Atomic emission and laser excited atomic fluorescence studies of the glow discharge for the analysis of microsamples

154 Pages·1992·4 MB·English

by  Marmolejo, Edison Becerra

ATOMICEMISSIONANDLASEREXCITEDATOMICFLUORESCENCE
STUDIESOFTHEGLOWDISCHARGEFORTHEANALYSISOF
MICROSAMPLES
By
EDISONBECERRAMARMOLEJO
ADISSERTATIONPRESENTEDTOTHEGRADUATESCHOOL
OFTHEUNIVERSITYOFFLORIDAINPARTIALFULFILLMENT
OFTHEREOUIREMENTSFORTHEDEGREEOF
DOCTOROFPHILOSOPHY
UNIVERSITYOFFLORIDA
1992
ACKNOWLEDGEMENTS
Iwanttothankallthemembersofthegroupfortheirsupportandfriendship
duringthewonderfultimeIspentattheUniversityofFlorida. Specialthanksgoto
DermisHueber,GiuseppePetrucci,RaulBadini,WellingtonMasambaandWilfredo
Restoforalltheirhelpandsuggestionsthroughoutthiswork.
IwishtoexpressmyspecialgratitudetoDr.JimDeavorforhisoutstanding
professionalhelpandencouragementduringthemostdifficultstagesofthiswork.
My appreciationalsogo toRudyintheglass shop andthepeopleinthe
machineshopfortheirexcellentcollaborationandassistance.
IwishtoexpressallmygratitudetoDr.JimD.Winefordnerforhisguidance,
adviceandencouragementaswellasforgivingmetheopportunitytoworkinhis
group.ToworkforJimhasbeenthebestacademicdecisionIhavehadinmylife.
Specialthanks also goto Dr. BenjaminW. Smithforhis constantand excellent
scientifichelpthroughoutthiswork.
MygratitudealsogoformyparentsTomasandTeresafortheircontinuous
encouragementandsupportduringmystudiesintheUnitedStates.
MyspecialthanksgototheUniversidaddelVallefortheirsupportduringmy
graduatestudiesattheUniversityofFlorida.
Finally,IwishtoexpressmyloveandgratitudetomywifeAnaMilenaand
my children Gustavo Adolfo, Brenda Cecilia, and Tomas Alberto. Their love,
understanding,patienceandsacrificemadethisworkpossible.
11
TABLEOFCONTENTS
ACKNOWLEDGEMENTS
ii
ABSTRACT v
CHAPTERS
1.HISTORICALPERSPECTIVES 1
3.
2. GLOWDISCHARGES 7
Introduction 7
FundamentalStudiesUsingGlowDischarges 9
DirectCurrentGlowDischarges 10
GlowDischargeProcesses 18
TheSputteringProcess 20
PulsedandRadioFrequencyGlowDischarges 20
AnalysisofSolutions 21
STUDYOFTHESPUTTERINGPROCESSOFMICROSAMPLES
BYGD-AESWITHWAVELENGTHMODULATION 23
Introduction 23
GlowDischargeAtomicEmissionSpectroscopy
withWavelengthModulation 24
Instrumentation 25
ResultsandDiscussions 34
MetallicElectrodes 35
StudyofMatrixInterferences 59
MatrixModifiers 60
DeterminationofGallium 74
111
4. PRINCIPLESOFLASEREXCITEDATOMICFLUORESCENCE
SPECTROSCOPY 79
AtomicFluorescence 79
BasicPrinciplesofLasers 86
PropertiesofLasers 88
Directionality 88
HighIrradiance 91
Monochromaticity 91
Coherence 92
TypesofPulsedLasers 92
TlieCopperVaporLaser 97
LaserExcitedAtomicFluorescenceSpectroscopy 100
5. STUDYOFTOESPUTTERINGPROCESSOFMICROSAMPLES
INGLOWDISCHARGEWITHLASEREXCITED
ATOMICFLUORESCENCESPECTROSCOPY 102
Introduction 102
MeasurementsofMetalsinSolutionbyGlow
DischargeLaserExcitedSpectroscopy 103
Instrumentation 104
ResultsandDiscussions Ill
StudyofMatrixInterferences 132
AtomicFluorescenceDeterminationofGallium 135
6. CONCLUSIONSANDFUTURESTUDIES 137
REFERENCES 139
BIOGRAPHICALSKETCH 147
IV
AbstractofDissertationPresentedtotheGraduateSchool
oftheUniversityofFloridainPartialFulfillmentofthe
RequirementsfortheDegreeofDoctorofPhilosophy
ATOMICEMISSIONANDLASEREXCITEDATOMICFLUORESCENCE
STUDIESOFTHEGLOWDISCHARGEFORTHEANALYSISOF
MICROSAMPLES
By
EdisonBecerraMarmolejo
August, 1992
Chairman:JamesD.Winefordner
MajorDepartment:Chemistry
