents and its transfer into the chiral columnby thermal desorption. To check for possible decompo-sition effects arising from the chemical nature of thesolid sorbents, experiments were performed with trapsfilled with graphitic carbons and Tenax TA. The latteradsorbent was selected as reference material in ourexperiments because it is the most widely used for thequantification of monoterpenes in plant emissions andit is sufficiently inert to prevent the thermal degradation

7.00 8.00 9.00 10.00 11.00

5000

10000

15000

20000

25000

30000

35000

Time-->

Abundance1

Fig. 1. Extracted ion chromatogram at m/z 91 of standard mixtureo-xylene (5).

of these components in air samples free from ozone(Larsen et al., 1997). The results obtained during theseexperiments are reported in Table 3. They show thatthe combination of carbon adsorbents and Tenax TAprovided consistent results that only slightly deviatedfrom theoretical values. Direct injection of pure com-pounds into the columns showed that these smalldiscrepancies were not originated by the thermal treat-ment of the sample but by changes in composition ofthe liquids stored in the bottles. Pure aromatic compo-nents showed a lower purity than that declared by thecompany, probably due to contamination or partial deg-radation. This was possible because liquids were storedfor long time in the laboratory (ca. 2 years) and oftenused for the preparation of standard mixtures ofdifferent aromatics.

12.00 13.00 14.00 15.00 16.00

2

3

4

5

of toluene (1), p-xylene (2), m-xylene (3), ethylbenzene (4) and

Table 3Comparison between the theoretical values of some BTEX-ratios in the standard mixture and those experimentallymeasured by using adsorption traps filled with graphiticcarbons and Tenax TA

Compounds Theoretical Graphiticcarbon

Tenax TA

p-Xylene/m-xylene

1.20 ± 0.05 1.27 ± 0.03 1.15 ± 0.03

Ethylbenzene/m-xylene

1.35 ± 0.05 1.42 ± 0.03 1.35 ± 0.03

Table 4Individual and total concentrations (lg m�3) of BTEXsrecorded in air tunnel and in ambient air at Viale di Liegi (V.di Liegi), Vila Ada (V. Ada) and at Montelibretti (MTL)

Compounds Tunnel V. di Liegi V. Ada MTL

Benzene 68 35 2.7 0.5Toluene 367 156 11.8 0.9p-Xylene 79 33 2.1 0.1m-Xylene 183 71 4.6 0.2Ethylbenzene 78 33 1.9 0.1o-Xylene 106 42 2.4 0.1

Total 881 370 25.5 1.9

Benzene/toluene 0.19 0.22 0.23 0.55p-Xylene/m-xylene 0.43 0.46 0.46 0.50Ethylbenzene/m-xylene 0.43 0.46 0.41 0.50

Benzene/toluene, p-xylene/m-xylene and ethylbenzene/m-xyleneratios are also reported.

Table 5BTEXs to benzene ratios obtained in air tunnel and in ambientair at at Viale di Liegi, Vila Ada and at Montelibretti

Sites Tunnel V. diLiegi

V. Ada MTL

Toluene 5.38 4.44 4.43 1.94p-Xylene 1.15 0.93 0.78 0.19m-Xylene 2.69 2.03 1.72 0.39Ethylbenzene 1.15 0.94 0.71 0.27o-Xylene 1.56 1.20 0.89 0.29Styrene 0.06 0.05 0.03 0.01Isopropylbenzene 0.08 0.06 0.05 0.011,3,5-Trimethylbenzene 0.47 0.32 0.26 0.05(1)-Methylethylbenzene 0.51 0.34 0.30 0.07(2+3)-Methylethylbenzene 1.08 0.76 0.69 0.16n-Propylbenzene 0.33 0.23 0.22 0.081,2,4-Trimethylbenzene 1.81 1.20 1.01 0.251,2,3-Trimethylbenzene 0.29 0.20 0.21 0.04


The repeatability of the sampling and analyticalmethod is here expressed as the relative standard devia-tion (RSD) determined in the basis of peak areasrecorded for more that triplicate samples. The RSDwas in general within an acceptable level and rangedfrom 3% to 10%.

Automobile sources are relatively difficult to estimatefrom laboratory experiments because of differences invehicle types, fuel, and vehicle operating modes underon-road driving conditions. A tunnel atmosphere pro-vides appropriate conditions for the in situ measurementof the average composition of vehicular emissions,because the measured concentrations of exhaust emis-sions are significantly higher than levels in ambient air.Tunnels also offer the advantage of providing an accu-rate appraisal of the traffic composition and the volumeinto which these emissions are released (Touaty andBonsang, 2000). For these reasons, the air tunnel wasselected in this study as representative of direct auto-vehicular emissions.

Individual and total concentrations of BTEX-aro-matics recorded in air tunnel and in ambient air at Vialedi Liegi, Vila Ada and at Montelibretti are summarizedin Table 4. At each investigated site and during the samejourney, three samples were collected at around 10 A.M.in the morning. In order to determine the impact of pho-tochemical processes on BTEX-compounds it is prere-quisite to consider specific BTEX-ratios. The benzene/toluene, the p-xylene/m-xylene, the ethylbenzene/m-xylene ratios were selected here and are reported inTable 4.

Regarding the content of BTEX-species in air tunneland on-road (Via di Liegi) toluene is the predominantaromatic followed by m-xylene. It is worth noting thatthe concentration of m-xylene that is supposed to bethe most reactive xylene isomer (Table 1) is almost dou-ble the corresponding value of p-xylene.

