Kusch, Knupp 539

Peter Kusch,Gerd Knupp

Fachhochschule Bonn-Rhein-Sieg, University of AppliedSciences, FachbereichBiologie, Chemie undWerkstofftechnik, von-Liebig-Str. 20, D-53359 Rheinbach,Germany

Analysis of residual styrene monomer and othervolatile organic compounds in expandedpolystyrene by headspace solid-phasemicroextraction followed by gas chromatographyand gas chromatography/mass spectrometry

A method for determination of residual styrene monomer and other volatile organiccompounds in expanded polystyrene (EPS) was developed using HS SPME and gaschromatography with FID. The extraction products were identified by GC/MS. Goodreproducibility of the measurements with RSD values between 3.2–3.6% was achiev-ed by extraction using a 75 lm Carboxen-polydimethylsiloxane fiber at 608C with15 min sample sonication. The contents of residual styrene monomer in two samplesof EPS were 153.2 and 65.7 mg/kg, respectively. Other compounds identified in EPSwere pentane, benzene, toluene, ethylbenzene, isomers of xylene, n-propylbenzene,1,2,4-trimethylbenzene, o-methylstyrene, benzaldehyde, benzyl alcohol, and aceto-phenone.

Key Words: Styrene; Expanded polystyrene (EPS); Headspace SPME; Carboxen-poly(dimethylsiloxane); GC/MS;

Received: October 4, 2001; revised: February 6, 2002; accepted: February 20, 2002

1 Introduction

EPS is an expanded polystyrene, produced by polymeriz-ing styrene monomer and adding pentane as a blowingagent. It is used for food packaging and for protection ofproducts against damage during transport and storage.EPS is also used in the building industry for insulation ofexterior walls and foundations.

It has been known for a long time that EPS emits residualstyrene monomer and other volatile organic compounds(VOCs) at ambient temperature. Styrene is harmful wheninhaled, is irritating to eyes, nose, throat, skin, and acts asa depressant on the central nervous system, causing neu-rological impairment [1].

VOCs have to be determined in mixtures in many areas ofanalytical endeavor, such as environmental, food, foren-sic, fragrance, oil, pharmaceutical, and polymer/copoly-mer analysis [2]. The method of choice for many of thesepolymer/copolymer analyses is static headspace sam-pling [3–4], dynamic headspace sampling [5–8], pyroly-sis [9, 10], and more recently solid-phase microextraction(SPME) [11–15] followed by GC and GC/MS determina-tion.

Several headspace-GC or -GC/MS methods have beenused to determine the level of styrene residues emittedfrom polystyrene [3, 16]. However, no previous work onthe analysis of residual styrene monomer and other vola-tile organic compounds in EPS by SPME-GC or SPME-GC/MS has been reported in the literature.

In this work, headspace SPME followed by GC and GC/MS has been applied to the analysis of residual styreneand other volatile organic compounds in EPS. The poly(di-methylsiloxane) (PDMS) SPME coating is one of the mostwidely used coatings for extracting volatile analytes fromenvironmental samples via absorption [17]. In contrast,the sensitivity of adsorptive solid SPME coatings, such asPDMS/divinylbenzene (PDMS/DVB) and Carboxen/PDMS (CAR/PDMS), was reported to be much highercompared to PDMS for extracting VOCs [18–19]. Wehave selected the CAR/PDMS fiber for our investigations.The fiber works very well in the headspace extractionmode. Carboxen has a similar surface to DVB. The majordifference is the much higher content of micropores of theCarboxen/PDMS fiber, making it the overwhelming choiceas material for the extraction of volatile and low molecularanalytes at trace levels. Carboxens differ from other por-ous carbons because the pores are not sealed but passentirely through the particle and allow analytes to desorbmore efficiently than in the case of the sealed pores com-monly found in charcoals and many carbon molecularsieves.

