Flame emission spectroscopy

1. Flame photometry or flame
emission photometry
By: Bijaya Kumar Uprety

2. Introduction
•Theabsorptionandemissionofradiantenergybyatoms
providepowerfulanalyticaltoolsforbothquantitativeand
qualitativeanalysisofsubstances.Flameemissionandatomic
absorptionspectroscopyisamethodofelementalanalysis.
•Thesemethodareparticularlyusefulfordeterminingtrace
metalsinliquidsandisalmostindependentofthemolecular
formofmetalinthesample.
•Thesemethodareverysensitiveandcandetectdifferent
metalsinconcentrationsaslowas1ppm.
•InFlameemissionspectroscopy,theconcentrationofthe
analytepresentinsampleisproportionaltotheintensityof
theemittedradiation.
•Inatomicabsorptionspectroscopy,theconcentrationof
analyteismeasuredbyabsorbancerelatingtothesignalby
Beer-Lambert’slaw.

3. •Thefundamentalfactofemissionspectroscopyisverysimple,whereintheatoms
presentinasampleundergoexcitationduetotheabsorptionofeitherelectricalor
thermalenergy.
•Subsequently,theradiationemittedbyatomsinanexcitedsampleisstudiedinan
elaboratedmannerbothqualitativelyandquantitatively.Therefore,emission
spectroscopyisconsideredtobeanusefulanalyticaltoolfortheanalysisof:
(i)elementalanalysisofmetals,
(ii)identificationandquantitativedeterminationofmetallicelements,
(iii)estimationofmetalloidse.g.,arsenic,silicon,selenium,presentisextremely
lowconcentrations,and
(iv)analysisofsolids,liquidsorgasesasfollows:
solids-assuchorevaporatedsolutions,
liquids-atomizedspray,analyzedoccasionally,and
gases-analyzedrarely.
•Inshort,emissionspectroscopyisconsideredtobethemostaccurate,preciseand
reliablemeansofquantitativeanalysisofelementsasondate.Ifproperskill,
precautionsandwisdomareappliedtogetherthismethodmaybeadoptedsafely
andconvenientlytoanalyzeapproximatelyseventyelementsfromthe‘periodic
table’ataconcentrationaslowas1ppm.

4. Theory and principle
•Inflameemissionspectrometry,thesamplesolutionis
nebulized(convertedintoafineaerosol)andintroducedinto
theflamewhereitisdesolvated,vaporized,andatomized,all
inrapidsuccession.
•Subsequently,atomsandmoleculesareraisedtoexcited
statesviathermalcollisionswiththeconstituentsofthe
partiallyburnedflamegases.Upontheirreturntoaloweror
groundelectronicstate,theexcitedatomsandmoleculesemit
radiationcharacteristicofthesamplecomponents.
•Theemittedradiationpassesthroughamonochromatorthat
isolatesthespecificwavelengthforthedesiredanalysis.A
photodetectormeasurestheradiantpoweroftheselected
radiation,whichisthenamplifiedandsenttoareadout
device,meter,recorder,ormicrocomputersystem.

5. •Combustionflamesprovideameansofconvertinganalytesinsolutionto
atomsinthevaporphasefreedoftheirchemicalsurroundings.
•Theenergyfromtheflamealsosuppliestheenergynecessarytomovethe
electronsofthefreeatomsfromthegroundstatetoexcitedstates.
•Theintensityofradiationemittedbytheseexcitedatomsreturningtothe
groundstateprovidesthebasisforanalyticaldeterminationsinFlame
EmissionSpectroscopy.
Detailfigurefromwhiteboard.

