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In-Line Detection with Microfluidic Bulk Acoustic Wave Resonator Gas Sensor for Gas Chromatography

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A microfluidic film bulk acoustic wave resonator gas sensor (mFBAR) adapted specifically as an in-line detector in gas chromatography was described. This miniaturized vapor sensor was a non-destructive detector with very low dead volume (0.02 μL). It was prepared by enclosing the resonator in a microfluidic channel on a chip with dimensions of only 15 mm × 15 mm × 1 mm. The device with polymer coating showed satisfactory performance in the detection of organophosphorus compound, demonstrating a very low detection limit (a dozen parts per billion) with relatively short response time (about fifteen seconds) toward the simulant of chemical warfare agent, dimethyl methylphosphonate. The in-line detection of the mFBAR sensor with FID was constructed and employed to directly measure the concentration profile on the solid surface by the mFBAR with the controlled concentration profile in the mobile phase at the same time. The difference of peak-maximum position between mobile phase and solid phase could be a convenient indicator to measure mass transfer rate. With the response of the mFBAR and FID obtained in one injection, an injection mass-independent parameter can be calculated and used to identify the analyte of interest.
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Sensors2021,21,6800.https://doi.org/10.3390/s21206800www.mdpi.com/journal/sensors
Article
InLineDetectionwithMicrofluidicBulkAcousticWave
ResonatorGasSensorforGasChromatography
JizhouHu,HemiQu*,WeiPang*andXuexinDuan*
StateKeyLaboratoryofPrecisionMeasuringTechnologyandInstruments,CollegeofPrecisionInstrument
andOptoElectronicsEngineering,TianjinUniversity,Tianjin300072,China;mrhjz1222@tju.edu.cn
*Correspondence:hemi.qu@tju.edu.cn(H.Q.);weipang@tju.edu.cn(W.P.);xduan@tju.edu.cn(X.D.);
Tel.:+862227401002(H.Q.&X.D.)
Abstract:Amicrofluidicfilmbulkacousticwaveresonatorgassensor(mFBAR)adaptedspecifically
asaninlinedetectoringaschromatographywasdescribed.Thisminiaturizedvaporsensorwasa
nondestructivedetectorwithverylowdeadvolume(0.02μL).Itwaspreparedbyenclosingthe
resonatorinamicrofluidicchannelonachipwithdimensionsofonly15mm×15mm×1mm.The
devicewithpolymercoatingshowedsatisfactoryperformanceinthedetectionoforganophospho
ruscompound,demonstratingaverylowdetectionlimit(adozenpartsperbillion)withrelatively
shortresponsetime(aboutfifteenseconds)towardthesimulantofchemicalwarfareagent,dimethyl
methylphosphonate.TheinlinedetectionofthemFBARsensorwithFIDwasconstructedandem
ployedtodirectlymeasuretheconcentrationprofileonthesolidsurfacebythemFBARwiththe
controlledconcentrationprofileinthemobilephaseatthesametime.Thedifferenceofpeakmaxi
mumpositionbetweenmobilephaseandsolidphasecouldbeaconvenientindicatortomeasure
masstransferrate.WiththeresponseofthemFBARandFIDobtainedinoneinjection,aninjection
massindependentparametercanbecalculatedandusedtoidentifytheanalyteofinterest.
Citation:Hu,J.;Qu,H.;Pang,W.;
Duan,X.InlineDetectionwith
MicrofluidicBulkAcousticWave
ResonatorGasSensorforGas
Chromatography.
Sensors2021,21,6800.
https://doi.org/10.3390/s21206800
AcademicEditor:
TomaszMarcinDymerski
Received:22September2021
Accepted:11October2021
Published:13October2021
Publisher’sNote:MDPIstaysneu
tralwithregardtojurisdictional
claimsinpublishedmapsandinstitu
tionalaffiliations.
Copyright:©2021bytheauthors.Li
censeeMDPI,Basel,Switzerland.
Thisarticleisanopenaccessarticle
distributedunderthetermsandcon
ditionsoftheCreativeCommons
Sensors2021,21,68002of14
Keywords:microfluidicchannel;multidimensionalgaschromatography;inlinedetection;bulk
acousticwaveresonator
1.Introduction
Gaschromatography(GC)isthepillartechnologyforgasphaseanalysisinavarietyof
applicationsincludingenvironmentalscience,clinicdiagnosis,petroleumproduction,etc.[1–
4].Togenerateinformativechromatogram,oneorseveraldetectorsareutilizedtomeasure
theanalyteselutingfromtheGCcolumn.Whiletheconventionaldetectors,suchasthermal
conductivitydetectors(TCDs)[5]andflameionizationdetectors(FIDs)[6],aregenerallyin
stalledattheterminalend,inlinedetectors(oroncolumndetectors)couldbeconfiguredin
thecapillaryline,demonstratingahighflexibilityinaserialcombinationwithotherdetectors
toprovidemultichanneldetectionwithcomplementaryinformation[7,8].Additionally,anin
linedetector,installedbetweenseparationcolumns,iscapableofgeneratingusefulrealtime
informationforthedecisionmakingofsubsequentseparationsinthemultidimensionalGC
(MDGC)[9–11].However,besidesmicrofluidicPID[12],inlinedetectionisrarelydeveloped
amongconventionaldetectors,possiblybecauseofthestringentrequirementsinthelowdead
volumeandnondestructivecharacters.
