Encyclopedia of Physical Science and Technology, 3e, Analytical Chemistry

600 Pages·2005·38.07 MB·English

by  Robert Allen Meyers

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Table of Contents
(Subject Area: Analytical Chemistry)

Pages in the
Article Authors
Encyclopedia
Ulrich J. Krull and Michael
Analytical Chemistry Pages 543-579
Thompson
Atomic Spectrometry Vahid Majidi Pages 765-786
Auger Electron
C. L. Briant Pages 787-792
Spectroscopy
Capillary Zone
Tim Wehr Pages 355-368
Electrophoresis
Electrochemistry Donald T. Sawyer Pages 161-197
Electron Spin
Larry Kevan Pages 331-345
Resonance
Electrophoresis S. P. Spragg Pages 363-378
Elemental Analysis,
T. S. Ma Pages 393-405
Organic Compounds
Gas Chromatography Milos Novotny Pages 455-472
Infrared Spectroscopy Norman B. Colthup Pages 793-816
Liquid
Neil D. Danielson Pages 673-700
Chromatography
Magnetic Resonance in
John F. Schenck Pages 959-981
Medicine
Mass Spectrometry Kenneth L. Busch Pages 145-158
Mass Spectrometry in
Jan Schuberth Pages 159-169
Forensic Science
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Microwave molecular
Robert L. Cook Pages 799-852
spectroscopy
Nuclear Magnetic
Bernard C. Gerstein Pages 701-720
Resonance (NMR)
Organic Chemistry, Raphael Ikan and Bernard
Pages 459-496
Compound Detection Crammer
Photoacoustic Konka Veeranjaneyulu and
Pages 1-13
Spectroscopy Roger M. Leblanc
Photoelectron G. Hohlneicher and A.
Pages 57-90
Spectroscopy Gildenpfennig
R. P. Van Duyne and C. L.
Raman Spectroscopy Pages 845-866
Haynes
Scanning Probe
C. Daniel Frisbie Pages 469-484
Microscopy
Sonoluminescence and
Kenneth S. Suslick Pages 363-376
Sonochemistry
Spectroscopy in
Michael B. Eyring Pages 637-643
Forensic Science
Thermal Analysis David Dollimore Pages 591-612
Tomography Z. H. Cho Pages 843-877
Ultrafast Spectroscopy M. Hayashi, Y. M. Chang, T.
Pages 217-226
and its Applications K. Wang, S. H. Lin and
X-Ray Analysis Ron Jenkins Pages 887-902
X-Ray Photoelectron Charles C. Chusuei and D.
Pages 921-938
Spectroscopy Wayne Goodman
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EncyclopediaofPhysicalScienceandTechnology En001f25 May26,2001 14:30
Analytical Chemistry
Ulrich J. Krull
Michael Thompson
University of Toronto
I. Classical Methods
II. Instrumental Methods
III. Computers in Analytical Chemistry
IV. Future Perspectives
GLOSSARY Quantitative analysis Process of determining the rela-
tiveamountsofoneormorecomponent(s)inasample
Accuracy Nearness of a measurement to its accepted ofmatter.
value. Resolution Theratiogivenbytheaveragemagnitudeof
Analyte Species present in a sample of matter about ananalyticalvariabledividedbythesmallestdifference
which chemical information is sought. of values of the analytical variable, where each value
Calibration Process of determining the precise quanti- providesanalyticalsignalsthatareconsidereddistinct
tative relationship between a known concentration of bystatisticaltechniques.
a chemical species and a physical property of that Selective Methodinanalyticalchemistrythatyieldsare-
species. sponseforagroupofchemicalspecies.
Interferences Elements or chemical compounds that Sensitivity Ratioofthechangeintheresponseofanin-
have similar properties to the analyte that prevent its strument with a corresponding change in the concen-
direct measurement. trationofachemicalspecies.
Limit of detection Concentration of a chemical species Specific Methodinanalyticalchemistrythatyieldsare-
that produces an analytical signal equal to twice the sponseforasinglechemicalentityonly.
standard deviation of the background signal. Standard Chemicalspecieswithwell-establishedphysi-
Matrix Matter present in a sample in which the species calpropertiesthatisemployedtocalibrateananalytical
being determined is dispersed. procedure.