J. Sep. Sci. 2002, 25, 539–542

Correspondence: Dr. Peter Kusch, Fachhochschule Bonn-Rhein-Sieg, von-Liebig-Str. 20, D-53359 Rheinbach, Ger-many.E-mail: peter.kusch@fh-bonn-rhein-sieg.deFax: +49 2241 8658513

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i WILEY-VCH Verlag GmbH, 69469 Weinheim 2002 1615-9306/2002/0806–0539$17.50+.50/0

2 Experimental

2.1 Materials

The fused silica capillary columns tested in this investiga-tions were obtained as follows: 30 m60.32 mm ID, filmthickness 4.0 lm, SPB-1 Sulfur from Supelco (Bellefonte,PA, USA); 60 m60.25 mm ID, film thickness 0.25 lm,Optima d-3 from Macherey-Nagel (Düren, Germany);60 m 6 0.25 mm ID, film thickness 0.25 lm, DB-5msfrom J&W Scientific (Folsom, CA, USA); 60 m60.25 mmID, film thickness 0.25 lm, BPX-50 from SGE (Mel-bourne, Australia); 60 m60.32 mm ID, film thickness0.50 lm, Rtx-225 from Restek (Bellefonte, PA, USA);50 m60.32 mm ID, film thickness 1.0 lm, PermaphaseCPMS/225 from Perkin-Elmer (Norwalk, CT, USA);30 m60.25 mm ID, film thickness 0.25 lm, SolGel WAXfrom SGE and 60 m60.32 mm ID, film thickness0.20 lm, Rtx-2330 from Restek.

The SPME fiber holder for manual use and the 75 lm Car-boxen-Polydimethylsiloxane (CAR/PDMS) fibers wereobtained from Supelco (Bellefonte, PA, USA). The fiberwas conditioned at 2808C for 30 min prior to use.

20-mL headspace glass vials with an aluminum-coatedsilicone rubber septum and pressure released aluminumseal, obtained from LABC Labortechnik (Hennef, Ger-many), were used.

A Sonorex Super RK 31H compact ultrasonic bath fromBandelin electronic (Berlin, Germany) was employed forsample agitation.

2.2 Chemicals

The chemicals used were BTEX standard solution N8 722372 obtained from Macherey-Nagel (Düren, Germany),styrene (stabilized) for synthesis obtained from Merck-Schuchardt (Hohenbrunn, Germany), ethyl methyl ketone(MEK, internal standard), and N,N-dimethylformamide(DMF), both of analytical grade, supplied by Merck (Darm-stadt, Germany).

2.3 Samples

Samples of commercially available expanded polystyrene(Styropor) from Germany (sample A) and from Australia(sample B) were used in our investigations.

2.4 Instrumentation

GC analyses were performed on a Perkin-Elmer (Nor-walk, CT, USA) AutoSystem gas chromatographequipped with split/splitless injector at 2508C and a flame-ionization detector (FID) operated at 2708C. Helium 5.0grade (Westfalen AG, Muenster, Germany) was used as

a carrier gas. The helium inlet pressure was 140 kPa andthe split flow was 50 mL/min. A SPME glass liner for split/splitless injector was used (Supelco).

The oven temperature was programmed from 808C (1 minhold) at 58/min to 1008C and then 88/min to 2208C (10 minhold) or from 608C (1 min hold) at 28C to 1008C and then88/min to 2008C (10 min hold). Chromatographic data wereprocessed with TurboChrom 4.0 software (Perkin-Elmer).

GC/MS measurements were made using an Thermo-Quest Trace 200 gas chromatograph (ThermoQuest CEInstruments, Milan, Italy) interfaced to a ThermoQuest/Finnigan Voyager quadrupole mass spectrometer (Ther-moQuest/Finnigan MassLab Group, Manchester, UK)with an ThermoQuest Xcalibur data system, the NIST 98spectra library, and a CombiPAL autosampler (CTC Ana-lytics AG, Zwingen, Switzerland).

The oven was programmed from 608C (1 min hold) at 58/min to 1008C and then 108/min to 2208C (10 min hold).Helium 6.0 grade (Westfalen) was used as a carrier gas.A constant pressure of 70 kPa helium was used during thewhole analysis. The temperature of the split/splitlessinjector was 2508C and the split flow was 10 mL/min. Thetransfer line temperature was 2708C. The ion source tem-perature was kept at 2008C. Ionization was induced byimpacting electrons with a kinetic energy of 70 eV. Thedetector voltage was 350 V. Mass spectra and recon-structed chromatograms (total ion current, TIC) wereobtained by automatic scanning in the mass range m/z46–204. Compound identification was carried out by com-parison of retention times and mass spectra of standards,study of the mass spectra, and comparison with data inthe NIST 98 spectra library.

