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


Intensity Distribution and Temperature Dependency versus Wavelength of Black Body Radiation Energy



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FTIR-Presentazione pptx

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

Movable mirror
Fixed mirror

Movable mirror
Fixed mirror

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

(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.

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