Noise Random fluctuations of analytical signal with time.
Precision Statistical measure of the distribution of a se-
ries of analytical determinations about the average ANALYTICAL CHEMISTRY is concerned with pro-
value of the same analytical determinations. vidingqualitativeandquantitativeinformationaboutthe
Qualitativeanalysis Processofidentifyingoneormore chemicalandstructuralcompositionofasampleofmat-
component(s)inasampleofmatter. ter.Ahugevarietyofsamples,fromhighconcentrationsof
543
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544 AnalyticalChemistry
elementsinalloysteelstopart-per-billionlevelsofdrugs ganicspeciesliesintheirchemicalandphysicalbehavior.
inbiologicaltissue,arehandledbytheanalyst.Thefield Forexample,reagentsareusedtoyielddistinctchemical
isfoundedontheconversionofameasuredphysicalprop- effectssuchastheproductionofcoloredsolutionsorpre-
erty of the species being examined to a usable signal. It cipitates,thegenerationofeasilyobservedgases,andthe
isgenerallydividedintotwocategories,classicalandin- dissolutionofpreviouslyinsolublesubstances.Inatypical
strumental,onthebasisofitshistoricaldevelopment.The analysis,theidentificationofseveralspeciesisrequired,
overall strategy is to prepare a sample correctly, choose andinthiscircumstance,itisdesirabletoemployselective
a particular method of analysis, and report the results reactionsforeachcomponentofthesystem.
in a meaningful format, which may include a statistical This type of chemistry is incorporated into a strategy
evaluation. that involves the separation of the original mixture into
several parts in order to avoid the buildup of a highly
complexarraywithaspecificsampleduetotheaddition
of a number of reagents. Each part is then subjected to
I. CLASSICAL METHODS
an analysis of a small number of species. In summary,
the analysis involves a set of sequenced separations and
A. SemimicroQualitativeAnalysis
identifications.
A complete system of qualitative analysis is usually re- Thestrategyfortheseparationofcationsinvolvestheir
stricted to the detection of a complex array of inorganic divisionintoasetofgroupsbytreatmentwithaparticular
cationsandanions.Inprinciple,schemesaredevelopedto selectivereagent.Aftersolutionoftheunknowninwater,
includelesscommonspeciessuchasrheniumandtellurate areagentisemployedtocauseallcationsofthegroupto
anions,butmoreoftenthannotattentionisconcentratedon precipitate, with all other ions remaining in solution. A
morecommoncationssuchasNa+andCu2+andoxyan- typical overall scheme (simplified) is depicted in Fig. 1.
ionssuchasSO2−andNO−.Thebasisforidentifyinginor- Afterseparation,theprecipitateisreexaminedforspecific
4 3
FIGURE1 Simplifiedstrategyforseparationofcationsintogroupsforqualitativeanalysis.
