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Applications of IR spectroscopy Yongsik Lee 2004. 6.

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Presentation on theme: "Applications of IR spectroscopy Yongsik Lee 2004. 6."— Presentation transcript:

1 Applications of IR spectroscopy Yongsik Lee

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 ( 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 =  2b/N =  Number if interference fringes  N between two known wavelengths   N = 2b/ 1 – 2b/ 2  b =  N /2( 1- 2)

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)   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 (< ) ► Evanescent wave can be absorbed by the sample

19 ATR instrumentation ► Figure ► 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  nm  – 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  m ► 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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