Senso s2020,20,1038;doi:10.3390/s20041038www.mdpi.com/jou nal/senso s
A icle
TheE ec o heO ien a ionTowa dsAnaly eFlow
onElec ochemicalSenso Pe o manceandCu en
Fluc ua ions
Pe Sedlák1,*andPe Kube ský2
1Facul yo Elec icalEnginee ingandCommunica ions,B noUni e si yo Technology,Technická10,
B no61600,CzechRepublic
2Facul yo Elec icalEnginee ing,RegionalInno a ionCen e o Elec icEnginee ing,
Uni e si yo Wes Bohemia,Uni e zi ni8,Plzen30100,CzechRepublic;[email p o ec ed]u.cz
*Co espondence:[email p o ec ed];Tel.:+420‐541‐146‐021
Recei ed:21Janua y2020;Accep ed:13Feb ua y2020;Published:14Feb ua y2020
Abs ac :Analy e lowin luences hepe o manceo e e ygassenso ; hus,mos o hesesenso s
usuallycon ainadi usionba ie (laye ,co e ,inle ) ha canp e en henega i eimpac o a
suddenchangeo di ec ionand/o he a eo analy e low,aswellas a iousunwan edimpac s
om hesu oundingen i onmen .Howe e ,se e almeasu emen echniquesuse hemodula ion
o he low a e oenhancesenso p ope ieso oex ac mo ein o ma ionabou hechemical
p ocesses ha occu onasensi i elaye o awo kingelec ode.Thepape dealswi h he
expe imen als udyonhow heanaly e low a eand heo ien a iono heelec ochemicalsenso
owa ds heanaly e lowdi ec ionin luencesenso pe o manceandcu en luc ua ions.
Expe imen swe eca iedou onasemi‐plana , h ee‐elec ode opology ha enabledadi ec
exposu eo hewo king(sensing)elec ode o heanaly ewi hou anya i icialdi usionba ie .
Thesenso was es edwi hin he low a e angeo 0.1–1L/minand heo ien a iono hesenso
owa ds heanaly e lowdi ec ionwasg aduallyse o he ou angles0°,45°,90°and270°in he
middleo he es chambe ,while hesenso wasalsoin es iga edin hes anda dposi iona he
bo omo hechambe .
Keywo ds:analy e low;cu en luc ua ions;ampe ome icsenso ;signal‐ o‐noise a io
1.In oduc ion
Chemicalgassenso s ep esen low‐cos ,easy‐ o‐usede iceswi hahighminia u iza ion
po en ial ode ec chemicalsubs ancesdispe sedinanen i onmen o awidespec umo
applica ions[1,2].Thesenso mechanismsand hei p ope iesa e igh lyboundwi helec iccha ge
anspo a /inac i eelec ochemicalin e aces[3],anda edelibe a elyin luencedbychanging
wo kingcondi ionso unin en ionallyby heou sideen i onmen .I shouldbeno ed ha se e al
measu emen echniquesusemodula iono he low a e[4–7], he empe a u e[8–12]o e en he
ligh [13–15] oex ac mo ein o ma ionabou hechemicalp ocesseso oenhancesenso
p ope ies,since heelec ochemicalsenso (suchasaconduc ome ico ampe ome icone)could
exhibi c oss‐in e e ence,i.e.,non‐selec i i y, o chemicallysimila molecules[1,16].
Theanaly e low a ein luences hepe o manceo e e ygassenso ; hus,mos o hesesenso s
usuallycon ainadi usionba ie (laye ,co e o inle ) ha mayp e en henega i eimpac o a
suddenchangeo di ec ionand/o he a eo analy e low,aswellas a iousunwan edimpac s om
hesu oundingen i onmen [17].Se e alau ho sdesc ibed hee ec o he low a eon hesignal
andsenso p ope ieso gassenso s[18–21].Ou p e iouspape [21]demons a edona ully‐p in ed
Senso s2020,20,10382o 17
ampe ome icgassenso ha hedi ec cu en (DC) h ough hesenso exhibi schangeswi hin he
angeo oneo de o anon‐ze o low a e,while hespec aldensi yo cu en luc ua ions
signi ican lychangesi sshapeand hele elo cu en luc ua ionsin hepowe spec um o he
o de sas he low a einc easesa acons an concen a iono hede ec edgas.Fu he mo e, he
signal‐ o‐noise a ioo esponsewasalmos in a ian ogasconcen a ionanddec easedwi h low
a einc ease.These esul sindica e heimpo anceo cu en luc ua ionmeasu emen s,makingi
possible oex ac newand aluablein o ma ion(pa ame e s) esul ingina luc ua ion‐enhanced
de ec ion echnique.