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AnalyticalChemistry 545
cations.Asoneexampleletusconsiderthesilvergroup. and therefore the error, on reagent precipitation. The fa-
TreatmentofthewhiteprecipitateofHg Cl ,AgCl,and cility with which a precipitate is removed from solution
2 2
PbCl withhotwaterresultsinsolubilizationofthelead isrelatedtotheparticlesizeofthesolidphase,whichin
2
compound to yield a colorless solution. The presence of turn is governed by the conditions that exist at the time
Pb2+isconfirmedbytheadditionofK CrO ,whichgives of formation of the precipitate. Thought to be important
2 4
a yellow precipitate of PbCrO . Addition of NH OH to arethesolubilityoftheprecipitate,temperature,reactant
4 4
Hg Cl andAgClresultsineitheragraymixtureofmer- concentration,andmixingphenomena.Theseparameters
2 2
cury and HgNH Cl, confirming Hg2+, and/or colorless controlthesupersaturationthatexistsinsolutionatapar-
2 2
solutionsofAg(NH )+Cl−.Acidificationofthelatterre- ticulartime.Althoughtheusualaimistoachieveaneas-
3 2
precipitatesAgCl,identifyingthepresenceofAg+. ilymanipulatedcrystallineprecipitate,colloidalparticles
Thedetectionofanionsintheoriginalsampledoesnot (10−6 to 10−4 mm in size) are sometimes obtained. In
proceedinthesamewayinthatthematerialisusuallysub- these cases, the individual particles must be coagulated
jectedtoaseriesofpreliminarytests.Moreover,thecation by an experimental procedure such as heating, stirring,
analysisdescribedabovecanoftenbeassessedtoinferthe andtheadditionofauxiliaryelectrolyte.Themechanism
presenceofcertainanions.Asamplesolutionisseparately ofthisprocessliesinthereductionofelectricalrepulsive
treatedwithAgNO andBaCl ,andthevariousprecipi- forcesontheparticles.Purer,moredenseprecipitatescan
3 2
tatesofsilverandbariumareusedtoverifythepresenceof oftenbeproducedbyprecipitationfromhomogeneousso-
anioniccomponents.Thisprocedureisthenfollowedby lution,wherethereagentisgeneratedinsolution.Finally,
theadditionofconcentratedcoldH SO totheunknown afterfiltrationagravimetricprecipitateisheateduntilits
2 4
solid,whichresultsintheliberationofcharacteristicgases weight becomes constant. Awide rangeof temperatures
(e.g.,I−givestheodorofH SandvioletfumesofI ),and (110–1200◦C)areusedforthispurpose,themostimpor-
2 2
byaseriesofconfirmatorytestsforeachanion. tantrequirementbeingweightconstancyandunequivocal
On a practical level the whole analysis is performed knowledgeofthecompositionoftheprecipitateatapar-
atthesemimicrolevel.Thismeansthatsamplesizesare ticulartemperature.
in the range of 10 mg and solutions are of the order of Someexamplesofinorganicandorganicprecipitation
1–2mlinvolume.Specialequipmentisrequiredtohan- agents for inorganic analytes are given in Table I.
dlelowvolumesofreagentsolutionsandtoavoidserious There are also methods available for the precipitation
losses in separative and transfer procedures. The sepa- of organic compounds through the reaction of a partic-
ration of precipitates is performed by centrifugation and ular functional group. For example, compounds with
decantation,andheatingisachievedinspeciallydesigned carbonyl functional groups can be precipitated with
testtubeheatingblocks. 2,4-dinitrophenylhydrazine according to the following
reaction:
B. GravimetricAnalysis RCHO+H NNHC H (NO ) →
2 6 3 2 2
Here,quantitativeanalysisisbasedonthemeasurementof R—CH NNHC H (NO ) ↓+H O.
6 3 2 2 2
theweightofasubstanceofpreciselyknowncomposition
that is chemically related to the analyte. Most often the In summary, the gravimetric method does not require
unknown is precipitated from solution by a reagent and, calibration, as is the case with many other analytical
after separation and drying, is weighed. Less frequently
thespeciesbeingdeterminedisvolatilized,andtheweight TABLEI TypicalInorganicandOrganicPrecipitat-
ofthecondensedgasorresidualsolidservestocomplete ingAgents
the analysis. The precipitate must be insoluble in water Elementprecipitated
(orothersolvent)inordertominimizeobviouslosses,be Agent (weighedform)
easilyfilteredandwashed,andbestableafterdryingorig-
nitionprocedures.Withrespecttothefirstrequirement,an NH3 Al(Al2O3),Fe(Fe2O3)