ComparedtotheconventionalGC,gassensingtechnologyoffersafast,onsite,andport
ablesolutionforthevaporanalysisinthefield.However,gassensorsimplementedalonein
evitablysufferfromfalsealarmsmostlyduetotheirlimitationsinselectivity.Recentstudies
demonstratedthatacombinationofgassensorswithGCtechnologywaspromisingtopro
ducesmallsizeandlowconsumptioninstrumentswithgoodanalyticalaccuracy.Therefore,
manyeffortshavebeendedicatedtoadaptgassensorsasGCdetectors,includingchemiresis
tors[13,14],chemicapacitor[15],surfaceplasmonresonance[16],Fabry–Pérotcavitysensor
[17],optofluidicringresonator[18],piezoelectricresonators[19],etc.Incomparisonwithcon
ventionalGCdetectors,thedetectorsbasedonmicrogassensorshavetheapparentadvantage
ofasmallfootprintandarepotentialcandidatesfortheinlinedetection.Forexample,Fabry–
Pérotcavitysensorscouldbedirectlyusedasinlinedetectors[20,21].Microgassensors,
sealedinamicrofluidicchannel,couldbeconvenientlyemployedforinlinedetectionwith
othertypesofdetectorsinaserialform[15,22].
AmonggassensorsemployedastheGCdetectors,oneapproachistomeasurethemass
changeinducedbytheadsorption/desorptionofanalyteintheefflux.Thistypeofgravimetric
sensorscontainsquartzcrystalmicrobalance(QCM)[23],asurfaceacousticwave(SAW)de
vice[24–27],andcantilevers[28].BeingsimilarwithQCM,thefilmbulkacousticwavereso
nator(FBAR)isamicroelectromechanicalsystem(MEMS)basedbulkacousticwavemicro
device.AccordingtotheSauerbreyequation,aFBARsensor,resonatingatthegigahertz
range,offershighersensitivitythanQCM,whichhastheresonatingfrequencyatseveralmeg
ahertz[29–33].WearequiteinterestedinapplyingFBARgassensorsasGCdetectorsandhave
firstlyreportedthefacilehyphenationoftheFBARswithGC[34,35].Inthisstudy,weaimed
todevelopamicrofluidicFBAR(mFBAR)gassensorforinlinedetectioninGC.ThismFBAR
basedGCdetectorhasanearlyzerodeadvolumeandaconcisefluidicdesign.Adirectcon
nectionofthemFBARwithFIDproducedatandemhybriddetectionsystem,whichgenerated
FIDchromatogramcontaininginformationofthemobilegasphaseandthemFBARchroma
togram,reflectingtheadsorption/desorptiononthesolidphase.Comparisonstudybetween
FIDandthemFBARchromatogramrevealsnotonlytheinherentcharacterofthemFBAR,but
alsotheadsorption/desorptionbehaviorofvaporsonthesurface.Intheend,todemonstrate
theutilityofanmFBARgassensorinMDGC,apreliminaryGC–GCsystem,withthemFBAR
installedbetweenseparationcolumns,wasconfiguredandtestedina11componentmixture.
2.MaterialsandMethods
2.1.DeviceFabrication
Attribution(CCBY)license
(http://creativecommons.org/li
censes/by/4.0/).
Sensors2021,21,68003of14
The2.44GHzFBARswerefabricatedinthecleanroombyMEMStechnology.Thefabri
cationprocesshasbeenreportedinourpreviouswork[36]andisbrieflydescribedasinthe
followingsteps(seeFigureS1).Step1:anaircavitywasetchedonthesiliconsubstrateby
deepreactiveionetching(DRIE).Step2:thesacrificematerial(phospho‐silicateglass,PSG)
wasfilledintotheaircavitybychemicalvapordeposition(CVD).Step3:athinlayerofmo
lybdenum(Mo)wasdepositedonthesiliconsubstrateasthebottomelectrodebyevaporation.
Step4:piezoelectricaluminumnitride(AlN)film(390nm)wasdepositedbysputtering.Step
5:alayerofMowasfabricatedtoformthetopelectrodebyaliftoffprocess.Step6:viaholes
wereetchedtoconnecttothebottomelectrodeandthedilutedHFsolutionwasintroducedto
removethesacrificialmaterial.FBARswerefabricatedonthewaferscaleandthesizeofa
singledevicewasapproximately1mm×1mm.Toenhancevaporanalytesorption,asolution
ofpolymer(Polyethyleneimine,PEI)couldbesprayedontothetopelectrodetoformathin
sorptivelayerbyinkjetprinting(Jetlab4,MicroFab,Plano,TX,USA).
