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

Intensity Distribution and Temperature Dependency versus Wavelength of Black Body Radiation Energy
2
5
20
10
105
104
103
102
10
1
10-1
10-2
10-3
10-4
0.1
0.2
0.5
1
50
100
Wavelength m
6000K
4000K
2000K
1000K
500K
300K
200K
Spectral irradiance W
IR light source
IR Light Source
FTIR seminar
Interferometer
He-Ne gas laser
Fixed mirror
Movable mirror
Sample chamber
Light
source
(ceramic)
Detector
(DLATGS)
Beam splitter
FT Optical System Diagram
Fixed mirror
Ｂ
Movable mirror
Fixed mirror
Ａ
Movable mirror
Fixed mirror
Ｃ
Movable mirror
Same-phase interference
wave shape
Opposite-phase
interference
wave shape
Same-phase interference
wave shape

0
Movable mirror
Ｄ Interference pattern of light manifested by the optical-path difference
Continuous phase shift
Signal strength
Ｉ
(X)
-2
-
0

2
-2
-
0

2
FTIR seminar
Interference of two beams of light
Relationship between light source spectrum and the signal output from interferometer

• Monochromatic

light
(b) Dichroic light

• Continuous

spectrum light

All intensities are standardized.
Light source spectrum
Signal output from interference wave
Time t
Time t
Time t
I(t)
I


Wavenumber


Wavenumber


Wavenumber


I
Az
Az
FTIR seminar
Interference is a superpositioning of waves
FTIR seminar
Interferometer interferogram
Output of a Laser interferometer
Primary interferometer interferogram that was sampled
Optical path difference x
Sampling of an actual interferogram
4000
400
SB
Fourier transform
Optical path difference[x]
(Interferogram)
(Single beam spectrum)
Wavenumber[cm-1]
Single strength
Time axis by FFT Wavenumber
Fourier Transform
FTIR seminar
TGS
Operates at room temperature
MCT
Operates at the temperatur
of liquid nitrogen
D* (, f) (cmHz1/2W-1)
1010
109
108
Wavenumber[cm-1]
4000
600
Detector Properties
1.Better sensitivity and brightness
- Allows simultaneous measurement over the entire wavenumber range
- Requires no slit device, making good use of the available beam
2.High wavenumber accuracy
- Technique allows high speed sampling with the aid of laser light interference fringes
- Requires no wavenumber correction
- Provides wavenumber to an accuracy of 0.01 cm-1
3. Resolution
- Provides spectra of high resolution
4. Stray light
- Fourier Transform allows only interference signals to contribute to spectrum.
Background light effects greatly lowers.
- Allows selective handling of signals limiting intreference
5. Wavenumber range flexibility
- Simple to alter the instrument wavenumber range
CO2 and H2O sensitive
FT-IR Advantages and Disadvantages

FT-IR Advantages

• Fellgett's (multiplex) Advantage
• FT-IR collects all resolution elements with a complete scan of the interferometer. Successive scans of the FT-IR instrument are coadded and averaged to enhance the signal-to-noise of the spectrum.
• Theoretically, an infinitely long scan would average out all the noise in the baseline.
• The dispersive instrument collects data one wavelength at a time and collects only a single spectrum. There is no good method for increasing the signal-to-noise of the dispersive spectrum.

FT-IR Advantages

• Connes Advantage
• an FT-IR uses a HeNe laser as an internal wavelength standard. The infrared wavelengths are calculated using the laser wavelength, itself a very precise and repeatable 'standard'.
• Wavelength assignment for the FT-IR spectrum is very repeatable and reproducible and data can be compared to digital libraries for identification purposes.

FT-IR Advantages

• Jacquinot Advantage
• FT-IR uses a combination of circular apertures and interferometer travel to define resolution. To improve signal-to-noise, one simply collects more scans.
• More energy is available for the normal infrared scan and various accessories can be used to solve various sample handling problems.
• The dispersive instrument uses a rectangular slit to control resolution and cannot increase the signal-to-noise for high resolution scans. Accessory use is limited for a dispersive instrument.