importantquantitativeparameteristhesolubilityproduct H2S Zn(ZnO),Ge(GeO2)
of the compound produced from the analyte. For exam- H2SO4 Pb(PbSO4),Ba(BaSO4)
ple,fortheprecipitationofSO2−withBa2+,thesolubility HCl Ag(AgCl)
4
product(K )forBaSO isdefinedby (NH4)2CO3 Bi(Bi2O3)
Kspsp =[Ba2+]4(cid:2)SO24−(cid:3)=1.3×10−10 BMagCCl2l2,NH4Cl PSOO2434−−((BMagS2OP24O) 7)
for a saturated solution. Clearly, this value can be used 8-Hydroxyquinoline(HQ) Al(AlQ3)
to compute the loss of analyte (remaining in solution), Dimethylglyoxime(DMG) Ni(NiDMG2)
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546 AnalyticalChemistry
methods(seelater).Accordingly,itisstillfrequentlyused ion, bases are standardized against potassium hydrogen
asa“standardizing”techniqueforinstrumentalmethods. phthalate (KHC H O ). The end point in a strong acid–
8 4 4
Gravimetry, however, can be rather time consuming, es- strongbaseneutralizationtitrationisusuallyfoundfrom
peciallyifalargenumberofsamplesareinvolved. theinsitubehaviorofanaddedindicator,whichisgener-
allyaweakorganicacidorbasethatundergoeschemical
changesexhibitingdifferentcolors.Forexample,wecan
C. Titrimetric(Volumetric)Analysis
writefortheacid-typeindicatorHIn
In titrimetric analysis, which is often termed volumetric
H O+HIn=H O++In−,
analysis, we obtain the volume of a standard reagent re- 2 3
quiredtoconsumeananalytecompletely.Onapractical (color1in (color2in
basisastandardsolutionofreagent,theconcentrationof acidsolution) basicsolution)
whichisknownaccurately,isaddedbyaburetuntilitis
Thus,itisveryimportanttoknowtherangeofhydrogen
decidedthattheanalyteisjustusedup.Thisconditionis
ionconcentration(i.e.,pH)inwhichachangefromcolor
usually called the equivalence point. Since it is difficult
1tocolor2canbeobserved.Generally,thisoccurswithin
toobservethispointexperimentally,itisusuallyapprox-
approximately±1pHunitofthepK oftheindicator.
imatedbythedistinctionofanendpoint,whichisasso- a
The titration of a strong acid solution by additions of
ciatedwithdetectablephysicalchangesatequivalence.It
a standard strong base solution from a buret would re-
isgenerallythecasethatstandardsolutionsarecalibrated
sult theoretically in the lower graph shown in Fig. 2 (the
against solutions of a primary standard that is a highly
titrationcurve).Accordingly,wewouldrequireanadded
pureandstablereferencesubstance.
indicatortorespondtotheabruptapproximately3–10pH
Inthepresentdescriptionweintroducefourtypesofvol-
change.Inthiscase,phenolphthaleinwouldbeappropri-
umetricanalysis.Oneinvolvestheneutralizationofacidor
ate, since its change from colorless to purple is easily
base,thesecondisconcernedwithprecipitation,thethird
detected by the eye. The titration curve for a weak acid
with complex formation, and the fourth with oxidation–
(withstrongbase)dependsverymuchonthedissociation
reduction reactions. Before doing this we must consider
constantoftheacid.Basically,additionsofbasesetupa
severaldefinitions.Theequivalentweightofasubstanceis
buffer zone as in the set of curves shown in Fig. 2 (upper
generallytheweightthatcombineswithastandardamount
curve).Thechoiceofanindicatorforthistypeoftitration
ofreactant.Theparameterisbasedentirelyonthenature
issignificantlymorelimitedthanforastrongacid,since
ofaparticularreactionand,therefore,canhavenomore
there is now no abrupt change in pH. Not surprisingly,
precisemeaningatthisstage.Titrationreferstothepro-
theappropriatecurvesforpolyproticweakacidsareeven
cessofgradualadditionofstandardreagent,whereasthe
morecomplex.
titerofasolutionistheweightofasubstancethatischem-
Neutralization methods are employed wherever inor-
icallyequivalentto1mlofthesolution.Finally,oneoften
ganicororganicsubstancespossessacidicorbasicgroups.
sees the term normality, which expresses the number of
An important application is the conversion of elements
milliequivalentsofsolutecontainedin1mlofsolution.
The end point in titrimetric methods is usually based
onawayofdynamicallydistinguishinganalyteorreagent
concentration.Themostwidelyusedmethodischangein
color due to reagent, analyte, or indicator, but the moni-
toringofelectricalpotentialandcurrent,andturbidity,are
sometimesused.
