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Flame emission spectroscopy

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Flame emission spectroscopy

  1. 1. Flame photometry or flame emission photometry By: Bijaya Kumar Uprety
  2. 2. Introduction •Theabsorptionandemissionofradiantenergybyatomsprovidepowerfulanalyticaltoolsforbothquantitativeandqualitativeanalysisofsubstances.Flameemissionandatomicabsorptionspectroscopyisamethodofelementalanalysis. •Thesemethodareparticularlyusefulfordeterminingtracemetalsinliquidsandisalmostindependentofthemolecularformofmetalinthesample. •Thesemethodareverysensitiveandcandetectdifferentmetalsinconcentrationsaslowas1ppm. •InFlameemissionspectroscopy,theconcentrationoftheanalytepresentinsampleisproportionaltotheintensityoftheemittedradiation. •Inatomicabsorptionspectroscopy,theconcentrationofanalyteismeasuredbyabsorbancerelatingtothesignalbyBeer-Lambert’slaw.
  3. 3. •Thefundamentalfactofemissionspectroscopyisverysimple,whereintheatomspresentinasampleundergoexcitationduetotheabsorptionofeitherelectricalorthermalenergy. •Subsequently,theradiationemittedbyatomsinanexcitedsampleisstudiedinanelaboratedmannerbothqualitativelyandquantitatively.Therefore,emissionspectroscopyisconsideredtobeanusefulanalyticaltoolfortheanalysisof: (i)elementalanalysisofmetals, (ii)identificationandquantitativedeterminationofmetallicelements, (iii)estimationofmetalloidse.g.,arsenic,silicon,selenium,presentisextremelylowconcentrations,and (iv)analysisofsolids,liquidsorgasesasfollows: solids-assuchorevaporatedsolutions, liquids-atomizedspray,analyzedoccasionally,and gases-analyzedrarely. •Inshort,emissionspectroscopyisconsideredtobethemostaccurate,preciseandreliablemeansofquantitativeanalysisofelementsasondate.Ifproperskill, precautionsandwisdomareappliedtogetherthismethodmaybeadoptedsafelyandconvenientlytoanalyzeapproximatelyseventyelementsfromthe‘periodictable’ataconcentrationaslowas1ppm.
  4. 4. Theory and principle •Inflameemissionspectrometry,thesamplesolutionisnebulized(convertedintoafineaerosol)andintroducedintotheflamewhereitisdesolvated,vaporized,andatomized,allinrapidsuccession. •Subsequently,atomsandmoleculesareraisedtoexcitedstatesviathermalcollisionswiththeconstituentsofthepartiallyburnedflamegases.Upontheirreturntoalowerorgroundelectronicstate,theexcitedatomsandmoleculesemitradiationcharacteristicofthesamplecomponents. •Theemittedradiationpassesthroughamonochromatorthatisolatesthespecificwavelengthforthedesiredanalysis.Aphotodetectormeasurestheradiantpoweroftheselectedradiation,whichisthenamplifiedandsenttoareadoutdevice,meter,recorder,ormicrocomputersystem.
  5. 5. •Combustionflamesprovideameansofconvertinganalytesinsolutiontoatomsinthevaporphasefreedoftheirchemicalsurroundings. •Theenergyfromtheflamealsosuppliestheenergynecessarytomovetheelectronsofthefreeatomsfromthegroundstatetoexcitedstates. •TheintensityofradiationemittedbytheseexcitedatomsreturningtothegroundstateprovidesthebasisforanalyticaldeterminationsinFlameEmissionSpectroscopy. Detailfigurefromwhiteboard.
  6. 6. Instrumentation •Flameemissionspectrometers(flamephotometers)areprobablythesimplesttypeofatomicspectrometers.Theyarenormallydesignedtomakemeasurementsonupto10differentelements,usuallythosefromgroupIandIIoftheperiodictable. Thisisachievedusinginterchangeablecolouredfiltersordiffractiongratings, whichisolateanareaofthespectrumcontainingthechosenemissionline. •The sequence of events occurring is as follows: 1.Solution is introduced into the flame as a fine spray. This is normally achieved using an aspirator. Solution is drawn out of the sample holder using a pump and fed into the gas stream through a thin nozzle creating an aerosol spray . 2.Solvent evaporate leaving the dehydrated salt. 3.Salt dissociate into free gaseous atoms in the ground state. 4.A certain fraction of atoms absorbs energy and are raised into excited state. 5.These excited atoms on returning to ground state emits photons of characteristic wavelength. 6.The emission from the flame passes through a conventional monochromator which filters out all emitted light except the wavelength of our interest. 7.A photoelectric detector measures the intensity of the filtered light.