Themicrofluidicchannelwasfabricatedonatransparentglassthroughlaserinduced
thermaletching(DPU10,ShenzhenLaserTechnology,Shenzhen,China).Thischannelwasa
cuboidgroovealongonesidewithtwothroughholestotheotherside.Thedimensionsofthe
groovewere4mminlength,250μminwidth,and20μmindepth.Theshallowgrooveinthe
glasschipwasoppositelyflipattachedtotheFBARschipandthejointwassealedwithepoxy
glue,thusencapsulatingtheFBARsensor(Figures1bandS1).Thegasinletandoutletwere
formedbyinsertingcapillarytubesinthetwothroughholesandthejointswerealsosealed
withepoxyglue.ThismicrofluidicchannelcappedFBARschipwithcapillaryconnectionwas
wirebondedonanevaluationboard(EVB)tomFBARgassensorsandusedasthedetectorin
subsequentgaschromatographyexperiments(Figure1c).
Figure1.SimplifiedinstrumentconfigurationdiagramshowingthefacilelyhyphenatedGC(a).(b)Car
toonshowingtheprototypemicrofluidicFBARdetectorinagaschromatographysystem.(c)Thedigital
photoandscanningelectronmicroscopepictureshowingthemicrofluidicFBARdetector.
2.2.GasSensingExperiments
Adynamicgasdistributioninstrument(MF3D,NationalInstituteofMetrology,
China)wasusedtoproduceethanolvapors(12,24,48,72ppm),acetone(8.65,17.3,34.6,
51.9ppm),heptane(4.6,9.2,18.4,27.6ppm),toluene(4.6,9.2,18.4,27.6ppm)andDMMP
(dimethylmethylphosphonate,thesimulantofchemicalwarfareagentsarin)vapors
(0.332,0.664,1.328,1.992ppm).Thevaporsarethenguidedintothetestingchamber,
whereweplacethebareandPEIcoatedFBARsensors.TheresponseofFBARswasmeas
uredbyavectornetworkanalyzerandrecordedbyaMATLABprograminrealtime.The
nitrogengasissequentiallypurgedintothechamberuntiltheequilibriumresponse.
Sensors2021,21,68004of14
2.3.GC–mFBAR–FIDSystem
AcommercialbenchtopGCapparatus(7890C,Agilenttechnology,SantaClara,CA,
USA)wasusedtocharacterizemicrofluidicFBARdetectors.AsshowninFigureS2,The
GCexperimentswereperformedwithanautomaticsplit/splitlessinjector,aseparation
column,aninlinedetector,andaflameionizationdetector(FID).Heliumwasusedasthe
carriergaswithflowratesbetween0.5and10mLmin1.Theinjectorwasoperatedat250
°Cwithaninjectionvolumeof1.0μLatasplitratioof~10:1,unlessotherwisestated.The
oventemperaturewassetasTinitial=50°C,ramp=10°Cmin1,Tfinal=230°C.HP5ms(30
m×0.32mmID,0.5μm,Agilenttechnology)wasusedasaseparationcolumnintheoven.
ThemicrofluidicFBARdetectorwasinstalledimmediatelyafterthecolumnasaninline
detectoroutsidetheoven.TheinletandoutletofmicrofluidicFBARdetectorwerecon
nectedtotheseparationcolumnandFIDthroughtransferlineswithpressfitunions(5190–
6979,Agilenttechnology,SantaClara,CA,USA).Thetransferlinewasadeactivatedfused
silicacolumn(0.3m×0.32mm).TheresonantfrequencyoftheFBARdetectorwascontin
uouslymonitoredbyavectornetworkanalyzer(E5061B,Keysight,SantaRosa,CA,USA)
andrecordedthroughacustommadeMATLABprogramonapersonallaptop.Thedata
acquisitionrateoftheFBARresonancefrequencyis20pointspermin.or60pointsper
min.,toensure10pointsperpeakatleastinthechromatogram.TheFIDwasoperatedat
aconstanttemperatureof250°Cwithairflowof350mLmin1andhydrogenflowof35
mLmin1.
