InfraredSpectroscopyTransmissionVsATR红外光谱透射比.pptx

OverviewIR spectroscopy theoryTransmission vs.Attenuated Total Reflectance(ATR)techniquesBased on study by:Based on study by:Koulis,Cynthia,et.al.Comparison of Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Transmission and Internal Reflection Infrared Spectra of Cocaine.Journal of Forensic Spectra of Cocaine.Journal of Forensic Sciences,2001.Sciences,2001.What is Spectroscopy?SpectroscopySpectroscopy-study of the interaction of atoms -study of the interaction of atoms and molecules with electromagnetic radiationand molecules with electromagnetic radiationProbe the world at an atomic and molecular Probe the world at an atomic and molecular levellevelqualitative analysisqualitative analysis Bond lengthsBond lengths Bond Force ConstantsBond Force Constants Molecular structuresMolecular structuresquantitative analysis quantitative analysis Quantum StatesAbsorption of electromagnetic radiation Absorption of electromagnetic radiation excitation to higher energy statesexcitation to higher energy stateshv=|Ehv=|E2 2 E E1 1|Electronic Vibrational RotationalElectronic Vibrational RotationalInfrared RadiationFrequencies-12800 to 10 cmFrequencies-12800 to 10 cm-1 -1 (cm(cm-1-1=wavenumbers)=wavenumbers)Divided into three Regions Divided into three Regions Near:12800 to 4000 cmNear:12800 to 4000 cm-1-1 Middle:4000 to 200 cmMiddle:4000 to 200 cm-1-1 Far:200 to 10 cmFar:200 to 10 cm-1-1Radiation of most use for spectroscopy falls in the mid-infrared region between 4000 to 670 cm-150Infrared AbsorptionAbsorption of IR radiation Absorption of IR radiation excitation of excitation of vibrational and rotational energy levelsvibrational and rotational energy levelsnot sufficient energy for electronic excitationnot sufficient energy for electronic excitationVibrational energy states are of greatest Vibrational energy states are of greatest importance for IR spectroscopyimportance for IR spectroscopyDynamic Dipole MomentAbsorption of IR radiation is only observed in Absorption of IR radiation is only observed in molecules with a dynamic dipole momentmolecules with a dynamic dipole momentAn induced dynamic dipole moment is An induced dynamic dipole moment is observed in polar moleculesobserved in polar molecules Magnitude of dipole moment depends on overlap of Magnitude of dipole moment depends on overlap of electron densities in moleculeelectron densities in molecule Vibration results in change in internuclear distance Vibration results in change in internuclear distance and therefore fluctuations in magnitude of dipole and therefore fluctuations in magnitude of dipole Homonuclear diatomic molecules(e.g.HHomonuclear diatomic molecules(e.g.H2 2,N,N2 2,OO2 2)dont absorb IR radiation,b/c of no dynamic)dont absorb IR radiation,b/c of no dynamic dipole moment dipole moment IR absorptionIF vibrational frequency of molecule=frequency IF vibrational frequency of molecule=frequency of radiation THEN energy is absorbedof radiation THEN energy is absorbed Results promotion to higher energy level and increase Results promotion to higher energy level and increase in amplitude of molecular vibrationin amplitude of molecular vibrationMolecular VibrationsMolecular Vibrational and Bending modesSingle absorption peak appears for each Single absorption peak appears for each mode with a dynamic dipole momentmode with a dynamic dipole momentInterpreting the SpectraPeaks are identified by:-Intensity-Shape-PositionCorrelation charts are used to aid in identifying functional group absorption bandsDetermining peak positionUsing the model of a simple harmonic oscillator the frequency vibration can be predicted Wavenumber=(2c)-1(k/)Reduced mass()Force constant(k)C-C C=C CC Qualitative AnalysisEnergy spectrum for each Energy spectrum for each chemical is uniquechemical is unique“Fingerprint”“Fingerprint”Functional groups can be Functional groups can be identified identified Uncertainty lies in Uncertainty lies in overlapping of group overlapping of group frequenciesfrequenciesCatalogs of IR spectra of Catalogs of IR spectra of known compounds used known compounds used for final identificationfor final identificationMS and NMR for further MS and NMR for further confirmationconfirmationInstrumental Setup:Transmission TechniqueIncident radiation is passed through sampleIncident radiation is passed through sampleRadiation of the right frequency will be absorbed Radiation of the right frequency will be absorbed while the rest is transmittedwhile the rest is transmittedDetector placed directly behind sample measures Detector placed directly behind sample measures the resulting radiationthe resulting radiationAbsorbance=-log TransmittanceAbsorbance=-log TransmittanceInstrumental Setup:Attenuated Total Reflectance(ATR)TechniqueIR radiation passes through IR radiation passes through(IRE-internal reflection(IRE-internal reflection element)crystal and hits element)crystal and hits sample at a 45 degree anglesample at a 45 degree angle IRE made of high refractive IRE made of high refractive index material(zinc selenide,index material(zinc selenide,diamond,germaniumdiamond,germaniumIncident radiation