Interaction of radiation & matterInteraction of radiation & matter
Electromagnetic Electromagnetic radiation in different radiation in different regions of spectrum regions of spectrum can be used for can be used for qualitative and qualitative and quantitative quantitative informationinformation
Different types of Different types of chemical informationchemical information
Energy transfer from photon to molecule or atomEnergy transfer from photon to molecule or atom
At room temperature most molecules At room temperature most molecules are at lowest electronic & vibrational are at lowest electronic & vibrational statestate
IR radiation can excite vibrational levels that IR radiation can excite vibrational levels that then lose energy quickly in collisions with then lose energy quickly in collisions with surroundingssurroundings
UV Visible SpectrometryUV Visible Spectrometry
absorption - specific energyabsorption - specific energyemission - excited molecule emission - excited molecule
emitsemitsfluorescence fluorescence phosphorescencephosphorescence
What happens to molecule after excitationWhat happens to molecule after excitation
collisions deactivate vibrational collisions deactivate vibrational levels (heat)levels (heat)
emission of photon emission of photon (fluorescence)(fluorescence)
intersystem crossover intersystem crossover (phosphorescence)(phosphorescence)
General optical spectrometerGeneral optical spectrometer
Wavelength Wavelength separationseparation
Photodetectors Photodetectors
Light source - hot Light source - hot objects produce “black objects produce “black body radiationbody radiation
Black body radiationBlack body radiation
Tungsten lamp, Globar, Nernst glowerTungsten lamp, Globar, Nernst glower Intensity and peak emission wavelength are Intensity and peak emission wavelength are
a function of Temperaturea function of Temperature As T increases the total intensity increases As T increases the total intensity increases
and there is shift to higher energies (toward and there is shift to higher energies (toward visible and UV)visible and UV)
Temp(K)
max.int.
Rel. int
1000 3000 nm 0.0003
2000 1600 nm 0.01
3000 1100 nm 0.1
4000 700 nm 0.4
UV sources UV sources
Arc discharge lamps with electrical discharge Arc discharge lamps with electrical discharge maintained in appropriate gasesmaintained in appropriate gases
Low pressure hydrogen and deuterium lampsLow pressure hydrogen and deuterium lamps Lasers - narrow spectral widths, very high Lasers - narrow spectral widths, very high
intensity, spatial beam, time resolution, intensity, spatial beam, time resolution, problem with range of wavelengthsproblem with range of wavelengths
Discrete spectroscopic- metal vapor & hollow Discrete spectroscopic- metal vapor & hollow cathode lampscathode lamps
Why separate wavelengths?Why separate wavelengths?
Each compound absorbs different Each compound absorbs different colors (energies) with different colors (energies) with different probabilities (absorbtivity)probabilities (absorbtivity)
SelectivitySelectivityQuantitative adherence to Beer’s Law Quantitative adherence to Beer’s Law
A = abc A = abcImproves sensitivityImproves sensitivity
Why are UV-Vis bands broad?Why are UV-Vis bands broad?
Electronic energy states give band Electronic energy states give band with no vibrational structurewith no vibrational structure
Solvent interactions Solvent interactions (microenvironments) averaged(microenvironments) averaged
Low temperature gas phase molecules Low temperature gas phase molecules give structure if instrumental give structure if instrumental resolution is adequateresolution is adequate
Wavelength DispersionWavelength Dispersion
prisms (nonlinear, range prisms (nonlinear, range depends on refractive index)depends on refractive index)
gratings (linear, Bragg’s gratings (linear, Bragg’s Law, depends on spacing of Law, depends on spacing of scratches, overlapping scratches, overlapping orders interfere)orders interfere)
interference filters interference filters (inexpensive)(inexpensive)
MonochromatorMonochromatorEntrance slit - provides narrow optical Entrance slit - provides narrow optical
imageimageCollimator - makes light hit dispersive Collimator - makes light hit dispersive
element at same angleelement at same angleDispersing element - directionalDispersing element - directionalFocusing element - image on slitFocusing element - image on slitExit slit - isolates desired color to exitExit slit - isolates desired color to exit
ResolutionResolution
The ability to distinguish different wavelengths The ability to distinguish different wavelengths of light - R=of light - R=
Linear dispersion - range of wavelengths spread Linear dispersion - range of wavelengths spread over unit distance at exit slitover unit distance at exit slit
Spectral bandwidth - range of wavelengths Spectral bandwidth - range of wavelengths included in output of exit slit (FWHM)included in output of exit slit (FWHM)
Resolution depends on how widely light is Resolution depends on how widely light is dispersed & how narrow a slice chosendispersed & how narrow a slice chosen
Filters - inexpensive alternativeFilters - inexpensive alternative
Adsorption type - glass with dyes to Adsorption type - glass with dyes to adsorb chosen colorsadsorb chosen colors
Interference filters - multiple reflections Interference filters - multiple reflections between 2 parallel reflective surfaces - between 2 parallel reflective surfaces - only certain wavelengths have positive only certain wavelengths have positive interferences - temperature effects interferences - temperature effects spacing between surfacesspacing between surfaces
Wavelength dependence in spectrometerWavelength dependence in spectrometer
Source Source MonochromatorMonochromatorDetectorDetectorSample - We hope so!Sample - We hope so!
