Spectrophotometric sensor system based on a liquid waveguide capillary cell for the determination of titanium: Application to natural waters, sunscreens and a lake sediment

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Spectrophotometric

sensor

system

based

on

a

liquid

waveguide

capillary

cell

for

the

determination

of

titanium:

Application

to

natural

waters,

sunscreens

and

a

lake

sediment

Ricardo

N.M.J.

Páscoa

a

_,

_Ildikó

_V.

_Tóth

b

_,

_Agostinho

_A.

_Almeida

b

_,

_António

_O.S.S.

_Rangel

a,∗

a_CBQF/Escola_Superior_de_{Biotecnologia,}_Universidade_Católica_Portuguesa,_Rua_Dr._António_Bernardino_de_Almeida,_4200-072_Porto,_Portugal b_REQUIMTE,_Departamento_de_Química,_Faculdade_de_Farmácia,_Universidade_do_Porto,_Rua_Aníbal_Cunha,_164,_4050-047_Porto,_Portugal

Keywords: Titanium

Multi-pumpingflowsystem Liquidwaveguidecapillarycell Spectrophotometry

Traceanalysis

a

b

s

t

r

a

c

t

Ananalyticalprocedureforthespectrophotometricdeterminationoftitaniumattracelevelswas devel-oped.Theprocedureinvolvestheuseofamulti-pumpingflowsystem(MPFS)coupledwithaliquid waveguidecapillarycell(LWCC)with1.0mpathlength,550mmi.d.and250mLinternalvolume,which enabledtoenhancethesensitivityofthedeterminationandthusavoidcomplexandtime-consuming pre-concentrationsteps.Thedeterminationisbasedonthecolorimetricreactionoftitaniumwith chro-motropicacid.Thelimitofdetection(3)was0.4mg/Landalinearresponseupto100mg/Lwithasample throughputof46h−1_,_and_a_low_reagent_{consumption/effluent}_production_was_achieved._The_developed

procedurewasappliedtonaturalwaters,sunscreenformulationsandonecertifiedlakesedimentsample.

1. Introduction

Titaniumistheninthmostabundantelementinearth’scrust, anaturalconstituentofrocks,soilsandsediments[1].Thelevels ofTiinrocksarelowerthan2%(w/w)[2].TheTiconcentrations foundinriverine,estuarineandcoastalwatersrangefrom0.005 tomorethan4.8mg/L[3]andfrom0.2to17ng/Linoceanwaters

[1].TheTimineralsareveryresistanttochemicalweatheringin soilandsedimentaryenvironments,allowingthecommonuseof Ti asa guideelement tocomparethe mobilityof thedifferent elements[1].

Inrecentyears,Tihasacquiredgrowingimportanceinseveral industrial fields because of its particular physical and chemi-cal characteristics.Themaincommercially availablecompound, titaniumdioxide,isusedinsolarenergycells[4],asa photocat-alystinsterilization,aircleaningandwaterpurificationprocesses

[5,6],asaningredientofsunscreens,cosmetics,toothpastes,paints and plastics, andin themanufacture of building materials, air-crafts and missiles [7,8]. It also shows great promise in the developmentofantitumoragents[9],andfordrugdelivery, envi-ronmental cleanup and computer manufacture [10]. Titanium dioxidenanoparticlesalsoshowsdurablephotocatalyticactivity, inducedbyUV-light,causingphotochemicaldegradationoforganic

∗ Correspondingauthor.Tel.:+351225580064;fax:+351225090351. E-mailaddress:aorangel@esb.ucp.pt(A.O.S.S.Rangel).

compounds[6,11],suggestingapotentialuseinwastewater treat-mentplants[8].

TiO2intheformofnanoparticlesisnotconsideredasnew

mate-rials,asbulkTiO2hasbeenincorporatedintovariousproductsasa

whitepigmentfordecades;itisthereforecategorizedasanewform ofanexistingsubstance.Nanoparticlesareultrafineparticleswith lengthintwoorthreedimensionsgreaterthan1nmandsmaller than100nm[10,12].Despitethelargescaleandincreasing pro-ductionofthesematerials,fewstudieshaveaddressedthepossible environmentalthreatposedbynanoparticles.