penetrates Incident radiation penetrates into sample(1 micrometer)into sample(1 micrometer)where it can be absorbedwhere it can be absorbedAttenuated radiation is Attenuated radiation is reflectedreflected Spectra ComparisonResulting peaks from ATR are very similar in Resulting peaks from ATR are very similar in intensity and wavelength to transmittance intensity and wavelength to transmittance techniquetechniqueKoulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine.Journal of Forensic Sciences,2001.Infrared Spectra of Cocaine.Journal of Forensic Sciences,2001.ATR Peak ShiftSmall variations in peak intensity and position occur:Small variations in peak intensity and position occur:Carbonyl band Absorption of cocaine shows ATR peak Carbonyl band Absorption of cocaine shows ATR peak shiftshiftKoulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Spectra of Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine.Journal of Forensic Sciences,2001.Cocaine.Journal of Forensic Sciences,2001.Why the Shift?Amount of reflected radiation is dependent of the index of refraction Amount of reflected radiation is dependent of the index of refraction of the crystal and sampleof the crystal and sampleAnomalous dispersion occurs at wavelengths of absorption results Anomalous dispersion occurs at wavelengths of absorption results in:in:Change in refractive index of sampleChange in refractive index of sample Change in penetration depth of radiation into sampleChange in penetration depth of radiation into samplePeak shifts are optical effects due to changes in refractive index Peak shifts are optical effects due to changes in refractive index Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine.Journal of Forensic Sciences,2001.Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine.Journal of Forensic Sciences,2001.Figure:Penetration depth and refractive index for cocaine at carbonyl absorption bandSample preparation MethodsTransmissionSolids:KBr PelletSolids:KBr PelletLiquids:NaCl PlatesLiquids:NaCl Plates Quick press KBr pellet pressTransmission vs.ATRTransmission:AdvantagesHigh quality spectraSatisfactory for qualitative analysisWide variety of spectra libraries availableTransmission vs.ATRTransmission:Transmission:DisadvantagesDisadvantagesSolid(KBr pellet)Solid(KBr pellet)Time consumingTime consuming Particle size radiation wavelength to avoid Particle size radiation wavelength to avoid scattering scattering Spectra dependent on sample thicknessSpectra dependent on sample thicknessLiquid(NaCl Plates)Liquid(NaCl Plates)Water in samples causes plates to fogWater in samples causes plates to fogSpectra not particularly reproducibleSpectra not particularly reproducibleSample cant be recovered after analysisSample cant be recovered after analysisSample preparation Methods ATRLiquids and solids Liquids and solids loaded directly onto loaded directly onto crystalcrystalArm Applies pressure Arm Applies pressure to solids for uniform to solids for uniform contact with crystalcontact with crystal PSI can be controlledPSI can be controlledTransmission vs.ATRATRAdvantagesAdvantagesHigh Quality Spectrum for qualitative analysisHigh Quality Spectrum for qualitative analysisMinimal sample preparationMinimal sample preparation Non destructiveNon destructive Time efficientTime efficientSpectra not affected by sample thicknessSpectra not affected by sample thickness Radiation penetrates only a few micrometersRadiation penetrates only a few micrometersHighly reproducible resultsHighly reproducible resultsWide variety of sample typesWide variety of sample types Threads,yarns,fabrics,fibers,pastes,powders,suspensions,Threads,yarns,fabrics,fibers,pastes,powders,suspensions,polymers,rubberspolymers,rubbers Transmission vs.ATRATR:DisadvantagesNew techniqueLess spectra catalogs availableLess spectra catalogs availableSpectral artifactsPeak shift and intensity differences Peak shift and intensity differences ATR forensic applicationsDrug analysisFiber analysisPaint chip analysisInk analysisPaper analysisBiological analysisIn ConclusionThere is an increasing trend toward the use of There is an increasing trend toward the use of ATR:ATR:High Quality SpectrumHigh Quality Spectrum Ease of sample preparationEase of sample preparation Wide variety of sample typesWide variety of sample typesMust be noted that ATR spectra and transmission spectra are differentVariations in peak position and intensity must be understoodNew libraries of spectra for ATR are needed.ReferencesKoulis,Cynthia,et.al.Comparison of Transmission and Internal Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine.Journal of Forensic Sciences,Reflection Infrared Spectra of Cocaine.Journal of Forensic Sciences,2001.2001.Skoog,Holler,and Nieman.Principles of Instrumental Analysis.5th Skoog,Holler,and Nieman.Principles of Instrumental Analysis.5th edition,1998edition,1998Engel and Reid.Physical Chemistry.Pearson Education,2006.Engel and Reid.Physical Chemistry.Pearson Education,2006.http:/wwwchem.csustan.edu/Tutorials/INFRARED.HTMhttp:/wwwchem.csustan.edu/Tutorials/INFRARED.HTMhttp:/ UniversityLewis UniversityDr.Salim M.DiabDr.Salim M.DiabAcknowledgmentTHE ENDQuestions?