Theglowdischargeasanatomizerforatomicspectroscopyoffersexcellent
advantagesthathavenotbeenfullyexploited. Inthisworkfundamentalstudies
aboutthesputteringprocessofliquidmicrosampleswasinvestigated. Thisworkwas
donebytwodifferentapproaches:AtomicEmissionSpectroscopyusingawavelength
modulationtechniqueforsimultaneousbackgroundcorrection, andLaserExcited
AtomicFluorescenceSpectroscopy. Inthefirstapproach,thesampleholderwasa
cooledcopperhollowelectrode. Thehollowdepthaswellastheatomizeroperating
parameters: current,voltage, andpressurewereoptimized inordertofollowthe
temporalbehavioroftheanalyte. Inthesecondapproach,thesampleholderwasa
hotplanarcopperelectrode. Here,theatomicexcitationprocesswasproducedby
a copper vapor pumped dye laser at a repetition rate of 6000 Hz, and the
V
fluorescencesignalwascollectedat90degreesfromthelaserpath. Intheatomic
fluorescencestudiesacomparisonofthepulsedandnon-pulsedoperatingmodesof
theglowdischargewas carriedout. Also, informationabouttheverticalatomic
distributioninthechamberareawasobtained. Calibrationcurvesandfiguresof
meritforleadandgalliumusingthe atomicemissionsystemandleadusingthe
atomicfluorescencesystemweredetermined. Theeffectofsodiumchlorideasa
matrix interferant was investigatedusing both techniques. The results ofthese
studiesusingthetwoapproachesalongwithbriefdiscussionsabouttheirtheoretical
aspectswillbepresentedinthis dissertation.
VI
CHAPTER1
HISTORICALPERSPECTIVES
Thediscoverythatthespectrumemittedbyanatomwascharacteristicofthat
atom was the starting point of a new area in analytical chemistry called
spectrochemicalanalysis. Arcandsparkspectrography,bothbasedonemissionof
radiationofexcitedatoms,werethefirsttechniqueswhichgainedwideacceptance
amongtheanalyticalcommunity(1),performingbothqualitativeandquantitative
elementaldeterminations.
Later,withtheseminalpapersofWalsh(2)andAlkemadeandMilatz(3),a
new analytical techniquebased onthe absorption ofresonance radiationby an
atomizedsamplewasshowntobeanexcellentapproachfortraceanalysisofliquid
samplesinawidevarietyofmatrices. Newadvancesin electronics,aswellasthe
introductionofelectrothermalvaporizationbyL’vov(4),placedatomicabsorption
spectrometryasthemostsensitiveprocedureforelementalanalysisduringthe1960s
and1970s.
WendtandFassel(5,6)intheUnitedStatesandGreenfieldandcoworkers
(7,8)intheUnitedKingdomdevelopedInductivelyCoupledPlasmaOpticalEmission
Spectroscopy(ICP-OES)asananalyticaltechniquefortracemetalanalysis. ICP-
OES offers the possibility of performing simultaneous or sequential elemental
1
2
determinations,withreducedmatrixinterferencesandsuperiorsensitivitiesandlimits
ofdetectionformanyelements(9)comparedwithflameAAS. Theseadvantages
madethisalternativetechniquethemethodofchoiceforelementaldeterminations
ofsamples.
Theglowdischarge(GD)isanotherexcitationsourceforatomicspectroscopy.
In 1916, Paschen studied a glow discharge configuration as a cathode discharge
(10,11). Thepopularityoftheotheranalyticaltechniquesalreadymentioned,aswell
assometechnicalproblems,especiallythesampleintroductionsystem,overshadowed
the progress and applications of the glow discharge as an atomizer. Several
importantandfundamentalstudiesandapplicationsoftheglowdischargeapproach
weredescribedintheearlyliteraturefortheanalysisofsolidsamples(12-14)andto
alesserdegree fordeterminations ofseveral elementsinpreviouslydriedliquid
solutions(15-19).
