Equilibrium-Sorptive Enrichment: A Novel Technique for TraceAnalysis in AirCarme Aguilar*

Universitat Rovira i Virgili, Analytical and Organic Chemistry Department, Pl. Imperial Tarraco 1, 43005 Tarragona, Spain

Hans-Gerd Janssen, Carel A. Cramers

Eindhoven University of Technology, Laboratory of Instrumental Analysis, PO Box 513, 5600 MB Eindhoven, The Netherlands

Ms received: December 12, 1998; accepted: January 27, 1999

Key Words:Trace analysis; air samples; BTX; equilibrium-sorptive enrichment

1 Introduction

The analytical methods currently used for the preconcentrationof volatile organic compounds (VOCs) in air samples are mainlybased on adsorptive enrichment of the analytes on a suitableadsorbent material [1–3]. The most popular adsorption materialsare carbon-based materials, such as activated carbon [1–5], andporous organic polymers, such ase.g. Tenax [3, 4]. After theadsorption step, the analytes are desorbed using either an extrac-tion solvent (liquid desorption) [1] or by heating the adsorbenttrap (thermal desorption) [2, 3].

Both liquid and thermal desorption have their own advantagesand disadvantages. Liquid desorption can be performed at roomtemperature, in that way minimizing the risk of thermal degrada-tion of labile analytes. One of the main drawbacks of this proce-dure, however, is that it is rather tedious. Moreover, a preconcen-tration step is usually necessary in order to obtain the desiredsensitivity. Large volumes of ultra pure solvents are required.Finally, this desorption method is difficult to automate [3].

Thermal desorption is carried out by heating the adsorbent trapto a high temperature. The analytes are then transferred to theanalytical column where, if necessary, they are refocused in acryo-trap. This makes it possible to transfer the entire sample tothe GC column. The sensitivity achieved using this desorptionmethod is hence generally better than that obtained using solventdesorption. Problems can arise from the high temperatures usedfor desorption. In the resulting chromatograms artifact peaks canappear, due to degradation of either the adsorbent or the analytes.Also, the analytes can be irreversible adsorbed on the surface ofthe adsorbent [6, 7].

The newly developed equilibrium-sorptive enrichment techniquecan overcome the major drawbacks of the classical desorptionmethods mentioned above [8]. In this new technique, a gaseoussample is drawn through an enrichment column containing asorption material. Typically, one could think of the enrichmentcolumn as a thick film GC column. Sampling is continued untilthe equilibrium between the sorption phase and the gas phase hasbeen established. After equilibrium is achieved, the air is elimi-nated from the enrichment column by briefly purging it withhelium. Next, the trap is closed and the system is heated from the(low) sampling temperature to the (high) desorption temperature.Finally, the total volume of enriched sample, or a homogeneoustime slice of it, can be injected onto the analytical column.

The sorptive preconcentration technique described above hasseveral advantages over the conventional techniques. For exam-

ple, even very volatile compounds can be enriched at ambienttemperature, the new method uses a highly inert adsorption mate-rial, does not require a cryogenic refocusing step, and requiresonly simple instrumentation. Additionally, if a suitable sorptionmaterial is selected,e.g. polydimethylsiloxane, high humiditysamples can be processed without water problems. In previouswork we have successfully used the technique of equilibrium(ab)sorption for sample enrichment in the analysis of volatileorganic compounds in air using portable GC instruments [8, 9].In these papers the influence of several parameters such as thesampling flow rate, the enrichment temperature, and the analyteconcentration on the equilibrium method for sample enrichmentis described.

In this short communication, the coupling of this new preconcen-tration technique to a conventional gas chromatograph isdescribed. The advantages of the new approach over standardmethods for on-line air enrichment are summarized. Finally, theapplicability of the new method is demonstrated by the analysisof various environmental samples. An important application thatis addressed is the analysis of benzene and other hydrocarbons inair.

2 Experimental

2.1 Experimental Set-Up

A schematic diagram of the experimental set-up used for theexperiments is shown inFigure 1. The gas enrichment systemwas placed inside the oven of an HP 5890 GC (Hewlett-Packard,Little Falls, DE, USA). It consists of a 1 m61 mm65 lm CPSil-5 CB (Chrompack) open-tubular column and a high tempera-ture six-port switching valve (VICI Valco, Schenkon, Switzer-land). Valve 1 in Figure 1 is a three-way flow selection valvewhich directs either the air sample, or the helium purge gas tothe trapping column. An on/off valve (valve 3) in the waste linecan be used to obtain stop-flow conditions during the heatingstep of the trap. With this valve in the open position, air can besampled through the trap by applying vacuum to the outlet of thewaste line. A pressure gauge (P) is installed in the waste line tobe able to measure the pressure drop over the enrichment trap.

