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Applications of IR spectroscopy

Applications of IR spectroscopy. Yongsik Lee 2004. 6. IR spectrum. Sample handling. Most time-consuming part is sample preparation Gases fill gas cell transparent windows (NaCl/KBr) long path length (10 cm) - few molecules Liquids fill liquid cell solute in transparent solvent

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Applications of IR spectroscopy

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  1. Applications of IR spectroscopy Yongsik Lee 2004. 6

  2. IR spectrum

  3. Sample handling • Most time-consuming part is sample preparation • Gases fill gas cell • transparent windows (NaCl/KBr) • long path length (10 cm) - few molecules • Liquids fill liquid cell • solute in transparent solvent • Not in water (attacks windows) • short path length (0.015-1 mm) - solvents absorb

  4. Solution sample • Solvents • Water, alcohols – NO • Check the background absorption

  5. Demountable IR cell for liquid • Cells • 0.01 – 1.0 mm path length • Narrower than UV/VIS • Sample concentration 0.1 – 10% • Demountable cells with Teflon spacers • Variation in path length

  6. Determination of cell thickness • Determination of thickness(b) • Empty cell in the light path • Interference fringe • 2b/N = l • Number if interference fringes DN between two known wavelengths • DN = 2b/l1 – 2b/l2 • b = DN /2(n1-n2)

  7. Solid sample • Solid samples • make semi-transparent pellet with KBr • Halide salts get transparent when pressured • grind and mix with Nujol (hydrocarbon oil) to form mull • Grind size < the radiation wavelength • 1-2 drop(s) between NaCl plates.

  8. Qualitative Analysis • Step One : Identify functional groups (group frequency region) • 1200 - 3600 cm-1 • Step Two : Compare with standard spectra containing these functional groups • fingerprint region – sensitive to the structure • 600 – 1200 cm-1

  9. Group frequency and fingerprint region

  10. Group frequencies • Approximately calculated from masses and spring constants • Variations due to coupling • Compared to correlation charts/databases • Bond force constant

  11. Group frequecy table for organic groups

  12. Correlation chart

  13. Computer search system • Spectra pattern search • Position • Relative magnitude • Sadtler IR collection (1980) • Over 130,000 spectra • Spectra coding Algorithm • Location of its strongest abs peak • Then each additional strong band in 10 regions • 40 second for 25,000 compound search

  14. Quantitative Analysis • IR more difficult than UV-Vis because • narrow bands (variation in e) • complex spectra • weak incident beam • low transducer sensitivity • solvent absorption • IR mostly used for rapid qualitative but not quantitative analysis • Beer’s law failure • Long optical path-length required • Regular FT-IR is worse than UV-VIS • Exception - Tunable IR laser, quantum cascade laser, OPO

  15. Reflectance spectroscopy • Types • Specular reflection • Diffuse reflection • Internal reflection • Attenuated total reflection

  16. DRIFTS • Diffuse reflectance IR FT spectroscopy • Fine powder sample • Specular reflections from all randomly oriented surfaces of the powder • The intensity of the reflection is roughly independent of the viewing angle • Analysis • Using mathematical models • Kubelka and Munk • Fuller and Griffiths

  17. Instrumentation of DRIFTS • Adaptor in cell compartment • Reference sample • Finely grounded KBr • Mirror • ABS vs. DRIFTS • Peak locations are same • Relative intensities are different • Figure 17-10

  18. ATR • Attenuated total reflection • Sample – wide variety of types • solids of limited solubility • Films • Threads • Pastes • Adhesives • Powers • Principles of the method • At a certain angle, total reflection can occur • Depth of penetration when reflected (< l) • Evanescent wave can be absorbed by the sample

  19. ATR instrumentation • Figure 17-11 • High refrative index ATR crystal • Thallium bromide • Thallium iodide • Germanium and ZeSe plate • Adjustment of incident angle • ATR crystal can be dipped into the liquid

  20. Applications of ATR

  21. 17C Photoacoustic IR Spectroscopy • History • 1880 Alexander Graham Bell • Photo absorption effect • Chopped Photon -> sample -> microphone

  22. Photoacoutstic (PA) IR • Advantages • Scattered & reflected light = no microphone signal • FT method is possible • CO2 laser PA IR spectroscopy • Tunable CO2 laser source • PA cell • 10 gases (sensitivity 1 ppb) in 5 minutes

  23. 17D Near IR • Spectrum • 770-2500 nm • 13000 – 4000 cm-1 • Overtone or combination of fundamental stretching modes • C-H, N-H, O-H • Weaker absorption than fundamental bands • Disadvantages • Low molar absorption coefficient • Detection limit 0.1% • Application • Mostly qualitative analysis • Water, protein, low mw carbohydrates, food, petro

  24. 17G IR microspectrometry • Introduced in 1980s • IR ABS or reflection spectra • Sample dimensions in 10 -500 mm • Instrument • Ordinary optical microsocpe • FT-IR with small IR beam size • LN2 MCT (mercury/cadmium/telluride) PC

  25. Nicolet Magna 760 with Nic-Plan IR Microscope

  26. Tabletop Optical Module (foreground), • Nicolet Magna 550 spectrometer, • Right Auxiliary Experimental Module • Olympus IX70 inverted microscope

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