4. Theory and principle
â˘Inflameemissionspectrometry,thesamplesolutionisnebulized(convertedintoafineaerosol)andintroducedintotheflamewhereitisdesolvated,vaporized,andatomized,allinrapidsuccession.
â˘Subsequently,atomsandmoleculesareraisedtoexcitedstatesviathermalcollisionswiththeconstituentsofthepartiallyburnedflamegases.Upontheirreturntoalowerorgroundelectronicstate,theexcitedatomsandmoleculesemitradiationcharacteristicofthesamplecomponents.
â˘Theemittedradiationpassesthroughamonochromatorthatisolatesthespecificwavelengthforthedesiredanalysis.Aphotodetectormeasurestheradiantpoweroftheselectedradiation,whichisthenamplifiedandsenttoareadoutdevice,meter,recorder,ormicrocomputersystem.
22. Types of flame used
Themostcommoninstrumentsuseairastheoxidant.Thetemperatureoftheflamesproducedisrelativelylowsothetechniqueisonlysuitableforelementsthatareeasilyexcitedsuchasalkaliandalkaliearthelements.Whenoxygenornitrousoxideisusedamuchhighertemperaturecanbeobtained.
FlameTemp/°C
Gas/Air1700â1900
Gas/O22700â2800
H2/Air2000â2100
H2/O22550â2700
C2H2/Air2100â2400
C2H2/O23050â3150
C2H2/N2O2600â2800
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 C2H2/ N2O 0.01
Ba 553 C2H2/ N2O 0.001
K 766 C2H2/ O20.001
Li 671 C2H2/ N2O 0.0001
29. Structure of Flames
â˘Flamesarenotuniformincomposition,lengthorcrosssection.Thestructureofapremixedflame,supportedasalaminarflowisshowninFig.7.7.
â˘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
33. Limitation of Flame Emission Photometry
â˘Asnaturalgasandairflameisemployedforexcitationthetemperatureisnothighenoughtoexcitetransitionmetals,thereforethemethodisselectivetowardsdetectionofalkaliandalkalineearthmetals.
â˘Thelowtemperaturemakesthismethodsusceptibletocertaindisadvantages, mostofthemrelatedtointerferenceandthestabilityoftheflameandaspirationconditions.Fuelandoxidantflowratesandpurity,aspirationrates,solutionviscosity,affectthese.Itisthereforeveryimportanttomeasuretheemissionofthestandardandunknownsolutionsunderidenticalconditions.
â˘Therelativelylowenergyavailablefromtheflameleadstorelativelylowintensityoftheradiationfromthemetalatoms,particularlythosethatrequirelargeamountofenergytobecomeexcited.
â˘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.
â˘Only liquid samples can be used. In some cases, lengthy steps are necessary to prepare liquid samples.