2.5 Procedure

A standard solution of MEK (internal standard) was pre-pared by making up 100 mg of the substance with DMF to100 mL. A 30 lL aliquot of this solution (containing 30 lgof the internal standard) was added from a 100 lL syringeto 50 mg of the crumbled EPS sample sealed in a 20 mLheadspace glass vial with aluminum-coated silicone rub-ber septum. The septum of the vial was pierced with theneedle of the SPME device and the fiber was exposedapproximately 10 mm above the sample. Afterwards theglass vial with the SPME injector was placed in an ultraso-nic bath and agitated by sonication for 1 to 30 min at ambi-ent temperature, at 408C and at 608C. Furthermore, EPSsamples were heated in the heating block of the Combi-PAL autosampler equilibrated at 808C without agitation.

After a sorption time of 1 to 30 min in the headspaceabove the sample, the fiber was retracted into the protec-tive sheath and removed from the vial. It was transferredwithout delay into the injection port of the gas chromato-

540 Kusch, Knupp J. Sep. Sci. 2002, 25, 539–542

graph or the GC/MS. The fiber was thermally desorbed inthe injector at 2508C for 3 min and the run was started.The samples for GC/MS identification were analyzed with-out using an internal standard solution.

For the determination of the HS SPME response factor50 lL (0.0530 g) of styrene and 50 lL (0.0394 g) of MEKwere placed a 10 mL flask and the flask was filled up withDMF. 1 mL of the solution obtained was then diluted 1:10with DMF. 2 mL of the diluted solution was transferred tothe headspace vial for adsorption onto the CAR/PDMSfiber and than analyzed by GC after thermal desorption ofthe fiber in the injector. The HS SPME conditions and theanalytical conditions were the same as in the determina-tion of EPS samples.

3 Results and discussion

The studies were conducted to establish the optimumconditions for extraction, desorption, chromatographicseparation, and identification of the VOCs from theexpanded polystyrene using headspace SPME followedby GC and GC/MS. Extraction temperature and time,sample agitation technique, and desorption temperatureand time are very important parameters in the optimiza-tion of the SPME. In general, the highest possible tem-perature should be used. In headspace SPME, anincrease in extraction temperature leads to an increase ofanalyte concentration in the headspace, and helps to facil-itate faster extraction [17]. However, at high temperature,coating headspace partition decreases and the fiber coat-ing begins to lose its ability to adsorb analytes. Thus thetemperature effect between 25–808C was studied.

The most efficient agitation method evaluated to date forSPME applications is direct sample sonication, which canprovide very short extraction times [17]. In this work wehave used sonication as sample agitation method.

The choice of the desorption temperature of the GC injec-tor is also a critical point. The results obtained show thatafter 3 min at 2508C a complete desorption of all com-pounds was achieved.

For the chromatographic separation of residual VOCsfrom samples of the expanded polystyrene, several fusedsilica capillary columns with stationary phases of differentpolarity were examined. The best results were achievedusing the Rtx-225 (60 m, 0.32 mm, 0.5 lm) or Perma-phase-CPMS/225 (50 m, 0.32 mm, 1.0 lm) columns withthe (50%-cyanopropenyl)-dimethylpolysiloxane station-ary phase. Figure 1 shows the GC/FID chromatograms ofboth the investigated EPS samples handled by SPMEwith the 75 lm CAR/PDMS fiber in the headspace modeat ambient temperature and agitated by sonication for15 min. The peaks were identified by GC/MS and by usingthe mixture of BTEX standards.