Animportantdevelopmentwhichincreasedinterestinthedevelopmentof
new analytical applications of glow discharges was initiated in 1967 with the
introductionbyGrimmofanewlowpressuredischargelampwhichwasusedeither
as anatomiccellfor elementalanalysisoras ahollowcathode lampsourcefor
atomicabsorptionspectroscopy(20). In1972,Boumanns(21)reportedanimportant
evaluationoftheGrimmtypeglowdischargeforthedeterminationofaluminum,
copper, thallium, tungsten, and zirconiumin solids using emission spectroscopy.
Sputteringratesforthoseelementsweredeterminedatvariousoperatingconditions.
Inadditiontostudiesthathavebeenfocusedintheareaofsurfaceanalysis(22-26),
3
otherpapersreportedstudiesofthecharacteristicsofthesource(27-34),spectral
interferencesandafewhavebeenorientedtotheanalysisofsolutions. Duringthe
1980sseveralimportantreviewsonglowdischargeswerewrittenbyPillow(35)and
Swift (36), Ferreira and coworkers (37), Mavrodineau (14), Caroli (10), and
Broekaert(38). Allofthemclearlyshowedthecapabilitiesoftheglowdischargeas
ananalyticaltoolforspectrochemicalanalysis.
Theatomicvaporinaglowdischargeisproducedthroughasputteringprocess
inwhichtheatomsareejectedfromthesurfacewhenhighlyenergeticatomsorions
comingfromthefillergasstrikethecathodewall. Althoughmostofthegenerated
atomsareinthegroundstate,asmallfractionareionized(39). Thisobservationled
toextendingtheuseofthisatomizerasanionsourceformassspectrometry(40).
Inthelasttwodecades,importantworkhasbeendonebyHarrisonandcoworkers
inthisarea(41-44). Aglowdischargemassspectrometricsystemespeciallydesigned
fortheanalysisofsurfacesiscommercially availablefromatleastonecompany(45).
Acathodicsputteringatomicsourceforatomicfluorescencespectroscopywas
introducedbyGouchetal.(46). Intheirwork,ahighintensityhollowcathodelamp
wasusedastheexcitationsourceforthedeterminationofnickel, chromiumand
copper. Althoughthelimitsofdetectionwereonlyoftheorderofapartpermillion,
theirworkdemonstratedthefeasibilityofextendingtheuseoftheglowdischargein
atomicspectroscopytofluorescencewherebetterselectivityisachieved.
Theadventoflasertechnologyinthe1960swithitsspecialcharacteristicslike
highspectralirradiance,tunability,andmonochromaticityopenedanewavenueof
4
research in atomic fluorescence called Laser Excited (or Enhanced) Atomic
FluorescenceSpectroscopy(LEAFS),wheretheanalyteinducedemissionintensity
depends onthelaserirradiance. Inthepast20years, anumberofpublications
reporttheuseofdifferenttypesoflasersaslinesourcesforLEAFSusingflames(47-
49),inductivelycoupledplasmas(50-53),electrothermalatomizers(54-58),andglow
discharges(59-64)asatomreservoirs.
TheuseofalowpressureglowdischargeasanatomiccellforLEAFShas
someuniquecharacteristicsthatshouldbementioned. First,thesourceproduces
sharp absorption lines. Second, quenching of fluorescence and chemical
interferencesareminimizedbythepresenceofaninertgas. Forthesereasonsthe
glowdischargebehavesasanearlyidealatomreservoirforLEAFS.
Theglowdischargecanbeoperatedinacontinuousorpulsedmode. When
operatedinthecontinuousmode,strongbackgroundemissionreducesthedetection
capabilityofthedeterminations. TheuseofapulsedglowdischargebySmithetal.
(60)inwhichthelaser(at20Hz)wastriggeredwhentheglowdischargewasoff
permittingmeasurementsofthefluorescenceinaregionwherethebackgroundnoise
was totally eliminated. Usingthis approach, limits ofdetectionofthe order of
femtogramswerereportedforleadusingaglowdischargewithahollowcathode
graphiteelectrodeandanitrogenandcoppervaporlaserpumpeddyelaser(61,65).
ResearchinLEAFSisstillactive. Manyofthesehavebeenevaluatedin
excellentreviewspublishedinthelastfewyears(47,66). Fromthesereviews,itis
evident that the possibility of performing ultra-trace determinations in various