Fourier Transform Infrared (ft-ir) Spectroscopy Theory and Applications

Infrared Spectroscopy

The bonds between atoms in the molecule stretch and bend, absorbing infrared energy and creating the infrared spectrum.

Symmetric Stretch
Antisymmetric Stretch
Bend
A molecule such as H2O will absorb infrared light when the vibration (stretch or bend) results in a molecular dipole moment change

Energy levels in Infrared Absorption

Infrared absorption occurs among the ground vibrational states, the energy differences, and corresponding spectrum, determined by the specific molecular vibration(s). The infrared absorption is a net energy gain for the molecule and recorded as an energy loss for the analysis beam.
h
Excited states
Ground (vibrational) states
h(1 - 0 )
h(1 - 0)
h(2 - 1) (overtone)
Infrared Absorption and Emission
1
2
0
3

Infrared Spectroscopy

A molecule can be characterized (identified) by its molecular vibrations, based on the absorption and intensity of specific infrared wavelengths.

Infrared Spectroscopy

For isopropyl alcohol, CH(CH3)2OH, the infrared absorption bands identify the various functional groups of the molecule.

Capabilities of Infrared Analysis

• Identification and quantitation of organic solid, liquid or gas samples.
• Analysis of powders, solids, gels, emulsions, pastes, pure liquids and solutions, polymers, pure and mixed gases.
• Infrared used for research, methods development, quality control and quality assurance applications.
• Samples range in size from single fibers only 20 microns in length to atmospheric pollution studies involving large areas.

Applications of Infrared Analysis

• Pharmaceutical research
• Forensic investigations
• Polymer analysis
• Lubricant formulation and fuel additives
• Foods research
• Quality assurance and control
• Environmental and water quality analysis methods
• Biochemical and biomedical research
• Coatings and surfactants
• Etc.

To separate IR light, a grating is used.
Grating
Light source
Detector
Sample
Slit
To select the specified IR light,
A slit is used.
Dispersion
Spectrometer
In order to measure an IR spectrum,
the dispersion Spectrometer takes
several minutes.
Also the detector receives only
a few % of the energy of
original light source.
Fixed CCM
B.S.
Moving CCM
IR Light source
Sample
Detector
An interferogram is first made by the interferometer using IR light.
The interferogram is calculated and transformed
into a spectrum using a Fourier Transform (FT).
FTIR
In order to measure an IR spectrum,
FTIR takes only a few seconds.
Moreover, the detector receives
up to 50% of the energy of original
light source.
(much larger than the dispersion
spectrometer.)
Comparison Beetween Dispersion Spectrometer and FTIR
Interferogram
is made by an interferometer.
Interferogram
is transformed
into a spectrum using a FT.
BKG
SB
3000
2000
1000
[cm-1]
Sample
SB
Sample
3000
2000
1000
[cm-1]
Sample/BKG
IR spectrum
%T
3000
2000
1000
[cm-1]
The Principles of FTIR Method
FTIR seminar