Sensors2021,21,68005of14
3.ResultsandDiscussion
3.1.GasSensingPerformance
TheresponseofaFBARdetectorisgovernedbytheSauerbreyequation,whichde
finesthatthefrequencychangeisdirectlyproportionaltotheaddedmassonthetopelec
trode,asfollows[37,38]:
𝑓
 
∆𝑚,(1)
𝑓
𝐵∙𝑚,(2)
where∆𝑓and𝑓
arethefrequencyshiftandtheresonantfrequencyofFBAR,respec
tively;𝜇and𝜌aretheelasticmodulusandthedensityofpiezoelectricmaterial,re
spectively;𝐴istheareaoftheplate;∆𝑚istheaddedmass;𝐵isaconstantforaspecific
FBARdevice.TheSauerbreyequationisvalidaslongastheaddedmassisrelativelysmall
incomparisonwiththemassofpiezoelectriclayerandrigid.Theadsorptionquantityon
thesolidsurfaceisexpressedbythefollowingequation[39]:
𝐾
∆
∆
∙∙,(3)
where𝐾istheequilibriumconstantatthetemperatureofT;𝐶and𝐶aretheanalyte
concentrationsonthesolidsurfaceandinthegasphase,respectively;𝐴istheexposed
areaofthetopelectrodeofFBARinthemicrofluidics.
InordertoevaluatetheperformanceofFBARsensorsinthevaporanalysis,wein
vestigatedthesensitivity,linearity,limitofdetection(LOD)andstability.Thepolymer
PEIwaschosenasthecoatingmaterialinthesensorsincethispolymeriscommercially
availableandcapableofprovidinghydrogenbondingbylargenumbersofiminegroups.
Inaddition,ourpreliminaryresultshaveshownthatitcouldbeapotentialcandidateas
sensitivematerialsforthedetectionofchemicalwarfareagents[40].Theresponsehereis
definedasthemaximumresponseoftheanalyte.
InFigure2a,weplottheresponseofthePEIcoatedmFBARwithvaporconcentration
forfiveanalytes.Thevaporconcentrationwasvariedfrompartperbillion(ppb)topart
permillion(ppm).ThesensitivityofthePEIcoatedmFBARtowardethanol,acetone,hep
tane,tolueneandDMMPwere0.84038,1.09487,2.7376,1.98344and178.7276kHzppm1.
Figure2bcomparedtherealtimeresponseofuncoatedFBARandPEIcoatedFBARto
wardDMMPofvariedconcentration.ThesensorwithPEIcoatingdemonstratestheen
hancedDMMPsensitivityofnearlythreetimesashighastheuncoatedmFBAR.However,
thesensitivitytowardtheothervaporsdidnotshowsignificantchangeaftercoating.As
canbeseeninTable1,thecalculatedsensitivityofthePEIcoatedmFBARisverycloseto
thecalculatedsensitivityofuncoatedFBARtowardethanol,acetone,heptaneandtoluene.
ThistrendindicatesafavorableinteractionbetweenPEIandDMMP.
Figure2.(a)ThelinearrelationshipbetweentheresponseofPEI–coatedFBARsensorandtheconcentrationofvarious
vapors.(b)ReversibleresponseofuncoatedFBARandPEIcoatedFBARsensortoDMMPwithconcentrationsat0.332,
0.664,1.328and1.992ppm.(c)StabilityofuncoatedFBARsensoratambientconditionin9months.
0 1020304050
-400
-300
-200
-100
0
Uncoated FBAR
PEI coated FBAR
F, kHz
time, min
Analyte: DMMP
0.332 ppm 0.664 ppm 1 .328 ppm 1.992 ppm
0369
1.328 ppm
0369
-90
-60
-30
0
30
1.328 ppm
1st test
after 2 months
after 9 months
0369
1.328 ppm
ΔF, kH z
ti me, min
// //
4.74 kHz
1.6 kHz
0 20406080
0
100
200
300
400
Device: PEI coated FBAR
Peak height, kHz
Concentration, ppm
Ethanol Acetone
Heptane Toluene
DMMP
(a) (b) (c)
Device: Uncoated FBAR
Analyte: DMMP
Sensors2021,21,68006of14
Table1.SensitivityandLODParametersforFBARdetectorsinthedetectionoffiveVOCs.
DevicePEI—CoatedFBARUncoatedFBAR
ParametersSensitivity,kHzppm1LOD,ppmSensitivity,kHzppm1LOD,ppm
Ethanol0.8402.8560.5514.356
Acetone1.0952.1920.8702.759
Heptane2.7380.8772.5000.960
Toluene1.9831.2101.7911.340
DMMP178.7280.01360.3130.040
Then,weinspectedthelinearity.InFigure2aandFigureS3,theresponsesofthetwo
FBARsensorswiththeconcentrationshowexcellentlinearitywithanR2of0.97592–
0.99774andnegligibleinterception(lessthan3kHz)inthelinearregressionanalysisfor
allvapors.Thislinearityindicatesaconstantvalueof𝐾atthevalueoflessthan5%of
saturationvaporpressure(p0).Itshouldbenotedthatthevalueof𝐾couldvarygradu
allyforthehigherconcentrationaccordingtotheLangmuirmodelandtheFreundlich
model[41].