Thenanoparticlestoxicitydifferswithparticletype,size, sur-faceareaandfunctionalgroupsattached[6,12].Oberdörsteretal. revealedthatthesmallerthesizeofthenanoparticles,strongerthe exertedtoxicityis[13],althoughtherelationbetweenthe physic-ochemicalpropertiesofnanoparticlesandtheirtoxicityappearsto bemorecomplex[14].

Inarecentstudy[15],basedonthealreadyexistingquantitative toxicitydata(ex.LC50 orEC50)fortheevaluationofthe

poten-tial hazardous effects of nanoparticles [16], TiO2 was classified

as“harmful”.Thisclassificationisbasedonstudiesthatinvolved differentgroupsof organisms(crustaceans,bacteria, algae,fish, nematodesandyeasts)andconcludedthatalgaearethemost sen-sitiveones.InthecaseofbulkTiO2thetoxicityfallsintothesame

classification,showingalowestLC50valueforalgae,similartothe

valuefoundforthenanoparticlesformulation.

Theaquifersmaybetheprincipalreceiverofnanotechnology industrydischarges.Therefore,ecotoxicologystudiesonwater

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col-umnorganismsandacrossseveraltaxonomicgroupsareofgreat interestforthecomprehensiveeffectassessmentofthe nanoparti-clesintheaquaticenvironment[5,7,14,17,18].

The development of analytical methods focused on under-standingthefateofnanoparticlesintheaquaticenvironment is important,asthelackofdetection/quantificationtoolshampers theadvancementsonotherareasrelatedtoitstoxicity.

In general,flow systemsarevery suitablefor wateranalysis because of increased accuracy, good repeatability, low equip-mentcost,highsamplethroughput,simplifiedsamplehandling, reducedcontaminationrisks,highdegreeofautomationand reduc-tion in sample/reagents consumption and effluent production

[19,20].

RegardingflowmethodologiesforthedeterminationofTi,the majority arebased onmolecularabsorptionspectrophotometry

[2,8,21,22],althoughtherearesomeworksexploitingother detec-tiontechniquessuchaschemiluminescence[23],ICP-AES[24]and ICP-MS[25].

Thespectrophotometricproceduresaresimple,fastandrobust andtherearesomecolorimetricreagentsusedfortitanium deter-mination aschromotropicacid, tiron,sulfosalicylic acid [2]and 4,40_{-diantipyrylmethane}_[21]_._However,_to_reach_the_{determination}

oftitaniumattracelevelswithspectrophtotometricprocedures, apreconcentrationstepisnecessary[26],significantlyincreasing thecomplexityoftheflowsystemsmanifoldanddecreasingthe determinationrate.

Toovercomethesedifficulties,inthisworkweproposetheuse ofaliquidwaveguidecapillarycell(LWCC),asamplecellwhere theopticalpathlengthisincreasedwithoutlightattenuation[27], whichenablestosignificantlyincreasesensitivity,inordertoattain thedirectdetermination(i.e.,withoutapre-concentrationstep)of tracelevelsoftitaniuminsamplesofdifferenttypes(asnatural waters,sunscreensandlakesediments).Thisapproachwasalready successfullyusedinthedeterminationofironatlowconcentration levels[28].Thecharacteristicsofthisdevicepermittotalinternal reflectionofthelightonthewallsasthelightconductingpathis transparenttothewavelengthofinterestandhasarefractiveindex higherthanthewallsmaterial.Thus,lightiskeptintheoptically densercore.

Tocarryoutthein-linesamplereactionneededforthe spec-trophotometricdeterminationofTi,anoptionwasmadeforthe utilizationofa manifold basedontherecentlyproposed multi-pumpingflowtechnique.Thisischaracterized byapulsedflow capable of producing an improved sample/reagent mixing and reactionzonehomogenization[29].Multi-pumpingflowsystems (MPFS)usemultiplelow-costsolenoidmicro-pumpsstrategically positionedinthemanifold,which,whencontrolledbycomputer software, provide easy and versatile automated fluid handling operations.Thesolenoidmicro-pumpsrequirelow-powersupply voltagetoworkandhave asmallsizewhen comparedto peri-stalticpumps,makingitanadvantageousalternativeforportable equipment/insituanalysis.

