Hauser, New fully portable instrument for the versatile determination of cations and anions by capillary electrophoresis with contactless conductivity detection, Electroanalysis 19 2007
Trang 1Automated dual capillary electrophoresis system with hydrodynamic
a
University of Basel, Department of Chemistry, Spitalstrasse 51, Basel 4056, Switzerland
b
Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, Nguyen Trai Street 334, Hanoi, Viet Nam
c
Department of Analytical Chemistry, Physical Chemistry and Chemical Engineering – University of Alcalá, Ctra Madrid-Barcelona km 33.6, Alcalá de
Henares, Madrid 28871, Spain
H I G H L I G H T S G R A P H I C A L A B S T R A C T
Concurrentdeterminationofcations
and anions was carried out by
electrophoreticseparation
Optimizedconditionsforeachclass
ofanalystes was possibleby using
separatecapillaries
Simultaneous hydrodynamic
injec-tionwascarriedout
Pneumatic actuation was used for
flushingandsamplehandling
Thedenitrificationofdrinkingwater
wassuccessfullydemonstrated
A R T I C L E I N F O
Article history:
Received 5 March 2014
Received in revised form 15 May 2014
Accepted 25 May 2014
Available online xxx
Keywords:
Dual-capillary electrophoresis
Capacitively coupled contactless
conductivity detection (C 4
D) Simultaneous separations
Cations
Anions
A B S T R A C T Thecapillaryelectrophoresisinstrumentdevelopedfortheconcurrentdeterminationofcationsand anionsfeaturestwoseparatecapillariesandindividualdetectorstoallowindependentoptimizationfor eachgroupofions.Thecapillariesarejoinedinacommoninjectorblock.Thesampleisdrawnintothe injectorwithasmallmembranepumpandautomatedsimultaneousinjectionintobothcapillariesis achievedbypressurizationofthefluidwithcompressedair.Flushingoftheinjectorandofthecapillaries withthebackgroundelectrolyteisalsocarriedoutautomaticallybythesamemeans.Thebufferconsisted
of12mMhistidineand2mM18-crown-6adjustedtopH4withaceticacidandwassuitableforthe contactless conductivity detectionemployed Thesystem was optimized forthe determination of cationicNH4 andanionicNO3 andNO2,andlinearcalibrationcurvesfromabout20mMuptoabout 1.5mMwereobtainedfortheseions.Inatestrunover8h,thereproducibilityforthepeakareaswas within7%.Fordemonstration,theinstrumentwassuccessfullyappliedtotheconcurrentmonitoringof theconcentrationsofthethreeionsduringthebiologicalremovalofammoniumfromcontaminated groundwaterinasequencingbatchreactor,whereNO3 andNO2areformedasintermediateproducts
ã2014ElsevierB.V.Allrightsreserved
1.Introduction Capillaryelectrophoresis(CE)isarelativelysimplemethodas basicallyonly a capillary and a high voltagepower supply are
* Corresponding author Tel.: +41 612671003; fax: +41 61 267 1013.
** Corresponding author Fax: +84 4 3858 8152.
E-mail addresses: phamhungviet@hus.edu.vn (H.V Pham),
Peter.Hauser@unibas.ch (P.C Hauser).
http://dx.doi.org/10.1016/j.aca.2014.05.046
0003-2670/ã 2014 Elsevier B.V All rights reserved.