In Table 5, BTEXs were used to form ratios withbenzene, a compound emitted anthropogenically, pre-dominantly through exhaust processes. Photochemi-cal influences seemed to lead to a more rapid decayfor the faster reacting BTEXs resulting in decreasingBTEXs/benzene ratios going from emission sources

(air tunnel) to rural forest site (Montelibretti). In fact,benzene is of relatively low importance in terms of pho-tochemical activity. Though found in considerableamounts in ambient urban air its low reaction ratesmakes it a BTEX species of almost marginal importancein urban air at least as far as its photochemical relevanceis concerned. However, benzene remains a hazardouscompound due to its well-documented carcinogenicity.Table 5 also demonstrates the importance of the atmo-spheric photochemical reactivity of aromatic com-pounds. Their high reactivity was mirrored by thelowest values of BTEXs/benzene ratios recorded atMontelibretti. Owing to its fast build up, m-xyleneexhibited the strongest spatial variation with respect tothe others. In particular, the value of m-xylene/benzeneratio in Montelibretti during is �6 times lower than that


in air tunnel. Whilst the combustion processes favourthe formation of benzene through cracking high aro-matic compounds and since xylene isomers could haveadditional sources other than auto-exhaust emission,the best way could be obtained only by considering com-pounds having the same origin. According to Nelsonet al. (1983), besides traffic emissions primary sourcesfor ethylbenzene and the xylene isomers are releases ofsolvents. They are often co-emitted and have source fin-gerprints that do not differ a lot (Nelson and Quigley,1983). It appears therefore that only p-xylene/m-xyleneand ethylbenzene/m-xylene ratios may reflect photo-chemical processes in the systematic way. It appears alsofrom Table 4 that these ratios provide a good tool forestimating the ageing of hydrocarbons. During daytimethe ethylbenzene/m-xylene and p-xylene/m-xylene ratioswere higher due to photochemical processes. Moreover,the both ratios exhibited the similar trends and overallsimilar values. Since m-xylene reacts significantly fasterwith OH-radical than p-xylene, the ethylbenzene/m-xylene ratio alone should be even more sensible withrespect to photochemical processes.

On the other hand, since m- and p-xylenes do nothave biogenic sources thus differences between urban,suburban and rural sites should be significant. Thoughm- and p-xylenes median values appear to be an ade-quate quantity for site discrimination, this approach isdifficult to assess when the fast build up of m-xylene isconsidered. At rural sites m-xylene is often depleted dur-ing daytime making it impossible to calculate mean diur-nal variation based on a minimum data set. The diurnalpatterns reflect not only chemical removal but also emis-sions, transport and dilution. Chemical removal ofBTEXs will only occur through reaction with the OH-radical during daytime and the important contributionto the enhancement of ozone production by the oxida-tion of m-xylene may not be ruled out. The photochem-ical impact of the other BTEX-aromatics are estimatedto be between the potentials of m-xylene and benzene.

Indeed, the emission of anthropogenic compoundswill mostly follow traffic patterns, while transport anddilution are influenced by the synoptic weather circula-tion, and the spatial and temporal variation of themixing height. Usually urban areas are marked byVOC-limitation of ozone formation due to relative highemissions of NOx. Ozone formation in rural areas, how-ever, is determined by NOx-limitation. Previous investi-gations carried out in Montelibretti which is situatedinside the Tiber Valley have shown that transport ofanthropogenically polluted air masses from the urbanarea of Rome is the main source of VOCs in that site(Ciccioli et al., 1999) and that local sources are notstrong enough to explain the content of BTEXs in air.This implies that the urban/rural transition zones aresubject to VOC-limitation, at least as long as the urbanplume stretches over these areas.

This is in good agreement with theoretical assump-tion made by Bowman and Seinfeld (1994). Accordingto their investigations aromatic compounds, especiallyxylene isomers and toluene are most effective in termsof photochemistry in areas where VOC-limitation pre-vails. As a matter of fact, xylene isomers were verylow at Montelibretti and one could not expect significantdifference in ethylbenzene/m-xylene ratios between emis-sion sources, on road and rural site.

The benzene/toluene ratio, a specific ratio often asso-ciated with traffic related processes and used as a tool forcharacterizing the distance from vehicular emissionsources (Gelencser et al., 1997), lies in very low rangein tunnel (ca. 0.19) and in the similar values in on-roadat Viale Liegi, and at Vila Ada and Montelibretti duringnighttime (�0.22–0.23). Typical values for benzene/tolu-ene ratios for exhaust processes are �0.3 for gasoline(QUARG, 1992). The highest value of this ratio wasreached at Montelibretti during daytime again indicat-ing the effective photochemical activity during dayhours.

4. Conclusions

The combined use of adsorption traps, thermaldesorption and GC–MS on b-cyclodextrin capillary col-umns has made possible the positive identification andquantitative determination of monoaromatic hydrocar-bons as well as xylene isomers in the atmosphere. Aro-matic hydrocarbon ratios were found to be a usefultool to investigate photochemical processes. In particu-lar, the BTEX-compounds seem to take part effectivelyin photochemcial processes even in areas that are moredistant to primary emittents.

Within the BTEX-compounds m-xylene could beselected as a key compound in discriminating differentlocation. Ethylbenzene/m-xylene ratio was found to bea good indicative of the impact of anthropogenicallyrelated hydrocarbon atmospheric chemistry.

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Isomeric Analysis of BTEXs in the Atmosphere

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