2. Experimental

2.1. Reagentsandsolutions

All solutions were prepared with analytical reagent-grade chemicalsanddeionizedwater.Titanium(IV)stockstandard solu-tion(100mg/L)waspreparedbydilutingtherespective1000mg/L atomic absorption standard (Fluka, 04689) with0.01mol/LHCl solution.Calibratingsolutionsintherangeof10–100mg/Lwere dailypreparedbyrigorousdilutionoftheTistocksolutionwith 0.01mol/LHCl. V1 V2 P₃ P2 P1 RC c W SL W LWCC S H₂O R PC V1 V2 P₃ P2 P1 RC c W SL W LWCC S H₂O R PC

Fig.1. Multi-pumpingflowsystemforthedeterminationoftitanium.Pi,pumps;Vi, solenoidvalves;SL,sampleloop(200mL);RC,reactioncoil(50cm);c,confluence; LWCC,detector(100cmopticalpath,425nm);PC,computer;W,waste;S,sample orstandard;andR,colourreagent(chromotropicacid)andbuffersolution(acetate bufferwithascorbicacid).

Acertifiedreferencematerial(CRM),alakesedimentsample ref.a_IAEA-SL-1,_was_analysed_in_order_to_evaluate_the_accuracy_of

thedevelopedanalyticalprocedure.

Fivesunscreensampleswerealsoanalysedbythedeveloped methodandtheresultsobtainedwerecomparedwiththe alterna-tiveICPMSprocedure.

TheTiO2 nanoparticles (<100nm)used intherecoverytests

performedwiththeCRMsamplewerefromSigma–Aldrich(Ref. 634662).

Thesolutionsusedin interferencestudieswerepreparedby dilutingcommercialatomicabsorptionstandardsolutions (Spec-trosol,BDH)ofFe,Al,CuorPbbydissolvingtherespectivesalts NH4VO3,K2Cr2O7andNaF(Merck),inthecaseofV,CrandF.

Chromotropicacidcolourreagentwasdailypreparedby dis-solving0.025gofC10H6Na2O8S2·2H2O(Sigma–Aldrich)and1.0g

of ascorbic acid (VWR International) in 100mL of a 0.2mol/L aceticacid–sodiumacetatesolution,resultinginasolutionwith 1.25mmol/L and0.113mol/Lof chromotropicacid andascorbic acid,respectively.

2.2. Apparatus

ThemanifoldusedforthedeterminationofTiisshowninFig.1

andcomprisedthreemicro-pumps(Bio-ChemValveInc.,Boonton, NJ,USA,Ref.120SP1220and120SP1230),withnominal dispens-ingvolumes of20 and30mLper stroke,andtwo commutation valves(NResearch,Caldwell,NJ,USA,Ref161T031)withaninternal volumeof27mL.Tocontrolbothmicro-pumpsandcommutation valves,anI/Odigitalcardwitheightoptocoupleddigitalinput chan-nelsandeightdigitalrelayoutputchannelswasused.Thiscardwas connectedtoapersonalcomputerthroughanRS485/RS232 inter-faceandcanindependentlycontroltheoperationofuptoeight micro-pumpsand/orcommutationvalves.Thesedevicesareseton amotherboardconnectedtoaprotectioninterface,whichis con-nectedtotherelayoutputsandanadditionalpowersourceof12V isrequiredtoactivatethesolenoiddevices.Inordertominimize theheatgenerationandextendthelifetimeofthevalves,asolenoid protectionsystem(Sciware,Mallorca,Spain)wasused.Thepower source,theRSserialinterfaceandtheI/Ocardswereintegratedinto auniquemodule(Ref.Module1Sciware).Thevolumesdispensed bythemicro-pumpsineachstrokewereverifiedandareshownin

Table1.