Photodetectors - photoelectric effect E(e)=hPhotodetectors - photoelectric effect E(e)=h
For sensitive detector we need a small For sensitive detector we need a small work function - alkali metals are bestwork function - alkali metals are best
Phototube - electrons attracted to anode Phototube - electrons attracted to anode giving a current flow proportional to giving a current flow proportional to light intensitylight intensity
Photomultiplier - amplification to Photomultiplier - amplification to improve sensitivity (10 millionimprove sensitivity (10 million))
Spectral sensitivity is a function of photocathode materialSpectral sensitivity is a function of photocathode material
Ag-O-Cs mixture Ag-O-Cs mixture gives broader range gives broader range but less efficiencybut less efficiency
Na2KSb(trace of Na2KSb(trace of Cs)has better response Cs)has better response over narrow rangeover narrow range
Max. response is 10% Max. response is 10% of one per photon of one per photon (quantum efficiency)(quantum efficiency)
300nm 500 700 900300nm 500 700 900
Na2KSbNa2KSb
AgOCsAgOCs
Photomultiplier - dynodes of CuO.BeO.Cs or GaP.CsPhotomultiplier - dynodes of CuO.BeO.Cs or GaP.Cs
Cooled PhotomultiplierCooled PhotomultiplierTubeTube
Dynode arrayDynode array
Photodiodes - semiconductor that conducts in one direction only when light is present
Photodiodes - semiconductor that conducts in one direction only when light is present
Rugged and smallRugged and smallPhotodiode arrays - allows Photodiode arrays - allows
observation of a number of observation of a number of different locations (wavelengths) different locations (wavelengths) simultaneouslysimultaneously
Somewhat less sensitive than PMTSomewhat less sensitive than PMT
T=I/IoA= - log T = -log (I/Io)
Calibration curve
T=I/IoA= - log T = -log (I/Io)
Calibration curve
One million photons impinge ona sample in a UV-visspectrometer and800,000 of the photons passthrough to the detector, theremaining photonshaving been absorbed.
How many photons will passthrough the sample ifthe concentration is doubled?
Beer’s Law •• A=A=abcabc•• A=A=absorbanceabsorbance
•• a=a=absorbtivityabsorbtivity(depends on species(depends on speciesand wavelength)and wavelength)
•• b=b=pathlength pathlength ininsamplesample
•• c=concentration ofc=concentration ofabsorbing speciesabsorbing species
One million photons impinge ona sample in a UV-visspectrometer and800,000 of the photons passthrough to the detector, theremaining photonshaving been absorbed.
How many photons will passthrough the sample ifthe concentration is doubled?
