Infrared Spectroscopy: Comparison of Transmission and ATR Techniques

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Infrared Spectroscopy Comparison of
Transmission and ATR Techniques

• Matt Herring
• Lewis University

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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
  Spectra of Cocaine. Journal of Forensic
  Sciences, 2001.

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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
• quantitative analysis

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Quantum States

• Absorption of electromagnetic radiation ?
  excitation to higher energy states
• hv E2 E1
• Electronic gt Vibrational gt Rotational

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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-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
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Infrared Absorption

• Absorption of IR radiation ? excitation of
  vibrational and rotational energy levels
• not sufficient energy for electronic excitation
• Vibrational energy states are of greatest
  importance for IR spectroscopy

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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
• Vibration 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

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IR absorption

• IF vibrational frequency of molecule frequency
  of radiation THEN energy is absorbed
• Results promotion to higher energy level and
  increase in amplitude of molecular vibration

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Molecular Vibrations

• Molecular Vibrational and Bending modes
• Single absorption peak appears for each mode with
  a dynamic dipole moment

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Interpreting the Spectra
Peaks are identified by -Intensity -Shape
-Position
Correlation charts are used to aid in identifying
functional group absorption bands
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Determining peak position

• Using the model of a simple harmonic oscillator
  the frequency vibration can be predicted
• Wavenumber (2pc)-1v(k/µ)
• Reduced mass (µ)
• Force constant (k)
• C-C lt CC lt CC

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Qualitative Analysis

• 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

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Instrumental Setup Transmission Technique

• Incident 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

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Instrumental Setup Attenuated Total Reflectance
(ATR) Technique

• IR radiation 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

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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.

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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.

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Why the Shift?

• Amount of reflected radiation is dependent of the
  index of refraction 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

Figure Penetration depth and refractive index
for cocaine at carbonyl absorption band

• Koulis, Cynthia, et. al. Comparison of
  Transmission and Internal Reflection Infrared
  Spectra of Cocaine. Journal of Forensic
  Sciences, 2001.

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Sample preparation Methods

• Transmission
• Solids KBr Pellet
• Liquids NaCl Plates

Quick press KBr pellet press
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Transmission vs. ATR

• Transmission
• Advantages
• High quality spectra
• Satisfactory for qualitative analysis
• Wide variety of spectra libraries available

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Transmission vs. ATR

• Transmission
• Disadvantages
• Solid (KBr pellet)
• Time consuming
• Particle size lt 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

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Sample preparation Methods

• ATR
• Liquids and solids loaded directly onto crystal
• Arm Applies pressure to solids for uniform
  contact with crystal
• PSI can be controlled

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Transmission vs. ATR

• ATR
• Advantages
• High Quality Spectrum for qualitative analysis
• Minimal sample preparation
• Non destructive
• Time efficient
• Spectra not affected by sample thickness
• Radiation penetrates only a few micrometers
• Highly reproducible results
• Wide variety of sample types
• Threads, yarns, fabrics, fibers, pastes, powders,
  suspensions, polymers, rubbers

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Transmission vs. ATR

• ATR
• Disadvantages
• New technique
• Less spectra catalogs available
• Spectral artifacts
• Peak shift and intensity differences

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ATR forensic applications

• Drug analysis
• Fiber analysis
• Paint chip analysis
• Ink analysis
• Paper analysis
• Biological analysis

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In Conclusion

• There is an increasing trend toward the use of
  ATR
• High Quality Spectrum
• Ease of sample preparation
• Wide variety of sample types
• Must be noted that ATR spectra and transmission
  spectra are different
• Variations in peak position and intensity must be
  understood
• New libraries of spectra for ATR are needed.

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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, 1998
• Engel and Reid. Physical Chemistry. Pearson
  Education, 2006.
• http//wwwchem.csustan.edu/Tutorials/INFRARED.HTM
• http//www.micromemanalytical.com/ATR_Ken/ATR.htm
• Lewis University
• Dr. Salim M. Diab

Acknowledgment
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• THE END
• Questions?