Forthesolenoidvalves,theexchangeoptionswereclassifiedin on/offlines.The“on”linewasassignedtotheflowmanifoldand

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Table1

VolumesactuallypropelledbytheMPFSmicro-pumps. Micro-pump Nominalvolumeper

stroke(mL)

Actualvolumeper stroke(mL)a

P1 20 14.0 ±0.1

P2 20 17.2±0.1

P3 30 20.4±0.4

a_Mean_and_standard_deviation_of₂₀_replicates.

the“off”linetothesolutionsflasks(representedwithasolidline anddottedline,respectively,onFig.1).

Theconnectionofthedifferentcomponentsoftheflowsystem wasmadewithPTFEtubeswith0.8mmid(Omnifit,Cambridge, UK). End-fitting and connectors (Gilson, Villiers-le-Bel, France) werealsoused.Thesampleloop(SL)andthereactioncoil (RC) were40and50cmlong,respectively.

ApersonalcomputerPentiumII,runningAutoAnalysisversion 5.0.3.5 software (Sciware),controlledthemicro-pumpsandthe commutationvalves.

The spectrophotometric measurements were carried out at 425nm.Referencewavelengthforminimizingtheschliereneffect wassetat800nm[30].Theprincipleofthesignalcompensationis tomeasuresimultaneouslythesignalattwodifferentwavelengths: oneatwhichthereactionproductabsorbspredominantlythelight andareferencewavelengthatwhichnoabsorbancechangedueto thechemicalreactionisobserved.

Asdetectionsystem,anOceanOpticsPC2000-ISA spectropho-tometer (Dunedin, FL, USA), two 200mm fibre optic cables, a DH-2000 deuterium halogen light source(Top Sensor Systems, Eerbeek,TheNetherlands)andaliquid waveguidecapillarycell (LWCC2100;WorldPrecisionInstruments,Sarasota,FL,USA)with 1.0mpathlength,250mLinnervolumeand550mminner diame-terwasused.DataacquisitionwasalsoperformedbytheSciware AutoAnalysissoftware.

AnAntonPaar(Graz,Austria)Multiwave3000microwaveoven, witha rotatingturntable(a16MF100/HF100 rotorwithsixteen PTFEvesselsof50mLmaximumcapacity),a2455MHzmagnetron andanominalexitpowerof1400Wwasemployedforacid diges-tionofthesamples.

AVGElemental(Winsford,UK),PlasmaQuad3ICP-MS instru-ment, equipped witha Meinhard type A pneumatic concentric nebulizer, aquartz watercooledimpact-bead spraychamber, a standardquartztubetorchandnickelsampleandskimmercones, wasusedinthecomparisonmethod.Boththespraychamberand samplinginterfacewerecooledto10◦_C_by_circulating_water._Argon

of 99.9999% purity (Alphagaz 2, supplied by Air Liquide, Maia, Portugal)wasusedasplasmogenicgas.ForICP-MSsample intro-ductionandwastedraining,aGilsonMinipuls3peristalticpump wasused.ThemainoperatingconditionsfortheICP-MS determina-tionofTiareindicatedinelectronicSupplementarydatainTable S1.Theisotopes(m/zratios)47_Ti_(as_analyte)_and45_Sc_(as

inter-nalstandard)weremonitored.BoththeICP-MSinstrumentcontrol anddataacquisitionwereaccomplishedbyusingtheVGElemental PlasmaLabsoftware.

2.3. MPFSconfigurationandprocedure

TheMPFS (Fig. 1)was designed toallow thedetermination of titaniumin waters, cosmetic and otherenvironmental sam-plesdigestsatverylowlevels.Theset-upincludedthreesolenoid pumpsandtwocommutationvalves.Thevolumeofthesample introducedintotheMPFScanbesetbycontrollingthevolume dis-pensedineachstrokeand/orthefrequencyofthestrokes,however, theanalyticalrepeatabilityisaffectedbythevariationofthe dis-pensedvolume.Therefore,anoptionforavolume-basedsample injectionwastaken,insteadofatime-basedinjectionapproach.To implementthisstrategy,twocommutationvalveswereusedand thesamplevolumewascontrolledbythelengthofthesampleloop placedbetweenthem.Aconfluencepointwasaddeddownstream topromotemixingbetweensample/standardandcolourreagent.

