1. Fourier Transformation Infra-red
(FT-IR)

2. Theory
Principle
Analysis by IR spectroscopy is based on the fact that molecules have specific frequencies associated with internal vibrations/rotational of groups of atoms
IR region 4000 cm-1 to ~ 200 cm-1

3. Theory
IR radiation does not have enough energy to induce electronic transition
Absorption of IR is restricted to compounds with small energy difference in the possible vibrational and rotational states.

4. In a molecule, the atoms are not held rigidly apart
they can move, as if they are attached by a spring of equilibrium separation Re
For a molecule to absorb IR ,the vibration or rotation must cause a net change in the dipole moment of the molecule.

5. The alternating electrical field of the radiation interact with fluctuations in the dipole moment of the molecule.
If the frequency of the radiation matches the vibrational of the molecule
 the radiation will be absorbed and excite the molecule to a higher vibrational state

6. An IR spectrometer measures the frequencies of the absorbed radiation at specific wavelength
The result is a plot of absorbed energy versus frequency and is called IR spectrum of the material
Molecular Rotation – not important especially in solid and liquid.
Molecular Vibration: vibrations fall into two main categories – stretching and bending

7. In a simple diatomic molecule, there is only one direction of vibrating, stretching
This means there is only one band of infra red absorption.
Weaker bonds require less energy, as if the bonds are springs of different strengths.
More atoms will give more modes of vibrations
For a linear molecule with "n" atoms, there are 3n-5 vibrational modes, if it is nonlinear, it will have 3n-6 modes.

8. For example, water (H20), has 3 molecules, and is non linear: therefore it has (3*3)-6 = 3 modes of vibration
There is one important restriction, the molecule will only absorb radiation if the vibration is accompanied by a change in the dipole moment of the molecule.
A dipole occurs when there is a difference of charge across a bond

9. To calculate the frequency of light absorbed, requires Hookes law:
If the two appositely charged molecules get closer or further apart as the bond bends or stretches, the moment will change
To calculate the frequency of light absorbed, requires Hookes law:
Where: k = force constant indicating the strength of the bond
m1 and m2 are the masses of the two atoms

10. From this equation, we can see that if there is a high value of k, i.e. the bond is strong, it absorbs a higher frequency of light.
a C=C double bond would absorb a higher frequency of light than a C-C single bond
the larger the two masses, the lower the frequency of light absorbed.

11. Experimental procedure:
If the sample is a liquid, it can be tested straight away
If it is a solid, then it is ground up to a fine powder, and mixed with a few drops of liquid paraffin (Nujol) to form a paste
A thin layer of the liquid or paste is then spread between two sodium chloride plates, and placed in the machine

12. Sodium chloride is used as it does not absorb strongly in the infrared region whereas glass does, however it does dissolve readily in water, so it must be cleaned with CH2CL2.

13. The reference cell is an identical rock salt plate, with a similar amount of Nujol (if used in sample).
The detector then compares the two beams it receives, and can remove any peaks due to the Nujol or the plates.

14. Interpreting the spectrum
The graph produced show percentage transmission against wave number
If no radiation is absorbed at a particular frequency, then the line on the graph will be be at 100% at the corresponding wave number
Identification possible due to differences in the chemical structure of materials  characteristic vibration and yield unique IR spectra (fingerprint)

15. In addition, the magnitude of the absorption related to concentration  quantitative
Different types of bonds have characteristic regions of the spectrum where they absorb:

16. Most functional groups absorb above 1500 cm-1.
The region below 1500 cm-1 is known as the "fingerprint region".
Every molecule produces a unique pattern here, so if an unknown sample produces a spectrum which matches that of a known compound, the sample can be confirmed to be that compound.

17. Example spectrum:
A sample with chemical formula (C3H6O2) gives the above spectrum

18. This shows a strong absorption at just over 1710 cm-1 and a medium absorption at 2500-3300 cm-1.
these bonds correspond to the C=O and O-H groups found in a carboxylic acid.
This information + the chemical formula tells us the structure must be propanoic acid:

19. For decades, dispersive spectrometers were the workhorse of IR analysis.
From 1980s onwards FTIR spectrometers almost entirely replaced dispersive
The most significant feature of FTIR spectrometers is that radiation from all wavelength is measured simultaneously.

20. Applications
IR spectroscopy – major tool determining structure, identification & quantitative analysis.
There are applications requiring special techiniques.

21. Surface Analysis
IR Microspectroscopy
Chromatography-FTIR
Time-resolved studies

22. Surface Analysis
The are 3 available techniques and each provide different depth resolution and types of sample.
a. ATR ( attenuated total reflectance)
probes the surface layer in the range ~0.5 to 3m

23. light entering prism undergoes internal reflection
Sample (2)
Prism (1) 
Attenuated light
light entering prism undergoes internal reflection
at the point of reflection light penetrates a small distance into the thin film
the depth depends on  and 
The light exiting the prism attenuated at specific freq. Corresponding to absorption by sample.

24. Applications of ATR Investigating adhesion problem
surface treatment / modification
weathering of plastic parts, polymer films and fiber
identification very thin layer contiminants on surface

25. b. PAS ( photoacoustic spectroscopy)
sampling depth of ~1 to 10 m
IR energy absorbed converted to heat  induces expansion of gas in contact - acoustic signal
the incident beam intensity is modulated at a frequency in the acoustic range.
microphonic detection –yields IR spectrum.
useful in analysis samples w/o preparation –powders, pellets,films
surface coatings,alumina-based catalysis, etc

26. c. GIR ( grazing incidence reflectance)
detection ultrathin films ( ar least monolayer coverage)
IR beam intersects the surface at a high angle of incidence
very sensitive, applicable only to metallic surfaces
Other applications
IR Microspectroscopy
Chromatography-FTIR
Time-resolved studies