Fourier Transform InfraRed

Spectroscopy ; FT-IR

Jintara Padchasri

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Outline

Basic Ideas Wavelength and Wavenumber What is the FT-IR ? Why Infrared Spectroscopy? Older Technology Theory and Instrumentation Mathematics Applications Advantages and Disadvantages

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Basic Ideas Wavelength and Wavenumber What is the FT-IR ? Why Infrared Spectroscopy? Older Technology Theory and Instrumentation Mathematics Applications Advantages and Disadvantages

Basic Ideas; Energy Level

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About 15 micron radiationBasic Global Warming:The C02 dance

Wavelength and Wavenumber Wavelength = 1 / Wavenumber

For the IR, wavelength is in microns.

Wavenumber is typically in 1/cm, or cm-1.

5 microns corresponds to 2000 cm-1.

20 microns corresponds to 500 cm-1.

15 microns corresponds to 667 cm-1. Much‘terrestrial’ IR energy at the wavenumber.

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Wavelength = 1 / Wavenumber

For the IR, wavelength is in microns.

Wavenumber is typically in 1/cm, or cm-1.

5 microns corresponds to 2000 cm-1.

20 microns corresponds to 500 cm-1.

15 microns corresponds to 667 cm-1. Much‘terrestrial’ IR energy at the wavenumber.

What is FT-IR?

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What is FT-IR?what information can FT-IR provide? It can identify unknown materials It can determine the quality or consistencyof a sample It can determine the amount of componentsin a mixture

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what information can FT-IR provide? It can identify unknown materials It can determine the quality or consistencyof a sample It can determine the amount of componentsin a mixture

Why Infrared Spectroscopy?

IR spectrum ;fingerprint of a sample withabsp. peaks which correspond to the f ofvibrations between the bonds of the atomsmaking up the material. Because each different material is a uniquecombination of atoms, no two compounds producethe exact same IR spectrum IR can result in a positive identification ofevery different kind of material. Size of peaks in the spectrum is a directindication of amount of material.

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IR spectrum ;fingerprint of a sample withabsp. peaks which correspond to the f ofvibrations between the bonds of the atomsmaking up the material. Because each different material is a uniquecombination of atoms, no two compounds producethe exact same IR spectrum IR can result in a positive identification ofevery different kind of material. Size of peaks in the spectrum is a directindication of amount of material.

Older TechnologyFT-IR spectroscopy is preferred over dispersive orfilter methods of infrared spectral analysis forseveral reasons:

It is a non-destructive technique It provides a precise measurement method whichrequires no external calibration It can increase speed, collecting a scan every second It can increase sensitivity – one second scans can be co-added together to ratio out random noise It has greater optical throughput It is mechanically simple with only one moving part

It is a non-destructive technique It provides a precise measurement method whichrequires no external calibration It can increase speed, collecting a scan every second It can increase sensitivity – one second scans can be co-added together to ratio out random noise It has greater optical throughput It is mechanically simple with only one moving part

Theory and InstrumentationThe Michelson interferometer principle1. Monochromatic light

Movable mirror δ = Optical Path Difference

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Detector

Stationary MirrorBeamsplitter

Interference

δ = (n + ½) λ

δ = n λ

2. Dichromatic source

II

Theory and Instrumentation

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10

vδ

- l -l/2 0 l/2 l

Moveable mirror

3. Broadband source

I I

Theory and Instrumentation

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v

Continuous IR spectrum Interferogram

δ0

FT

Mathematics

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Time domain: I vs. δ Frequency domain: I vs. v

δ

I

v

I

Mathematics

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Optical path difference is

Intensity of the detector has maxima at

and minima at

)(I 2,1,0, nn )2/1( n

MathematicsThe resulting interferogram is described as an infinitely long

cosine wave

where =intensity as F(v)

For non-monochromatic source treat each frequency as if itresulted in a separate cosine train.

)2cos()(B )(B

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The resulting interferogram is described as an infinitely longcosine wave

where =intensity as F(v)

For non-monochromatic source treat each frequency as if itresulted in a separate cosine train.

ApplicationsApplications are vast and diverse.Some of them are

Compositional analysis of organic, inorganic and polymers

Biological and biomedical fields like detection of water inbiological membranes

Analysis of Aircraft exhausts

Measurement of toxic gas in fuels

Combustion

Gas analysis

and lots more

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Compositional analysis of organic, inorganic and polymers

Biological and biomedical fields like detection of water inbiological membranes

Analysis of Aircraft exhausts

Measurement of toxic gas in fuels

Combustion

Gas analysis

and lots more

Advantages and Disadvantages FT – IR can take wavelength readings across the whole IRregion simultaneously and smoothly, making this a very rapidtechnique.

The technique is non-invasive and non-destructive. Its willnot give the same detailed structural information that NMR,MS, or X-ray crystallography.

Spec. in the freq. domain can never be eyeballedconclusively. They are always subject to some sort ofmanipulation, leading some to believe that the data can saywhatever the experimenter wants it to say depending on how itis manipulated.

Greater wavenumber accuracy. Most FT instruments havean accuracy of +/- 0.01 cm-1

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FT – IR can take wavelength readings across the whole IRregion simultaneously and smoothly, making this a very rapidtechnique.

The technique is non-invasive and non-destructive. Its willnot give the same detailed structural information that NMR,MS, or X-ray crystallography.

Spec. in the freq. domain can never be eyeballedconclusively. They are always subject to some sort ofmanipulation, leading some to believe that the data can saywhatever the experimenter wants it to say depending on how itis manipulated.

Greater wavenumber accuracy. Most FT instruments havean accuracy of +/- 0.01 cm-1

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