Beer’s Law •• A=A=abcabc•• A=A=absorbanceabsorbance
•• a=a=absorbtivityabsorbtivity(depends on species(depends on speciesand wavelength)and wavelength)
•• b=b=pathlength pathlength ininsamplesample
•• c=concentration ofc=concentration ofabsorbing speciesabsorbing species
Deviations from Beer’s LawDeviations from Beer’s Law
High concentrations (0.01M) distort High concentrations (0.01M) distort each molecules electronic structure & each molecules electronic structure & spectraspectra
Chemical equilibriumChemical equilibriumStray lightStray lightPolychromatic lightPolychromatic light InterferencesInterferences
Interpretation - quantitativeInterpretation - quantitative
Broad adsorption bands - considerable Broad adsorption bands - considerable overlapoverlap
Specral dependence upon solventsSpecral dependence upon solventsResolving mixtures as linear Resolving mixtures as linear
combinations - need to measure as combinations - need to measure as many wavelengths as componentsmany wavelengths as components
Beer’s Law .htmlBeer’s Law .html
Resolving mixturesResolving mixtures
Measure at different wavelengths and solve Measure at different wavelengths and solve mathematicallymathematically
Use standard additions (measure A and then Use standard additions (measure A and then add known amounts of standard)add known amounts of standard)
Chemical methods to separate or shift Chemical methods to separate or shift spectrumspectrum
Use time resolution (fluorescence and Use time resolution (fluorescence and phosphorescence)phosphorescence)
Improving resolution in mixturesImproving resolution in mixtures
Instrumental (resolution)Instrumental (resolution)Mathematical (derivatives)Mathematical (derivatives)Use second parameter (fluorescence)Use second parameter (fluorescence)Use third parameter (time for Use third parameter (time for
phosphorescence)phosphorescence)Chemical separations (chromatography)Chemical separations (chromatography)
Fluorescence Fluorescence
Emission at lower energy than Emission at lower energy than absorptionabsorption
Greater selectivity but fluorescent yields Greater selectivity but fluorescent yields vary for different moleculesvary for different molecules
Detection at right angles to excitationDetection at right angles to excitation S/N is improved so sensitivity is betterS/N is improved so sensitivity is better Fluorescent tagsFluorescent tags
SpectrofluorometerSpectrofluorometer
Light sourceLight source
Monochromator to select excitationMonochromator to select excitation
Sample compartmentSample compartment
Monochromator toMonochromator toselect fluorescenceselect fluorescence
Photoacoustic spectroscopyPhotoacoustic spectroscopy
Edison’s observationsEdison’s observationsIf light is pulsed then as gas is If light is pulsed then as gas is
excited it can expand (sound)excited it can expand (sound)
Principles of IRPrinciples of IR
Absorption of energy at various frequencies Absorption of energy at various frequencies is detected by IRis detected by IR
plots the amount of radiation transmitted plots the amount of radiation transmitted through the sample as a function of through the sample as a function of frequencyfrequency
compounds have “fingerprint” region of compounds have “fingerprint” region of identity identity
Infrared SpectrometryInfrared Spectrometry
Is especially useful for qualitative analysisIs especially useful for qualitative analysis functional groupsfunctional groups other structural featuresother structural features establishing purity establishing purity monitoring ratesmonitoring rates measuring concentrationsmeasuring concentrations theoretical studiestheoretical studies
How does it work?How does it work?
Continuous beam of radiationContinuous beam of radiation Frequencies display different absorbancesFrequencies display different absorbances Beam comes to focus at entrance slitBeam comes to focus at entrance slit molecule absorbs radiation of the energy to molecule absorbs radiation of the energy to
excite it to the vibrational stateexcite it to the vibrational state
How Does It Work?How Does It Work?
Monochromator disperses radiation into Monochromator disperses radiation into spectrumspectrum
one frequency appears at exit slitone frequency appears at exit slit radiation passed to detectorradiation passed to detector detector converts energy to signaldetector converts energy to signal signal amplified and recordedsignal amplified and recorded
Instrumentation IIInstrumentation II
Optical-null double-beam instrumentsOptical-null double-beam instruments Radiation is directed through both cells by Radiation is directed through both cells by
mirrorsmirrors sample beam and reference beamsample beam and reference beam chopperchopper diffraction gratingdiffraction grating
Double beam/ null detectionDouble beam/ null detection
Instrumentation IIIInstrumentation III
Exit slitExit slit detectordetector servo motorservo motor Resulting spectrum is a plot of the intensity Resulting spectrum is a plot of the intensity
of the transmitted radiation versus the of the transmitted radiation versus the wavelengthwavelength
Detection of IR radiationDetection of IR radiation
Insufficient energy to excite electrons Insufficient energy to excite electrons & hence photodetectors won’t work& hence photodetectors won’t work
Sense heat - not very sensitive and must Sense heat - not very sensitive and must be protected from sources of heatbe protected from sources of heat
Thermocouple - dissimilar metals Thermocouple - dissimilar metals characterized by voltage across gap characterized by voltage across gap proportional to temperatureproportional to temperature
IR detectorsIR detectors
Golay detector - gas expanded by heat Golay detector - gas expanded by heat causes flexible mirror to move - measure causes flexible mirror to move - measure photocurrent of visible light sourcephotocurrent of visible light source
DetectorDetector
IR beamIR beam VisVissourcesource
GASGAS
Flexible mirrorFlexible mirror
Carbon analyzer - simple IRCarbon analyzer - simple IR
Sample flushed of carbon Sample flushed of carbon dioxide (inorganic)dioxide (inorganic)
Organic carbon oxidized by Organic carbon oxidized by persulfate & UVpersulfate & UV
Carbon dioxide measured in gas Carbon dioxide measured in gas cell (water interferences)cell (water interferences)
IR SourceIR Source IR SourceIR Source
ChopperChopper
SAMPSAMP
REFREF
Detector cellDetector cellFilterFilter
CO2CO2 CO2CO2Beam trimmerBeam trimmerPress. sens. det.Press. sens. det.