Thispape dealswi h heexpe imen als udyo how heanaly e low a eand heo ien a ion
o hesenso ,i.e., hesu acea eao hewo kingelec ode(WE) owa ds heanaly e lowdi ec ion,
impac o e allsenso pe o manceandcu en luc ua ions.The i s se o expe imen swasaimed
a acompa isono selec edsenso pa ame e s(sensi i i y, esponse/ eco e y ime,limi o de ec ion
and epea abili y)dependen onbo h hesizeo heWEsu acea eaand heanaly e low a e.Wi hin
hesecondse o expe imen s,a en ionwaspaid o hee ec o hemu ualanglebe ween heanaly e
lowdi ec ionand heWEsu acea eaonsenso pa ame e s,whe e heo ien a iono hesenso
owa ds heanaly e lowwasg aduallyse o he ou angles0°,45°,90°and270°in hemiddleo
he es chambe .The hi dse o expe imen swasaimeda hes udyo cu en luc ua ions
dependingonsenso o a ion o hea o emen ionedanglesbe ween heanaly e lowdi ec ionand
hesu acea eao hewo kingelec ode.Twodependenceswe es udiedsepa a ely o eachangle:
(i) hee ec o analy econcen a iona apa icula low a e,(ii) hee ec o analy e low a ea a
pa icula concen a ion.
2.Ma e ialsandMe hods
2.1.Senso P epa a ion
Allexpe imen swe eca iedou onawell‐es ablished, h ee−elec odesenso pla o m(see
Figu e1)basedonace amicsubs a ewi hapla inumcoun e (CE)andapla inumpseudo e e ence
elec ode(RE).Asolidpolyme elec oly e(SPE)con ained1‐e hyl‐3‐me hylimidazolium
bis( i luo ome hylsul onyl)imide[C
2
mim][NT
2
]ionicliquid,poly( inylidene luo ide)(PVDF)and
1‐me hyl‐2‐py olidone(NMP).Mo ein o ma ionabou hesenso pla o mandade ailed
desc ip iono SPEp epa a ioncanbe oundin[22–24]and[25,26], espec i ely.Aca bonwo king
elec odewasp epa edbysp aycoa ingamix u econsis ingo 300mgglassyca bonsphe icalpowde
(2−12μm,Sigma−Ald ich,Ge many)and1mLo e hanol.Th eedi e en wo kingelec odea eas
we ep epa edwi h ega d o heo e alldimensiono hebasicce amicsubs a e.The a ioo
geome ica easbe weenWEandCEwe eapp oxima ely1/5,1/2and1 o heWEsu acea ea2.9,
8.5and22.4mm
2
, espec i ely.
Figu e1.Senso opologywi h h eedi e en wo kingelec odea eas.
2.2.Expe imen alSe up
Theappa a us o senso es ingconsis edo PC‐con olledmass lowcon olle s
(Sma T ak100,Sie aIns umen s,Mon e ey,USA‐Cali o nia),gascylinde swi hsyn he icai and
Senso s2020,20,10383o 17
a e e encecalib a ionmix u e(100ppmNO
2
balancedinni ogen),a es chambe and eadou
elec onics.Fo u he desc ip iono heappa a us, heau ho s e e oap e iouss udy[21].Fo
he i s se o expe imen s,when he es edsenso swe eplacedon o he es chambe bo om,gold
sp ingp obeswe eused ocon ac all h eeelec odes.Fo hesecondand hi dse o expe imen s,
he es edsenso wascon ac ed iasil e wi es ha allowed he o a iono hesenso .A
p og ammableanalog on ‐end(AFE)po en ios a LMP91000(TexasIns umen ,Dallas,USA)was
usedas eadou elec onics o all hemeasu emen sexcep hemeasu emen o cu en luc ua ion.