6. Instrumentation
•Flameemissionspectrometers(flamephotometers)areprobablythesimplesttype
ofatomicspectrometers.Theyarenormallydesignedtomakemeasurementson
upto10differentelements,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. 1. Flame atomiser
•Theroleofatomizeristogeneratethevaporsofanalytewhichgetexcitedbythe
thermalenergyoftheflameandthenemitcharacteristicradiationthatis
measured.
•Theflameatomizerassemblyconsistsoftwocomponents.Thepriorisanebulizer
wherethesampleintheformofasolutionisdrawninandconvertedintoafine
mistoranaerosol.
•Itisthenpassedontothesecondcomponenti.e.theburneralongwithairor
oxygenandafuelgas.Intheflameanumberofprocessesoccurthatconvertthe
analyteintoexcitedspecies.
a.Nebulizer:Itisadeviceusedforsampleintroductionintotheflame.Theprocess
iscallednebulisationandconsistsofthermalvaporizationanddissociationof
aerosolparticlesathightemperaturesproducingsmallparticlesizewithhigh
residencetime.Anumberofnebulisationmethodsareavailable.Afewarelisted
below.
•Pneumaticnebulisation
•Ultrasonicnebulisation
•Electrothermalvaporization
•Hydridegeneration(usedforcertainelementsonly).

8. •However,wewoulddiscussaboutthepneumaticnebulisationonly.Itis
themostcommonlyemployednebulisationmethodinflame
photometers.
•Pneumaticnebulizeristhemostcommonlyusednebulizerfor
introducingaqueous/liquidsamples.Inthisthesamplesolutionisfedor
aspiratedintothenebulizerwhichconvertsliquidintoafinemist,or
aerosolwhichisthenfedintotheflame.Acommontypeofpneumatic
nebulizeriscalledconcentricpneumaticnebulizer,asshowninFig.7.9.
Theconcentricpneumaticnebulizerconsistsofafinecapillarysurrounded
byconcentrictubewithasmallorificenearoneendofthecapillary.The
capillaryisdippedintoasolutionoftheanalytewhiletheoutertubeis
connectedtoahighpressuregassupply.Theanalyteissuckedintothe
capillarybythehighpressuregasstreamflowingaroundthetipofthe
capillaryusingtheBernoullieffect.Theprocessiscalledaspiration.The
highvelocitygasbreaksuptheliquidintovarioussizedfinedroplets.The
othertypesofthepneumaticnebulizersalsoworkonthesameprinciple.

9. b.AtomiserBurnersFlamePhotometry
Thesampleisintroducedintheformofafinesprayatacontrolledrate
intotheflameofburnerwiththehelpofnebuliser.Intheburner,the
analyteundergoesanumberofprocessesasmentionedearlier.
•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,
Mg
2+
(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. iv)Excitation:Theatomsatthisstageareabletoabsorbenergyfromtheheatofthe
flame.Theamountofenergyabsorbeddependsontheelectrostaticforcesof
attractionbetweenthenegativelychargedelectronsandthepositivelycharged
nucleus.Thisinturndependsuponthenumberofprotonsinthenucleus.As
electronsabsorbenergytheymovetohigherenergylevelsandareintheexcited
state.
v)Emissionofradiation:Electronsintheexcitedstateareveryunstableandmove
backdowntothegroundstateoralowerenergystatequitequickly.Astheydoso,
theyemittheenergyintheformofradiationofcharacteristicwavelength,whichis
measuredbyadetector.
Forsomemetalsthisradiationcorrespondstowavelengthsoflightinthevisible
regionoftheelectromagneticspectrumandisobservedasacharacteristiccolour
oftheflame.
Aselectronsfromdifferentenergylevelsareabletoemitlightastheyrelax,the
flamecolourobservedwillbeamixtureofallthedifferentwavelengthsemittedby
thedifferentelectronsinthemetalatomunderinvestigation.

