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Technical/ApplicationArticle11Version2.114March2017WRHBenzene-SpecificMeasurementsUsingtheTigerSelect

IntroductionTheIonScienceTigerSelectPIDtogetherwiththebenzenepre-filtertubes,partno.861614(formerly5/FD-01),canbeusedtomeasurebenzenespecificallywhenpresentinacomplexmixtureofotherhydrocarbonssuchasgasoline.AsshowninFigure1,thepre-filtertubeoxidizesaromaticsandolefins,andadsorbsheavycomponents includingalkanes. Lighteralkanes thatpass through the tubedonot respondon the10.0eV lamp. Onlybenzenepassesthroughthetubeunoxidizedandunabsorbed,andisdetected.Thisarticledescribesthelimitingconditionswheninterferencesmightoccur.

Figure1.Benzeneisolationfromgasoline.

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†Benzeneispresentinrefinedfuelsandthusgivesaresponse.

*Higherconcentrationsmaydecreasebenzeneresponse.**Methanedoesnotdecreasetubecapacityorgivearesponse,butabove10,000ppmitwilldecreasebenzenereadings.STELTestsThreesampleswerepreparedatamajoroilrefineryinnorthern California. One sample was from a coolerwhereQAsamplesofrefinedfuelsarestored.Anotherwas prepared by injecting pure hexanes, heptane,toluene,andbenzeneliquidsintoa20-literTedlar©gasbagfilledwithair.Thethirdwasdilutedfromacylinderofcalibrationgasusedinthelaboratorytomonitorlightend streams. Table 2 shows the make-up of thiscalibration gaswithestimatedconcentrationsof eachcomponentbasedonthebenzenemeasurement.

Each sample was analyzed a) by granular activatedcarbon (GAC) using a Gillian sampling pump, withsubsequentlaboratoryanalysis,asareference,b)usingthe Tiger Select in standard benzene mode for twominutes, and c) continuing the Tiger SelectmeasurementinSTELmodefor15minutes.TheresultsaresummarizedinTable3andFigure2.

SelectivityQuantificationTheTigerSelectgivesaspotconcentrationreadinginabouttwominutes(timesvarydependingontemperature),butcanalsomeasureShort-TermExposureLimit(STEL)readingsaftera15-minutesamplingperiod.Eventuallythepre-filtertubewillbeusedup,andcomponentsofamixturewillthenbreakthroughandbereadasiftheywerebenzene. The following testswereconducted todeterminewhatconcentrationsofotherpotential componentsmightinterfere:Table1.TigerSelectBenzeneInterferenceTests.

Compound TestConc.(ppm)

2-minResult

Max.Conc.15-min

Toluene 400 0 100Xylene 200 0 100Ethylbenzene 200 0 100n-Octane 300 0 200n-Heptane 20 0 >20n-Hexane 100* 0 20Pentane 1500 0 Butane 100 0 Propane 1000 0 Methane 25000 0 10000**Cyclohexane 50* 0 Isobutylene 100 0 RefinedFuels 50 Noeffect† 50Acetone 400 0 200Ethanol 450 0 >400Isopropanol 400 0 >400EthylAcetate 400 0 400H2S 25 0 >25CO 100 0 >100Chlorobenzene 20 2.9 Trichloroethylene 100 1.8 Perchloroethylene 50 >11

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Table3.STELMeasurementsonRefinerySamples BenzeneResponse(ppm)

SampleTVOC†(ppm) byGAC

byTigerSelectInitial* STEL

Cooler:Refinedfuelsamples 90 0.38 0.37 0.38

StandardMix:20ppmHexane19ppmHeptane26ppmToluene0.3ppmBenz.‡

65 0.45 0.44 0.44

Cal.GasMix(seeTable2) 212 4.8 4.2 4.4†TotalVolatileOrganicCompounds*Spotreadingafterthefirsttwominutesofsampling‡Addedbenzeneamount,plusunknownamountspresentasatraceimpurityintheothersolvents

Table2.CalibrationGasMixtureComponent Vol% ppmPropane 22.9 48.7n-Butane 15.0 31.9Propylene 14.7 31.2Isopentane 11.9 25.3Isobutane 10.1 21.5Isobutylene 9.53 20.21-Butene 4.99 10.6n-Heptane 2.30 4.9Benzene 2.26 4.8n-Hexane 2.09 4.4cis-2-Butene 1.04 2.2trans-2-Butene 1.03 2.2n-Pentane 1.01 2.11,3-Butadiene 0.514 1.1Propadiene 0.492 1.03-Methyl-1-butene 0.0582 0.1Methylacetylene 0.0342 0.1Total 212

Table3showsthatbenzeneSTELcanbeaccuratelymeasuredatsub-ppmlevelsinrefinerymixturescontaining65ppmhexane,heptane,&toluene,andinvaporsfromsomerefinedgasolineanddieselfuelsupto90ppm.Itcanalsobemeasuredinover200ppmofmixedalkanesandalkenestypicaloflightendstreamsinarefinery(cal.gasmix).Figure2showstheprogressionofSTELreadingswithtime.Ifthesamplingendsbeforethe15minuteperiod,thereadingequalstheaverageconcentrationtimesthefractionofthe15minutessampled,asexpected.