Bias ol agebe weenREandWEwasse o0.5V o all heexpe imen sand hesenso esponse
cu en was eco dede e yo he second.
Themeasu emen se up o e alua iono cu en luc ua ionwe ebasedonou ownba e y‐ ed
de ice,whichimplemen s hepo en ios a ci cui inag ounded‐WEcon igu a ion[27]andalow‐noise
ansimpedanceampli ie wi hsepa a edal e na ingcu en (AC)andDCou pu s.Thesenso unde
es isconnec ed o hepo en ios a ci cui .TheAC ol ageou pu wasled oanampli ie wi hhighly
selec i e il e sAM22(3SSedlak,s. .o.,B no,CzechRepublic),andwasob ainedbya12‐bi AD
con e o HS5(TiePieenginee ing,Sneek,Ne he lands)aswellasDC ol ageou pu .Toelimina e he
ex e naldis u bance om heen i onmen , he es chambe wi h hesenso andou de iceisloca ed
ina a adaycageandallde iceso hemeasu emen chainwe epowe ed omba e ies.
3.Resul sandDiscussion
Toillus a eai lowbeha io in he es chambe a a ious low a es,simpli iedp elimina y
simula ionswe eca iedou wi hou heconside a iono asenso ,con ac needles,e c.Figu e2a
showscompu a ional luiddynamics(CFD)simula ions[28]o ai lowin hegaschambe a he low
a eso 0.1L/min,0.5L/minand1.0L/min.Thecompu a ionwasca iedou in heso wa e
en i onmen ANSYS17.0 o hechambe wi hou anysenso o con ac needles.TheCFD
simula ionsshow ha o icesa e o med o all a es, hus, he lowin hechambe is u bulen [29].A
0.1L/min, he lowalong helongi udinalmid‐planeexhibi minimalin luence om o ices,while
o highe low a es hedeg eeo u bulenceg ows.Twosigni ican o icesa e o med
symme ically o helongi udinalplaneo hechambe ,andchange hei posi ions om hele o
he igh as he low a einc eases.Oneo hese wo o icesissupposed osigni ican lycon ibu e
o hesignal esponseo asenso ,sincei di ec lyin luences he lowa ound hesenso .Figu e2b
illus a es heposi iono hesenso du ing hemeasu emen sin heai aswellass eamlinesa he
low a eo 1L/min.
Senso s2020,20,10384o 17
Figu e2.(a)Compu a ional luiddynamics(CFD)simula ionso ai lowin he es chambe a low a es
0.1L/min,0.5L/minand1.0L/min,(b)senso posi ionando ien a ion owa ds he lowa angle0°
and90°in hegaschambe .
Theo ien a iono hesenso owa ds heanaly e lowwasse g adually o he ou angleso 0°,
45°,90°and270°,while hesenso wasalsoin es iga edin hes anda dposi iona hebo omo he
gaschambe [21].Acco ding osimula ions, heanaly e low ominle seems oimpac di ec lyWE
elec odeo hesenso a angles0°,45°and90°.De ailedsimula ionso he luidiccondi ionsin he
icini yo hesenso we eno ca iedou due o hei highcomplexi y.Figu e3shows heloca ion
and o a iono hesenso o he0°angle.
Figu e3.Senso posi ionin hegaschambe a angle0°.
