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 Spectra 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 quantitative analysis

Quantum States Absorption of electromagnetic radiation  excitation to higher energy states hv = |E 2 – E 1 | Electronic > Vibrational > Rotational

Infrared Radiation Frequencies to 10 cm -1 –(cm -1 = wavenumbers) Divided into three Regions –Near : 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 50

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

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. H 2, N 2, O 2 ) don’t absorb IR radiation, b/c of no dynamic dipole moment

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

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 -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 = (2πc) -1 Wavenumber = (2πc) -1 √(k/μ) – –Reduced mass (μ) – –Force constant (k) C-C < C=C < C≡C

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

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

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

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 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 Transmission and Internal Reflection Infrared Spectra of Cocaine. Journal of Forensic Sciences, 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

Transmission 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 can’t be recovered after analysis

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

Transmission vs. ATR ATRAdvantages 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

Transmission vs. ATR ATR:Disadvantages New technique –Less spectra catalogs available Spectral artifacts –Peak shift and intensity differences

ATR forensic applications Drug analysis Fiber analysis Paint chip analysis Ink analysis Paper analysis Biological analysis

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.

References Koulis, Cynthia, et. al. Comparison of Transmission and Internal Reflection Infrared Spectra of Cocaine. Journal of Forensic Sciences, Skoog, Holler, and Nieman. Principles of Instrumental Analysis. 5th edition, 1998 Engel and Reid. Physical Chemistry. Pearson Education, Lewis University Dr. Salim M. Diab Acknowledgment

THE END Questions?