Figure2.STELvalueaccumulationinCoolersample.

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Used

Unused

Pre-filterTubeDiscolorationandRe-UseofTubesFigure3showshowVOCs(volatileorganiccompounds)turnthetubetoagreenish-browncolorasthereagentisconsumed.IonSciencerecommendsthatanewtubebeusedwitheachtubemeasurementforbestaccuracyandperformance.Tubescanbere-usedformorethanoneshort-term(~2min.)measurementprovidedthatthetubehasnotbeenexposedtohighhumidity,andtubediscolorationhasnotexceededthe¾markindicatedonthetube,asbelow,topreventinterferenceinthebenzenereading.Pleasenote,however,thatthere-useoftubesisdonesoattheuser’sownrisk.Iftubesarere-useditisimportantthattheyarepurgedincleanairfortwominutes,toremoveanybenzenefromthetubebeforestartinganotherbenzenemeasurement.Neveruseatubeformorethanone15-minuteSTELevenifthereisnodiscolorationofthetubeorthedisplayedreadingiszero.TheTigerSelectshouldnotbestoredwithtubesconnected.

Figure3.TubediscolorationwithVOCexposure.

HumidityConsiderationsVariationsinhumiditydonotaffectthebenzenereadingsbecausethetubeabsorbsmoisturefromtheair.However,tubesthathavebeenopentotheambientairformorethananhourorhavebeenusedformultipleairsampleswillturnadullorangecolorandhavereducedcapacityforremovingorganicinterferences.CAUTION:Donotusetubesthathaveexcessivemoistureabsorbedontothem,asliquidacidsmaybedrawnintotheinstrument,causingcorrosionandseveredamagetothePID.Again,itisalwayssafesttouseanewtubeforeachmeasurement,andneveruseatubeformorethanone15-minuteSTELtoavoidliquidacidformation.

HowWellDoestheTACReadingCorrelatetoActualTotalAromaticHydrocarbons?TheTigerSelectcanbeusedinTACmodetoscreenfortotalaromatichydrocarbonsbeforeapplyingapre-filtertubetomeasurebenzeneselectively.Figure4showsthecalculatedTACresponseona10.0eVlampforatypicalgasolinemixture. About 80% of the response is due to aromatic compounds, while 20% is due to alkanes and othercompounds.Thus,TACgivesareasonableupperlimitestimateoftruetotalaromatichydrocarbonconcentrationswhenmeasuringgasoline-typesamples. Table4showsasimilaragreement for theStandardMixtureofhexane,heptane&toluene,whichsimulatesgasoline.However,therewasnoagreementfortheCalibrationGasMixture,whichcontainsmoreolefinsthanaromatics.Theseolefins,likepropeneandbutadiene,respondonthe10.0eVlampwhen no pre-filter tube is present, and thus the so-called TAC reading is not representative of the aromaticconcentration.Nevertheless,ahighTACreadingisusefulinthatitprovidesanupperlimitandthusawarningsignaltoinsertatubetomeasurebenzeneselectively.

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Table4.TACMeasurementsonRefinerySamples

Sample TACCalc.orAdded TACReading

Cooler:Refinedfuelsamples --- 10

StandardMix:20ppmHexane19ppmHeptane26ppmToluene

26 19*

Cal.GasMix(seeTable2) 4.8 ~28

*Thisvaluewasmeasuredathighhumidityandmaybecloserto26ppmaftercorrectionforhumidityeffects.

Figure6.Linearresponseathighbenzenelevels.

Figure4.CalculatedTACresponseingasoline.EnvironmentalComplianceMeasurementsAlthoughtheTigerSelectisdesignedtomeasurebenzeneatthelowppmrangenearTWAlevels,forenvironmentalcomplianceconcentrationsof200ppmormoremayneedtobemeasured.Figure5showsthat200ppmbenzenecomes to a stable reading within 90 seconds, which is a normal response time for the Tiger Select. Figure 6demonstratesthatreadingsarelinearuptoatleast300ppmbenzene.However,thatatsuchhighbenzenelevels,the concentration of other hydrocarbons may also be high, and the capacity of the pre-filter tubes should beconsidered.Ifthetubesarediscoloredpastthe¾mark(seeFigure3),thebenzenereadingsmaybehigh,butaslongastheresponseisbelowthetestlimit(e.g.,200ppm)theresultstillwithinenvironmentalcomplianceandtheworkactivitycancontinue.

Figure5.Benzeneresponsetimeat200ppm.

AcknowledgmentsThankstoPatrickOwensforprovidingthevenueandmaterialsformuchofthiswork,andtoLongDamforassistanceintheexperiments.FormoreinformationcontactIonScience:E-mail:marketing@ionscience.com