3.1.E ec o heWo kingElec ode(WE)Su a ceA eaandAnaly eFlowRa eonSenso Pa ame e s
Wi hin he i s se o expe imen s,we es edsenso swi h h eedi e en wo kingelec ode
a eas(2.9,8.5and22.4mm
2
)andcompa ed hei basicsenso pa ame e s(sensi i i y,
esponse/ eco e y ime,limi o de ec ion, epea abili y)ino de ode e mine hein luenceo a
di e en wo kingelec odea eaonsenso pa ame e s.Eachsenso wasexposed o hesame es
p o ile ha consis edo as epwiseinc easeinni ogendioxideconcen a ion om0 o3ppm(1s ep
equaled1ppmNO
2
)wi hsubsequen h eeconsecu i eexposu es o hesameconcen a iono 3ppm
NO
2
.Weapplied hesame es p o ilewi h ou di e en o al low a eso analy e(0.1,0.5,0.8and
1L/min)ino de oobse e heimpac o he low a ele elonsenso pa ame e s o eachsizeo
hewo kingelec odea ea.Rela i ehumidi yand empe a u ewe econs an wi hinallexpe imen s
(298Kand40%RHunlesso he wises a ed)andeacho hesenso s es edwasplacedon hesame
posi ionin he es chambe obeexposed o hemos iden icalcondi ions.Figu e4shows ypical
senso esponses o he es cycle.Sensi i i ywasde e minedas heslopeo acalib a ioncu e.Fo
hecons uc iono hecalib a ioncu e,ana e age alueo senso cu en wasde e mined o each
concen a ion.A e age alues(illus a edby ed,g eenandbluesqua esinFigu e4)we ecalcula ed
om helas minu eo eachconcen a ionle el(highligh edby heg ayzone),whe esenso
esponseswe es able.Response/ eco e y ime(T
90
/T
10
)wascalcula edas he imepe iodnecessa y
oachie e90%o 10%o hes eadys a ecu en uponas epinc ease/dec easeinNO
2
concen a ion.
Fu he senso pa ame e s, helimi o de ec ion(LOD)and epea abili y,willbediscussedin he
ollowing ex .
Senso s2020,20,10385o 17
Figu e4.Senso esponse o he es cycle o sampleswi hdi e en wo kingelec ode(WE)su ace
a eas(condi ions:298K,40%RH( ela i ehumidi y),analy e low a e0.5L/min).
Figu e5showssensi i i ydependenceon heWEsu acea ea o di e en analy e low a es
(Figu e5a)andonanaly e low a es o di e en WEsu acea eas(Figu e5b).Senso sensi i i y
exhibi edalinea lyg owing endwi h heinc easeo bo h heWEsu acea eaand heanaly e low
a e.Theinc easeo sensi i i ype uni su acea eawas19,25.1,30.5and31.4nA/ppm o analy e
low a eso 0.1,0.5,0.8and1L/min, espec i ely.Thehighe he low a e, hehighe hechangeo
sensi i i ype uni su acea ea.Theinc easeo 100mL/mino analy e low a ecausedasensi i i y
inc easeo 3.9,9.8and31.5nA/ppm o heWEsu acea eao 2.9,8.5and22.4mm
2
, espec i ely.The
la ge heWEsu acea ea, hehighe changeo sensi i i ype uni analy e low a e.
Figu e5.(a)Sensi i i ydependenceon heWEsu acea ea;(b)sensi i i ydependenceon heanaly e
low a e.Thee o ba sinFigu e5 ep esen hes anda de o o heslopeo hecalib a ioncu e
(sensi i i y) ha wascons uc edbylinea eg ession.
Figu es6and7illus a e he esponse/ eco e y imedependenceson heWEsu acea ea o
di e en analy e low a es(Figu es6aand7a)andon heanaly e low a es o di e en WEsu ace
a eas(Figu es6band7b).I canbe easonablysaid ha hein luenceo WEsu acea eaonbo h he
esponse/ eco e y imewasnegligible.Di e ences omallmeasu emen s ellwi hin he angeo ±2s
wi hou anyclea endwi h espec o heWEsu acea ea.This ac isno su p isingbecause he
a eao hewo kingelec ode,whichisclosely ela edwi h hea eao heelec ochemicalac i e
in e acebe ween heSPElaye and hewo kingelec ode,usuallyin luence hesenso sensi i i y.