TheLODsforthetestedvaporsaredirectlycalculatedasminimumexposureconcen
trationrequiredtogeneratetheresponseatthesignaltonoiseratio(S/N)of3.Thenoise
isdefinedbythetwoparallellinesdrawnbetweenthepeaktopeakmaximaandminima
fromthebaselineoveraperiodoftime,whicharecalculatedto0.8kHzattheoperation
temperature.TheresultsarecalculatedandsummarizedinTable1.Thepolymercoated
mFBARintheexposureofDMMPshowsatheoreticalLODaslowas1ppbandaverified
valueof13.4ppbasGCdetector.TheinjectedmassinGCcouldbeconvertedtoaverage
gasphasedimensionlessconcentrationwiththefollowingequation[22]:
𝑐
∆ ,(4)
where𝑐isthemassdensityofDMMPintheliquidsample,𝑉istheliquidvolumeof
sampleinjectedintothecolumn,𝑉
(=24.0L/mol)isthemolarvolumeofanidealgasat
theoutlettemperature(20°C),𝑆istheinjectionsplitratio,𝑀isthemolecularweight
oftheanalyte,∆𝑡isthepeakwidthintime,and𝐹isthecolumnflowrate.Thedetectable
minimuminjectionmassofDMMPforthepolymercoatedmFBARis0.89ng,whichcor
respondstoanaveragegasphaseconcentrationof13.4ppbaccordingtoEquation(4).
ToassessthestabilityoftheFBARgassensor,thesamesensorwasrepeatedlytested
intheDMMPvaporwithinninemonths.Figure2cshowsthestabilitytestofuncoated
FBARgassensors.IncomparisonwiththeinitialresponseofsensorinDMMPvapor,the
sensorproducesaslightlylowersensingresponse(<4%)tothevaporsofidenticalconcen
tration,indicatinggoodstabilityofthesensor.Thisresultdemonstratedthatthedeviceis
stableatambientconditionandthestabilityinsensorresponseismainlydependenton
thestabilityofsorptionmaterials.FigureS4showsthestabilitytestofPEIcoatedFBAR
atambientcondition.Wedidnotobserveasignificantchangeinresponse,indicatinga
goodstabilityofPEIpolymercoatinginninemonths.
3.2.FlowProfileinmFBARSensor
Tounderstandtheinfluenceoftheinlinedetectoronthecapillaryflowprofile,we
carriedouttheCOMSOLsimulationtowardthegasflowpassingthroughthemFBAR
detector.Figure3showstheplotsoftheflowrateagainstthelengthofamicrofluidic
channel.Asexpected,themicrofluidicchannelwiththecrosssectioninasimilarsizeas
theinletholeoffersamoreevenflowprofile(Figure3b).Underthedeviceof4×0.25×
0.02mm3dimension,thevelocityinsidethemicrochannelisabout8ms1when3ms1of
heliumgasfromtheinletisapplied.Next,westudiedtheflowdisturbanceafterthein
troductionofthemicrofluidicFBARbyGCexperiments.ByusingFIDasaterminalde
tectortorecordchromatograms,wecomparedthesignalsbeforeinstallingthe
Sensors2021,21,68007of14
microfluidicFBARdetectorwiththatafterinstallingthemicrofluidicFBARdetector.As
showninFigureS5,thosetwochromatogramsarenearlyidenticalexceptforaminorshift
inretentiontime,demonstratingthattheeffectofintroducingamicrofluidicFBARdetec
torisequivalenttothatofintroducingatransferline.Thisquasi“transferline”istooshort
toaffecttheseparationefficiencyinoursystem.
Figure3.COMSOLsimulationofanalyte(ethanol)flowratemagnitudefordifferentsizesofgas
chambers.(a)4×1×0.08mm
3
.(b)4×0.25×0.02mm
3
.Thedatagraphbelowshowsthevelocity
changeofethanolvaporinthexaxisdirectionwithinthemicrochannel.Thediameteroftheinlet
andoutletis0.12mm.Inthebeginning,thechamberisfullofethanolhomogeneously.Helium,as
purginggas,isflowedinatt=0withaflowrateof3ms
1
topurgethegaschamber.Thedead
volume,definedasthespaceinsidethechamber,iscalculatedtobe0.32μLand0.02μL,respec
tively,for(a)and(b).
3.3.InLineDetectionofmFBARSensorwithFID
WithinlinedetectionofthemFBARsensorwithFID,boththeFIDchromatogram
andthemFBARchromatogramcanbeobtained.WhiletheFIDchromatogramrevealsthe
informationinthegasphase,themFBARchromatogramofferstheinformationonthe
solidsurface.ForaFIDdevice,themagnitudeofsignalisdirectlyproportionaltothe
concentrationofanalyteinthemobilegasphase.IncomparisonwithFID,theFBARde
tectoroperatesonmeasuringthechangeintheweightofadsorbatesonthesolidsurface
ratherthanthequantityofanalyteinthemobilephase.Thiskindofcomplementarityis
beneficialinunderstandingthethermodynamicanddynamicfortheinteractionbetween
gasanalytesandthesolidsurfacesincetheadsorption/desorptionprocessinvolvesthe
masstransferbetweenthegasphaseandsolidphase.
