introduction to measurements - instrumental · pdf fileintroduction to measurements skoog...
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INTRODUCTIONTOMEASUREMENTSSKOOG‘N’HOLLERCH1
CHEM314
TUESDAY,JANUARY19
DUE:UNGRADEDWRITINGASSESSMENT
LECTURETOPIC:INTRODUCTIONTOMEASUREMENTS
READING:CH1,MYTHANDREALITYOFSTANDARDADDITION
OBJECTIVES• Introducesimplifieddiagramofaninstrument:source,sample,phenomena,
detector
• Discussdatadomains
• Discussthenecessityofcalibra:on
• Comparetherela:vemeritsofcalibra:oncurve,standardaddi:on,andinternalstandardmethods
• Discusscriteriathatcanhelpinselec:onofanaly:caltechnique
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Wetchemistry
InstrumentaPon
SeparaPonàCharacterizaPonàGravimetricVolumetric
AnalysisColors
MelPngpointBoilingpointSolubilityOdor
OpPcalproperPes
DisPllaPonPrecipitaPonExtracPon
CLASSICMEASUREMENTS
AdvancesininstrumentaPonmeanswecanstudymorecomplex
phenomena
HOWAREMEASUREMENTSMADE?
Spectroscopy
Electrical
Chromatographymayalsobeappliedtoisolateaspecificanalyteinamixture.CombinewiththeuseoftheotherinstrumentsfordetecPon
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Chromatographymayalsobeappliedtoisolateaspecificanalyteinamixture.CombinewiththeuseoftheaboveinstrumentsfordetecPon
INSTRUMENTALMEASUREMENTSARESIMPLE
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NONELECTRICALDOMAIN
Whereameasurementstartsandends
Startwithphenomena:fluorescenceofquinineEndwith:quanPtyofquinine
NONELECTRICALDOMAIN
Whereameasurementstartsandends
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hbp://www.tradeget.com www.amazon.com www.comicvine.com
NONELECTRICALDOMAIN
WhereameasurementstartsandendsMeasurementscanremaincompletelyinnonelectricaldomain
nonelectrical electrical nonelectrical
NONELECTRICALDOMAINWhereameasurementstartsandendsMeasurementscanremaincompletelyinnonelectricaldomainMostmeasurementsgothroughtheelectricaldomain
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ELECTRICALDOMAIN
AnalogDigitalTime
www.delcity.net hbp://commons.wikimedia.orgwww.radioshack.com
ANALOGSIGNALSConPnuousinamplitudeandPme.
Magnitudeofvoltage,current,charge,orpower
ParPcularlysuscepPbletoelectricalnoise.
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TIMEDOMAINInformaPonstoredinPmedomain.Variablesofinterest:Frequency-cyclesofsignalperunitPmePeriod-PmebetweensuccessiveLOàHItransiPonsPulsewidth-PmebetweenLOàHIandHIàLOtransiPonsAmplitudeimportantonlyrelaPvetoHI/LOthreshold
TIMEDOMAINInformaPonstoredinPmedomain.VariablefrequencySignaliseitherhighorlowrelaPvetothearbitraryLimit(dashedline)Timerela:onshipsareinforma:onofinterest
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ELECTRICALDIGITALDATAEncodedusingatwostatesystemCanonlybeeninoneoftwostates:HIorLOOnoroff1or0Eachpieceofdataisabit(binarydigit)
DIGITALDATABothnonelectricalandelectricaldatacanbeconveyedusingnumbersAnynumericaldataisdigital
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MAKINGMEASUREMENTSCanbeconvertedbetweenseveraldomainsinthecourseofmakingameasurement.
SELECTINGANANALYTICALMETHOD
SKOOG‘N’HOLLERCH1CHEM314
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DEFININGTHEPROBLEM1. Whataccuracyisrequired?2. Howmuchsampleisavailable?3. WhatistheconcentraPonrangeoftheanalyte?4. WhatarepotenPalinterferes?5. WhatarethephysicochemicalproperPesofthesample
matrix?6. Howmanysamplesrequireanalysis?