ContentslistsavailableatScienceDirect
j o u r n al h o m ep a g e: w w w el s e v i e r c o m / l o c at e / a c a
Trang 2needed for the separation of the analyte ions It is, therefore,
possibletoconstructcompact and inexpensiveportable
instru-mentsforfieldanalysis[1–3].Sampleinjectionintotheseparation
capillarycaneasilybeautomatedbyemployingaflow-injection
analysis(FIA)front end[4–6].Theuseof asequential injection
analysis(SIA)manifoldasanalternativefluidhandlingmethodfor
capillaryelectrophoresis[7,8]isabitmorecomplex,butitshigher
degreeofversatilityallows,forexample,theimplementationof
extendedunattendedmonitoring[9],orautomated
preconcentra-tion[10]
In CE theseparation of both,cations andanions ispossible
However,ifbothtypesofionsmustbedeterminedinthesame
sample this usually has tobe donein two separate runs with
oppositepolarityoftheappliedvoltage.Inordertosimplifythe
analysis,i.e.toenableconcurrentseparationofbothtypesofions,
themethodof dualopposite end injectionhasbeendeveloped
[4,11–17].Inthisapproach,analytecationsandanionsmigratein
oppositedirectionthroughthecapillary.Theneedtoinjectatboth
endsis,however,acomplication.Itmaybeovercomebypumpinga
sampleplugfromonecapillaryendtotheotherbeforeseparation
[18].Nevertheless,thesemethodsrequirecarefuloptimizationin
ordertoavoidpeakoverlapsarisingfromtheoppositemovement
of cations and anions Alternatively, concurrent separation is
carried out in two separate capillaries, following simultaneous
injectionfromthesamesamplecontainer.Thiswasdemonstrated
by Bächmann et al in 1992 [19] employing two fluorescence
detectorsandsimultaneousmanualhydrostaticsampleinjection
Morerecently,Huangetal.[20]reportedadualcapillarysystem
forthedeterminationofinorganiccationsandanionsinanaerosol
sample.However,thissystemwasimprovisedandwasbasedon
twocompletelyseparateinjectionsintothetwocapillaries.Gaudry
etal.[21]reportedanautomateddualcapillarysystemconnected
toamanifoldbasedonaperistalticpumpandaminiaturepiston
pump.Theformerwasusedforflushingofthesystem,including
the capillaries, with background electrolyte, and the latter for
sampleaspirationfollowedbyconcurrentelectrokineticinjections
intobothcapillariesfromthesamesampleplug.Theapplicationof
theinstrumenttothemonitoringofinorganiccationsandanions
in industrial and municipal water samples was demonstrated
Contactlessconductivitydetection(C4D)wasemployedforbothof
thesesystems.Amongotheradvantages,oneofthefeaturesofthis
detectionmethodislowcost,sothattheneedfortwodetectorsin
dualcapillaryelectrophoresisisnotalimitation.Forfundamental
aspectsofC4Dsee,forexample,[22–28].ApplicationsofC4DforCE
havebeendescribedinseveralreviews[29–32]
ThealternativeautomateddualCE systemreportedhereinis
alsobasedon contactlessconductivitydetection,uses a simple
pneumaticmechanismforthepumpingofbackgroundelectrolyte
andasmallmembranepumpforsampleaspiration.Thepneumatic
pressurization system also allowed the implementation of
hydrodynamic injection This is generally preferred to the
electrokineticinjectionemployedbyGaudryet al.intheirdual
capillarysystem[21].Electrokineticinjectioniseasierto
imple-mentthan hydrodynamicinjectionbut suffers froma sampling
bias The system was successfully applied tothe simultaneous
monitoringoftheconcentrationsofNHþ4,NO3andNO2 duringthe
biologicalremovalofammoniumfromcontaminatedgroundwater
inHanoi,Vietnam
2.Experimental
2.1.Chemicalsandmaterials
Allchemicalswereofanalyticalorreagentgradeandpurchased
fromFluka(Buchs,Switzerland)orMerck(Darmstadt,Germany).For
thepreparationofthestocksolutions(10mM)ofchloride,nitrate,
sulfate and nitrite their sodium or potassium salts were used Similarly, those ofthe inorganic cations (NH4,Na+, Ca2+, Mg2+,K+, Li+) were prepared from the chloride salts The separation buffer consistedof12mML-histidine(His)and2mM18-crown-6adjusted