TheoperationsequenceoftheMPFSislistedinTable2.Thefirst stepconsistedoftheintroductionof500mLofsample/standardin theSLbypump2.Subsequently,thesample/standardandcolour reagent(100mL)werepropelledtotheconfluencebypump2and3, respectively.Inthefinalstep,theresultingmixturewaspropelled tothedetectorandtheanalyticalsignalwasregistered.

Attheendofaworkingday,theLWCCwassequentiallywashed incountercurrent withHCl(0.05mol/L) andNaOH(0.05mol/L) solutions,andfinallywithwater.

2.4. Microwavedigestionprocedure

The microwave digestion procedures used for each type of samples are summarized in electronic complementary data in

TableS2.SamplesweredirectlyweighedintothePTFEreactorsof themicrowavedigestionunitand,afteradditionofthedigestion reagents,themixturewassubjectedtothefollowingprogram:a pre-settimeperiodtoattainthemaximumpowerof800W, fol-lowedbyaholdingperiodatthispoweranda15mincoolingperiod at0W.Afterthat,andtocomplexfreefluoride,saturatedboric acidsolutionwasaddedataratioof6mlpereachmLofHFused inthedigestion.Theresultingclearsolutionswerequantitatively transferredtovolumetricflasksand,afterappropriatedilutionwith deionizedwater,theywereanalysedbothbythedevelopedandthe referenceprocedure.

3. Resultsanddiscussion

3.1. StudyofphysicalandchemicalparametersoftheMPFS

The optimizationstudies includedboth physical parameters (flowrate,plugsize,reactioncoillength,sampleandreagent vol-umes)andchemicalparameters(reagentsconcentrationandpH)of thewholeanalyticalprocedure.Theunivariatemethodwasused, whereonlyoneparameterwaschangedwhiletheotherswerekept constant.TheresultsaresummarizedinFig.1andinTable2.

Initially,thevolumesactuallydispensedbythemicro-pumps wereevaluated.Twodifferentflowrates(1and5mL/min)andtwo differentvolumes(1and5mL)wereselectedtoperformthis evalu-ationineachmicro-pump.Theresultsobtainedaresummarizedin

Table2

MPFSsequenceofoperationsforthedeterminationoftitanium.

Step Pump Volume(mL) Flowrate(mL/min) Valveposition Action V1 V2

1 P2 0.5 5 1 1 Sampling

2 P1 0.1 1 0 0 Propellingsamplewithcarriertotheconfluence

3 P1andP3 0.1C 0.1R 1 0 0 Propellingsamplewithcarrierandcolourreagenttothedetector

4 P1 5 5 0 0 Propellingthemixturetothedetectorandsignalregistration

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Table3

Interferencesstudy.Resultsexpressedastherelativedeviationfromtheabsorbance valueofa20mgL−1_titanium_standard_solution.

Speciestested Concentration (mg/L) Relative deviation(%) Irona ₂₀ _+5.4 Ironb ₂₀₀₀ −2.4 Aluminium 100 +4.5 Copper 500 +5.6 Lead 1000 +4.9 Vanadium 500 +5.6 Chromium 200 +2.9 Fluoride 200 +3.9

a_Without_ascorbic_acid. b_With_ascorbic_acid.

Table1andrepresentthemeanoftwentyreplicates(fivereplicates foreachflowrateandvolume).

Asreferredabove,thesamplevolumeintroducedinthe sys-temwascontrolledbythelengthofthetubeplacedbetweenthe twocommutationvalves(the“sampleloop”—SL),sincein prelimi-naryexperimentsanimprovementintherepeatabilitywasverified usingthissamplingmode.

TheSLvolumewasvariedwithin100and400mLand200mL wastheselectedvalue.Thereactioncoillengthwasstudiedinthe rangeof10–150cmand50cmwasthelengththatallowedbetter sensitivity.Theflowratewasvariedwithin0.5and5mL/mininall thestepsoftheanalyticalcycle(Table2).Inthefirstandfourth steps,thesensitivityandtheblankvalueswereindependentofthe flowrate,therefore5mL/minwasusedinordertomaximizethe samplingrate.Inthesecondandthirdsteps,thesensitivityandthe blankvaluesincreasedforlowerflowrates.Thebestcompromise betweensensitivityandblankvalueswasachievedat1mL/min.