  7. 7. 1. Flame atomiser •Theroleofatomizeristogeneratethevaporsofanalytewhichgetexcitedbythethermalenergyoftheflameandthenemitcharacteristicradiationthatismeasured. •Theflameatomizerassemblyconsistsoftwocomponents.Thepriorisanebulizerwherethesampleintheformofasolutionisdrawninandconvertedintoafinemistoranaerosol. •Itisthenpassedontothesecondcomponenti.e.theburneralongwithairoroxygenandafuelgas.Intheflameanumberofprocessesoccurthatconverttheanalyteintoexcitedspecies. a.Nebulizer:Itisadeviceusedforsampleintroductionintotheflame.Theprocessiscallednebulisationandconsistsofthermalvaporizationanddissociationofaerosolparticlesathightemperaturesproducingsmallparticlesizewithhighresidencetime.Anumberofnebulisationmethodsareavailable.Afewarelistedbelow. •Pneumaticnebulisation •Ultrasonicnebulisation •Electrothermalvaporization •Hydridegeneration(usedforcertainelementsonly).
  8. 8. •However,wewoulddiscussaboutthepneumaticnebulisationonly.Itisthemostcommonlyemployednebulisationmethodinflamephotometers. •Pneumaticnebulizeristhemostcommonlyusednebulizerforintroducingaqueous/liquidsamples.Inthisthesamplesolutionisfedoraspiratedintothenebulizerwhichconvertsliquidintoafinemist,oraerosolwhichisthenfedintotheflame.Acommontypeofpneumaticnebulizeriscalledconcentricpneumaticnebulizer,asshowninFig.7.9. Theconcentricpneumaticnebulizerconsistsofafinecapillarysurroundedbyconcentrictubewithasmallorificenearoneendofthecapillary.Thecapillaryisdippedintoasolutionoftheanalytewhiletheoutertubeisconnectedtoahighpressuregassupply.TheanalyteissuckedintothecapillarybythehighpressuregasstreamflowingaroundthetipofthecapillaryusingtheBernoullieffect.Theprocessiscalledaspiration.Thehighvelocitygasbreaksuptheliquidintovarioussizedfinedroplets.Theothertypesofthepneumaticnebulizersalsoworkonthesameprinciple.
  9. 9. b.AtomiserBurnersFlamePhotometry Thesampleisintroducedintheformofafinesprayatacontrolledrateintotheflameofburnerwiththehelpofnebuliser.Intheburner,theanalyteundergoesanumberofprocessesasmentionedearlier. •The following processes occur in the flame. i) Desolvation: The sample containing metal particles is dehydrated by the heat of the flame and the solvent is evaporated. ii) Vapourisation: The heat of the flame vapourises the sample constituents. No chemical change takes place at this stage. iii) Atomisation: At this stage the metal ions that were in the solvent are reduced to metal atoms. For example, Mg2+ (aq)+ 2e-Mg (g) By heat of the flame and action of the reducing gas (fuel), molecules and ions of the sample species are decomposed and reduced to give atoms.
  10. 10. iv)Excitation:Theatomsatthisstageareabletoabsorbenergyfromtheheatoftheflame.Theamountofenergyabsorbeddependsontheelectrostaticforcesofattractionbetweenthenegativelychargedelectronsandthepositivelycharged nucleus.Thisinturndependsuponthenumberofprotonsinthenucleus.Aselectronsabsorbenergytheymovetohigherenergylevelsandareintheexcitedstate. v)Emissionofradiation:Electronsintheexcitedstateareveryunstableandmovebackdowntothegroundstateoralowerenergystatequitequickly.Astheydoso, theyemittheenergyintheformofradiationofcharacteristicwavelength,whichismeasuredbyadetector. Forsomemetalsthisradiationcorrespondstowavelengthsoflightinthevisibleregionoftheelectromagneticspectrumandisobservedasacharacteristiccolouroftheflame. Aselectronsfromdifferentenergylevelsareabletoemitlightastheyrelax,theflamecolourobservedwillbeamixtureofallthedifferentwavelengthsemittedbythedifferentelectronsinthemetalatomunderinvestigation. .