to pH 4 with acetic acid Before use, the capillaries were preconditionedwith1MNaOHfor15min,0.1MNaOHfor5min anddeionisedwaterfor10minpriortoflushingwiththebuffer.The groundwater contaminatedwith ammoniumwas collectedfrom VanPhucvillage(Hanoi,Vietnam).Deionisedwaterpurifiedusinga system from Millipore (Bedford, MA, USA) was used for the preparationofallsolutionsandforsampledilutionifrequired 2.2.Instrumentation
ThesolenoidvalveswerepurchasedfromNResearch(116T021 and 116T031, Gümligen, Switzerland) and the micro-graduated needle valve from Idex (P-470, Oak Harbor, WA, USA) The membrane pumpforsample aspirationwasobtainedfromKNF (NF-5-DCB,Balterswil,Switzerland).Allfluidicconnectionswere madewith0.02in.i.d.and1/16in.o.d.TeflonPFAtubingunless otherwisestatedandwith1/4-280UNFfittings(Idex).Theinterface accommodating the capillaries and the ground electrode was machined in a PMMA (poly(methyl methacrylate))block (3cm
2cm2cm)andisamodificationofthesplitinjectorreported
byKubánetal.[5].Pneumaticpressurizationwasachievedwitha standardcylinderofcompressed nitrogenat200bar.Theoutlet pressurewasadjustedto1barwitharegulator
Theelectrophoresissectionwasbasedontwodualpolarityhigh voltagepowersupplies(SpellmanCZE2000,Pulborough,UK)with
30kV maximum output The high voltage electrodes were containedininsulatedcagesfittedwithsafetyswitches.Polyimide coatedfusedsilicacapillariesof50mmi.d.and365mmo.d.(from Polymicro,Phoenix,AZ,USA)wereusedfortheseparations.Thehigh voltageendsofthecapillarieswereisolatedwithsafetycagesmade fromPMMA,whichwereequippedwithmicroswitchestointerrupt thehighvoltageonopening.Detectionwascarriedoutwithtwo miniaturizedhigh-voltageC4Dcellsbuiltin-house.Forexcitation,a sinewaveof400kHzand20Vp–pwasproducedwithafunction generatorintegratedcircuit(XR2206,Exar,FremontCA,USA).This was boostedto200Vp–pusingpurpose-built transformersmade fromtwoE13/7/4,N87ferritecoreswithmatchingE13/7/4coil formers.Thesecomponentswere obtainedfromEPCOS(Munich, Germany) (product nos.B66305-G-X187 and B66306-C1010-T1) Theamplifiersonthepick-upside(OPA602andOPA2227)were obtained from Texas Instruments (Austin, TX, USA), and the synchronous detectors(AD630) fromAnalog Devices (Norwood,
MA,USA).Theresultingsignalswererecordedwithane-corder401 data acquisition system (eDAQ, Denistone East, NSW, Australia) connectedtotheUSB-portofapersonalcomputer
2.3.Systemcontrol Thesystem wascontrolledwitha personal computerviaits parallelport.Apurpose-builtelectronicinterfaceallowed switch-ingofthestop-valves,ofthe3-portvalvesandofthehighvoltage,
aswellastriggeringoftherecordingofelectropherograms.The solenoidvalveswerecontrolledviaaspecialdriverboardobtained fromthesupplierofthevalves(CoolDrive,116D5X12,NResearch) TheForthprogrammingpackageProForthfor Windows (Micro-Processor Engineering Limited, Southampton,UK) was used to write the control code Different modules were written to independentlycarryouttasksincludingflushingoftheinterface and capillaries, sample delivery, hydrodynamic injection and electrophoretic separations All modules were then assembled together to produce the instruction protocol for the entire analyticalmethod
Trang 3groundwater
A 30cm (width)30cm (depth)60cm (height) PMMA
sequencingbatch reactor (SBR) for biological nitrogen removal
withaholdingcapacityof54Lwasconstructedaccordingtothe
designbyLeeetal.[33].Aerationwascarriedoutwithanaquarium
airpump.Theseedsludgeusedinthisstudywastakenfroman
urbanwastewatertreatmentplantinHanoiandwasfirstcultivated
for15daysintap-watertowhichsugar,andNPK fertilizerwas
added prior to the ammonium removal experiments Each
sequenceofammoniumtreatmentintheSBRlastedfor6h.First,
the reactor was filled with 30L of ammonium-contaminated
groundwater Subsequently, aeration of the solution inside the
reactorwas implemented for 5husing anair pump Afterthis