Theanalyticalwavelengthwasevaluatedintherangefrom415 to445nmand425nmallowedthebestcompromiseregarding sen-sitivityandlowblankvalues.

Followingthisstudy,reagentvolumesfrom60to200mLwere testedand100mL(correspondingto5micro-pumppulses)were chosenasthisvolumeprovidedabetterrepeatability(RSD<3%). The chemical parameters evaluated in the optimization of the methodology included the concentration of the colour reagent (chromotropicacid)andreducing agent(ascorbic acid),andthe concentrationandpHofthebuffersolution(sodiumacetate/acetic acid).Firstly,thereagentconcentrationwasstudiedintherangeof 1.25×10−4_to_2.5_×₁₀−3_mol/L_and_the_best_sensitivity_with_a

rela-tivelylowblankvalue(0.030)wasobtainedfor1.25×10−3_mol/L.

Highblankvaluesshortentheapplicationrangeofthe method-ologysinceCCDspectrophotometersperformancedeterioratesfor absorbancevaluesover1.8[31].

Theconcentration(intherangeof0.05–1mol/L)andthepH(in therangeof4.2–5.0)oftheacetatebufferwerethenstudied.The bestsensitivitywasobtainedforaconcentrationof0.2mol/Landa pHof4.6,andthesevalueswereusedinsubsequentexperiments. Finally,theascorbicacidconcentrationwasevaluated.Ascorbic acid wasnecessarytoassurethecomplete reductionofiron(III) toiron(II),sinceiron(III)interfereswithtitaniumdetermination (Table3).Theascorbicacidconcentrationwasvariedbetween1 and4%(w/v)andaconcentrationof2%showedtoguaranteethe totaliron(III)reductionatconcentrationsupto2mg/L.

3.2. Interferencestudies

Theinterferenceofseveralionsonthedeterminationof tita-niumwastested.Solutionswithafixedconcentrationoftitanium (20mg/L)andincreasingconcentrationsofthetestedionswere pre-pared.Ionscausingdeviationshigherthan5%intheabsorbance valueofthepure20mg/Ltitaniumstandardsolution,were

consid-Table4

Figuresofmeritofthedevelopedmethodology.

Parameters Values

Limitofdetection(mg/L) 0.4 Limitofquantification(mg/L) 0.8 Analyticalworkingrange(mg/L) Upto100 Analyticalthroughput(/h) 46 Reagentconsumptionperassay(mmol)

Chromotropicacid 0.125

Ascorbicacid 5.68

Sodiumacetate 20

Aceticacid 20

Wasteproducedperassay(mL) 5.8

eredasinterferents.TheionsstudiedwereFe(III),Pb(II),Al(III),F−_,

Cu(II),V(V)andCr(VI)atconcentrationsof20,40,100,200,500, 1000and2000mg/L,respectively(Table3).Themajorinterference wasfromFe(III)atthesameleveloftitanium.Thisinterferencewas minimizedbytheadditionofascorbicacidtothecolourreagentas canbeseeninTable3.Theothermaininterferenceswerefrom aluminiumataconcentrationfivetimeshigherthantitaniumand fluorineandchromiumataconcentrationtentimeshigherthan titanium.

3.3. Figuresofmerit

Alltheanalyticalperformancecharacteristicsobtainedfor tita-nium determination are summarized in Table 4. The limit of detection(LOD)andlimitofquantification(LOQ)werecalculated astheconcentrationcorrespondingtothreeandtentimesthe stan-darddeviationoftheblank,respectively[32].

3.4. Applicationtowatersamples

Concludedtheoptimizationofallofthephysicaland chemi-calparameters,theproposedanalyticalprocedurewasappliedto differenttypesofwaterinordertoassessitsaccuracy.

Initially,recoverytestswerecarriedoutondifferenttypesof watersamples:surface(seaandriver)andground(wellandmine) waters.Table5summarizestheresultsobtainedforthreelevelsof analyteaddition(4,10and20mg/L).Theysuggestthatthematrixof watersamplesdoesnotappreciablyinterfereinthedetermination oftitanium.

