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Atmospheric Environment

Atmospheric Environment 42 (2008) 6144– 6151

1352-23

doi:10.1

� Cor

E-m

journal homepage: www.elsevier.com/locate/atmosenv

Technical note

Analysis of polycyclic aromatic hydrocarbons (PAHs) in airborneparticles by direct sample introduction thermal desorption GC/MS

Matthew Bates a, Paolo Bruno b, Mariangela Caputi b, Maurizio Caselli b,Gianluigi de Gennaro b,�, Maria Tutino b

a Markes International Ltd., Pontyclun, UKb Dipartimento di Chimica, Universita degli Studi di Bari, Via Orabona 4, 70126 Bari, Italy

a r t i c l e i n f o

Article history:

Received 4 September 2007

Received in revised form

19 March 2008

Accepted 31 March 2008

Keywords:

Thermal desorption

PAHs

10/$ - see front matter & 2008 Elsevier Ltd.

016/j.atmosenv.2008.03.050

responding author.

ail address: giangi@chimica.uniba.it (G. de G

a b s t r a c t

The technique of thermal desorption (TD)–GC/MS was evaluated for measuring airborne,

4–6 ring polycyclic aromatic hydrocarbons (PAHs) collected onto quartz filters. TD

provides a more readily automated and sensitive alternative to traditional solvent

extraction, decreasing the time/cost of analysis and reducing the risk of analyte loss or

sample contamination. The developed method was successfully applied to the analysis of

PAH standard solutions loaded on sorbent tubes packed with quartz wool and the

graphitized carbon black sorbent Carbograph2. The optimized method showed high

desorption efficiency over the whole range of target PAHs with good precision, linearity

and sensitivity. The proposed method was verified on an urban dust Standard Reference

Material (SRM 1649a); the experimentally determined concentrations agreed with the

certified ranges (95% confidence limit) for all target compounds except benzo[a]anthra-

cene, which fell just outside the narrow certified range. The desorption efficiency and the

reproducibility of the method was evaluated by analysing pieces of real sample filters

sampled from urban air for a period of 24 h. The results confirmed the homogeneity of the

filter and showed high recovery efficiencies for all target PAHs.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Airborne particulate matter (APM) contains numerousinorganic and organic species, many of which canadversely affect human health. Of these constituents, theclass of polycyclic aromatic hydrocarbons (PAHs) isnoteworthy because it is ubiquitous and includes severalpotent carcinogens (IARC, 2001; Cohen, 2000). In theatmosphere PAHs are known to be distributed betweenthe gaseous and particulate phases. The gas-to-particlepartitioning of PAHs was investigated (Bidleman, 1988;Masclet et al., 1988; Subramanyam et al., 1994; Odabasiet al., 1999) and showed that the phase partitioningdepends on atmospheric conditions, such as temperature

All rights reserved.

ennaro).

and relative humidity, on the vapour pressure of a givenPAH, and on the atmospheric concentrations of thesuspended particulate matter. The vapour pressure ofPAHs ranges from 101 to 10�10 Pa and decreases as afunction of increasing molecular weight or ring number.This phase partitioning must be taken into account duringsampling and analysis. The lighter 2–4-ring PAHs aremainly found in the gaseous-phase, whereas PAHs withfive rings or more are generally bound to APM.

Determination of particle-bound PAHs is very impor-tant because those with a higher number of rings have thehighest toxicological interest (Lyall et al., 1988; Pozzoliet al., 2004).

Several experimental studies showed that these com-pounds are mostly present in the finest fraction of APM(Van Cauwenberghe, 1985; Dockery et al., 1993; Gamble,1998; Oberdorster, 2001). So, to investigate the heavierPAHs, particulate matter in the size ranges PM10, PM2.5