InlinedetectionofthemFBARsensorwithFIDcouldhelptorevealthemasstransfer
processinthedynamicsofchromatography.Traditionally,chromatographersanalyzethe
dynamicswithouttheinformationofconcentrationprofileinthestationaryphase.Inline
detectionofthemFBARsensorwithFIDofferstheopportunityofthedirectmeasurement
oftheconcentrationprofileonthesolidsurfacebythemFBARwiththecontrolledcon
centrationprofileinthemobilephasebyFIDatthesametime.IntheGC–mFBAR–FID
system,thePEIcoatedmFBARdetectorisfirstlyexposedtotheanalytesintheGCefflu
ent,describingtheconcentrationprofileonthesolidsurfacebymeasuringthemass
changinginducedbytheadsorption/desorptionprocess.Later,theterminalFIDdirectly
ionizesthevaporanalytesandmeasurestheconcentrationprofileinthemobilephase.
Thepatterninthevaporconcentrationcanbeeasilyvariedbychangingtheflowrate.
Sensors2021,21,68008of14
Here,wecomparedthechromatogramofFIDwiththePEIcoatedmFBAR.Itshouldbe
notedthat,throughchangingthecoatingmaterial,thesameprocedurecouldbeapplied
toanalyzeinteractionofanalyteswithanyotherpopularstationaryphasematerialssuch
asOV1,SE30,etc.Figure4a,bshowsthesimulatedconcentrationprofileofthetwoana
lytesinthemobilegasphase(top)andinthestationaryphase(bottom)forthefastmass
transferandslowmasstransfer.Inthefastmasstransfer,thepeakmaximumsinthemo
bilephaseandsolidphasearelocatedatthesamepositionbecauseofreachingequilib
riumquickly.Intheslowmasstransfer,thepeakmaximumsarefarawayfromeachother,
wheretheequilibriumisnotachieved.InFigure4c,weshowthatthemasstransferof
DMMPbetweenthemobilegasphaseandthesolidsurfacereachesequilibriumquickly,
thusgivingthepeakmaximumattheclosedpositionintheconcentrationprofilewhen
theflowrateis1mLmin1.Theflowrateappearstohaveaninsignificanteffectonthe
masstransferbetweenDMMPandPEIpolymersincethereisnosignificantdifferenceof
concentrationprofilefrom1mLmin1to7mLmin1.AscanbeseeninFigure4c,the
differenceinpeakpositionremainsthesamewhentheflowrateis7mLmin1forDMMP.
Incomparison,showninFigure4d,theadsorption/desorptionofethanolbetweenthemo
bilegasphaseandthesolidsurfacedeviatesignificantlyfromtheequilibriumduetoa
relativelyslowmasstransferratewhentheflowrateis1mLmin1,givingthedifferent
peakmaximumpositionsfarawayfromeachother.Theflowrateappearstohaveasig
nificanteffectonthemasstransferbetweenethanolandPEIpolymer.Thedifferencein
peakpositionforethanolreducesfrom0.283(1mLmin1)to0.1(4mLmin1).Themass
transferofethanolwouldincreasetothesaturationwhentheflowrateincreasesto4mL
min1.After4mLmin1,themagnitudeofvariationforethanolisthesameasthatforthe
DMMP,indicatingasimilarmasstransferrate.Theamplitudeofdifferenceinpeakmax
imumpositionscouldbeaconvenientindicatorofmasstransferrate.Wesummarizedthe
valueofvariationforthetwovaporsatthedifferentflowratesinTableS1.
Sensors2021,21,68009of14
Figure4.ThemeasurementoftheconcentrationprofileonthesolidsurfacebythePEI–coated
mFBARwiththecontrolledconcentrationprofileinthemobilephasebyFIDatthesametime.(a,b)
Simulatedanalyteconcentrationprofileinthemobilegasphase(top)andinthestationaryphase
(bottom)forthefastmasstransfer(a)andslowmasstransfer(b).Kistheequilibriumconstant;𝐶
,
𝐶
,𝐶

,𝐶
,𝐶
and𝐶

aretheanalyteconcentrationonthesolidsurfaceandinthemobilephase,
respectively.(c,d)ThecomparisonofchromatogramfromFID(top)withthatfromthePEI–coated
mFBAR(bottom)atthedifferentflowratesforanalyteDMMP(c)andethanol(d).
