Infrared Spectroscopy Transmission Vs ATR红外光谱透射比ATR.ppt
Infrared Spectroscopy:Comparison of Transmission and ATR Techniques,Matt HerringLewis University,Overview,IR spectroscopy theoryTransmission vs.Attenuated Total Reflectance(ATR)techniquesBased on study by:Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine.Journal of Forensic Sciences,2001.,What is Spectroscopy?,Spectroscopy-study of the interaction of atoms and molecules with electromagnetic radiationProbe the world at an atomic and molecular levelqualitative analysisBond lengthsBond Force ConstantsMolecular structuresquantitative analysis,Quantum States,Absorption of electromagnetic radiation excitation to higher energy stateshv=|E2 E1|Electronic Vibrational Rotational,Infrared Radiation,Frequencies-12800 to 10 cm-1(cm-1=wavenumbers)Divided into three Regions Near:12800 to 4000 cm-1Middle:4000 to 200 cm-1Far:200 to 10 cm-1,Radiation of most use for spectroscopy falls in the mid-infrared region between 4000 to 670 cm-1,50,Infrared Absorption,Absorption of IR radiation excitation of vibrational and rotational energy levelsnot sufficient energy for electronic excitationVibrational energy states are of greatest importance for IR spectroscopy,Dynamic Dipole Moment,Absorption of IR radiation is only observed in molecules with a dynamic dipole momentAn induced dynamic dipole moment is observed in polar moleculesMagnitude of dipole moment depends on overlap of electron densities in moleculeVibration results in change in internuclear distance and therefore fluctuations in magnitude of dipole Homonuclear diatomic molecules(e.g.H2,N2,O2)dont absorb IR radiation,b/c of no dynamic dipole moment,IR absorption,IF vibrational frequency of molecule=frequency of radiation THEN energy is absorbedResults promotion to higher energy level and increase in amplitude of molecular vibration,Molecular Vibrations,Molecular Vibrational and Bending modesSingle absorption peak appears for each mode with a dynamic dipole moment,Interpreting the Spectra,Peaks are identified by:-Intensity-Shape-Position,Correlation charts are used to aid in identifying functional group absorption bands,Determining peak position,Using 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 Analysis,Energy spectrum for each chemical is unique“Fingerprint”Functional groups can be identified Uncertainty lies in overlapping of group frequenciesCatalogs of IR spectra of known compounds used for final identificationMS and NMR for further confirmation,Instrumental Setup:Transmission Technique,Incident radiation is passed through sampleRadiation of the right frequency will be absorbed while the rest is transmittedDetector placed directly behind sample measures the resulting radiationAbsorbance=-log Transmittance,Instrumental Setup:Attenuated Total Reflectance(ATR)Technique,IR radiation passes through(IRE-internal reflection element)crystal and hits sample at a 45 degree angleIRE made of high refractive index material(zinc selenide,diamond,germaniumIncident radiation penetrates into sample(1 micrometer)where it can be absorbedAttenuated radiation is reflected,Spectra Comparison,Resulting peaks from ATR are very similar in intensity and wavelength to transmittance technique,Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine.Journal of Forensic Sciences,2001.,ATR Peak Shift,Small variations in peak intensity and position occur:Carbonyl band Absorption of cocaine shows ATR peak shift,Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine.Journal of Forensic Sciences,2001.,Why the Shift?,Amount of reflected radiation is dependent of the index of refraction of the crystal and sampleAnomalous dispersion occurs at wavelengths of absorption results in:Change in refractive index of sampleChange in penetration depth of radiation into samplePeak 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.,Figure:Penetration depth and refractive index for cocaine at carbonyl absorption band,Sample preparation Methods,TransmissionSolids:KBr PelletLiquids:NaCl Plates,Quick press KBr pellet press,Transmission vs.ATR,Transmission:AdvantagesHigh quality spectraSatisfactory for qualitative analysisWide variety of spectra libraries available,Transmission vs.ATR,Transmission:DisadvantagesSolid(KBr pellet)Time consumingParticle size radiation wavelength to avoid scattering Spectra dependent on sample thicknessLiquid(NaCl Plates)Water in samples causes plates to fogSpectra not particularly reproducibleSample cant be recovered after analysis,Sample preparation Methods,ATRLiquids and solids loaded directly onto crystalArm Applies pressure to solids for uniform contact with crystalPSI can be controlled,Transmission vs.ATR,ATRAdvantagesHigh Quality Spectrum for qualitative analysisMinimal sample preparationNon destructiveTime efficientSpectra not affected by sample thicknessRadiation penetrates only a few micrometersHighly reproducible resultsWide variety of sample typesThreads,yarns,fabrics,fibers,pastes,powders,suspensions,polymers,rubbers,Transmission vs.ATR,ATR:DisadvantagesNew techniqueLess spectra catalogs availableSpectral artifactsPeak shift and intensity differences,ATR forensic applications,Drug analysisFiber analysisPaint chip analysisInk analysisPaper analysisBiological analysis,In Conclusion,There is an increasing trend toward the use of ATR:High Quality SpectrumEase of sample preparationWide 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.,References,Koulis,Cynthia,et.al.Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine.Journal of Forensic Sciences,2001.Skoog,Holler,and Nieman.Principles of Instrumental Analysis.5th edition,1998Engel and Reid.Physical Chemistry.Pearson Education,2006.http:/wwwchem.csustan.edu/Tutorials/INFRARED.HTMhttp:/UniversityDr.Salim M.Diab,Acknowledgment,THE ENDQuestions?,