SELECTINGANANALYTICALMETHOD
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METHODSOFCALIBRATION• Directcomparison
TitraPons• Internalstandards• ExternalcalibraPon-Preparedseparatefromthesample
Leastsquaresmethod(calibraPoncurve) StandardaddiPon
DIRECTCOMPARISON
DirectcomparisonTitraPons
hbp://minisite.proj.hkedcity.nethbp://community.asdlib.org
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INTERNALSTANDARDSCanaccountforrandomandsystemaPcerrors
• Addeduniformlytoallsamples,blanks,andstandards• Oramajorcomponentofsamplesassumedtobepresent
inconsistentamountsinallsamplesAccountsforfluctuaPonsininstrumentsandmethodsExamples:LossesduringsamplepreparaPonVariaPoninflowrateduringseparaPonsPlotraPoofsignalfromsampleandinternalstandard
INTERNALSTANDARDSPlotraPoofsignalfromsampleandinternalstandard
F=responsefactor
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INTERNALSTANDARDS-ASSUMPTIONSANDISSUES• AssumesignalraPobetweensampleandInternal
standardisconstant• Assumeinternalstandardisnotpresentin
samples(oristhesameinall)
• DifficulttoidenPfyasuitableinternalstandard-similarbutdifferenttoanalyte
• Difficulttointroducetheinternalstandardconsistentlytosamplesandstandards
EXTERNALCALIBRATION-CALIBRATIONCURVESUsedto:CalibrateinstrumentsVerifyproceduresLeastsquaresmethodAssumpPons:1. ThereisalinearrelaPonshipbetweeninstrumentresponseandanalyteconcentraPon.2.AnydeviaPonarisesfromerrorinthemeasurement
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CALIBRATIONCURVESSeekstominimizethesquareoftheresidual.
Sx=uncertaintyinxSy=uncertaintyinym=slopek=numberrepsinsamplesn=thenumberofstandards
EXTERNALCALIBRATION-ISSUES&ASSUMPTIONS• AssumethesameconcentraPongivesthesamesignal
• Signal=rawcountsvsblankcounts
• Nomatrixeffects
• AssumeuniformexperimentalcondiPons
• AssumenosystemaPcerrorsduringcalibraPon
• ProblemsiftoofewstandardsforgoodstaPsPcs
• PotenPalissueswithconcentraPonsatendsofcalcurve
• PotenPalissueswithrandomerror
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ERRORINCONCENTRATIONMEASUREMENTS
STANDARDADDITIONUsedtowhensamplematrixiscomplex
Addstandardtothesample
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STANDARDADDITION-ISSUES&ASSUMPTIONS• OnlyaddressesrotaPonalmatrixeffects
RotaPonal TranslaPonal
STANDARDADDITION-ISSUES&ASSUMPTIONS• OnlyaddressesrotaPonalmatrixeffects
TranslaPonalissuesmustbedealtwithseparately
• InherentlylessprecisethanexternalcalibraPon
involvesextrapolaPon
Buteffectsarenegligible
• BestresultsachievedifQandKarehigh
K=mulPpleforstandarddeviaPonoftheblank
Q=amountofstandardaddedrelaPvetoconcentraPoninsample
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ACASEFORREPEATEDMEASUREMENTSATASINGLESPIKETypicalSt.Addplot
Abebermethod?
RECOMMENDATIONSFOREXECUTINGST.ADD.1. DeterminelinearrangeofanalyPcalsystem2. DealwithtranslaPonalinterferencesseparately3. Usereplicatemeasurementsofonespike
concentraPon4. LettheconcentraPonofspikeaddedtobehigh.
4xtheconcentraPoninsoluPonifpossible
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YOURTASK:QUANTIFYRED#40INASODA1. Planexperiment• MakepreliminaryUV-Vismeasurementsofstockand
soda• CalculateconcentraPonofstock• DoyouneedacalibraPoncurve?• Howmanyspikesdoyouneed?• DecideonadiluPonscheme
2. Performexperiments• CompareavailableUV-Visinstruments• ArerotaPonalmatrixeffectsapparent?• EsPmateerrorassociatedwitheachresult