Applications Note VOC Analyzer

Introduction Volatileorganiccompounds(VOCs)areubiquitousfrombothbiogenicandanthropogenicsources.DetectionofsuchVOCiscriticalinenvironmental,industrial,medical,andmilitaryapplications.Despitetheirimportance,real-timedetectionofVOCsatthesub-parts-per-billionlevelinthefieldremainselusive.Thecurrent,gold-standardmethodforVOCmeasurementsisEPAMethodTO-15[1],whichinvolvescollectingdiscreteairsamplesinapassivated,evacuatedsteelcanister(e.g.Summacanister)andthenmeasuringthesesamplesatalaboratoryusinggaschromatographycoupledtomassspectrometry(GC/MS).Thissystemishighlysensitiveandextensivelyvetted;however,samplingislabor-intensiveandsparse,measurementsarecostly,andthesystemisnotamenabletoreactivespeciesthatcannotbereadilycapturedandtransported.Inordertoaddresstheselimitations,researchershavedevelopedseveralfield-deployableVOCanalyzers,includingportableGC/MS,field-portablemassspectrometry,andionmobilityspectrometry.Thesesystemsarecostlytooperate,requirefrequentcalibration,areinsufficientlysensitive,andutilizeextensiveconsumablegases.Moreover,sincemanyVOCanalyzersrequirehighvacuum,theirrobustnessislimitedandskilledoperatorsareneededforfielddeployments.

Technology Inordertoaddresstheselimitations,ABBhasdevelopedtheVOCAnalyzer(model909-0048),afield-deployable,sensitiveanalyzerbasedonitspatentedmid-infraredincoherentcavityrindownspectrometry(mid-IRiCRDS).Lightinthemid-infrared(5–12microns)isabsorbedatselectwavelengthbyparticularVOCs.Byexaminingtheabsorbedfrequenciesinthis“fingerprint”regionandthemagnitudeoftheabsorption,thespecificVOCanditsconcentrationcanbedetermined.AtlowVOCconcentrations,theamountofmid-infraredlightabsorptionisverysmallandconventionalmid-infraredabsorptionspectrometrycannotreadilydiscerntraceVOClevels.

ABB’ssolution,theVOCAnalyzer,shownschematicallyinFigure1,exploitsrecentlydevelopedExternalCavityQuantumCascadeLasers(EC-QCLs)andcavityringdownspectroscopy(CRDS)toprovidesimultaneousandselectivedetectionofmultipleVOCswithhighsensitivity[2,3].Awidelytunableexternalcavityquantumcascadelaseriscoupledintoahigh-finesseopticalcavityinanoff-axisfashion[4].Thelaserisoperatedinpulsedmodeandcantuneoverawidespectralrange(e.g.2–3microns)inthemid-infrared.Theparticularspectralrangecanbecustomizedtoaddresstheapplication,withthe8–10micronrangeoftenusedforVOCcharacterization.Thehigh-finesseopticalcavityconsistsofaTeflon-coated,stainlesssteelgascellboundedbytwohighreflectivitymirrors(R>99.9%).LaserlighttransmittingthroughthecavityisfocusedontoanamplifiedHgCdTe(MCT)photodetectorwithfastresponse.

Figure1:(left)Schematicofthepatented,widelytunablemid-infrarediCRDSsystemthatisutilizedin(right)theABBVOCAnalyzer.

Eachlaserpulsecreatesacavityringdowneventonthedetector.Manysuchevents(e.g.1000–10000ringdowns)areaveragedtogether,andtheresultantexponentialdecayisfittoyieldtheopticallossofthecavity.TheopticallossataspecificwavelengthismeasuredwhenthecavitycontainsbothVOC-freeairandthesamplegas.Thedifferencebetweenthesevaluesprovidestheopticalabsorptionofthegassampleatagivenlaserfrequency.Thelaseristhensteppedthroughitswavelengthtuningrangetocreateacompleteabsorptionspectrumofmeasuredcavityopticallossduetoabsorptionversuswavelength.Eachspectrumtakesapproximately5–15minutestoobtain,dependingonthewavelengthrangeandstepsize.

Theminimumdetectableabsorptionlossislimitedbythedetectornoiseonthemeasuredringdowntrace.Inordertoreducethisnoise,theeffectivelaserpowercoupledinthecavitymustbeincreased.Thelaserpowercoupledintothecavitywasincreasedbyusingare-injectionmirrortorepeatedlyre-injectthelaserbeamreflectedoffthefrontmirrorintothecavity.Thisprocesssignificantlyimprovesthemeasurementsignal-to-noiseratio.