The GC instrument used for the analysis of the enriched sampleswas a Varian 3400 GC (Sunnyvale, CA, USA) equipped with anFID detector. The chromatographic column was a CP Sil-8CBcapillary column (Chrompack, Middelburg, The Netherlands) of25 m60.22 mm i.d. with a film thickness of 0.12lm. The oven

J. High Resol. Chromatogr.1999, 22, (4) 231–234 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 1999 0935-6304/99/0404–0231$17.50+.50/0 231

Short Communications

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temperaturewasheld at 408C during the enrichmentof the gassamplesand2 minutesafterwards.Next,thetemperaturewaspro-grammedto 2008C at a rateof 108/min. The temperatureof thedetectorwassetat2258C.Heliumwasusedasthecarriergas.

2.2 SamplingandDesorption Process

Air sampleswerepulled throughthe trapby applyingvacuumtotheoutlet.Thesamplingpumpwasa portableair sampler(modelLFS113DS, Gillian InstrumentCorporation,West Candell, NJ,USA). Theflow rateduringsamplingwasapproximately 20 mL/min. The air pump was calibratedusing an electronic bubbleflow meter(Chrompack).After equilibrium wasestablished,thetrap was flushed with helium for a period of 0.3minutesat aflow rate of approximately 10mL/min to removethe air fromthe trap.Next, theGC ovenin which theenrichmentsystemwasinstalledis heatedto thedesorptiontemperatureunderstop-flowconditions.Once the desorptiontemperaturehasbeenreached,the six-port valve is switchedto the inject positionandback tothe load position againin order to transferthe enrichedsampleonto the analytical column. Becausethe enriched sample ishomogeneous,everyslice of it givesa correctrepresentationofthe original sample.Dependingon the time period the valve isleft in theinject position,eithertheentirevolumeof theenrichedgasor a homogeneoustime slice of it is injectedonto the chro-matographiccolumn.After the actual injection, the enrichmenttrap is flushed with helium to remove the remainderof theenrichedgasvolume or remainingcontaminantsfrom the trap.Oncethe trap is clean,it canbe cooledto the sorptiontempera-tureanda newenrichmentcanbecarriedout.Figure1 showsthesix-portswitchingvalvein thesamplingposition.

The enrichmentfactors obtainedwith the techniqueof equili-brium-sorptiveenrichmentarecompounddependent[8]. Hence,a calibration step is necessary. This can be carried out eitherfrom themeasurementof capacityfactors,or by usingcalibrationstandards.In the presentwork, calibration was carried out bymeansof standards.

The gaseousstandardsampleusedfor calibrationwasa mixtureof benzene,toluene,andp-xylene(BTX), eachat a concentrationof 25ppm in helium. For the optimizationof the parametersofthepreconcentrationprocess,a testsampleof benzenein heliumwasused.This samplewaspreparedin-houseby spiking liquidbenzeneinto a Tedlarbagfilled with helium.

3 Resultsand Discussion

A first seriesof experimentswasperformedin orderto establishthe optimum conditionsfor the equilibrium-absorption process.For theseexperiments,a standardmixture of benzenein heliumwasused.Thesamplewasenrichedat 508C anddesorptionpro-files were measuredat different temperatures.In theseexperi-ments, a short piece (approximately30 cm) of 0.22mm i.d.fused-silicatubing wasusedto directly connectthe outlet of theenrichmenttrapto theFID detector. To studythedesorption pro-file, the valve was left in the inject position for a few minutes.This allowed direct monitoring of the desorptionprofiles. Theflow-rate usedduring desorptionwas approximately7 mL/min.From the experimentsit wasfound that 25 minutesof samplingwererequiredto reachequilibriumfor benzene.

The effect of the temperatureusedfor desorptionwasevaluatedin a secondseriesof experiments.In theseexperiments,different

Figure 1. Schematicdiagramof theexperimentalset-up.Valve1 = three-wayflow selectionvalve,Valve2 = 6-portswitchingvalve,Valve3 = on/offvalve,P= pressuregauge.Thesix-portswitchingvalveis in thesamplingposition.

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desorptiontemperatures,in the rangeof 758C to 3008C, weretested.Table 1 shows the enrichmentfactors obtainedin thisstudy. Thedatain this tableclearlyshowthatthedesorptiontem-perature has a strong influence on the enrichment factorobtained. As expected,the enrichment factor increaseswithincreasingdesorptiontemperature,until it reachesa constantvalueat 2008C. All further experimentswereperformedusingadesorptiontemperatureof 2008C.

Figure 2 showsthedesorptionprofile obtainedfor benzeneafter25minutespreconcentrationat 508C followed by desorptionat2008C. Fromthis figure andTable1, it canbeconcludedthat thepreconcentrationtechniqueproposedis suitablefor gasenrich-ment for conventionalGC analysis.A homogeneousplug ofenrichedsampleis obtained.