1. NeutralizationTitrations
Inthistechniquetheconcentrationofacidorbaseisde-
terminedthroughtheabruptchangeofpHthatoccursat
thetitrationendpoint.Here,theequivalentweightofthe
acidorbaseistheweightthateithercontributesorreacts
with1molofhydrogenioninthatspecificreaction.The
standard reagents used in the titration are always strong
acidsorbases,althoughtheanalytemaybeaweakacid
orbase.A-standardacidsolutioncanbepreparedbytitra-
FIGURE 2 Neutralization titration curves for 50 ml of 0.05M
tionoftheacid,sayHCl,againstasolutioncontainingan
strongacid(lowercurve)andweakacid(uppercurve)with0.1M
accurately known weight of Na2CO3. In a similar fash- NaOHsolution.
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AnalyticalChemistry 547
in organic or biological systems to acid–base character. of Cl− and Br− is that of Mohr, who used chromate ion
Forexample,organicnitrogenisconvertedtoNH+inthe (CrO2−),theendpointbeingspecifiedbytheappearence
4 4
Kjedahlmethodtobedeterminedinaneutralizationpro- of Ag CrO , which is brick red in color. The essence of
2 4
cedureasliberatedNH . theprinciplehereisthatthesolubilityofsilverchromate
3
ismuchgreaterthanthatofsilverhalide.Accordingly,the
conditions of the titration can be adjusted such that the
2. PrecipitationTitrations formationofsilverchromateoccursonlyinthevicinityof
theendpoint.IntheVolhardmethodastandardsolutionof
Inthismethod,thereagentsolution,addedfromaburet,
thiocyanateion(SCN−)istitratedagainstAg+usingFe3+
isallowedtocauseprecipitationwiththeanalyte,andthe
astheindicator.AttheinitialexcessofSCN−,aredcolor
endpointisassociatedwithanabruptchangeinthecon-
duetoredFe(SCN)2+isproduced.Animportantapplica-
centrationofthereagentspecies.Theequivalentweightof
tionofthismethodisthe“backtitration”procedureforthe
aparticipantinaprecipitationtitrationistheweightthat
Cl−ion.Here,anexcessofstandardsilvernitratesolution
reactswithorprovides1gformulaweightofthereacting
is added to the unknown Cl− solution and then titrated
cation if it is univalent (0.5 for divalent, etc.). The tech-
with SCN− with the Volhard indicator in place. Finally,
niquecanbebestunderstoodbytheexampleprovidedby
adsorptionindicators(Fajansmethod)areemployedthat
thelong-standingsilverionprocedureforthedetermina-
tionofhalideanion,particularlyCl− (sometimestermed arebasedontheadsorptionofacoloredorganiccompound
totheprecipitate,onlyinthevicinityoftheendpoint.
argentometrictitration).Typicaltitrationcurvesforvari-
ous halide anions with silver cation are shown in Fig. 3.
Notice that the concentration of the free reagent cation
3. ComplexometricTitrations
presentataparticularpointinthetitrationisusuallyex-
pressedas−log[Ag+],thatis,pAg,inafashionanalogous Titrimetric methods based on the reaction of metal
to pH for hydronium ion. Before the equivalence point ions with a coordinating species (ligand) are more than
(withabruptchangeofpAg),theconcentrationofAg+is 100 years old. If the ligand is attached to the metal by
determined by the solubility product of the silver halide morethanonefunctionalgroup,itissaidtobepolyden-
concerned.Afterthispointwesimplyhavetheconcentra- tateandthecompoundproducediscalledachelate.The
tion of excess added reagent, which is not now reacting complexometricreagentthatiseasilythemostwidelyused
withhalideanion.Todetecttheendpointanindicatoris for the titration of unknown concentrations of metal ion
required that will respond to the particular range of abrupt isthehexadentateligandethylenediaminetetraaceticacid
pAgchangeshowninFig.3.Acommonmethodinthecase (EDTA),firstrecognizedbySchwarzenbachin1945.The
structureofthemoleculeis
HOOC−CH CH −COOH
2 2
\\ /
N−CH −CH −N
/ 2 2 \
HOOC−CH CH −COOH
2 2
andisusuallyabbreviatedtotheshorthandnotationH Y,
4
withHrepresentingthecarboxylichydrogenatoms.This
reagent is extremely important because it forms simple
1:1complexeswithametalion,thatis,onemetalatom
tooneEDTAmoleculeofveryhighstability.Notethatthe
latterisassociatedwiththerelativelylargenumber(6)of
points of attachment (4 × COO−+2×N) of the ligand
tothemetal.