ChromotropicacidreactswithTi(IV).However,TiO2isthemost

frequentformoftheanalytepresentinenvironmentalandcosmetic samples.Therefore,acomparisonstudyusingsimilar concentra-tionsofstandardsofTi(IV)(preparedbothfromthecommercial standardsolutionandfromthedissolved/digestedTiO2)was

per-formed.Thelinearrelationshipbetweenthemolarconcentrationof Ti(CTi(IV);CTiO2)obtainedusingTi(IV)standardsorTiO2standards

wasAbs425nm=0.0121(±2×10−4)CTi(IV)+0.0026(±0.0040)and

Table5

Recoverytests.Results(%)obtainedindifferenttypesofwaterforthreedifferent additionlevels.

Watertypes Concentrationadded

4mg/L 10mg/L 20mg/L

Well 95±3 106±1 105±1

Pollutedrivernotfiltered 89±2 94±1 98±2

Sea 92±1 106±3 106±2 Sea 96±8 102±6 96±4 Estuarine 88±3 102±1 107±1 Mine 96±3 94±1 99±1 Pollutedriver 90±4 96±1 98±1 Pollutedriver 91±2 93±2 97±1 Meanandstandarddeviationof5replicates.

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Table6

ComparisonoftheresultsobtainedinthedeterminationofTiO2infivecommercial

sunscreensproducts.

Sample TiO2%(w/w)

Developedmethoda _ICP-MSb

1 2.33±0.01 2.29±0.13

2 1.77 ±0.01 1.70 ±0.11

3 1.66±0.01 1.65±0.10

4 2.29±0.02 2.33±0.07

5 1.40±0.01 1.35±0.12

a_Mean_and_standard_deviation_of₅_replicate_analysis_of_the_same_digestion

solu-tion.

b_Mean_and_standard_deviation_of₂_replicate_analysis_of_the_same_digestion

solu-tion.

Table7

Recoverytests.Resultsobtainedwithacertifiedlakesedimentsample. Assaynumber ExpectedTiconcentration(mg/L) Recovery(%)a

1 41.1 97±1 2 29.7 105 ±1 3 18.9 97±1 4 26.2 108±1 5 25.5 92±1 6 37.6 93±1

a_Mean_and_standard_deviation_of₅_replicates.

Abs425nm=0.0122(±1×10−4)CTiO2+0.0052(±0.0011),

respec-tively. The equation parameters obtained for thetwo standard curvesshownosignificantdifference(standarderrorsonthe esti-matedparametersareindicatedbetweenbrackets).Theseresults demonstratethatthedevelopedmethodisapplicabletothe deter-minationofTiinbothchemicalforms.

3.5. Applicationtocommercialsunscreens

Althoughtheprimaryobjectiveoftheproposedmethodisto determineTiinwaters,itwasalsoappliedtodifferent commer-cialsunscreensamples,previouslysubmittedtoamicrowaveacid digestion.Toassessthequalityoftheresults,theywerecompared withthoseobtainedbyICP-MS(Table6).Thecalculatedt-values were belowthecritical t-value(6.31) andthelinear regression parametersshowednosignificantdifferencebetweentheresults obtainedbythedevelopedsystem(CMPFS)andbythealternative

procedure, CMPFS=0.94 (±0.15)CICP-MS+0.13 (±0.29),wherethe

valuesbetweenbracketsarethelimitsofthe95%confidence inter-vals.

3.6. Applicationtolakesediment

Thedevelopedanalyticalprocedurewasalsoappliedtoone ref-erencelakesedimentmaterial(IAEA-SL-1)withacertifiedvalueof 5170±₄₃₀_mg_Ti_per_kg_of_sediment._When_analysed_by_the devel-oped procedure theresultobtainedwas5155±113mg/kg.This resultcorrespondstotheaverageof5assaysandthehalfwidth ofthe95%confidenceinterval.Thesolutionobtainedinthesample digestionwasdiluted1000timesinordertoadjustits concentra-tiontothelinearresponserangeoftheMPFSanalyticalprocedure. Theresultshowsagoodaccuracyofthedevelopedprocedure.