Response/ eco e y imeo hesenso isusuallyin luencedby he anspo o heanaly e o he
elec ochemicallyac i e(elec ode‐elec oly e)in e ace.Wi hinou expe imen s,senso swe eno
in en ionallyp o ec edanydi usionba ie ha usuallycon ols he anspo o heanaly e h ough
hesenso housingandso heonly hicknesso hewo kingelec ode, h oughwhich heanaly e
pene a ed o hein e acewi h heSPE,couldsigni ican lyin luence esponse/ eco e y ime.
Senso s2020,20,10386o 17
Al houghwedono ha eexac alueso WE hicknesses o pa icula senso swi hdi e en WE
a easwesuppose ha pa icula wo kingelec odeswe enea ly hesame hicknesswhich esul ed
in henegligiblee ec o heWEsu acea eaon he esponse/ eco e y ime.Figu es6band7bshow
esponseand eco e y imedependenceon heanaly e low a e.Whileasligh inc easeinbo h
esponse/ eco e y imewasobse edwhen heanaly e low a ewasdec eased om1L/min o0.5
L/min,asubs an ialinc easeinbo h imeswasobse edwhen heanaly e low a ewasse o0.1
L/min.Howe e , his ac couldha epa iallycon ibu ed o he imepe iodwhichisnecessa y o
exchange hein e naldead olumeo he es chambe (90mL).
TheLODcalcula edas he a ioo he iples anda dde ia iono hebackg oundcu en noise
(a ze oconcen a ion)andsensi i i yisa a he heo e ical alue ha canusuallybe e i iedwi h
di icul y.Fi s ,anaccu a eand ep oduciblep epa a iono suchlowconcen a ionle els(ideally
equaling hecalcula edde ec ionlimi s)is e yp oblema ic.Inou case, he es ingappa a us
allowedaminimums epinc easeequaling100ppbNO
2,
which esul edinana e ageinc easeo 83nA
o he educ ioncu en I
CE
wi hin he angeo 0−1ppmNO
2
(seeFigu e8).
Figu e6.(a)Response imedependenceon heWEsu acea ea,(b) he esponse imedependenceon
heanaly e low a e.Thee o ba s ep esen s anda de o o hemean ha wascalcula ed om
h eeconsecu i eexposu es o hesameconcen a ion.
Figu e7.(a)Reco e y imedependenceon heWEsu acea ea,(b) eco e y imedependenceon he
analy e low a e.Thee o ba s ep esen S anda de o o hemean ha wascalcula ed om h ee
consecu i eexposu es o hesameconcen a ion.
Thus,i hesenso wasexposed o hecalcula edde ec ionlimi (uni so ppb o senso wi h he
WEsu acea eao 22.4mm
2
,seeFigu e9)anexpec edcu en esponsewouldbewi hin he ange
o uni so nanoampe e(nA).Al houghsuchalowcu en esponsecanbemeasu ed, he eisno
gua an eewhe he suchasmallchangein hemeasu edcu en canno bea ibu ed osome
Senso s2020,20,10387o 17
pa asi ice en s.Second, heac i a iono heelec odesu acemay equi easpeci icamoun o
analy eandso hecalcula edde ec ionlimi mayna u allydi e om heac ual alue.Ne e heless,
LODisindispu ablyasenso pa ame e ha con ibu es o hede e mina iono heo e allsenso
pe o manceandiswo hde e mining;especiallywhenwea eno p ima ilyin e es edin he
absolu e alueo LODbu a he ini spo en ialdependenceonWEsu acea eaand/o analy e low
a e(seeFigu e9).While heWEsu acea eahadamino impac onLOD o analy e low a eso
0.5,0.8and1L/min,asubs an ialinc easeo heLOD aluewasobse ed o hesmalles WEa ea
(2.9mm
2
)a helowes analy e low a e(0.1L/min).This ac wasp obablycausedbyacombina ion
o lowsensi i i yandnon‐ideal(non‐cons an )backg oundcu en ,which esul edinahighe alue
o hebackg oundcu en noise.Asimila esul canbeseeninFigu e9b,whe enegligibleLOD
dependenceonanaly e low a ewasobse ed,wi h heexcep iono hesenso wi h hesmalles
WEsu acea eaa helowes analy e low a e.
