5/3/16 1 THROUGHPUT OF AN OPTICAL INSTRUMENT CHEM 314 OBJECTIVES Calculate the number of photons present in a single beam UV-Vis: • At the source • Entering the monochromator • Incident on the diffracJon graJng • Emerging from the monochromator • Incident on the sample • Emerging from the sample • Incident on the detector Calculate the number of electrons generated by the photons on the detector. Examine detector current to voltage converter electronics. Calculate the voltage measured from the detector.
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5/3/16
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THROUGHPUTOFANOPTICALINSTRUMENT
CHEM314
OBJECTIVES
CalculatethenumberofphotonspresentinasinglebeamUV-Vis:
• Atthesource• Enteringthemonochromator• IncidentonthediffracJongraJng• Emergingfromthemonochromator• Incidentonthesample• Emergingfromthesample• Incidentonthedetector
Calculatethenumberofelectronsgeneratedbythephotonsonthedetector.
Examinedetectorcurrenttovoltageconverterelectronics.
Calculatethevoltagemeasuredfromthedetector.
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BUILDINGASPECTROSCOPICINSTRUMENT
Components1. StableradiaJonsource
2.WavelengthisolaJon-monochromator3.Transparentsampleholder/opJcs4.Detector5.Signalprocessing
TASKS:PARTSA-C1. Measuredistances(page2)onthespectrometer.
2. CutandweightheoutputspectrumtodeterminefracDonofemissionin450-550nmrange.
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ConJnuumsourceswithconstantoutputasafuncJonofwavelengthrequiredformolecularspectroscopy
SOURCES
TUNGSTENFILAMENT
Outputspectrumat2870K
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BLACKBODYRADIATION
h^ps://docs.kde.org/stable/en/kdeedu/kstars/ai-blackbody.html
Wein’sDisplacementLaw
λmax=maxoutputλT=temp(K)L=luminosityA=surfaceareaα=constant
A-MEASURINGTHEINSTRUMENT
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B-CUTTINGANDWEIGHING
C-PHOTONSCOLLECTEDININITIALOPTICS
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OBJECTIVES
CalculatethenumberofphotonspresentinasinglebeamUV-Vis:
• Atthesource• Enteringthemonochromator• IncidentonthediffracJongraJng• Emergingfromthemonochromator• Incidentonthesample• Emergingfromthesample• Incidentonthedetector
Calculatethenumberofelectronsgeneratedbythephotonsonthedetector.
Examinedetectorcurrenttovoltageconverterelectronics.
Calculatethevoltagemeasuredfromthedetector.
TASKS:PARTSD-E1. Calculatethephotonsenteringthemonochromator
2. CalculatethenumberofphotonsexiDngthemonochromator PointSource
Collimatedlight
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COLLIMATINGMIRRORSourcemirrorfuncDonstocollectphotonsfromthelightsourceandmakesthephotonstravelparalleltooneanother
Whereinthisdiagramisthelightsourcelocated?
Whatistheradiusofcurvatureofthesourcemirror?
R=2f
PointSource
Collimatedlight
OVERANDUNDERFILLEDOPTICS
Underfilledsourcemirror
Overfilledsourcemirror
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REFLECTIONGRATINGPERFORMANCE
MONOCHROMATORPERFORMANCE
λ1>λ2
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MONOCHROMATORCALCULATIONAnechelleMegraDngthatcontains1200blazespermmwasirradiatedwithapolychromaDcbeamatanincidentangleof29°tothegraDngnormal.Calculatetheangleatwhichfirstorder500nmlightwillappear.
OBJECTIVES
CalculatethenumberofphotonspresentinasinglebeamUV-Vis:
• Atthesource• Enteringthemonochromator• IncidentonthediffracJongraJng• Emergingfromthemonochromator• Incidentonthesample• Emergingfromthesample• Incidentonthedetector
Calculatethenumberofelectronsgeneratedbythephotonsonthedetector.
Examinedetectorcurrenttovoltageconverterelectronics.
Calculatethevoltagemeasuredfromthedetector.
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TASKS:PARTSG-F1. Determinefluxatdetectorwithablankandsample
2. CalculatethecurrentatthePMTanodewithablankandsample
3. Examinedetectorelectronics
OPTICSANDSAMPLEHOLDERS
01234
190 490 790 1090
Absorban
ce
Wavelength(nm)
Quartz PlasJc
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REFRACTIVEINDEX-OFGLASS
REFRACTIVEINDEX-OFWATER
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SAMPLE-CUVETTEINTERACTIONCalculatethepercentageofthebeamlosttoreflecDonat500nmwhenitpassesthroughthecuveMe.
Glass
Glass
AnalyteSoluJonA=0.4at500nm
AirAir
1. SoluJonfactors2. Non-monochromaJclight3. Notanalyzingatλmax4. Straylight5. Mismatchedcuve^es6. Instrumentnoise
Toomuchortooli^leabsorpJon
SOURCESOFNONLINEARITYOFBEER’SLAW
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DETECTORRESPONSE
PHOTOMULTIPLIERTUBE(PMT)
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UV-VISELECTRONICCIRCUIT
CURRENTTOVOLTAGECONVERTER
Measurevoltage
OperaJonalamplifier
• Lowresistancetosourcecurrent
• Preventsloadingerrorduringcurrentmeasurement• HighamplificaDonofsignal
OPERATIONALAMPLIFIERManyresistorsandtransistorsonasinglechip
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OPERATIONALAMPLIFIERS
OriginallyappliedtoanalogcomputerstodosimplemathfuncDons.
• Largeopenloopgains(104-106)-largeamplificaDon.
• Highinputimpedance(108-1015Ω)-doesn’tmesswithcurrentfromPMT.
• Lowoutputimpedance(0.001-1Ω)-doesn’tmesswithreadingvoltage.
• Verylowdarkcurrent-whichcanbeeasilyoffset.
CURRENTTOVOLTAGECONVERTER
Currenttovoltageconverter
Currentshunttoground
Voltagereadout
Darkcurrentadjustment
Capacitorstabilizesoutputvoltage
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