  11. 11. Twotypesofatomisationburnershavebeenusedinflamephotometrywhicharegivenbelow. a)Pre-mixorLundegarhburner b)Totalconsumptionburner
  12. 12. 2.Monochromator •Generallyagratingoraprismmonochromatorisemployed.Theroleofthemonochromatoristodispersetheradiationcomingfromtheflameandfallingonit.Thedispersedradiationfromtheexitslitofthemonochromatorgoestothedetector. •Incasealowtemperaturesflameisused,thespectrallinesfromonlyafewelementsareemitted.Insuchacase,formostroutineanalyses,afiltercanbeusedasamonochromatortoisolateaparticularspectralline. •Filtersaregenerallymadefrommaterialswhicharetransparentinasmallselectivewavelengthregion.Thefilterchosenisonewhichhasawavelengthrangeinwhichitistransparenttoemissionfromtheelementofinterest. •Insuchacase,acondenserlenssystemisemployedtocollecttheemittedlightandsendtheraysthroughthefilterasanapproximatelycollimated(parallel)beamtoreachthedetector.Filtershavebeendesignedforuseinthedeterminationoflithium,sodium,potassium,calciumandotherelements.
  13. 13. 3. Detector •Thefunctionofadetectoristomeasuretheintensityofradiationfallingonit. •Photoemissivecellsorphotomultipliertubesarecommonlyemployedforthepurpose. •ThesedetectorsarealsousedinUV-VISspectrophotometers. •ReadindetailaboutthephotoemissivecellfromUVspectroscopynotes.
  14. 14. 4. Amplifier and Readout Device •Theoutputfromthedetectorissuitablyamplifiedanddisplayedonareadoutdevicelikeameteroradigitaldisplay. •Thesensitivityoftheamplifiercanbechangedsoastobeabletoanalyzesamplesofvaryingconcentrations. •Nowadaystheinstrumentshavemicroprocessorcontrolledelectronicsthatprovidesoutputscompatiblewiththeprintersandcomputerstherebyminimizingthepossibilityofoperatorerrorintransferringdata.
  15. 15. QualitativeApplications •Flamephotometricmethodsarewidelyusedforthedeterminationofalkaliandthealkalineearthmetalsinsamplesthatareeasilypreparedasaqueoussolutions. •Someoftheseelementscanbedetectedvisuallybythecolorintheflame, e.g.sodiumproducesyellowflame.However,thismethodisnotveryreliable. •Thebestmethodistouseflamephotometerwithafilterormonochromatortoseparateradiationwiththewavelengthscharacteristicofthedifferentmetalsfromotherradiationspresent. •Iftheradiationofthecharacteristicwavelengthisdetected,itwillindicatethepresenceofthemetalinthesample. •Themethodtocarryoutdetectionofelementsbyflamephotometryisfast,simpleandifcarriedoutwithcare,quitereliable.However,therearesomedifficulties. •Itdoesnotprovideinformationaboutthemolecularstructureofthecompoundpresentinthesamplesolution.Nonradiatingelementsuchascarbon,hydrogenandhalidescannotbedetected.Thesecanonlybedeterminedunderspecialcircumstances.
  16. 16. QuantitativeMeasurements. •Theintensityofthespectrallinebeingmeasuredisdirectlyproportionaltothesolutionconcentrationoftheanalyte. •Quantitativemeasurementsaremadebyreferencetoapreviouslypreparedcalibrationlineorbythemethodofstandardaddition. •Theresponselinearityofmostinstrumentsisrestrictedtoconcentrationsbetween10and100ppmwhichisfairlylimiting.TypicalelementsthatthistechniqueisusedforareCa,Ba,K,Li,Na,Mg,Al. •Oneofthemethodofquantitativemeasurementinvolvesthepreparationofcalibrationcurvebymeasuringtheintensityofemissionforaseriesofsolutionsofdifferentconcentrationspreparedbyusingastandardsolutionandplottingagraphbetweenemissionintensityversusconcentrationoftheionicspeciesoftheelementofinterest.Theconcentrationoftheelementintheunknownsamplecanthenbefoundoutfromthestandardplotasisdoneinvisiblespectrophotometry. Further,itisveryimportanttomeasuretheemissionfromthestandardandunknownsolutionsunderconditionsthatareasnearlyidenticalaspossible.