Inthenowfamiliarpatterndiscussedabove,thetitration
involvestheburetadditionofEDTAsolutiontothemetal
ion solution, which generates a titration curve with an
abrupt change in −log[Mn+] (pM). This is governed by
the equilibrium constant for the formation of the metal–
FIGURE3 Precipitationtitrationcurvesfor25mlof0.1M solu-
tionsofvariousanionswith0.1MAgNO3solution. EDTAcomplex:
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548 AnalyticalChemistry
(cid:2) (cid:3)
MY(n−4)+ TABLE II Common Oxidizing and Reducing Agents Em-
KMY = [Mn+][Y4−], ployedasStandardSolutionsinOxidation–ReductionTitra-
tions
Note the analogy of this result with that outlined for
Oxidizing Reduction
the effect of solubility product in precipitation titrations Reagent species Product
discussed above. A great many compounds have been
proposedasindicatorsformetalionsinEDTAtitrations. Oxidizing
Thesespeciesaregenerallyorganiccompoundsthatform Potassiumpermanganate,KMnO4 MnO−4 Mn2+
colored chelates with metal ions in a range of pM that Potassiumbromate,KBrO3 BrO−3 Br−
is characteristic of the cation and dye. One example is Ceriumammoniumnitrate, Ce4+ Ce3+
EriochromeblackT,whichisblueatpH7andredwhen Ce(NO3)42NH4NO3
complexedwithavarietyofmetalions. Potassiumdichromate,K2Cr2O7 Cr2O27− Cr3+
EDTA titrations are still widely used because of their Potassium,iodate,KlO3 IO−3 I−
greatversatilitywithrespecttotheanalysisofalargenum- Reducing Oxidation
berofdifferentmetalcations.Furthermore,thetechnique species product
can be made more selective by adjusting the pH or by
Reducing
tchaetiounsesforfocmomthpeotuitnradtsiothna(tmeaffsekcitnivgealgyernetm).oTvheeinmteerthfeordinigs SFoerdriouumsathmiomsounlfiautme,Nsual2faSt2eO,3 SF2eO223+− SF4eO326+−
inexpensiveandreasonablyaccurate. Fe(NH4)2(SO4)2·6H2O
4. Oxidation–ReductionTitrations
derive
Here, we are dealing with the reaction of an oxidizing
(reducing) reagent as titrant with a reducing (oxidizing) E0 +5E0
species as unknown. The equivalent weight of a partici- Eeq = Fe3+ MnO−4
6
pant in this type of system is the weight that directly or (cid:2) (cid:3)
indirectlyproducesorconsumes1molofelectrons.Thus, −0.0591log 5 MnO(cid:2) −4 [Mn(cid:3)2+] .
theequivalentweightforthepermanganateion(MnO−) 6 5[Mn2+] MnO− [H+]8
4 4
initsoxidationofoxalateanioninthereaction
Thus,
5C→2O241−0C+O22M+nO2M−4 n+2+16+H8+H2O Eeq = EF0e3+ +65EM0nO−4 − 0.06591log[H1+]8.
isthemolecularweightofMnO−dividedby5. In much the same manner as for the other types of
4
A great variety of both oxidizing and reducing agents titrationdescribedabove,itistheelectrodepotentialdur-
have been employed for this type of titration, and some ing the procedure that exhibits an abrupt change on ad-
commoncompoundsaregiveninTableII.Theequivalence ditionofreagent.Accordingly,werequireindicatorsthat
point of oxidation–reduction titrations can be computed changecolorduringthischange(i.e.,thatshowoxidation–
fromaknowledgeofsolutionconcentrationsandelectri- reductionbehaviorthemselves).Twoexamplesare1,10-
calpotentials.Forexample,inthetitrationofferrousion phenanthroline–iron (II) complex, which changes from
againstKMnO accordingtothefollowingreaction pale blue to red at an electrical potential of+1.11 V (in
4
1M H SO ), and diphenylaminesulfonic acid, which
5Fe2++MnO−4 +8H+ →5Fe3++Mn2++4H2O, change2sfro4mcolorlesstovioletat0.85V.