period, brown sugar was added to the reactor to provide an
endogenousorganiccarbonsourceasrecommendedbyGuoetal
[34].Sedimentationwasthencarriedoutforonemorehourunder
ananoxiccondition.Everyhourduringthis6-htreatmentprocess,
a5mLaliquotwaswithdrawnfromthereactor,filteredthrougha
0.45-mmmembraneandanalyzedwithoutdilution
3.Resultsanddiscussion
3.1.Systemdesignandoperation
AschematicdrawingofthesystemisgiveninFig.1.Thefluid
propulsion and handling system was adopted from an earlier
design[35].Itisbasedonpneumaticpumping(pressurizationofa
reservoirofbackgroundelectrolytewithcompressedair)and
two-andthree-portvalvestodirecttheflow.Sampleisaspiratedintoa
sample loop, located between two 3-port valves, by a small
membranepump(withdimensionsofapproximately6cm2cm
2cm).Thisisthentransportedtoasplitinjectorblockmadefrom
PMMAwheretheendsofbothseparationcapillariesarelocated
Someofthesampleplugispushedintothecapillaries
hydrody-namicallybyclosingavalveattheexitoftheinterfacetocreatea
backpressure for a controlled length of time The desired
backpressureissetwithanadjustableneedlevalve.Moredetails
canbefoundintheearlierpublication[35].Bothcapillariessharea
common electrical ground electrode for the application of the
electrophoresisvoltage,whichisalsolocatedintheinjectorblock
Theseparationvoltagesareappliedatthedetectionendsofthe
two capillaries, using two high voltage modules set to either
negative or positive polarity for the separation of cations and
anions,respectively.Thecapillaryendsareplacedinbuffervials
together with the high voltage electrodes Note that the
electrolysisoccurringattheelectrodesleadstoaslowchangeof
thecompositionof thebufferinthesecontainers.Thistendsto affectthebaselineduetothemigrationofionsintothecapillary fromthefarend.Forthisreason,theelectrolyteinthesecontainers needstobeexchangedoccasionally.Thisoperationhasnotbeen automatedinthecurrentsystem.Forsafety,thevialswiththehigh voltageelectrodesareenclosedinPMMAcageswhicharefitted withmicroswitchestointerruptthepoweronopening
Thefactthatthehighvoltagesareappliedatthedetectionends
ofthecapillaries(ratherthantheinjectionendasisusuallythe case) isnot aproblem withC4D Thetwodetectorcells canbe positioned freely on the respective capillaries for independent optimizationasitisnotnecessarytoremovethepolyimidecoating
atthedetectionpoint,aswouldbenecessaryforopticaldetection Thedetectorswerebuiltin-houseandareamorecompactandless expensivemodificationofourprovendesign[36,37].Mechanically, thearrangementhasbeenborrowedfromFranciscoanddoLago [38]andisbasedonastackofprintedcircuitboardswhichholdthe circuitryaswellasthetubularelectrodesandactasFaradaicshield betweenthetwohalfcells.Moredetailsonthemechanicalcell
set-upcanbeseeninpreviouspublications[39,40].Ablockdiagramof thecircuitryisgiveninFig.2andconsistsofsinewavegenerator, booster,tubularelectrodepair,pick-upamplifier,rectifier,lowpass andoffsetcircuitry.Thishasnowbeenimplementedcompletelyin surface mount technology, and the cells feature a built-in miniature transformerto boost the excitationvoltage to 200V peak-to-peak(400kHz)forahighsignal-to-noiseratio.Anentire detectorcouldbehousedinasmallcaseof10cm length6cm width4cm depth.Thepreviousarrangementrequiredamuch largercasetocontaintheelectroniccircuitrywhichneededtobe separatefromthecellcontainingtheelectrodes.Theperformance
ofthenewdevicewasfoundtobecomparabletothatofourearlier design
TheoperationalsequencefortheinstrumentisgiveninTable1 Theprotocolstartswiththerinsingoftheinterfacebyallowingthe flowofthepressurizedseparationbufferthroughthesampleloop
Fig 1 Schematic drawing of the dual-capillary electrophoresis system HV, high voltage; GND, electrical ground; V1, V2, electrically actuated 3-port valves; V3, V4, electrically actuated stop valves; Pt: platinum electrodes.