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M. Bates et al. / Atmospheric Environment 42 (2008) 6144–6151 6145

and PM1 must be sampled. In the literature there arenumerous methods for sampling these particles depend-ing on conditions of aspiration flow (Jacob, 1995; Caricchiaet al., 1999; Bruno et al., 2000, 2002; Zheng et al., 2000;Romero et al., 2002; Shimmo et al., 2004). The particlesare collected onto filters of various dimensions made froma range of materials such as PTFE, glass-fibres, quartz,PTFE-coated polystyrene and polycarbonate. After thesampling and extraction steps, clean-up, detection andquantification procedures follow. All these stages areinterrelated since the choice of extraction techniquegreatly influences the clean-up step and both procedureswill place demands on the performance of the analyticaltechnique (usually GC–MS or HPLC). Conventional meth-ods for the determination of PAHs require intensivesample preparation, involving many manual steps andcomparatively large amounts of toxic solvents. Theseprocedures are generally time consuming, labour inten-sive, expensive, prone to contamination and environmen-tally unfriendly. Additionally, more recent hyphenatedtechniques, although accurate and precise, require ex-pensive equipment and an experienced operator (Shimmoet al., 2004). In this study thermal desorption (TD) withGC/MS was evaluated for measuring airborne PAHs with4–6 rings collected on quartz-fibre filters. With the TDtechnique, no sample preparation is required and itprovides a more readily automated and more sensitivealternative to solvent extraction. As a consequence thisdecreases the time/cost of analysis and reduces the risk ofanalyte loss or sample contamination.

TD is commonly employed for extracting volatile andsemivolatile species from adsorbing matrices such assampling tubes or solid-phase extraction (SPE) devices(Clement, 1997; Pankow et al., 1998; Harper, 2000; Caputi,2001). Only a few applications of TD–GC/MS for theanalysis of organic compounds associated with particulatematter were published (Sigman and Ma, 1999; Watermanet al., 2000; Neusuess et al., 2000). These applicationsrefer to the analysis of PAHs with 4–6 rings in standardreference material (SRM 1649a National Institute ofStandards and Technology [NIST]) but not on recentlycollected samples (Hall et al., 1999; Falkovich and Rudich,2001). The experiments described in this paper weredesigned to demonstrate the efficiency, reproducibility,sensitivity and ability of the TD–GC/MS technique toquantify PAHs with 4–6 rings (listed in Table 1) in urbanAPM collected on filters.

Table 1List of PAHs analytes

Compounds Extracted ion No. of rings

Pyrene (PY) 202 4

Benzo[a]anthracene (BaA) 228 4

Chrysene (ChR) 228 4

Benzo[bj]fluoranthene (BbjF) 252 5

Benzo[k]fluoranthene (BkF) 252 5

Benzo[a]pyrene (BaP) 252 5

Indeno[1,2,3-cd]pyrene (Ip) 276 6

Benzo[g,h,i]perylene (BgP) 276 6

Dibenzo[a,h]anthracene (DbA) 278 5

2. Materials and methods

2.1. Chemicals and materials

Standard solutions of PAHs were prepared by dilutionof an EPA PAH mix containing each PAH at 500 ng ml�1 inmethylene chloride (EPA 525 PAH Mix A, Supelco,Bellefonte, PA, USA). Dichloromethane (99.9%, Supelco,Bellefonte, PA, USA) was used as solvent for the dilution.

Carbograph-2TDTM and quartz wool were supplied byMarkes (Markes International Ltd., Pontyclun, UK); Carbo-graph-2 may be considered equivalent to Carbotrap-Cwith the main difference being the mesh size: Carbotrap-Cis 20/40 mesh whereas Carbograph 2TDTM is 40/60 mesh.The NIST Urban Dust (organics) Standard ReferenceMaterial used was SRM 1649a (National Institute ofStandards and Technology, Gaithersburg, USA).