  17. 17. •Thecalibrationcurvemethodhelpsinfindingtheconcentrationofunknownsamples.However,forsomesamples,itisdifficulttopreparestandardswhicharesufficientlysimilar.Thismayoccurwhenthesamplescontainhighandvariableconcentrationsofmatrixmaterials,orwhenthesamplescontainsolidswhoseeffectonabsorptionishardtoduplicate.Insuchcasesweneedtoresorttoanyofthefollowingtwomethods. 1.Standardadditionmethod 2.Internalstandardmethod StandardAdditionMethod •Inthismethod,knownamountofastandardsolutionisaddedtoidenticalaliquotsofthesampleandtheabsorbanceismeasured. •Thefirstreadingistheabsorbanceofsamplealoneandthesecondreadingisabsorbanceofsamplecontaininganalyteplus,aknownamountofanalyteandsoon.Similarly,inflamephotometricdeterminations,increasingamountsofastandardsolutionofthesaltoftheelementtobedeterminedisaddedtoaseriesofsolutionsofthesample. •Theintensityofemissionforallthesesolutionsisthenmeasured.Acurveofintensityvs.concentrationoftheaddedelementisobtainedandextrapolatedtozerovalueofintensitytogiveconcentrationoftheelementinthesample.Letustakeanexampleofthestandardadditionmethod.
  18. 18. InternalStandardMethod •Inthismethod,aconstantamountofanothermetalwhichisnotpresentinthesampleisaddedtoboththeunknownsampleandaseriesofstandardsolutionsoftheelementbedetermined. •Thisiscalledinternalstandard,forexample,lithiumisaddedinthedeterminationofsodiummetal.Sinceboththeelementandtheinternalstandardareinthesamesolution,theemissionreadingsatthewavelengthsofboththeinternalstandardandtheelementtobeenclosedaresimultaneouslydetermined. •Theintensityratioforthetwoelementsisthenplottedagainstconcentrationofthestandardsolution.Fromtheobservedratioforthesample,theconcentrationoftheelementinitcanbedetermined.Letustakeanexampletounderstandit.
  19. 19. Types of flame used Themostcommoninstrumentsuseairastheoxidant.Thetemperatureoftheflamesproducedisrelativelylowsothetechniqueisonlysuitableforelementsthatareeasilyexcitedsuchasalkaliandalkaliearthelements.Whenoxygenornitrousoxideisusedamuchhighertemperaturecanbeobtained. FlameTemp/°C Gas/Air1700–1900 Gas/O22700–2800 H2/Air2000–2100 H2/O22550–2700 C2H2/Air2100–2400 C2H2/O23050–3150 C2H2/N2O2600–2800
  20. 20. A higher temperature will tend to increase the number of atoms in the excited state and hence the signal. Some detection limits for flame emission methods. Element Spectral Detection line Limits /nm Flame /ppm Al 396 C2H2/ N2O 0.01 Ba 553 C2H2/ N2O 0.001 K 766 C2H2/ O20.001 Li 671 C2H2/ N2O 0.0001
  21. 21. Interference in Quantitative analysis •Thesuccessofthequantitativedeterminationdependsonhowaccuratelytheintensityoftheemittedradiationrepresentstheconcentrationoftheanalyte. •Ithasbeenfoundthatnumberoffactorsbesidestheanalyteaffecttheintensityoftheemittedradiation.Theanalyticalsignalsmeasuredoftenincludecontributionsfromconstituentsotherthantheanalyte.Theconstituentsarecalledthematrixconstituents. •Thecontributionsareknownasinterferencesandarefoundtoinfluencetheoutcomeoftheanalyticalprocedure.Thesecanbecorrectedbysubtractingtheircontributionstothesignal.Thecontributionoftheinterferentcanbecalculatedfromthemagnitudeoftheinterferenceandtheconcentrationoftheinterferent. Theinterferencesencounteredcanbeclassifiedasfollows. 1.Spectralinterferences 2.Ionisedinterferences 3.Chemicalinterferences •Letuslearnaboutdifferenttypesofinterferencesencounteredinquantitativedeterminationsbyflamephotometry.
