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Dong-Sun Lee / cat - lab / SWU 2010Fall versionChapter28Atomic spectroscopyAtomic spectroscopyAtomicspectroscopydealswiththeabsorption,emission,orfluorescenceby atomorelementaryions.Tworegionsofthespectrumyieldatomic information- the !-visible and the "-ray.Asatomshavenorotationalorvibrationalenergy,transitionsoccuronly between electronic levels and bandwidths in atomic spectra are very narrow.Atomic spectroscopic methods normally are classified according to the type of spectral process involved and the method of atomi#ation used.Absorption, emission, fluorescence$chematic representation of absorption, emission, and fluorescence.Origin of three sodium emission lines (resonance line)(a) Partial absorption spectrum for sodium vapor.(b) Electronic transitions responsible for the absorption lines in (a)Atomic energy level diagram.Atomic absorption spectrometryAtomicabsorptionistheprocessthatoccurswhenagroundstateatomabsorbs energyintheformofelectromagneticradiationataspecificwavelengthandis elevatedtoane%citedstate.Theatomicabsorptionspectrumofanelement consistsofaseriesofresonancelines,alloriginatingwiththegroundelectronic state and terminating in various e%cited states. sually the transition between the groundstateandthefirste%citedstateisthelinewiththestrongestabsorptivity, and it is the line usually used.Transition between the ground state and e%cited state occur only when the incident radiationfromasourceise%actlye&ualtothefre&uencyofaspecifictransition. 'artoftheenergyoftheincidentradiationPoisabsorbed.Thetransmitted radiation ' is given by P ( Poe)kb*where k is the absorption coefficient of the analyte element and b is the hori#ontal path length of the radiation through the flame. Atomic absorption is determined by the difference in radiant power of the resonance line in the presence and absence ofanalyteatomsintheflame.Thewidthofthelineemittedbythelightsource must be narrower than the width of the absorption line of the analyte in the flame. The concentration value in the AA$ is based on the +ambert-,eer law- with -. - e%tinction, T - light transmission e%pressed as a percentage )transmission*, /0 - intensity of the transmitted signal, /0 - intensity of the original signal, e - e%tinction coefficient )proportionality factor*, c - concentration l-layerthic1ness)(distancetraversedbythelightraythroughthe flame2atomic cloud*. Atomic emission spectrometry with flameAsolutionifthesampleissprayedintoaflamepossessingthethermalenergy re&uired to e%cite the element to a level at which it will radiate its characteristic line emission spectrum. For an atom or molecule in the ground electronic state to be e%cited to a high electronic energy level, it must absorb energy from the flame viathermalcollisionswiththeconstituentsofthepartiallyburnedflamegases. pontheirreturntoalowerorgroundstate,thee%citedatomsandmolecules emit radiation characteristic of the sample components.,andspectraarisefromelectronictransitionsinvolvingmolecules.Foreach electronictransitiontherewillbeawholesuiteofvibrationallevelsinvolved. Thiscausestheemittedradiationtobespreadoveraportionofthespectrum. ,andemissionsattributedtotriatomichydro%ides)3a45*at667nmand mono%ides)Al4,strongestbandat787nm*arefre&uentlyobservedand occasionallyemployedinF.$.Theborono%idesystemgivesverysensitive bands at 618 and 679 nm.Aphotodetectormeasurestheradiantpoweroftheselectedradiationthatis correlated with the concentration of analyte in the sample and in standards.Basic components of an atomic spectrophotometer Thermo .lemental $4+AA: ;9; AA spectrometers http-22www.thermoelemental.com2instruments2aa2solaar