we can write Nernst electrode potentials for each
oxidation–reductionsystem:
II. INSTRUMENTAL METHODS
[Fe2+]
E = E0 −0.0591log
Fe3+ [Fe3+] A. AbsorptionofElectromagneticRadiation
0.0591 [Mn2+] Matter interacts with incident electromagnetic radiation
E = EM0nO−4 − 5 log(cid:2)MnO−(cid:3)[H+]8, bythethreedistinctprocessesoftransmission,scattering,
4 orabsorption.Thenatureofanyinteractionisafunctionof
At the equivalence point we know that these two poten- thepropertiesoftheradiation,suchasenergy,phase,po-
tials,nowcalled E ,areequal;therefore,onaddingwe larization,andthechemicalpropertiesofthematterunder
eq
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AnalyticalChemistry 549
FIGURE4 Analyticalpotentialforabsorptionspectroscopyacrosstheelectromagneticspectrum.
investigation.Chemicalevaluationisderivedfromobser- concentrationoftheanalyte,andArepresentsavaluefor
vationoftheextentofinteractionbymeasurementofthe absorption.
energy and intensity of transmitted, scattered, absorbed,
orlatentlyreleasedradiation.
1. AtomicAbsorption
Absorption is said to occur when radiation passes
through matter and interacts with the ions, atoms, or Theabsorptionofradiationbyasampleofatomicparti-
moleculesconstitutingthesampleinsuchawaythatthey cles, created by vaporizing the sample, represents a rel-
gainenergyandmovefromaquantizedlow-energystate atively simple spectral situation that has great practical
toahigherenergystate.Analytically,thisphenomenonis value for elemental identification and concentration de-
observedasareductionoftheintensityofradiationafter termination.Theabsorptionspectrumobservedwithpoly-
passagethroughasampleofmatter.Twodistinctparam- chromatic light contains only a few areas of reduced in-
eters can be investigated. The reduction of intensity is a tensityoccurringatverywell-definedfrequenciesdueto
functionoftheprobabilityofinteractionoftheradiation thesmallnumberofenergystatesavailabletotheatoms.
withappropriateanalytespeciesandindicatesconcentra- Thenaturalwidthofsuchabsorptionlinescanbelessthan
tion.Also,theenergyoftheabsorbedradiationindicates 10−4 nm, but broadening often occurs due to collisional
thequantizedenergylevelsinwhichenergyisdeposited andDopplereffectsinthesamplematrix.Thequantized
(Fig. 4) and therefore assists in species identification. The energy transitions responsible for atomic absorption are
Beer–Lambertlawgenerallyappliestoallabsorptionpro- ofelectronicorigin,indicatingthatelectromagneticspec-
cessesatlowconcentrationswhenmonochromaticradia- trumenergiesfromX-raystoultraviolet–visiblerediation
tionisused.Itisconventionallywritten aresufficienttoobservethisprocess.
The technique known as atomic absorption spec-
P
log 0 =εbc= A, troscopyisofparticularanalyticalimportanceforthede-
P
termination of metals due to its sensitivity and potential
where P istheinitialpowerofanincidentbeamofradi- forselectivitybyvirtueofthenarrowatomicabsorption
0
ation, P isthefinalpower(decreasedduetoabsorption), lines.Aschematicrepresentationofthespectrophotome-
εrepresentsavaluecharacteristicoftheextentofabsorp- ter is shown in Fig. 5. To take advantage of characteristic
tionexpectedforacertainspeciesatadefinedwavelength selectivity, a special radiation source must be provided
in a defined matrix, b is the sample path length, c is the to produce extremely monochromatic radiation with a
FIGURE5 Representationofthedesignofaconventionalatomicabsorptionspectrophotometer.