InlinedetectionofthemFBARsensorwithFIDcouldbeusedtogenerateanalyte
specificparameters,whichcouldbefurtherusedforanalyteidentification.Firstly,wecal
culatedtheamountofanalytesinthemobilegasphasebyusingFIDpeakareawiththe
equation:
𝑃
𝑅∙𝑛
,(5)
where𝑃
isthepeakareaofanalyte,𝑅isresponsefactor,whichisaconstantforaspec
ifiedGC,𝑛
istheamountofanalytes.Then,wecomparedtheresponseofFIDandthe
responseofthemFBARtogiveanalytespecificparameter𝑆asthefollowing:
𝑆
∆
∙
∙∆
∙
,(6)
where∆𝑓isthepeakheightofanalyte.Inthisequation,theobtained Sisrelatedtothe
molecularweightoftheanalyte𝑀
andresponsefactor𝑅.Inaddition,itisobserved
that∆𝑓and𝑃
arelinearlyproportionaltotheinjectedmassintherangeof3.14–94.2ng
Sensors2021,21,680010of14
forDMMPand19.7–591ngforethanol,andthecurvespassthroughtheoriginpoint(see
FigureS6).Therefore,theobtainedSisalsoinjectionmassindependentinthelinear
ranges.ItcouldbecalculatedinoneinjectionthankstotheinlinedetectionofthemFBAR
withFID.InTable2,wesummarizedtheobtainedSvalues.Amongthem,DMMPgave
thelargestvalueandheptaneshowedtheminimum.Thedependenceofthisparameter
(S)ontheflowrateisalsoinvestigated.ForethanolandDMMP,thevariationislessthan
5%whentheflowrateisintherangeof1–7mLmin1(SeeTableS2).Therefore,wecould
usethisparametertoidentifytheanalyteofinterestwhenSisknownforthisspecific
analyte.
Table2.Svaluesforthesixvapors.
VaporsEthanolAcetoneTolueneHeptaneDMMPMS
S(107)2.701.601.481.112.912.37
Hz(A·s)1
Figure5displaystwosimultaneouschromatogramswhichwereobtainedfromthe
mFBARsensoraswellasthedownstreamFID.Combinedwiththediverseresponsepat
ternofthemFBARandFIDwithGCseparation,therecognitionabilityforvariousana
lyteswassignificantlyimprovedbyusingtheanalytespecificparameterS.Ninerepeated
injectionsofsixchemicalswereintroducedintotheinlineGCsystem.Theresultingdata,
includingresponsesfrombothdetectorsandretentiontimepassingthroughthecolumn,
wasrecordedandpresentedinathreedimensionalspace,subsequentlyprojectedonto
theleftplanegeneratedbyemployingonlytheresponsesfromFIDandFBAR.Asshown
inFigure6,thesixanalytesoccupieddifferentandisolatedvolumesinthreedimensional
andeventwodimensionalrecognitionspace,indicatingafacileidentificationbyuseof
theanalytespecificparameterS.
Figure5.ChromatographicseparationandmFBARsensoraswellasFIDdetectionofsixmixed
chemicalcompounds.SimultaneouschromatogramtracesfromamFBARsensor(blue,bottom)and
adownstreamFID(black,top).1:Ethanol;2:Acetone;3:Toluene;4:Heptane;5:DMMP;6:MS.The
proportionofthesixanalyteswas1μL:1μL:1μL:1μL:0.1μL:0.1μL,splitratio=10:1.Theoven
temperaturewassetasTinitial=50°C,ramp=15°Cmin–1,Tfinal=230°C.Constantheliumgasflow=
1mLmin–1.
0
1E8
2E8
3E8
4E8
FID signal, pA
FID
0369121
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Sensors2021,21,680011of14
Figure6.Threedimensionalrecognitionandtwodimensionalprojectionplotsofthe6analytesus
ingthemFBARpeaksensorresponse,FIDresponse,andtheGCretentiontimeasthethreeaxes.
Eachclustercorrespondstonineindependentmeasurements.TheSvaluewaslabeledalongsidethe
analytecluster.
3.4.ApplicationinGC–GCSeparation
TodemonstratetheusefulnessofthemFBARdetectorintheMDGC,weconstructed
apreliminaryGC–GCsystem.Thesystemsetupwasdescribedinthesupportinginfor
mationandshowninFigureS7.Figure7ashowsthe1Dseparationchromatogramof11
compoundsbyusingthemFBARdetectorasaninlinedetector.Itshouldbenotedthat
thischromatogramcontainsseparationinformationoftheheartcutportionsinthe1D
separationcolumn,whilethoseinformationelementsaregenerallymissedinconven
tionalheartcut2DGC.Withtherealtimeinformationprovidedbytheinlinedetector,
theheartcuttingtimescanbedeterminedandthenusedinthetimeorientedsequential
controlprogramtoautomaticallycutthedesired1Deffluentportionsintoa2Dseparation
column.Figure7bshowsthe2Dseparationchromatogramoftwoheartcutwindows
fromthe1Dseparationcolumn.AsshowninFigure7b,twopairsofanalytes,difficultto
beseparatedinthe1Dcolumn,areseparatedthoroughlybyafast2Dseparationcolumn
operatedintheheartcutmodelafterinlinedetection.Additionally,weobservedthatthe
ratiooftheintensitychangedafter2Dseparationforthetwopairs(1and2,9and10).This
changeiscausedbyinjectorofthe2Dchromatography,wheretheconcentratorshowsthe
differenttrappingefficiencytowardtheanalytes(1and2,9and10)andsomevaporspen
etratedtheconcentrator.