After optimization of the preconcentrationparameters,themethodwasappliedto theanalysisof a standardmixtureof ben-zene,toluene,andp-xylenein helium.Theconditionsusedwerethe onespreviouslyidentified to be the optimumparametersforbenzeneenrichment.It should be emphasizedhere that theseconditionsmight not be the optimumparametersfor enrichmentof toluene and xylene. Figure 3.A shows the chromatogramobtainedfor the original sampleand Figure 3.B the chromato-gram obtainedfor the enrichedBTX sample.A comparisonofthe peak areasobtainedwith and without enrichmentshowedthat the systemis capableof achievinga 21-fold enrichmentforbenzeneincreasingto a 94-fold enrichmentfor p-xylene. Thesensitivityobtainedwith the techniquecanbe increasedby leav-ing thevalvein theinject positionfor a longerperiodof time. Atthis longerinject time a larger fraction of theenrichedsampleistransferredto thecolumn.Becausetheenrichedvolumeis homo-geneous,thereis no needof transferringtheentirevolumeof the

enrichedgas to the GC column as is the casein standardgasenrichmenttechniques.Evidently, larger injection volumespro-ducewider input bandwidths.Hence,a compromiseshouldbemadebetweensensitivity and loss of resolution.In its currentset-up,the systemhad an analytical repeatabilityof peakareasrangingfrom 5% to 11% (n = 5), which areacceptablevaluesattheselow concentrationlevels.

After the promisingresultsobtainedwith the standardmixture,themethodwasappliedto theanalysisof realsamples.Air sam-pleswereobtainedfrom differentareassuchasthelaboratory, oran undergroundparkinggaragein the centerof the city of Eind-

Table1. Experimental enrichment factors for benzene on the1 m61 mm65 lm CP Sil-5 CB trappingcolumn obtainedat differentdesorptiontemperatures.Samplingtemperature508C.

Enrichmentfactor

758C 1008C 1508C 2008C 2508C 3008C

1.5 3.7 12.4 21.6 21.1 20.4

Figure 2. Desorptionprofile obtained for benzeneafter equilibrium(ab)sorptiveenrichmenton a 1 m61 mm65 lm CP Sil-5 CB trappingcolumn. Samplingtime 25min at a flow rate of 20mL/min, samplingtemperature50 8C, desorptiontemperature200 8C.

Figure 3. Chromatogramsobtainedfor a calibration mixture of aro-matic hydrocarbons(BTX, 25ppm each)in helium before(A) andafterenrichment(B) on a 1 m61 mm65 lm CP Sil-5 CB trappingcolumn.Sampling flow rate: 20mL/min, sampling time: 25min. Chromato-graphicconditions:25m 6 0.22mm i. d.6 0.12lm CP Sil 8CB capil-lary column,column temperature:408C (2 min) to 2008C at 10 8/min.Peakassignment:(1) benzene,(2) toluene,(3) p-xylene.

Figure 4. Chromatogramof an air samplefrom a parkinggaragesitu-ated in the centre of the city of Eindhoven. Trapping column:1 m61 mm65 lm CP Sil-5 CB, sampling at 308C, desorption at2008C. Samplingtime 25min at a flow rate of 20mL/min. GC condi-tions: 25m60.22mm i. d.60.12lm CP Sil 8CB capillary column,col-umn temperature:408C (2 min) to 2008C at 108/min. For peakassign-ment,seeFigure3.

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hoven.The chromatogramobtainedfor the parkinggaragesam-ple is shownin Figure 4. Theidentificationof someof thepeaksthat appearedin the chromatogramasbenzene,tolueneor m/p-xylenewascarriedout on thebasisof retentiontimesby compar-isonwith standardsamples.Theconcentrationof benzenefoundin theair samplewasapproximately0.2ppm.

Theexperimentsdescribedherehaveconvincinglydemonstratedthe feasibility of the equilibrium-sorptiveenrichmentmethodasa preconcentrationtechniquefor capillary GC. The principle ofthemethodis straightforward.All that is requiredis anopen-tub-ular enrichment trap and a temperatureprogrammableoven/valve.Becausethe techniquegenerateshomogeneouslyenrichedsamples,no cryo trap is needed.Futurework will focus on thesimplification of the instrumentation.Ideally, the techniquecanbe performedusinga standardgasinjection valve with a coatedsampleloop andequippedwith a heatedvalveenclosure.

4 Conclusions

In the presentwork, the recently developedpreconcentrationtechniqueof equilibrium-(ab)sorptionis successfullycoupledtoa conventionalGC instrument.With a simple experimentalset-up it is possibleto obtainenrichmentfactorsrangingfrom 21 to94 in the analysisof aromatichydrocarbonsin air. The precon-centrationtechniquedescribedin this studyis a goodalternativeto conventionalair preconcentrationmethods.

The most important advantagesof the equilibrium-sorptiveenrichmenttechniquedescribedherearethat it canbecarriedoutat ambienttemperatures,giveshomogeneousdesorptionprofiles,usesa highly inert materialfor enrichmentanddoesnot requiretheuseof a cryogenicrefocusingstep.

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