11. Twotypesofatomisationburnershavebeenusedinflamephotometrywhich
aregivenbelow.
a)Pre-mixorLundegarhburner
b)Totalconsumptionburner

12. 2.Monochromator
•Generallyagratingoraprismmonochromatorisemployed.Theroleof
themonochromatoristodispersetheradiationcomingfromtheflame
andfallingonit.Thedispersedradiationfromtheexitslitofthe
monochromatorgoestothedetector.
•Incasealowtemperaturesflameisused,thespectrallinesfromonlya
fewelementsareemitted.Insuchacase,formostroutineanalyses,a
filtercanbeusedasamonochromatortoisolateaparticularspectralline.
•Filtersaregenerallymadefrommaterialswhicharetransparentinasmall
selectivewavelengthregion.Thefilterchosenisonewhichhasa
wavelengthrangeinwhichitistransparenttoemissionfromtheelement
ofinterest.
•Insuchacase,acondenserlenssystemisemployedtocollecttheemitted
lightandsendtheraysthroughthefilterasanapproximatelycollimated
(parallel)beamtoreachthedetector.Filtershavebeendesignedforusein
thedeterminationoflithium,sodium,potassium,calciumandother
elements.

13. 3. Detector
•Thefunctionofadetectoristomeasuretheintensityof
radiationfallingonit.
•Photoemissivecellsorphotomultipliertubesarecommonly
employedforthepurpose.
•ThesedetectorsarealsousedinUV-VISspectrophotometers.
•ReadindetailaboutthephotoemissivecellfromUV
spectroscopynotes.

14. 4. Amplifier and Readout Device
•Theoutputfromthedetectorissuitablyamplifiedand
displayedonareadoutdevicelikeameteroradigitaldisplay.
•Thesensitivityoftheamplifiercanbechangedsoastobe
abletoanalyzesamplesofvaryingconcentrations.
•Nowadaystheinstrumentshavemicroprocessorcontrolled
electronicsthatprovidesoutputscompatiblewiththeprinters
andcomputerstherebyminimizingthepossibilityofoperator
errorintransferringdata.

15. QualitativeApplications
•Flamephotometricmethodsarewidelyusedforthedeterminationof
alkaliandthealkalineearthmetalsinsamplesthatareeasilypreparedas
aqueoussolutions.
•Someoftheseelementscanbedetectedvisuallybythecolorintheflame,
e.g.sodiumproducesyellowflame.However,thismethodisnotvery
reliable.
•Thebestmethodistouseflamephotometerwithafilteror
monochromatortoseparateradiationwiththewavelengthscharacteristic
ofthedifferentmetalsfromotherradiationspresent.
•Iftheradiationofthecharacteristicwavelengthisdetected,itwillindicate
thepresenceofthemetalinthesample.
•Themethodtocarryoutdetectionofelementsbyflamephotometryis
fast,simpleandifcarriedoutwithcare,quitereliable.However,thereare
somedifficulties.
•Itdoesnotprovideinformationaboutthemolecularstructureofthe
compoundpresentinthesamplesolution.Nonradiatingelementsuchas
carbon,hydrogenandhalidescannotbedetected.Thesecanonlybe
determinedunderspecialcircumstances.

16. QuantitativeMeasurements.
•Theintensityofthespectrallinebeingmeasuredisdirectlyproportional
tothesolutionconcentrationoftheanalyte.
•Quantitativemeasurementsaremadebyreferencetoapreviously
preparedcalibrationlineorbythemethodofstandardaddition.
•Theresponselinearityofmostinstrumentsisrestrictedtoconcentrations
between10and100ppmwhichisfairlylimiting.Typicalelementsthatthis
techniqueisusedforareCa,Ba,K,Li,Na,Mg,Al.
•Oneofthemethodofquantitativemeasurementinvolvesthepreparation
ofcalibrationcurvebymeasuringtheintensityofemissionforaseriesof
solutionsofdifferentconcentrationspreparedbyusingastandard
solutionandplottingagraphbetweenemissionintensityversus
concentrationoftheionicspeciesoftheelementofinterest.The
concentrationoftheelementintheunknownsamplecanthenbefound
outfromthestandardplotasisdoneinvisiblespectrophotometry.
Further,itisveryimportanttomeasuretheemissionfromthestandard
andunknownsolutionsunderconditionsthatareasnearlyidenticalas
possible.