ThespectrumofVOC-free,dryairisthensubtractedfromthespectraofthegassamplestodeterminetheabsorptionspectrumofthesamplegas.Thismeasuredabsorptionspectrumisfittoasumofabsorptionspectraofdiscretecompoundsviaalinearleast-squaresfit.Theindividualspeciesabsorptionspectraareeitherdirectlymeasuredortakenfrompublisheddatabases(e.g.theNorthwestInfraredspectrallibraryofquantitativeinfraredabsorptionspectrafromthePacificNorthwestNationalLaboratory).Asamplemeasuredspectraof97.5ppbtetrachloroethylene(PCE)and88.7ppbtrichloroethylene(TCE)inambientairandtheassociatedfitsareshowninFigure2.Theformeralsoincludesopticalabsorptionbetween940–1000cm-1duetoFreon-134ausedinthechemicaldustersinthelaboratory.

Figure2: Spectraoftetrachloroethylene(PCE)andtrichloroethylene(TCE)measuredbytheABBVOCAnalyzer.TheanalyzeriscapableofdetectingPCEandTCEto<1ppb.ThePCEspectrawascontaminatedwithFreon-134afoundintheambientlaboratoryair.

Advantages of ABB VOC Analyzer TheABBVOCanalyzerhasseveraladvantagesoverconventionaltechnologiesincluding:

Field-PortableDeploymentTheABBinstrumentishighlyrobustandcanbereadilyfield-deployedinawidearrayofenvironmentalconditions.Turnkeyoperationassuresthatitdoesnotrequireatrainedoperatororonsitesupport.TheanalyzerisalsoequippedwithremoteaccesstoallowforinstrumentverificationanddataaccessthroughanInternetconnection.UltrasensitiveDetectionABB’spatented,cavity-enhancedtechnologyenablesthesystemtomeasuresub-ppblevelsofseveralkeyVOCs,includingtrichloroethylene,tetrachloroethylene,refrigerants,andothercriticalenvironmentalhazards.Theanalyzerdetectionlimitcanbefurtherenhancedbyemployingapre-concentrationsystem,ifnecessary.Fast,ContinuousMeasurementsTheinstrumentprovidesaVOCmeasurementevery5–15minutes.Unlikeconventionalflaskmethods,whicharetypicallytakenatverylong,discreteintervals(e.g.,monthly),theanalyzerprovidescontinuousmeasurements,allowingforquantificationoftransienteventslikediurnalcycling,pollutionmigration,andremediationefforts.

NoConsumables

PeriodicmeasurementsofVOC-free,dryaircanbeprovidedbypassingambientairthroughaNafiondryercoupledtoaVOCscrubber.Thisconfigurationobviatestheneedforanyconsumables,extendingfielddeployments,reducingserviceneeds,andloweringcostofownership.

Cost-Effective

TheABBVOCAnalyzerissubstantiallymoreeconomicalthancompetinganalyzersthatutilizemassspectrometry.Additionally,theinstrument’srobustnessandlackofconsumablesresultinamarkedlylowercostofownership.

Applications TheABBVOCAnalyzercanbeusedinawidevarietyofapplications.ThesystemcandetectmanyenvironmentalpollutantsincludingBTEXcompounds(benzene,toluene,ethylbenzene,andxylenes),PCE,andTCE.Thismakesitidealformonitoringpollutedarea,includingEPASuperfundandDOEremediationsites.Likewise,theinstrumentcanbeusedtodetecthighlytoxicspecies,includingmanychemicalweapons,forpersonnelsafety.VOCdetectionhasalsobeenimplicatedinnon-invasivemedicaldiagnosticsandisanexcitingprospectforbreathmonitoring.Thesystemiscapableofmeasuringhundredsofcompoundsthatabsorblightinthemid-infraredandABBcantailorasolutiontomeetmanycustomerneeds.

Field Deployment 1 – Superfund Site Monitoring TheMiddlefield-Ellis-Whisman(MEW)SuperfundStudyAreausedtohouseseveralindustries,includingsemiconductormanufacturing,drycleaning,andmetalfinishingfacilities.Chemicalsfromtheseindustrieswereleachedintothesoilandgroundwater,resultinginsignificantTCEandPCEcontaminationlevels.

TheanalyzerwasdeployedinBuilding10oftheNASAAmesResearchCenterandsampledalternativelyfrom2locationsinthebuilding:thebreathingzoneandtunneltube.Forcedaircirculationwasusedinthetunneltubeaspartofremediationefforttopreventthebuildupofairtoxins.Thesampleinletwascycledbetweenthetunnelair,breathingzoneair,andzeroairwithaspectrummeasurementevery20minutes.MeasurementsofPCEandTCEversustimeforbothdeploymentsareshowninFigure3.