Fig 2 Simplified electronic circuit diagram of the miniaturized high-voltage C 4
D.
Trang 4(designatedasV3andV4).Thentheelectrolyteisforcedthrough
thecapillariesforflushingbyclosingbothstopvalvesattheoutlet
oftheinjectorblock.Subsequently,valvesV1andV2areturned
andsampleisaspiratedintothesampleloopbyactivationofthe
membranepump.ValvesV1andV2arethenturnedbacktothe
originalposition,andthesampleplugispushedintotheinjection
interfaceby the pressurized buffer A split injection into both
capillaries is then performed byturning onthe required
back-pressure,which issetwiththeneedlevalve,byclosingonlyV3
while leaving V4 open The interface is then flushed again to
replacethesamplebybackgroundelectrolytebefore
commence-ment of the separation Bothhigh voltage power supplies are
turnedonatthesametimefortheconcurrentseparationsofthe
anionic and cationic analytes in the respective capillaries The
commonelectrodeintheinterfaceremainsgroundedatalltimes
3.2.Performance
A slightly acidic background electrolyte (pH 4), which was
basedonhistidine,aceticacidand18-crown-6andhadbeenused
successfully for the separation of inorganic cations as well as
anions by CE-C4D [9,35], was employed to investigate the
performance of the dual CE system Atthe relatively lowpH,
theelectro-osmoticflow(EOF)issuppressed;therefore,noEOF
modification is needed 18-crown-6 was included to facilitate
baselineseparationofK+andNH4 Anexampleoftheconcurrent
analysisofastandardmixtureofcationsandanionsinthetwo
capillariesisshowninFig.3.Asdiscussed,forexample,in[35],a
CE-C4Dsystemmaybeoptimizedeitherforfastseparations,for
low limits of detection or for high separation efficiency, and
compromiseshavetobemade.Inviewoftheapplicationexample
discussed below, the system was set up for high separation
efficiency by injecting relatively short plugs of sample The
calibrationdataforthethreeionsofinterest(ammonium,nitrite
andnitrate)isgiveninTable2.ForNH4 linearityupto2000mM
wasachieved.Fortheanionsnitriteandnitrate,thelinearranges
were somewhat shorter (up to 1500mM) The correlation
coefficientsobtained werebetter than0.999 for allthreeions
The reproducibilities of the measurements of peak areas and
migrationtimeswerebetterthan5%andaround1%,respectively
Thesystemwas thensetupforasupervisedtest runovera
period of 8h, during which repeated measurements of the
standardmixturewerecarried outautomatically atintervalsof
15min.Themembranepumpalsoenabledautomaticaspirationof
thesampleforeachmeasurement.Theresultsforpeakareasare
showninFig.4.Themaximumdeviationsarelessthan7%,which
is deemed acceptable considering that these are due to the
accumulationoftheerrorsofalloperations,i.e sampleloading,
delivery,injection,separationandtemperaturefluctuations.Adrift
inpeakareasisnotevidentfromthedataforthis8hrun,which
demonstrates the suitability of the system for unattended
operation
3.3.Monitoringoftheconcentrationsof NHþ4, NO3and NO2during biologicalremovalofammoniumfromcontaminatedgroundwater
In Hanoi, groundwater, which is an important source for drinking water, is often contaminated by ammonium [41] Chronicconsumptionof thiscontaminatedgroundwaterresults
inammoniumaccumulationinthebody,whichinturncanleadto theproblemsofmethemoglobinemiaininfantsandtheformation
of carcinogenicnitrosamines.Biologicalremoval ofammonium fromcontaminatedwaterusingasequencingbatchreactorisa reliable,inexpensive,andsimplemethod[34,42,43],whichhas beenpracticedinHanoi Thetreatmentprocessconsistsoftwo mainsteps.Inthefirststep,NH4 ismicrobiallyoxidizedtoNO2 andNO3 underaerobicconditions.Inthesecondstep,denitri fica-tion of these ions to molecular nitrogen occurs under anoxic conditions.Aschematicdrawingofthesmallexperimentalreactor
Table 1
Typical operation sequence.