Additionally,recoverytestswereperformedonthecertifiedlake sedimentsample,towhichTiO2nanoparticleswereaddedintheir

solid form.Therecoveryresults, presentedin Table7,are rela-tivetotheexpectedconcentrationsinthedigestsafterspikingthe sedimentsample.

4. Conclusions

Thisworkpresentsareliablespectrophotometric determina-tionoftitaniumatlowlevelsapplicabletodifferentwatersamples, digested commercial sunscreens and sediments. The proposed methodologyis relatively simple and inexpensive,witha good analyticalthroughputandlowreagentconsumption/effluent pro-duction(greenchemistryapproach).TheemploymentofaLWCC allowsthedeterminationofTiatverylowlevelswithoutusinga preconcentrationstep.

Whencomparedtopreviousworks,thisanalyticalprocedure allowstitaniumdeterminationsindifferenttypesofsampleswitha lowdetectionlimit,lowreagentconsumptionandhighthroughput.

Acknowledgements

TheauthorsthankFundac¸ãoparaaCiênciaeaTecnologia(FCT) for financial support through Projects PTDC/AMB/64441/2006 andPTDC/AAC-AMB/104882/2008.RicardoPáscoaacknowledges financialsupportfromFCTthroughthegrantSFRH/BD/30621/2006. I.V.TóththanksFSEandMinistériodaCiência,TecnologiaeEnsino Superior(MCTES)forthefinancialsupportthroughthePOPH-QREN program.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,atdoi:10.1016/j.snb.2011.03.025.

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Biographies

RicardoN.M.J.PáscoaisaPhDstudentatUniversidadeCatólicaPortuguesa-Escola SuperiordeBiotecnologia,Porto.HehasMScdegreefromtheFacultyofPharmacyof theUniversityofPortoonEnvironmentalAnalyticalChemistry(2006)anda univer-sitydegreeonMicrobiologyfromUniversidadeCatólicaPortuguesa-EscolaSuperior deBiotecnologia.HehasbeenworkingwithflowanalysismethodologiesinhisPhD andworkedwithchemometricsinhisMScdegree.Heisthefirstauthorofafew numberofpapersinbothfields.

IldikóV.TóthisanassistantresearcherattheRequimte,FacultyofPharmacyat theUniversityofPorto.Shereceivedheruniversity(1993)degreeinFood Engi-neeringfromtheUniversityofHortofruticultureandFoodIndustry(Budapest), andaPhD(2001)inBiotechnology,specialityinChemistry,fromtheUniversidade CatólicaPortuguesa-EscolaSuperiordeBiotecnologia(Porto).Sheisaco-authorof 30papersininternationalpeerreviewedjournals.Hercurrentresearchinterests includechemicalsensorsforemergingenvironmentalcontaminantsandlaboratory automation.

AgostinhoA.AlmeidaisanauxiliaryprofessorattheFacultyofPharmacyof theUniversityofPorto.AfterhisgraduatestudiesinPharmaceuticalSciences,he receivedhisPhDdegreeinAnalyticalChemistryfromtheUniversityofPortoin 2000.Hiscurrentresearchinterestsarethedevelopmentandvalidationof ana-lyticalmethodologiesfortraceelementanalysisinclinical,toxicological,forensic andenvironmentalfields,particularlybyusinggraphitefurnaceatomicabsorption spectrometry(GFAAS)andinductivelycoupledplasma-massspectrometry(ICP-MS) techniques.

António O.S.S. Rangel is associate professor at the Universidade Católica Portuguesa-EscolaSuperiordeBiotecnologia,Porto.HegothisdegreeinChemistry fromUniversityofPortoandhisPhDdegreeinBiotechnology(specializationin Chemistry)fromUniversidadeCatólicaPortuguesa-EscolaSuperiorde Biotecnolo-gia.Heco-authoredmorethan100papersininternationaljournals,heismemberof theEditorialAdvisoryBoardofAnalyticaChimicaActaandisPresidentofthe Analyt-icalChemistryDivisionofthePortugueseChemicalSociety.Hisresearchinterests arefocusedonthedevelopmentofflowmethodsfortheautomationand minia-turizationof(bio)chemicalassays,withapplicationsinthefoodandenvironmental sectors.