18. •Thecalibrationcurvemethodhelpsinfindingtheconcentrationofunknown
samples.However,forsomesamples,itisdifficulttopreparestandardswhichare
sufficientlysimilar.Thismayoccurwhenthesamplescontainhighandvariable
concentrationsofmatrixmaterials,orwhenthesamplescontainsolidswhose
effectonabsorptionishardtoduplicate.Insuchcasesweneedtoresorttoanyof
thefollowingtwomethods.
1.Standardadditionmethod
2.Internalstandardmethod
StandardAdditionMethod
•Inthismethod,knownamountofastandardsolutionisaddedtoidenticalaliquots
ofthesampleandtheabsorbanceismeasured.
•Thefirstreadingistheabsorbanceofsamplealoneandthesecondreadingis
absorbanceofsamplecontaininganalyteplus,aknownamountofanalyteandso
on.Similarly,inflamephotometricdeterminations,increasingamountsofa
standardsolutionofthesaltoftheelementtobedeterminedisaddedtoaseries
ofsolutionsofthesample.
•Theintensityofemissionforallthesesolutionsisthenmeasured.Acurveof
intensityvs.concentrationoftheaddedelementisobtainedandextrapolatedto
zerovalueofintensitytogiveconcentrationoftheelementinthesample.Letus
takeanexampleofthestandardadditionmethod.

20. InternalStandardMethod
•Inthismethod,aconstantamountofanothermetalwhichis
notpresentinthesampleisaddedtoboththeunknown
sampleandaseriesofstandardsolutionsoftheelementbe
determined.
•Thisiscalledinternalstandard,forexample,lithiumisadded
inthedeterminationofsodiummetal.Sinceboththeelement
andtheinternalstandardareinthesamesolution,the
emissionreadingsatthewavelengthsofboththeinternal
standardandtheelementtobeenclosedaresimultaneously
determined.
•Theintensityratioforthetwoelementsisthenplotted
againstconcentrationofthestandardsolution.Fromthe
observedratioforthesample,theconcentrationofthe
elementinitcanbedetermined.Letustakeanexampleto
understandit.

22. Types of flame used
Themostcommoninstrumentsuseairastheoxidant.Thetemperature
oftheflamesproducedisrelativelylowsothetechniqueisonlysuitable
forelementsthatareeasilyexcitedsuchasalkaliandalkaliearth
elements.Whenoxygenornitrousoxideisusedamuchhigher
temperaturecanbeobtained.
Flame Temp/°C
Gas/Air 1700–1900
Gas/O
2 2700–2800
H
2/Air 2000–2100
H
2/O
2 2550–2700
C
2H
2/Air 2100–2400
C
2H
2/O
2 3050–3150
C
2H
2/N
2O 2600–2800

23. 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 C
2H
2/ N
2O 0.01
Ba 553 C
2H
2/ N
2O 0.001
K 766 C
2H
2/ O
2 0.001
Li 671 C
2H
2/ N
2O 0.0001

25. Interference in Quantitative analysis
•Thesuccessofthequantitativedeterminationdependsonhowaccuratelythe
intensityoftheemittedradiationrepresentstheconcentrationoftheanalyte.
•Ithasbeenfoundthatnumberoffactorsbesidestheanalyteaffecttheintensity
oftheemittedradiation.Theanalyticalsignalsmeasuredofteninclude
contributionsfromconstituentsotherthantheanalyte.Theconstituentsarecalled
thematrixconstituents.
•Thecontributionsareknownasinterferencesandarefoundtoinfluencethe
outcomeoftheanalyticalprocedure.Thesecanbecorrectedbysubtractingtheir
contributionstothesignal.Thecontributionoftheinterferentcanbecalculated
fromthemagnitudeoftheinterferenceandtheconcentrationoftheinterferent.
Theinterferencesencounteredcanbeclassifiedasfollows.
1.Spectralinterferences
2.Ionisedinterferences
3.Chemicalinterferences
•Letuslearnaboutdifferenttypesofinterferencesencounteredinquantitative
determinationsbyflamephotometry.