2.2. TD– GC– MS methodology

The analytical system utilizes a UNITYTM ThermalDesorber equipped with an UltrA-TDTM Multi-tube Auto-sampler (Markes International Ltd.) and coupled to a gaschromatograph (Agilent GC-6890 PLUS) and a massselective detector (Agilent GC/MS 5973N). The TD devicecomprises a desorption oven connected to a Peltier-cooledsorbent packed cold-trapping system. The sample isinserted into empty stainless steel or glass tubes whichare 89 mm long and have an outer diameter (o.d.) of6.4 mm with an internal diameter (i.d.) of either 5 mm(stainless steel) or 4 mm (glass). The tubes are placed intothe desorption oven at ambient temperature. After a leaktest and ambient temperature purge to remove oxygen;the samples are thermally desorbed using temperatureand a flow of inert gas (generally helium) to extractanalytes from the sample (this is the ‘‘primary desorption’’stage). The desorbed analytes from the sample arerefocused on a small electrically cooled adsorbent trap(Fig. 1). The trap is constructed of quartz and contains a2 mm diameter�60 mm long bed of sorbent(s). For PAHanalyses a trap packed with a 0.8 mm i.d.�15 mm quartzcollar, 15 mm of quartz beads, 10 mg of quartz wool and30 mg of Carbograph-2TDTM was used. Once the primarydesorption is complete, the focusing trap is rapidly heated(�100 1C s�1) and back-flush desorbed with completeextraction typically achieved in as little as 100–200 ml ofvapour (secondary desorption) (Fig. 1). This small ‘‘plug’’of vapour is introduced to the GC via uniformally heated,uncoated, deactivated fused-silica transfer line whichleads to minimal band spreading and thus provides goodchromatographic separation. The thermal desorber con-tains a two-stage Peltier cell which uniformly cools thefocusing trap adsorbent bed without need for liquidcryogen. The system operates in back-flush mode fordesorption, i.e. the sample gas stream enters and leavesthe cold trap through the narrow-bore/restricted end.Back-flush desorption is essential when multi-sorbentpacked traps are used. These traps allow the analysis ofwide volatility range samples in a single run: high boilingcompounds are retained by, and quantitatively desorbed

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sam

ple

tube

cold

trap

split

tube

heated valve

to column

from bypass

cold

trap

split

tube

heated valve

to column

optional split

optional split

inert carrier gas inert carrier gas

Primary (tube) Desorption

Secondary (trap) Desorption

Fig. 1. UNITY configuration during primary and secondary desorption.

Table 2Parameters for thermal desorption

Parameter Value

Adsorbent tube desorption

Temperature 320 1C

Time 10 min

Temperature of the focusing trap 30 1C

Trap packing Quartz collar/quartz beads/quartz

wool and carbograph 2

Split 30 ml min�1

Flow through trap 50 ml min�1

Split ratio 100% of the sample reaches the

trap

Desorption from the focusing trap

Temperature of the focusing trap 360 1C

Time 4 min

Split 30 ml min�1

Column flow (Oven 50 1C) 2.6 ml min�1

Split 8% of the sample retained by the

cold trap reaches the GC

Auxiliary parameters

FP 220 1C

Total split 8% of sample reaches column and

detector

GC cycle time 33 min

M. Bates et al. / Atmospheric Environment 42 (2008) 6144–61516146

from, the weaker sorbents (quartz collar, beads or wool),without ever coming into contact with the strongeradsorbent (Carbograph-2TD) behind. According to themass of analytes collected, the desorbed samples may besplit as they are transferred from the primary sample tubeto the focusing trap and/or during subsequent transferfrom the focusing trap to the GC. On the equipment usedin this work it is possible to quantitatively re-collect eitheror both the split portions onto another adsorbent tube.This allows quantitative reanalysis of the sample and is anessential tool for method validation. The optimizedparameters for TD of PAHs are listed in Table 2; thosefor GC/MS are listed in Table 3. From the analyses of realsamples it was observed that the real matrix is morecomplex and that there are many other organic andinorganic compounds present alongside the PAHs. Whenanalysing real samples the desorption temperature of thetube is a compromise in order to avoid desorption of otherheavier compounds that can interfere with the quality ofthe PAHs analysis. The quantitative determination wascarried out in Full Scan mode followed by the extraction ofthe molecular ion of the different PAHs: quantificationwas based on peak areas. Compounds, their respectiveions and number of rings are listed in Table 1.

2.3. Calibration tubes

The calibration solution was introduced via a glasstube packed with a sorbent bed of 25 mg of quartz wooland 350 mg of Carbograph 2TDTM. It is important toquantitatively and reproducibly purge the solventfrom the tube prior to analysis whilst quantitativelyretaining the compounds of interest on the sorbent beds.A Calibration Solution Loading Rig (CSLRTM, Markes

International Ltd.) (Fig. 2) was used for introducing thecalibration solution onto tubes and allowing a controlledvaporization and purging of the solvent. The CSLRTM

consists of an unheated injector port with a controlledcarrier gas supply and a sorbent tube connection point.The sampling (grooved) end of a packed sorbent tube isconnected to the CSLR via a 1/4 in brass nut and combined(one-piece) PTFE ferrule. The carrier gas flow was set, via aneedle valve, at 100 ml min�1. This sweeps the injection