Fig 3 Concurrent separations of inorganic anions and cations (A) Cations: NH 4 ,
50mM; K +
, 75mM; Ca 2+
, 75mM; Na +
, 225mM; Mg 2+
, 75mM; Li +
, 50mM (B) Anions:
Cl(400mM); NO3 (75mM); NO2 (75mM); SO24 (75mM) Electrolyte: 12 mM histidine and 2 mM 18-crown-6 adjusted to pH 4 with CH 3 COOH Capillaries: fused silica, 50mm i.d., 40 cm effective length and 55 cm total length Sample loop: 50mL Gas pressure: 0.8 bar Separation voltage: +15 kV for anion- and –15 kV for cation-separation.
Trang 5used for this study is shown in Fig 5 Activated sludge and
groundwaterwerefirstaddedtothetank.Tocreatetheaerobic
conditions,airwaspassedinwithasmallaquariumpump,andthis
also led toan effective mixing During the second step, when
aeration had stopped, the sludge slowly settled (within about
15min.),andthesupernatantcleanwatercouldthenbedrawnoff
Tomonitortheprocess,theconcentrationsoftheindicative
nitrogen-ions,namely,NHþ4,NO3andNO2,needtobedetermined
periodically.AnalysesofNO3 andNO2 havemostlybeencarried
outwithionchromatography,whereasNH4 hasfrequentlybeen
determinedbyspectrophotometryusingNesslersreagent.These
methods,whileworkingwellfordiscretesamples,arecostlyand
laboriousiffrequentsamplinganddeterminationofthesepositive
and negative ions are needed Here, we propose a simple and
inexpensive method for the concurrent determination of NHþ4,
NO3 andNO2 usingthedevelopeddual-channelCE system.The
rawgroundwatersampleandthosewithdrawnfromthebiological
reactorwere filtered and fed intothe dual CE system without
dilution.Notethatwhiletheinstrumentiscapableofautomated sampleaspirationandoperation(seeSection3.2),thisfeaturewas notmadeuseofforthisdemonstrationbecauseofthehighburden withsuspendedsolidsduringmostoftheprocess.Itwouldhave beenfairlydifficulttosetupreliableon-linefiltering.NHþ4 was determined in one channel and NO3 and NO2 in the other Electropherogramsofgroundwatersamplestakenbefore,during andafterthebiologicalammoniumremovalprocessareshownin Fig 6 As can be seen, the groundwater contained abundant concentrationsofCl,SO24 ,Ca2+andNa+which,ofcourse,stayed constant during the biological ammonium treatment.The con-centrations of NHþ4, NO3 and NO2 on the other hand varied considerablyduringthetreatmentprocess.Plotsofthe concen-trationsoftheseionsovertimearegiveninFig.7.Atthebeginning (t=0),anextremelyhighconcentrationofammonium(1300mM) wasrecorded,whereasonlyaminoramountofnitritewasfound Thenitrate concentrationwasbelowthedetectionlimit.Asthe collectedgroundwaterwasunderananoxicenvironment,nitrogen speciesinthisgroundwatershouldbepresentinthemostreduced form,whichisNH4 ratherthantheoxidizedproductsNO3 and
NO2.ArapiddecreaseinNH4 concentrationsduringthefirst5hof thetreatmentwhenaerationtookplacewasclearlyobserved.At thesametime,theconcentrationofNO3 increasedaccordingly,
reflectingtheoccurrenceofnitrification.Themaximum concen-trationofNO3 wasobservedat240mMattheendoftheaeration process.TheconcentrationofNO2,theintermediateproductwhen
Table 2
Calibration ranges, limits of detection and reproducibilities for the concurrent determination of NHþ4, NO3 and NO2 Electrolyte: 12 mM histidine and 2 mM 18-crown-6 adjusted to pH 4 with acetic acid.