26. SpectralInterferences
•Theserefertotheinterferencesthataffectthespectralintensityorresolution.Thereare
severaltypesofspectralinterferenceswhichareexplainedbelow.
•Thefirsttypeofinterferenceariseswhentwoelementsexhibitspectra,whichpartially
overlap,andbothemitradiationatsomeparticularwavelength.Thedetectorcannot
distinguishbetweenthesourcesofradiationandrecordsthetotalsignal,thusresultingin
incorrectanswer.Suchinterferencesaremorecommonathighflametemperaturesbecause
numerousspectrallinesareproducedathightemperatures.Forexample,theFelineat
324.73nmoverlapswiththeCulineat324.75nm.Suchinterferencecanbeovercomeeither
bytakingmeasurementsatanalternativewavelengthwhichhasnooverlap,ifavailable,orby
removingtheinterferingelementbyextraction.Alternatively,onemaymakeacalibration
curve,whichispreparedfromasolutionhavingsimilarquantitiesoftheinterferingelement.
•Thesecondtypeofspectralinterferencedealswithspectrallinesoftwoormoreelements
whichareclosebuttheirspectradonotoverlap.Thistypeofinterferencebecomesa
problemwhenafilterisusedasthedevicetoisolatespectrallines.Afiltermayallowspectral
linesseparatedby5.0-10.0nmtopassthrough,thusresultinginanerrorintheanalysis.Such
interferencescanbereducedbyincreasingtheresolutionofthespectralisolationsystem.
However,theinterferencecannotbeeliminatedentirelyduetothefinitewidthofthe
spectralisolationsystemandthefiniteslitwidthinsuchsystems.
•Athirdtypeofspectralinterferenceoccursduetothepresenceofcontinuousbackground
whicharisesduetohighconcentrationofsaltsinthesample,especiallyofalkaliandalkaline
earthmetals.Someorganicsolventsalsoproduceacontinuousbackground.Thistypeof
interferencecanbecorrectedbyusingsuitablescanningtechnique.

27. IonisationInterferences
•Insomecases,hightemperatureflamemaycauseionizationofsomeofthemetalatoms,e.g.,in
caseofsodium,itcanbegivenasfollows.
Na→Na
+
+e
-
•TheNa
+
ionpossessesanemissionspectrumofitsownwithfrequencies,whicharedifferentfrom
thoseofatomicspectrumoftheNaatom.Thisreducestheradiantpowerofatomicemission.This
interferencecanbeeliminatedbyaddingalargequantityofapotassiumsalttothestandardsas
wellassamplesolutions.Theadditionofpotassiumsaltsuppressestheionisationofsodium,as
thepotassiumatomitselfundergoesionisationduetolowionisationenergy.Thus,thesodium
atomemissionisenhanced.Thistypeofinterferenceisrestrictedtoalkalimetals.
ChemicalInterferences
Thechemicalinterferencesariseoutofthereactionbetweendifferentinterferentsandthe
analyte.Theseareofdifferenttypes.Someofthesearegivenbelow.
•Cation-anioninterference:Thepresenceofcertainanions,suchasoxalate,phosphate,sulphateand
aluminate,inasolutionmayaffecttheintensityofradiationemittedbyanelement,resultingin
seriousanalyticalerror.Forexample,calciuminthepresenceofphosphateionformsastable
substance,asCa
3(PO
4)
2whichdoesnotdecomposeeasily,resultingintheproductionoflesser
atoms.Thus,thecalciumsignalisdepressed.Anothersimilarexampleisthatofdeterminationof
bariuminpresenceofsulphateforminginsolubleBaSO
4.Thistypeofinterferencecanberemoved
eitherbyextractionoftheanionorbyusingcalibrationcurvespreparedfromstandardsolutions
containingsameconcentrationsoftheanionasfoundinthesample.