In order to optimize the extraction temperature, the con-tent of styrene monomer in EPS was determined at fourdifferent temperatures. The internal standard method wasused for the quantitative determination according toEq. (1):

c i = A i N f i N Wst N 106/A st N Wp (1)

where A i refers to the peak area of styrene, and A st refersto the peak area of the internal standard (MEK), Wst is themass of MEK, Wp is the mass of the sample (EPS), and fi

is the HS SPME response correction factor of styrene(see Section 2.5). The HS SPME response correction fac-tor of styrene was calculated according to Eq. (2):

J. Sep. Sci. 2002, 25, 539–542 Analysis of styrene and other VOCs in EPS 541

Figure 1. SPME/GC/FID chromatograms of a headspace ofEPS at ambient temperature, agitated for 15 min by sonica-tion. Top chromatogram: sample A, bottom chromatogram:sample B. Fused silica capillary column: Rtx-225,60 m60.32 mm ID, film thickness 0.5 lm. Column tempera-ture: programmed from 808C (1 min hold) at 58/min to 1008Cand then 88/min to 2208C (10 min hold). Split/splitless injec-tor: 2508C, split flow 50 mL/min, FID: 2708C. Peak identifica-tion: 1 = pentane (3.95 min); 2 = non-aromatics (5.88–6.32 min); 3 = benzene (6.63 min); 4 = toluene (7.05 min);5 = ethylbenzene (7.60 min); 6 = m/p-xylene (7.81 min);7 = o-xylene (8.48 min); 8 = styrene (9.10 min); 9 = n-propyl-benzene (9.29 min); 10 = 1,2,4-trimethylbenzene (9.51 min);11 = o-methylstyrene (12.19 min); 12 = benzaldehyde(13.66 min); 13 = acetophenone (16.15 min); 14 = benzylalcohol (16.80 min).

fi = c i N A st/c st N Ai (2)

where c st and c i are the content of MEK and styrene in themixture, respectively, and A st and A i are the peak area ofMEK and styrene, respectively. The value obtained was0.2 l 0.01 (n = 20).

The results obtained and precision (repeatability) of theextraction for both the EPS samples are summarized inTable 1. As can be seen from Table 1, six replicate SPMEextractions at 608C with 15 min sonication followed by GCmeasurements showed RSD within 3.6% for EPS sampleA and within 3.2% for EPS sample B. The repeatability ofthe extraction at ambient temperature and at 408C wasnot satisfactory (RSD = 7.7%–11.7%). The resultsobtained by extraction at 808C without sonication indicatea degradation process of the EPS (retropolymerization).Therefore, the extraction temperature of 608C was cho-sen as the optimum. In order to determine the equilibrationtime, different extraction times between 1 and 30 min at

608C with sonication were studied. Figure 2 shows theeffect of the extraction time at 608C on the peak area ofstyrene determined in EPS, sample A. The extraction timeof 15 min was chosen for subsequent analyses.

4 Conclusions

The proposed HS-SPME-GC procedure is appropriate fordetermination of residual styrene monomer and otherVOCs in expanded polystyrene. The rapid, simple, andcost-effective method with good repeatability can beapplied to the process control of EPS production and tomeasure and screen styrene and VOCs in the environ-ment.

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542 Kusch, Knupp J. Sep. Sci. 2002, 25, 539–542

Table 1. Results of the GC/FID determination of residualstyrene in expanded polystyrene (EPS) after headspaceSPME for 15 min at different temperatures. Samples at tem-peratures 25, 40, and 608C were agitated by sonication.

Styrene, mg/kgEPS (A) EPS (B)

i 258C 408C 608C 808C 258C 608C

1 125 137 154 210 43 692 111 128 157 194 41 663 127 143 161 192 34 674 141 112 145 221 38 665 131 134 147 199 47 636 139 117 155 198 48 63n 6 6 6 6 6 6

Mean 129.0 128.5 153.2 203.3 41.8 65.75RSD,% 7.7 8.5 3.6 5.0 11.7 3.2

Figure 2. Effect of the extraction time on the peak area ofstyrene determined in EPS, sample A at 608C, agitated bysonication for 15 min. Analytical conditions: fused silica capil-lary column Permaphase-CPMS/225 (50 m60.32 mm ID,film thickness 1.0 lm), column temperature programmedfrom 608C (1 min hold) at 28/min to 1008C and then 88/min to2008C (10 min hold), split/splitless injector at 2508C, splitflow 50 mL/min, FID at 2708C.