Ion Linear range (mM) a
Correlation coefficient, r 2
Limit of detection b
(mM) Reproducibility of peak area (RSD%) c
Reproducibility of migration time (RSD%) c
a
5 Concentrations.
b
Concentrations corresponding to peak heights of 3 times the baseline noise.
c
Relative standard deviation in %, n = 6.
Fig 4 Stability test The concentrations of the ions and other conditions were as for
Fig 3
Fig 5 Schematic drawing of the activated sludge reactor During the treatment process, samples were drawn from tap 2, and at the end, after the sludge had settled, clean water was taken from tap 3.
Trang 6ammoniumis oxidizedtonitrate,sharplyincreasedin thefirst
hour, but then slightly diminishedwhen thetreatment further
proceeded.In the last hour, when theenvironment inside the
reactorwasswitchedtoanaerobicconditions,theNH4 content
remained almost unchanged while those of NO3 and NO2
decreased.Thisis becausedenitrificationof thegenerated NO3
andNO2 leadstotheirconversiontogaseousnitrogen.After6hof
treatment,theammoniumcontentwasdecreasedby88%,73%of
whichwasconvertedintogaseousnitrogen(calculatedbasedon
themolarratioofdecreasingNH4 tooxidizednitrogenionsNO3
andNO2).Thenitrogenremovalefficiencywasslightlylowerthan
thatachievedwithastep-feedsequencingbatchreactorinwhich
real-time control of the pH, oxidation reduction potential and
dissolvedoxygenwasimplementedinordertooptimize
ammoni-umelimination[34]
4.Conclusions
Adual-channelCEsystemfortheconcurrentdeterminationof cationsandanionswasconstructedandsuccessfullydemonstrated forthemonitoringofbiologicalnitrogenremovalfromammonium contaminatedgroundwater.Theinstrumentisinexpensive,simple
inconstructionandcanthereforebeassembledwithlittleeffort Thestate-of-the-artcontactlessconductivitydetectorscanbebuilt withmodestexpertiseinelectronics.Theonlyitemthatrequired engineering workshop facilities was the injection block, but it shouldbepossible tosubstitutethis withcommercialcapillary connectors [21] Pneumatic actuation proved to be a facile approachtotheimplementationofhydrodynamicinjection,which
isessentialinordertoavoidabiaswhichotherwiseoccurswhen samplesofvaryingbackgroundconductivityaretobeanalysed Further integration and miniaturization in order to obtain an readilyportableinstrumentandbatteryoperationarepossible Acknowledgements
The authorswould liketothank theSwiss NationalScience Foundation (Grant No 200020-137676/1) and the National FoundationforScienceandTechnologyDevelopmentofVietnam (NAFOSTED,GrantNo.104.04-2013.70)forfunding,aswellasthe SwissFederalCommissionfor ScholarshipsforForeignStudents (ESKAS)foragranttoThiThanhThuyPham(GrantNo.2010.0331) TheauthorsalsowouldliketoacknowledgeVanTangNguyenand VanQuanNguyen(CETASD,HanoiUniversityofScience)forhelp withsomeinstrumentalandenzyme-culturingoperations References
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