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Table 3Parameters for GC/MS analysis

Parameter Value

Carrier gas He

ColumnJ&W Scientific Columns, DB-XLB,

30 m�0.25 i.d. �0.25 mm

Constant pressure 21.49 psi

Oven temperature (start) 50 1C per 0.5 min

First ramp 20 1C min�1 until 160 1C

Second ramp 10 1C min�1 until 325 1C

Third ramp 25 1C min�1 until 340 1C

Oven temperature (end) 340 1C per 5 min

Interface temperature (MS) 280 1C

Mode Constant pressure

Fig. 2. The calibration solution loading rig.

M. Bates et al. / Atmospheric Environment 42 (2008) 6144–6151 6147

port and passes down through the sorbent tube to vent.The calibration solution is introduced through the injectorseptum using a standard GC syringe. The solvent vaporizesin the flow of inert gas and reaches the sorbent bed in thevapour phase. Carrier gas (200 ml) was allowed to passthrough the tube such that the solvent was almostcompletely purged from the tube.

2.4. Aerosol sample collection

Sampling of particulate matter with an aerodynamicdiameter less than 10 mm (PM10) was carried out by usingan HYDRA low volume (LV) sampler (FAI Instruments s.r.l.,Roma, Italy) at a volumetric flow of 1 m3 h�1. The sampleswere collected on QM-A Whatman (Maidstone, UK) quartzfilters (o.d. 47 mm) with sampling periods of 24 h. Piecesof the filters (1/4 of the filter) were weighed into emptyTD sample tubes before carrying out the analysis. Theresults obtained analysing only 1/4 of the filter arerepresentative of the sample as a whole as demonstratedin our previous work (Tutino, 2005; Bruno et al., 2007).Nonetheless, the homogeneity of the filter was also testedand proved during this work by analysing all four quartersof one filter.

3. Results and discussion

A series of tests were carried out to evaluate theperformance of the TD–GC/MS technique for the quanti-fication of PAHs with 4–6 rings collected on filters. Whencalibrating any piece of analytical equipment it isimportant that the loading and analysis of standardsreplicates, as closely as possible, that of the samplesthemselves. For this reason a TD–gas chromatograph(TD/GC) system was calibrated by loading the standardonto a sample (calibration) tube and desorbing it throughthe system so that it is subjected to the complete two-stage TD process. Calibration tubes and real samples ofparticulate matter were both used for determining thedesorption efficiency, reproducibility and sensitivity of theoptimized method.

3.1. Calibration tubes

A simple test for complete desorption is to repeat thedesorption process and check for carryover of componentsof interest, i.e. the appearance of a significant percentageof analytes in a second analysis. TD recovery of all targetcompounds was investigated at various loading levels. TheTD recoveries were higher than 99.999% for all targetcompounds. In the range between 1 and 20 ng loaded onthe calibration tube the TD recovery was independentof the loaded amount. A comparison between the first andthe second analysis of a calibration tube loaded with PAHsstandard solution (10 ng) is shown in Fig. 3.

A thermal blank was performed after each sample(after the first and the second desorption) in order toverify if there was memory effect; the results showed thatno memory effect was present. Moreover it was verifiedthat the chromatogram of the second TD corresponded tothe chromatogram of the thermal blank. The limitsof quantification (LOQ), expressed in ng/tube (total massof each PAH loaded in to the tube), were determined fromsequential analyses of tubes loaded with diluted standardsolution with a signal to noise ratio of 10 (Table 4).

Method repeatability was evaluated by loading a totalof 30 tubes packed with quartz wool and Carbograph 2TDwith 1ml of three different solutions of all targetcompounds in dichloromethane. The results, listed inTable 4, show that percentage relative standard deviationsare less than 10% for all target compounds. Linearity ofresponse was evaluated as a function of injected masson the tubes by loading 1ml of four standard solutionsof all target compounds in dichloromethane (1, 5, 15,20mg ml�1) onto 12 tubes packed with quartz wool andCarbograph 2TD. Fig. 4 displays, as an example, thecalibration line for the heaviest PAHs. Under the opti-mized method, all the analytes exhibited good linearityover the explored range. The range of 1–20 ng allowsquantification of the real samples investigated in thisstudy. The linear correlation coefficients, R2, were betterthan 0.997 except for dibenzo[a,h]anthracene. The lowervalues of R2 of dibenzo[a,h]anthracene could be explainedwith the low response near to the quantification limit forthis compound.