28. •Cation-cationinterference:Inmanycases,mutual
interferencesofcationshavebeenobserved,resultingin
reducedsignalintensityoftheelementbeingdetermined.
Theseinterferencesareneitherspectralnorionicinnature
andthemechanismofsuchinterferencesisnotwell
understood.Thus,forexample,aluminuminterfereswith
calciumandmagnesium.Also,sodiumandpotassiumshow
cation-cationinterferenceononeanother.
•Interferenceduetooxideformation:Thistypeofinterference
arisesduetotheformationofstablemetaloxideifoxygenis
presentintheflame,resultinginreducedsignalintensity.The
alkalineearthmetalsaresubjecttothistypeofinterference.
Thistypeofinterferencecanbeeliminatedbyeitherusing
veryhighflametemperaturetodissociatetheoxidesorby
usingoxygen-deficientenvironmenttoproduceexcitedatom.

29. Structure of Flames
•Flamesarenotuniformincomposition,lengthorcross
section.Thestructureofapremixedflame,supportedasa
laminarflowisshowninFig.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

31. •Thefirstortheinnermostregionoftheflameisthepreheatingzone
wherethecombustionmixtureisheatedtotheignitiontemperatureby
thermalconductionfromtheprimaryreactionzone.
•Thesecondzoneistheprimaryreactionzoneorinnerzone.Thiszoneis
about0.1mmthickatatmosphericpressureandisvisiblebyvirtueofits
bluegreenlightascribedtoradicals
.
C2and
.
CH.Thereisno
thermodynamicequilibriuminthiszoneandtheconcentrationofionsand
freeradicalsisveryhigh.Thisregionisnotusedforflamephotometry.
•Immediatelyabovetheprimaryreactionzoneliesthethirdorinterconal
zoneorthereactionfreezonewhichcanextenduptoconsiderable
height.Themaximumtemperatureisachievedjustabovethetipofthe
innerzone.Thehighertemperaturefavoursbothproductionoffreeatoms
andmaximumexcitationforatomicemissionspectroscopy.Therefore,this
zoneisusedforflamephotometry.
•Theoutermostfourthzoneisthesecondaryreactionzone.Withinthis
zone,theproductsofthecombustionprocessesareburnttostable
molecularspeciesbythesurroundingair.

32. •Theshapeofanunmixedflameisgenerallydifferent.Theinnerzonecan
stillberecognised,butitisveryvagueandisthickened.Alaminarflame
makesastronghissingnoisewhichgetslouderwhenaliquidisatomised
intoit.Weshallnowlookintothereactionswhicharetakingplacewhen
theelementisplacedinflame.

33. Limitation of Flame Emission Photometry
•Asnaturalgasandairflameisemployedforexcitationthetemperatureisnothigh
enoughtoexcitetransitionmetals,thereforethemethodisselectivetowards
detectionofalkaliandalkalineearthmetals.
•Thelowtemperaturemakesthismethodsusceptibletocertaindisadvantages,
mostofthemrelatedtointerferenceandthestabilityoftheflameandaspiration
conditions.Fuelandoxidantflowratesandpurity,aspirationrates,solution
viscosity,affectthese.Itisthereforeveryimportanttomeasuretheemissionofthe
standardandunknownsolutionsunderidenticalconditions.
•Therelativelylowenergyavailablefromtheflameleadstorelativelylowintensity
oftheradiationfromthemetalatoms,particularlythosethatrequirelargeamount
ofenergytobecomeexcited.
•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.

34. Applications
•Flamephotometersarewidelyusedinqualitycontrol
whereasimpleandquickdeterminationofalkalior
alkaliearthelementsisrequired.Theyhavethe
advantageofbeingsignificantlylowerpricedthan
mostotheratomicspectrometers.
•Biological/medicalapplications–notable
applicationsarethedeterminationsofNa,K,Caand
Mginbodyfluidsandotherbiologicalsamples.
•Foodindustry–determinationofcalciumandironin
beer.