The document discusses atomic emission spectroscopy (AES) and two specific techniques: flame photometry and inductively coupled plasma atomic emission spectroscopy (ICP-AES). Flame photometry uses a low temperature flame to atomize samples and determine the presence and concentration of sodium, potassium, lithium, and calcium. ICP-AES uses a plasma torch to produce excited atoms and ions from samples. The plasma is much hotter than a flame and allows for more complete atomization and a wider dynamic range of analysis.

1. Atomic Emission Spectroscopy
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By: Bijaya Kumar Uprety

2. Introduction
•Atomicemissionspectroscopy(AES)isamethodofchemicalanalysisthatusestheintensityoflightemittedfromaflame,plasma,arc,orsparkataparticularwavelengthtodeterminethequantityofanelementinasample.Thewavelengthoftheatomicspectrallinegivestheidentityoftheelementwhiletheintensityoftheemittedlightisproportionaltothenumberofatomsoftheelement.
•TheprincipleofAESisthatatoms(orsometimesions)arethermallyexcitedtohigherelectronicenergylevels,andthenrelaxbacktolowerelectronicenergylevelsbyemittingradiationintheUV-visibleregion.
•Theemittedradiationisdetectedandusedtodeterminewhichelementsarepresent,andtheirconcentration.TheinstrumentationforAESrangesfromtheverysimpleandcompactflamephotometer,tosophisticatedandexpensiveplasmaspectrometers.
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3. 1. Flame Photometry
•FlamephotometryisthesimplesttechniqueinAES, havingtheadvantageofcheapnessandsimplicity.Itisapplicabletosolutionsonly,andisverylimitedintherangeofelementswhichcanbeanalysed,thesebeingprincipallyNaandKandalsoLiandCa.
•Theseelementsareeasilyionisedandthereforeonlyrequirelowtemperatureatomisationmethods.ThemajorapplicationofflamephotometryisinclinicallaboratoriesfordeterminingNa,K,LiandCainbodyfluids,althoughthistechniqueisbeingsuperseded(replaced)byion-selectiveelectrodes.Adiagramofaflamephotometerisgivenbelow.
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4. •Thesamplesolutionisdrawnupintothenebuliserbyairflowingatconstantpressure,whichhasasuctioneffect.Theairandthenebulisedsamplesolutionaremixedwiththefuel(whichisusuallypropane),inthemixingchamberbeforepassingintotheburner.Theemittedradiationisfocussedbyalens,thenpassesthroughanopticalfilterwhichtransmitsonlyonestrongresonancelineoftheelementofinterest.Thedetectorisusuallyaphotocell.
•Theadvantagesofflamephotometryarethelowcost,beingconsiderablycheaperthananyotherAAorAEspectrometer; freedomfromspectralinterferences,thelowflametemperaturemeaningthatonlythegroup1andgroup2elementsproducelinesofintensity;andfreedomfromionisationinterferences,becauseionisationbecomesaproblemonlyathighertemperatures. 4

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6. 2. Inductively Coupled plasma Atomic Emission Spectroscopy (ICP-AES)
•Therearethreeplasmaatomisationmethodswhichhavebeendeveloped, namelyinductively-coupledplasmaAES(ICP);direct-currentplasmaAES(DCP);andmicrowaveplasmaAES.Ofthethree,ICPisnowthemostwidelyused,DCPislesscommonandmicrowaveplasmaAESisseldomused.
•Inductivelycoupledplasmaatomicemissionspectroscopy(ICP-AES)usesaninductivelycoupledplasma(orplasmatorch)toproduceexcitedatomsandionsthatemitelectromagneticradiationatwavelengthscharacteristicofaparticularelement.
•Itissimilartoflameemission.However,aplasmatorchisusedforexcitationratherthanaflame.Theplasmaismuchhotterandhasseveralotheradvantages.
•Aplasmaisaconductinggaseousmixturecontainingasignificantquantityofcationsandelectrons.Theargonionsandelectronsformtheprincipalchargecarriersthoughthesamplecationsalsocontribute.
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7. •Oncetheplasmahasformed,theargonionswithinitarecapableofabsorbingsufficientenergyfromanexternalsourcetomaintainatemperaturewherefurtherionizationcansustaintheplasmaindefinitely.Plasmatemperaturesashighas10,000Kelvincanbereached.
•TheICPtorchconsistsofthreeconcentricquartztubeswhicharecalledthe‘outertube’,‘intermediatetube’and‘innertube’or‘carriergastube’.
•Thesamplesolutionisnebulisedbyaflowofargonwhichcarriesthesampleupinsidetheinnertube.Anangledflowofargonbetweentheouterandmiddletubeskeepsthesampleflowandplasmastable.
•Atthetopofthetorchthereisaninductioncoilwhichgeneratestheplasma;oncethishasformed,theplasmabecomesself-sustaining.Thesampleintheargonflowreachestheplasmawheretheveryhightemperaturesatomisethesamplecompletely.
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8. •Thetemperatureofplasmavariesfrom2000Katthetopofthetorchto10000Katthebottomwhichismuchmoregreaterthanwegetusingaconventionalflame.
•Thismeansatomisationofthesamplewilloccurtoamuchgreaterextent(nearlytocompletion).
•ICP-AESalsohastheadvantageofhavingalargedynamicrange,i.e.sampleswithconcentrationsoverseveralordersofmagnitudecanbeanalysedfromthesamecalibrationcurve.
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9. Fig showing Plasma torch
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10. •Thepreviousslidefigureshowsaninductivelycoupledplasmatorch.Therearethreeconcentricquartztubesthroughwhichargonflows,thecombinedflowratebeing11-17litersperminute.
•Thetopoftheoutertubeissurroundedbyawater-cooledinductioncoil,whichispoweredbyaradio-frequencygeneratorcapableofproviding2kWofenergyatafrequencyof27MHz.
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11. •Plasmainitiation-TheionisationoftheflowingargonhastobeinitiatedbyasparkprovidedbyaTeslacoil.Theresultingionsandelectronsfromthesparkinteractwiththefluctuatingmagneticfield.Thiscausesthemtoflowinclosedannularpaths.Astheenvironmentbecomesmoreconductivethereisresistancetothechargemovementthatresultsinthegenerationofohmicheating. TheheatgeneratedisgreatenoughtoformaplasmawhichwillbesustainedprovidedthereiscontinuedpowerfromtheRFsourceandsufficientargon.Thetemperaturewouldbesufficienttodamagethequartzwallsbutthisisovercomebyhavingasecondaryflowofargon,whichcirclestheinnertubeandcoolsthewall.Thisflowalsohelpstostabilizetheplasmaradially.
•Anewformofignitionusesahightensioncoil(60Kv)surroundingthegasinlet.Thisvoltageissufficienttoionizetheargonwhichstartstomoveincloseannularpathsonceitreachesthemagneticfield.
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