Withactiveremediation,themeasurementsshowtypicalambientPCEandTCElevelsof<50ppbinthetunnelzonethatreduceto<2ppbinthebreathingzone.Whentheremediationeffortsweredeactivated,thelevelsinthebreathingzoneincreasedto75–150ppb,withlittletonocorrespondingincreaseinthebreathingzone.Moreover,thereal-timemeasurementsdemonstratedthatthebuild-uptimeconstantfortheairtoxinswasabout14hours,allowingresearcherstomeasurevaporintrusionratesandobtainanimmediategaugeofremediationefficacy.ThemeasurementswerecomparedtodiscretesamplestakenaccordingtotheTO-15protocol,andfoundtoagreetowithinthespreadofthediscretesampling.

Figure3: MeasurementsoftetrachloroethyleneandtrichloroethyleneattheMiddlefield-Ellis-WhismanSuperfundsite(yosemite.epa.gov/r9/sfund/r9sfdocw.nsf/ViewByEPAID/CAD982463812).Thecontinuousmeasurements,whichwereinexcellentagreementwithconcurrentEPAdata,showdiurnalcyclingandvaporintrusiondynamics.

Theinstrumentreadilyresolvedsub-ppbchangesinPCEandTCElevelsinboththetunnelandbreathingzones,showingcleardiurnalcontaminantcycling.Increasedexposureswereobservedwhenthetunnelwasopened.SincetheABBVOCAnalyzersimultaneouslydetectsmanyVOCs,italsodetectedanunexpecteddimethyletherleakduringthedeploymentperiod.

Field Deployment 2 – Chemical Weapon Simulants TheABBVOCAnalyzerhasalsobeenusedtodetectavarietyoftoxicgasesandhazards.Forexample,theinstrumenthasbeendeployedataDoDtestfacilitytomeasurechemicalweapons.Preliminarymeasurementsoftracelevelsofdimethylmethylphosphonate(DMMP),asarinsimulant,areshowninFigure4.

Figure4: Spectraofdimethylmethylphosphonate(DMMP),asarinsimulant,measuredbytheABBVOCAnalyzeratsingle-digitppm-levelconcentrations.

ThemeasurementssuggestaDMMPdetectionlimitof150parts-per-trillion.Manychemicalweapons(e.g.sarin)havesimilarmid-infraredabsorptioncross-sectionsandasimilardetectionlimitisanticipatedforsuchweapons.Notethatthisdetectionlimitisfarbelowthemaximumsarinexposurelimitof3–6ppb,suggestingthatfalsepositivesorfalsenegativeswouldoccurveryrarely(e.g.>3years)atanalarmlimitof1ppb.FurthertestinganddeploymentiscurrentunderwaywithDoDresearchers.

Field Deployment 3 – Bacterial Off-Gas ABBscientistshaveworkedwithStanfordUniversityresearcherstoemploytheABBVOCAnalyzerforavarietyofbacterialandmedicalbreathdiagnostics.Onesuchtestinvolvedmeasuringbacterialoff-gasofseveraldifferentstrains.TheinstrumentwasconnectedtoabacterialgrowthchamberinarecirculatingfashionasshowninFigure5.

Figure5:(left)ExperimentsetupinwhichtheABBVOCAnalyzer(iCRDSCell)wasconnectedtoabacterialgrowthchamberingasrecirculationmode.

Figure6:(right)Spectraofoff-gasfromvariousbacterialstrainsmeasuredbytheABBVOCanalyzer.

Themid-infraredabsorptionspectraofseveralbacterialstrainsareshowninFigure6.Thestrainsclearlyexhibitsomesimilaroff-gaschemicalcomponents,butdiffersubstantiallyintheratiosandamountsofsuchchemicals.Thesedifferencescanbeexploitedtousethebacterialoff-gastospeciateculturetypes.

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Compounds(VOCs)inAirCollectedInSpecially-PreparedCanistersandAnalyzedbyGasChromatography."MassSpectrometry(GC/MS).USEPA.ReportnrEPA/625/R-96/010b(1999):1-67.

2. Leen,J.B.,Gupta,M.,andD.S.Baer.“DetectingExplosiveandChemicalWeaponsUsingCavity-EnhancedAbsorptionSpectrometry.”Chapter11inPellegrino,PaulM.,EllenL.Holthoff,andMikellaE.Farrell,eds.Laser-BasedOpticalDetectionofExplosives.Vol.40.CRCPress,2015.

3. Leen,J.Brian,andAnthonyO’Keefe."Opticalre-injectionincavity-enhancedabsorptionspectroscopy."ReviewofScientificInstruments85,no.9(2014):093101.

4. Baer,D.S.,Paul,J.B.,Gupta,M.,andA.O’Keefe,“Sensitiveabsorptionmeasurementsinthenear-infraredregionusingoff-axisintegratedcavityoutputspectroscopy,”Appl.Phys.B.75(2002)261.