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TIC: 21110606.DTIC: 21110605.D (*)

20000

40000

60000

80000

100000

120000

140000

160000

180000

200000

220000

240000

260000

280000

300000

Time-->

Abundance

Fig. 3. Comparison between the first and the second analysis of a calibration tube loaded with PAHs standard solution (10 ng).

Table 4Limits of quantification (LOQ) in ng/tube and repeatability of the proposed method

PAHs LOQ 2 ng 8 ng 20 ng

(ng/tube) Area (n ¼ 10) %RSD (n ¼ 10) Area (n ¼ 10) %RSD (n ¼ 10) Area (n ¼ 10) %RSD (n ¼ 10)

PY 0.23 358,707 7 1,383,001 10 3,602,092 7

BaA 0.29 253,081 4 1,412,519 4 3,685,324 2

ChR 0.23 327,075 6 1,564,844 5 4,009,314 2

BbjF 0.27 215,854 5 1,344,311 5 3,606,982 2

BkF 0.24 287,849 3 1,633,496 4 4,303,617 3

BaP 0.42 149,018 8 1,118,632 4 3,094,786 2

IP 0.59 82,816 9 862,423 6 2,662,709 1

BgP 0.47 116,606 10 854,548 5 2,554,087 3

DbA 0.92 54,784 10 585,751 7 1,864,548 3

The average peak area and RSD% for 2, 8, and 20 ng injected were reported.

M. Bates et al. / Atmospheric Environment 42 (2008) 6144–61516148

3.2. Certified reference material SRM1649a

The proposed method was tested on the urban dustStandard Reference Material (SRM1649a). Three samplesof approximately 1.5 mg of certified dust standard wereanalysed by TD–GCMS.

SRM was weighed by using a balance with thesensibility of 0.001 mg and was introduced via a glasstube packed with quartz wool. About 15 cm of quartz woolwere introduced at the head and the end of the SRM inorder to block it and to avoid the loss of the standardreference material.

Before the introduction of the SRM, the tubes wereanalysed for verifying the blank level for the quartz wool.The results of the comparison between the calculatedconcentrations and the certificate values are listed inTable 5. All compounds except for benzo[a]anthraceneshowed good agreement; the maximum relative percen-tage difference was 15%. The quantified concentrationswere close to the certified values and all of the calculatedranges (95% confidence) overlap with the certified rangesexcept for benzo[a]anthracene.

3.3. Real samples

The optimized method was applied to real samplescollected in a street with moderate traffic near to theDepartment of Chemistry, University of Bari (Bari, Italy).The desorption efficiency, the homogeneity of the filter,and the reproducibility of the method for the analysis ofPAHs collected on filters by LV samplers were allevaluated. The filter homogeneity was tested by analysingall four quarters of a filter. The results showed that thefilters can be considered homogenous; the RSD% calcu-lated on four pieces were less than 9% for all targetcompounds (Table 6). TD recoveries were investigated byrepeating the desorption process on the same sampledfilter and comparing the area of the peaks of thecomponents of interest. The results obtained for all PAHsare listed in Table 6. During the method development, ablank run and a secondary sample desorption wereperformed for each sample; no significant blank contam-ination was observed. Nevertheless several tests werefurther performed to verify the cleanliness of the totalinstrument. Blank runs were carried out after 10, 15, and