Figu e8.Senso esponse os epwiseinc easeinNO
2
concen a ionswi hin he angeo 0−1ppm(1s ep
equaled100ppb),WEsu acea eao 22.4mm
2
condi ions:298K,40%RH,analy e low a e1L/min.
Figu e9.(a)Limi o de ec ion(LOD)dependenceon heWEsu acea ea,(b)LODdependenceon
heanaly e low a e.
Repea abili ymayp o idein o ma ionabou hesho ‐ e ms abili yo senso esponse.This
pa ame e wasde e minedasa a ioo he iples anda dde ia iono h eeconsecu i e
measu emen s(seeFigu e4)when he es edsenso wasexposed o hesameconcen a ionle eland
sensi i i y.Figu es10a,bshow epea abili ydependenceson heWEsu acea eaand heanaly e
low a e.Themeasu emen o senso epea abili ywasna u allyin luencedby he epea abili yo
heac ualconcen a ionle elin he es chambe i sel .Because heac ualconcen a ionle ela
Senso s2020,20,10388o 17
epea edexposu escould luc ua ewi hin he angeo enso ppb( hewo s alueis±40ppba low
a e0.5L/min,gi enby he epea abili yo hepa icula low h ough hemass lowcon olle ),i
isnecessa y o ake his ac in oaccoun anddiscussonlyclea gene al ends.Figu e10aindica es
changeso epea abili ywi hou aclea endindependenceonWEsu acea ea o he low a eso
0.5,0.8and1L/min,whileconside ableg owingdependencewasobse ed o he low a eo 0.1
L/min,which esul edinwo se epea abili y o ala ge WEsu acea ea.Figu e10bshows ela i ely
mino changesinsenso epea abili ywi hin he angeo 0.5−1L/min,whilesubs an iallywo se alues
o epea abili ywe eobse ed o allWEsu acea easwhenanaly e low a ewasdec eased o0.1
L/min.The esponseo hesenso sshowedasigni ican dependenceon hemu ualo ien a iono he
senso and heanaly e lowdi ec ion,aswellason heloca iono hesenso in he es chambe .
Conside ingp e iousmeasu emen s, u he expe imen s ega dingsenso o ien a ionandloca ion
we e ocusedonlyon hesenso wi h hela ges wo kingelec ode.
Figu e10.(a)Repea abili ydependenceon heWEsu acea ea,(b) epea abili ydependenceon he
analy e low a e.
3.2.E ec o heSenso Ro a iononSenso Pa ame e s
Thesecondse o expe imen swasaimeda hesenso wi h hela ges WEsu acea ea,whe e
a en ionwaspaid o hee ec o hemu ualo ien a iono heWEsu acea eaand heanaly e low
di ec iononsenso pa ame e s.Thesenso wasconnec ed iasil e wi esandplacedapp oxima ely
5mmabo e hechambe bo om(seeFigu e3).Weexposed hesenso oa es cycle(Figu e4)unde
ou di e en mu ualo ien a ionso analy e lowdi ec ionandsenso su ace(angleso 0°,45°,90°
and270°,seeFigu e11).Senso sensi i i yg aduallyinc easedas hesenso was o a ed om he0°
angleposi ion(analy e lowdi ec ionwaspa allel o heWEsu acea ea) o he90°angleposi ion
(analy e lowdi ec ionwaspe pendicula o heWEsu acea ea).Themu ualangleo 270°( he
senso was acingaway om heanaly e lowdi ec ion)causeda apiddec easeinsensi i i y.An
in e es ing ea u ewasobse edwhen hesensi i i yo hesenso placeda hechambe bo om( ed
poin inFigu e11a,mu ualanglenea 0°isexpec ed)wascompa edwi h he esul so he o a ed
senso omFigu e11a.I wasobse ed ha hesenso posi ioninzaxis,i.e., heheigh o hesenso
abo e hechambe bo om,wasimpo an andal houghbo hsensi i i ies o heseposi ionscould
