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1、Infrared Spectroscopy: Comparison of Transmission and ATR Techniques,Matt Herring Lewis University,Overview,IR spectroscopy theory Transmission vs. Attenuated Total Reflectance (ATR) techniques Based on study by: Koulis, Cynthia, et. al. Comparison of Transmission and Internal Reflection Infrared Sp
2、ectra of Cocaine. Journal of Forensic Sciences, 2001.,What is Spectroscopy?,Spectroscopy - study of the interaction of atoms and molecules with electromagnetic radiation Probe the world at an atomic and molecular level qualitative analysis Bond lengths Bond Force Constants Molecular structures quant
3、itative analysis,Quantum States,Absorption of electromagnetic radiation excitation to higher energy states hv = |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-1 Middle: 4000 to 200 cm
4、-1 Far: 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 levels not sufficient energy for electronic excitation Vibrational energy states a
5、re of greatest importance for IR spectroscopy,Dynamic Dipole Moment,Absorption of IR radiation is only observed in molecules with a dynamic dipole moment An induced dynamic dipole moment is observed in polar molecules Magnitude of dipole moment depends on overlap of electron densities in molecule Vi
6、bration 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 a
7、bsorbed Results promotion to higher energy level and increase in amplitude of molecular vibration,Molecular Vibrations,Molecular Vibrational and Bending modes Single absorption peak appears for each mode with a dynamic dipole moment,Interpreting the Spectra,Peaks are identified by: -Intensity -Shape
8、 -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
9、,Energy spectrum for each chemical is unique “Fingerprint” Functional groups can be identified Uncertainty lies in overlapping of group frequencies Catalogs of IR spectra of known compounds used for final identification MS and NMR for further confirmation,Instrumental Setup: Transmission Technique,I
10、ncident radiation is passed through sample Radiation of the right frequency will be absorbed while the rest is transmitted Detector placed directly behind sample measures the resulting radiation Absorbance = -log Transmittance,Instrumental Setup: Attenuated Total Reflectance (ATR) Technique,IR radia
11、tion passes through (IRE-internal reflection element) crystal and hits sample at a 45 degree angle IRE made of high refractive index material (zinc selenide, diamond, germanium Incident radiation penetrates into sample (1 micrometer) where it can be absorbed Attenuated radiation is reflected,Spectra
12、 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
13、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 refracti
14、on of the crystal and sample Anomalous dispersion occurs at wavelengths of absorption results in: Change in refractive index of sample Change in penetration depth of radiation into sample Peak shifts are optical effects due to changes in refractive index,Koulis, Cynthia, et. al. Comparison of Transm
15、ission 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,Transmission Solids: KBr Pellet Liquids: NaCl Plates,Quick press KBr pellet press,Transmis
16、sion vs. ATR,Transmission: Advantages High quality spectra Satisfactory for qualitative analysis Wide variety of spectra libraries available,Transmission vs. ATR,Transmission: Disadvantages Solid (KBr pellet) Time consuming Particle size radiation wavelength to avoid scattering Spectra dependent on sample thickness Liquid (NaCl Plates) Water in samples causes plates to fog Spectra not particularly reproducible Sample cant be recovered after analysis,Sample preparation Methods,AT