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Benzo[k]fluoranthene

y = 321616.858x - 182542.750R2 = 1.000

0

2000000

4000000

6000000

8000000

0ug/ml

sign

al

Benzo[b]fluoranthene

y = 263758.323x - 78451.397R2 = 0.997

0

2000000

4000000

6000000

sign

al

Benzo[g,h,i]perylene

y = 227329.197x - 308020.332

0

2000000

4000000

6000000

sign

al

Benzo[a]pyrene

y = 295854.634x - 363134.870

0

2000000

4000000

6000000

sign

al

Dibenzo[a,h]anthracene

y = 152762.792x - 240729.563R2 = 0.995

0

1000000

2000000

3000000

4000000

sign

al

Indeno[1,2,3-cd]pyrene

y = 166858.885x - 237519.724

0

1000000

2000000

3000000

4000000

sign

al

2 4 6 8 10 12 14 16 18 20 22 24 0ug/ml

2 4 6 8 10 12 14 16 18 20 22 24

0ug/ml

2 4 6 8 10 12 14 16 18 20 22 24 0ug/ml

2 4 6 8 10 12 14 16 18 20 22 24

0ug/ml

2 4 6 8 10 12 14 16 18 20 22 24 0ug/ml

2 4 6 8 10 12 14 16 18 20 22 24

R2 = 0.997

R2 = 0.998

R2 = 0.997

Fig. 4. Calibration curves for the heaviest PAHs.

Table 5Comparison of calculated concentration (ng mg�1) of PAHs over three runs, each using about 1.5 mg of SRM 1649a, Urban Dust

PAHs (E�C)/C *100aExperimental values Certificate values

ng mg�1 Confidence 95% ng mg�1 Confidence 95%

PY 7 5.654 70.362 5.29 70.25

BaA 23 2.714 70.245 2.21 70.073

ChR 15 3.503 70.076 3.049 70.06

BkF 7 2.047 70.265 1.913 70.031

BaP 8 2.708 70.499 2.509 70.087

IP 13 3.592 71.081 3.18 70.072

BgP 13 4.53 70.846 4.01 70.091

a (E�C)/C *100 ¼ (experimental values�certified values)/certified values*100.

M. Bates et al. / Atmospheric Environment 42 (2008) 6144–6151 6149

20 samples. Even though non-significant blank contam-ination was observed, a condition of the cold trap and ofthe GC column were performed after lot samples (about50–70 samples). In this way it can preserve the instru-ments from possible contaminations that do not due as

much to a systematic memory effect but rather to thecomposition of some particular real samples. The analysesof real samples showed that the real matrix is quitecomplex; there are many other compounds presentalongside the PAHs however the recovery was high and

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Table 6Homogeneity and desorption recovery for four quarters of filter A

sampled for 24 h

PAHs RSD% Recovery % Recovery % Recovery % Recovery %

BaA 6 98.8 98.7 99.1 99.1

ChR 9 98.7 98.6 99.1 99.1

BbjF 7 97.3 97.7 98 97.7

BkF 7 97.4 97.8 97.8 97.7

BaP 8 98.2 98.3 98 98.2

IP 5 98.2 97.4 97.5 97.5

BgP 3 97 96 95.5 96.3

DbA o LOQ o LOQ o LOQ o LOQ o LOQ

M. Bates et al. / Atmospheric Environment 42 (2008) 6144–61516150

the peak shape for the PAHs was good. Some particularsamples can cause a drastic reduction of the performance,i.e. of the sensitivity and resolution of the measure.

4. Conclusions

The technique of TD–GC/MS was evaluated for measur-ing airborne PAH with 4–6 rings collected onto quartzfilters. With the TD technique, no sample preparation isrequired. TD provides a readily automated and moresensitive alternative to solvent extraction, decreasingthe time/cost of analysis and reducing the risk ofanalyte losses or sample contamination. The methodwas successfully applied to the analysis of PAHs loadedon adsorbent tubes packed with quartz wool andCarbograph2. The optimized method has shown highdesorption efficiency over the whole range of PAHs,good precision, linearity and sensibility. The proposedmethod was tested on an urban dust Standard ReferenceMaterial (SRM 1649a); the calculated concentrationsof PAHs with 95% confidence limit coincide with thecertified ranges apart from benzo[a]anthracene,which fell just outside the narrow certified range. Thedesorption efficiency for real samples was evaluatedanalysing pieces of filters sampled for a period of 24 h.The results have confirmed the homogeneity of the filterand have showed high TD recovery for all PAHs collectedon filter.

Disclaimer (conflict of interest)

The authors state that an author belongs to a Company(Markes International, UK) that produces thermal deso-rber. The input, help and advice of Matthew Bates wasfundamental in adapting instrumentation and the methoddevelopment stages of this work.

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