beexpec ed obesimila , heydi e edma kedly.Conside ing hese ac s,i isob ious ha he
numbe o de ec edgasmolecules,whichhi di ec ly hesu aceo WE,pe secondinc easeswi h
heangle( om0°up o90°), hecu en iasenso g ows(desc ibedinde ailin heSec ion3.3.)and
hesensi i i yishighe (seeFigu e11).Thus,wecouldassume ha hesensi i i yis ela ed o he o
hep essu eexe edonsenso ,i.e., o luidiccondi ionsa ound hesenso .Response/ eco e y ime
dependenceisshowninFigu e11b.Gene ally, esponse imeswe elowe han eco e y imes o all
mu ualo ien a ionanglesbe weenanaly e lowand heWEsu acea ea.Bo h he esponse/ eco e y
imeshowedasligh lydec easing endas hemu ualanglewasinc eased.When hesenso was
acingaway om heanaly e low(270°angle)bo h imesinc easedsligh ly.Theonlyexcep ionwas
obse edwi h espec o hea o emen ioneddesc ip ion.Thechangeo mu ualangle om45° o90°
Senso s2020,20,10389o 17
esul edinmino inc easeo he alueo eco e y imeincon as o hecon inualdec easing end
o esponse ime.This ac doesno ha eaclea explana ionandi couldbecausedbyspeci ic luidic
condi ionsa ound hesenso o a ibu ed oano qui es ablecu en esponseo hesenso a he
angleo 45°.
Theimpac o senso o a ionon helimi o de ec ionand he epea abili yo senso esponse
isshowninFigu e12.LODdec easedas heangleo o a ioninc eased,excep heangle alueo
270°.BecauseLODwascalcula edas he a ioo iples anda dde ia iono backg oundcu en
noiseandsensi i i y,i canbe easonablyexpec ed ha heLODdependenceon he o a ionangle
would ollowexac ly heopposi e end osensi i i ydependenceinFigu e11.Repea abili ywas
de e io a ingwi h heinc easingangleo o a ion om0° o90°.When heangleo 270°wasse , he
alueo epea abili ydec eased oalmos heo igin aluea he0°angle.
Figu e11.(a)Sensi i i ydependenceon hemu ualo ien a iono heanaly e lowdi ec ionand he
WEsu acea ea(22.4mm
2
);(b) esponse/ eco e y imedependenceon hemu ualo ien a iono he
analy e lowdi ec ionand heWEsu acea ea(22.4mm
2
);condi ions:298K,40%RH,analy e low
a e1L/min.Thee o ba sinFigu e11a ep esen hes anda de o o heslopeo hecalib a ion
cu e(sensi i i y) ha wascons uc edbylinea eg ession.Thee o ba sinFigu e11b ep esen
s anda de o o hemean ha wascalcula ed om h eeconsecu i eexposu es o hesameconcen a ion.
Figu e12.(a)LODdependenceon hemu ualo ien a iono analy e lowdi ec ionand heWE
su acea ea(22.4mm
2
);(b) epea abili ydependenceonmu ualo ien a iono analy e lowdi ec ion
andWEsu acea ea(22.4mm
2
);condi ions:298K,40%RH,analy e low a e1L/min.
3.3.TheE ec o heSenso O ien a iononDi ec Cu en (DC)ResponseandCu en Fluc ua ionsunde
Equilib iumCondi ions
Thesignal esponseso hela ges wo kingelec odewe e u he in es iga edunde
equilib iumcondi ionswhen hesenso wasbeingkep a pa icula condi ions(concen a ionand
Senso s2020,20,103816o 17
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©2020by heau ho s.LicenseeMDPI,Basel,Swi ze land.Thisa icleisanopenaccess
a icledis ibu edunde he e msandcondi ionso heC ea i eCommonsA ibu ion
(CCBY)license(h p://c ea i ecommons.o g/licenses/by/4.0/).