NDIR detector - no monochromatorNDIR detector - no monochromator
LimitationsLimitations
Mechanical couplingMechanical coupling
Slow scanning / detectors slowSlow scanning / detectors slow
Limitations of Dispersive IRLimitations of Dispersive IR
Mechanically complexMechanically complex Sensitivity limitedSensitivity limitedRequires external Requires external
calibrationcalibrationTracking errors limit Tracking errors limit
resolution (scanning fast resolution (scanning fast broadens peak, decreases broadens peak, decreases absorbance, shifts peakabsorbance, shifts peak
Problems with IRProblems with IR
c no quantitativec no quantitative H limited resolutionH limited resolution D not reproducibleD not reproducible A limited dynamic rangeA limited dynamic range I limited sensitivityI limited sensitivity E long analysis timeE long analysis time B functional groupsB functional groups
LimitationsLimitations
Most equipment can Most equipment can measure one measure one wavelength at a timewavelength at a time
Potentially time-Potentially time-consumingconsuming
A solution?A solution?
Fourier-Transform Infrared Spectroscopy (FTIR)Fourier-Transform Infrared Spectroscopy (FTIR)
A Solution!A Solution!
FTIRFTIR
Analyze all wavelengths simultaneouslyAnalyze all wavelengths simultaneously signal decoded to generate complete signal decoded to generate complete
spectrumspectrum can be done quicklycan be done quickly better resolutionbetter resolution more resolutionmore resolution However, . . .However, . . .
FTIR FTIR
A solution, yet an A solution, yet an expensive one!expensive one!
FTIR uses FTIR uses sophisticated sophisticated machinery more machinery more complex than generic complex than generic GCIRGCIR
Fourier Transform IRFourier Transform IRMechanically Mechanically
simplesimple Fast, sensitive, Fast, sensitive,
accurateaccurate Internal calibrationInternal calibrationNo tracking errors No tracking errors
or stray lightor stray light
IR Spectroscopy - qualitativeIR Spectroscopy - qualitative
Double beam required to correct Double beam required to correct for blank at each wavelengthfor blank at each wavelength
Scan time (sensitivity) Vs Scan time (sensitivity) Vs resolution resolution
Michelson interferometer & Michelson interferometer & FTIRFTIR
Advantages of FTIRAdvantages of FTIR
Multiplex--speed, sensitivity (Felgett)Multiplex--speed, sensitivity (Felgett)Throughput--greater energy, S/N (Jacquinot)Throughput--greater energy, S/N (Jacquinot)Laser reference--accurate wavelength, Laser reference--accurate wavelength,
reproducible (Connes)reproducible (Connes)No stray light--quantitative accuracyNo stray light--quantitative accuracyNo tracking errors--wavelength and No tracking errors--wavelength and
photometric accuracyphotometric accuracy
New FTIR ApplicationsNew FTIR Applications
Quality control--speed, accuracyQuality control--speed, accuracyMicro, trace analysis--nanogram Micro, trace analysis--nanogram
levels, small sampleslevels, small samplesKinetic studies--millisecondsKinetic studies--millisecondsInternal reflectionInternal reflectionTelescopicTelescopic
Attenuated Internal ReflectionAttenuated Internal Reflection
Surface analysisSurface analysis Limited by 75% Limited by 75%
energy lossenergy loss
New FTIR ApplicationsNew FTIR Applications
Quality control--speed, accuracyQuality control--speed, accuracyMicro, trace analysis--nanogram Micro, trace analysis--nanogram
levels, small sampleslevels, small samplesKinetic studies--millisecondsKinetic studies--millisecondsInternal reflectionInternal reflectionTelescopicTelescopic