  22. 22. SpectralInterferences •Theserefertotheinterferencesthataffectthespectralintensityorresolution.Thereareseveraltypesofspectralinterferenceswhichareexplainedbelow. •Thefirsttypeofinterferenceariseswhentwoelementsexhibitspectra,whichpartiallyoverlap,andbothemitradiationatsomeparticularwavelength.Thedetectorcannotdistinguishbetweenthesourcesofradiationandrecordsthetotalsignal,thusresultinginincorrectanswer.Suchinterferencesaremorecommonathighflametemperaturesbecausenumerousspectrallinesareproducedathightemperatures.Forexample,theFelineat324.73nmoverlapswiththeCulineat324.75nm.Suchinterferencecanbeovercomeeitherbytakingmeasurementsatanalternativewavelengthwhichhasnooverlap,ifavailable,orbyremovingtheinterferingelementbyextraction.Alternatively,onemaymakeacalibrationcurve,whichispreparedfromasolutionhavingsimilarquantitiesoftheinterferingelement. •Thesecondtypeofspectralinterferencedealswithspectrallinesoftwoormoreelementswhichareclosebuttheirspectradonotoverlap.Thistypeofinterferencebecomesaproblemwhenafilterisusedasthedevicetoisolatespectrallines.Afiltermayallowspectrallinesseparatedby5.0-10.0nmtopassthrough,thusresultinginanerrorintheanalysis.Suchinterferencescanbereducedbyincreasingtheresolutionofthespectralisolationsystem. However,theinterferencecannotbeeliminatedentirelyduetothefinitewidthofthespectralisolationsystemandthefiniteslitwidthinsuchsystems. •Athirdtypeofspectralinterferenceoccursduetothepresenceofcontinuousbackgroundwhicharisesduetohighconcentrationofsaltsinthesample,especiallyofalkaliandalkalineearthmetals.Someorganicsolventsalsoproduceacontinuousbackground.Thistypeofinterferencecanbecorrectedbyusingsuitablescanningtechnique.
  23. 23. IonisationInterferences •Insomecases,hightemperatureflamemaycauseionizationofsomeofthemetalatoms,e.g.,incaseofsodium,itcanbegivenasfollows. Na→Na++e- •TheNa+ionpossessesanemissionspectrumofitsownwithfrequencies,whicharedifferentfromthoseofatomicspectrumoftheNaatom.Thisreducestheradiantpowerofatomicemission.Thisinterferencecanbeeliminatedbyaddingalargequantityofapotassiumsalttothestandardsaswellassamplesolutions.Theadditionofpotassiumsaltsuppressestheionisationofsodium,asthepotassiumatomitselfundergoesionisationduetolowionisationenergy.Thus,thesodiumatomemissionisenhanced.Thistypeofinterferenceisrestrictedtoalkalimetals. ChemicalInterferences Thechemicalinterferencesariseoutofthereactionbetweendifferentinterferentsandtheanalyte.Theseareofdifferenttypes.Someofthesearegivenbelow. •Cation-anioninterference:Thepresenceofcertainanions,suchasoxalate,phosphate,sulphateandaluminate,inasolutionmayaffecttheintensityofradiationemittedbyanelement,resultinginseriousanalyticalerror.Forexample,calciuminthepresenceofphosphateionformsastablesubstance,asCa3(PO4)2whichdoesnotdecomposeeasily,resultingintheproductionoflesseratoms.Thus,thecalciumsignalisdepressed.AnothersimilarexampleisthatofdeterminationofbariuminpresenceofsulphateforminginsolubleBaSO4.Thistypeofinterferencecanberemovedeitherbyextractionoftheanionorbyusingcalibrationcurvespreparedfromstandardsolutionscontainingsameconcentrationsoftheanionasfoundinthesample.