Sensors2021,21,680012of14
Figure7.(a)Real–timeresponseofelevengasesdetectedby1DGC.1:Ethanol;2:DCH;3:Acetone;
4:TCH;5;Hexane;6:Benzene;7:Toluene;8:Heptane;9:DMMP;10:DIMP;11:MS.Theproportion
oftheelevenanalyteswas0.3μL:0.45μL:0.3μL:0.1μL:0.1μL:0.3μL:0.1μL:0.1μL:0.05μL:0.05
μL:0.05μL.TheoventemperaturewassetasT=230°C.Constantheliumgasflow=0.5mLmin–1.
(b)Heart–cut2Ddetectionoftwopairsofunseparatedanalytes.
4.Conclusions
Inthiswork,wedevelopedanovelmicrofluidicFBARfortheinlinedetectioninGC.
Inthisdetector,FBARisenclosedinthemicrofluidicchannelasthedetectionelementand
worksonanadsorption/desorptionmechanism.Comparativeexperimentsandsimula
tiondemonstratedthatinstallinganadditionalmicrofluidicFBARinthecapillarylinedid
notcauseanysignificantflowdisturbanceandthusdeteriorateseparation.IntheGC–
mFBAR–FIDsystem,wedemonstratedthatinlinedetectionofthemFBARsensorwith
FIDcoulddirectlymeasuretheconcentrationprofileonthesolidsurfacebythemFBAR
withthecontrolledconcentrationprofileinthemobilephasebyFIDatthesametime.The
calculatedparameterbasedontheresponseofthemFBARandFIDwasobservedtobe
nearlyconstantforaspecificanalyteregardlessofconcentrationsandflowratesinthe
linearresponserange.Thisworkshowsthepotentialforchromatographerstoanalyzethe
dynamicswiththeinformationoftheconcentrationprofileinthemobilegasphasebyFID
andonthesolidsurfacebythemFBARaswellastoutilizethemFBARwiththetime
orientedsequentialcontrolprogramtoautomaticallycutthedesired1Deffluentportions
intoa2Dseparationcolumn.
SupplementaryMaterials:Thefollowingareavailableonlineatwww.mdpi.com/1424
8220/21/20/6800/s1,FigureS1:FabricationprocessofmFBAR,FigureS2:GC–mFBAR–FIDsystem
setup,FigureS3:LinearityofuncoatedFBARsensorforvariousvapors,FigureS4:Stabilitytestof
PEIcoatedFBAR,FigureS5:Flowdisturbance,TableS1:Variationvalueofpeakmaximumposi
tionsforthetwovaporsatthedifferentflowrates,FigureS6:ResponseofmFBARandFIDfor
ethanolandDMMP,TableS2:ThedependenceofSvalueontheflowratesforethanolandDMMP.
AuthorContributions:Conceptualization,J.H.,H.Q.andX.D.;methodology,H.Q.andX.D.;vali
dation,H.Q.,W.P.andX.D.;formalanalysis,J.H.andH.Q.;investigation,J.H.;resources,W.P.and
X.D.;datacuration,J.H.andH.Q.;writing—originaldraftpreparation,J.H.;writing—reviewand
editing,H.Q.;fundingacquisition,X.D.Allauthorshavereadandagreedtothepublishedversion
ofthemanuscript.
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(b)
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Sensors2021,21,680013of14
Funding:ThisresearchwasfundedbytheNationalNaturalScienceFoundationofChina(NSFC
No.21861132001),NationalKeyR&DProgramofChina(2018YFE0118700),TianjinAppliedBasic
ResearchandAdvancedTechnology(17JCJQJC43600),the111Project(B07014),andtheFoundation
forTalentScientistsofNanchangInstituteforMicrotechnologyofTianjinUniversity.
InstitutionalReviewBoardStatement:Notapplicable.
InformedConsentStatement:Notapplicable.
DataAvailabilityStatement:Notapplicable.
Acknowledgments:TheauthorsgratefullyacknowledgefinancialsupportfromtheNationalNat
uralScienceFoundationofChina,NationalKeyR&DProgramofChina,TianjinAppliedBasicRe
searchandAdvancedTechnology,the111Project,andNanchangInstituteforMicrotechnologyof
TianjinUniversity.
ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.
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