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 = 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)
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 Analysis
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 Disadvantages Application 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 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