  24. 24. •Cation-cationinterference:Inmanycases,mutualinterferencesofcationshavebeenobserved,resultinginreducedsignalintensityoftheelementbeingdetermined. Theseinterferencesareneitherspectralnorionicinnatureandthemechanismofsuchinterferencesisnotwellunderstood.Thus,forexample,aluminuminterfereswithcalciumandmagnesium.Also,sodiumandpotassiumshowcation-cationinterferenceononeanother. •Interferenceduetooxideformation:Thistypeofinterferencearisesduetotheformationofstablemetaloxideifoxygenispresentintheflame,resultinginreducedsignalintensity.Thealkalineearthmetalsaresubjecttothistypeofinterference. Thistypeofinterferencecanbeeliminatedbyeitherusingveryhighflametemperaturetodissociatetheoxidesorbyusingoxygen-deficientenvironmenttoproduceexcitedatom.
  25. 25. Structure of Flames •Flamesarenotuniformincomposition,lengthorcrosssection.Thestructureofapremixedflame,supportedasalaminarflowisshowninFig.7.7. •As seen in the figure, the flame may be divided into the following regions or zones. i) Preheating zones ii) Primary reaction zone or inner zone iii) Internal zone iv) Secondary reaction zone
  26. 26. •Thefirstortheinnermostregionoftheflameisthepreheatingzonewherethecombustionmixtureisheatedtotheignitiontemperaturebythermalconductionfromtheprimaryreactionzone. •Thesecondzoneistheprimaryreactionzoneorinnerzone.Thiszoneisabout0.1mmthickatatmosphericpressureandisvisiblebyvirtueofitsbluegreenlightascribedtoradicals.C2and.CH.Thereisnothermodynamicequilibriuminthiszoneandtheconcentrationofionsandfreeradicalsisveryhigh.Thisregionisnotusedforflamephotometry. •Immediatelyabovetheprimaryreactionzoneliesthethirdorinterconalzoneorthereactionfreezonewhichcanextenduptoconsiderableheight.Themaximumtemperatureisachievedjustabovethetipoftheinnerzone.Thehighertemperaturefavoursbothproductionoffreeatomsandmaximumexcitationforatomicemissionspectroscopy.Therefore,thiszoneisusedforflamephotometry. •Theoutermostfourthzoneisthesecondaryreactionzone.Withinthiszone,theproductsofthecombustionprocessesareburnttostablemolecularspeciesbythesurroundingair.
  27. 27. •Theshapeofanunmixedflameisgenerallydifferent.Theinnerzonecanstillberecognised,butitisveryvagueandisthickened.Alaminarflamemakesastronghissingnoisewhichgetslouderwhenaliquidisatomisedintoit.Weshallnowlookintothereactionswhicharetakingplacewhentheelementisplacedinflame.
  28. 28. Limitation of Flame Emission Photometry •Asnaturalgasandairflameisemployedforexcitationthetemperatureisnothighenoughtoexcitetransitionmetals,thereforethemethodisselectivetowardsdetectionofalkaliandalkalineearthmetals. •Thelowtemperaturemakesthismethodsusceptibletocertaindisadvantages, mostofthemrelatedtointerferenceandthestabilityoftheflameandaspirationconditions.Fuelandoxidantflowratesandpurity,aspirationrates,solutionviscosity,affectthese.Itisthereforeveryimportanttomeasuretheemissionofthestandardandunknownsolutionsunderidenticalconditions. •Therelativelylowenergyavailablefromtheflameleadstorelativelylowintensityoftheradiationfromthemetalatoms,particularlythosethatrequirelargeamountofenergytobecomeexcited. •Flame photometry is a means of determining the total metal concentration of a sample; it tells us nothing about the molecular form of the metal in the original sample. •Only liquid samples can be used. In some cases, lengthy steps are necessary to prepare liquid samples.
  29. 29. Applications •Flamephotometersarewidelyusedinqualitycontrolwhereasimpleandquickdeterminationofalkalioralkaliearthelementsisrequired.Theyhavetheadvantageofbeingsignificantlylowerpricedthanmostotheratomicspectrometers. •Biological/medicalapplications–notableapplicationsarethedeterminationsofNa,K,CaandMginbodyfluidsandotherbiologicalsamples. •Foodindustry–determinationofcalciumandironinbeer.

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