Fourier Transform IR Spectroscopy

Saptarshi Basu

Introduction• Absorption peaks in an infrared absorption spectrum arise

from molecular vibrations• Absorbed energy causes molecular motions which create a

net change in the dipole moment.

What is FTIR• Fourier-transform infrared spectroscopy is a vibrational

spectroscopic technique, meaning it takes advantage of asymmetric molecular stretching, vibration, and rotation of chemical bonds as they are exposed to designated wavelengths of light.

• Fourier transform is to transform the signal from the time domain to its representation in the frequency domain

Theory and Instrumentation

• Light enters the spectrometer and is split by the beam splitter. The figure above shows what is referred to as the Michelson interferometer

Theory and Instrumentation(contd.)

• The light originates from the He-Ne laser• Half of the light is reflected 90 degrees and hits a fixed

mirror, while the other half passes through the beam splitter and hits the moving mirror

• The split beams are recombined, but having traveled different distances, they exhibit an interference pattern with each other

• As they pass through the sample, the detector collects the interfering signals and returns a plot of response v. mirror displacement known as an interferogram

Mathematics

• Optical path difference is

• Intensity of the detector has maxima at

and minima at

)(I

)(I

2,1,0, nn

)2/1( n

Mathematics (contd.)• The resulting interferogram is described as an infinitely long cosine wave • where =intensity as F(v) • For non-monochromatic source treat each frequency as if it resulted in a

separate cosine train.

)2cos()(B )(B

Mathematics (contd.)• An infrared source is typically approximated as a black body

radiator and the summation can be replaced by an integral.

• At = 0, signal always has a strong maxima called Centreburst• Outwards from the centreburst the cosine waves cancel and

reinforce and the amplitude of the interferogram dies off.

dBI )2cos()()(0

Mathematics (contd.)

• Spectroscopists are interested in the spectrum in the frequency domain i.e intensity versus wavenumber

• If the mathematical form of the interferogram is known,spectrum in the frequency domain can be calculated by Fourier Transformation

dIB )2cos()()(

For mathematical validity this integration must be carried out over all possible values of delta i.e + to –

Measurement TechniquesThough some emission spectroscopy is done most of the

work is in the absorption spectra area using FTIR.• 1.     If there is no sample present the spectrum is that of

a black body radiator modified by any transmission characteristics of the interferometer components.

• 2.     If the interferometer chamber is not evacuated or purged with dry gas some absorption from the atmospheric Co2 and H20 is observed. This is called the background spectrum.

• 3.     When the sample is introduced the spectrum now is a superposition of the absorption bands of the sample on an uneven background.

• 4.     To obtain %T with wavenumber we ratio the single beam sample spectrum with the background spectrum.

Measurement Techniques(contd.)

Apodization

Energy transitions

For the vibrational motion of a diatomic molecule, the permanent electric dipole moment can be expanded in a Taylor series about the equilibrium internuclear separation

Hence can be expressed as

Energy transitions(contd.)

Therefore, diatomic molecules will only exhibit a vibrational absorption spectrum if there is a change in dipole moment with the vibrational motion

Theory also dictates transition between 1

Energy Transitions• A typical IR spectrum will have absorbance bands

which can be attributed to the presence of individual chemical groups in the molecule under study and a "fingerprint" region distinctive of the individual compound.

ApplicationsApplications are vast and diverse.Some of them are

•Compositional analysis of organic,inorganic and polymers

•Biological and biomedical fields like detection of water in biological 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 IR region

simultaneously and smoothly, making this a very rapid technique. • The technique is non-invasive and non-destructive. Its resolution

of .125 cm-1 is not spectacular in comparison to other vibrational techniques and it will not give the same detailed structural information that NMR, MS, or X-ray crystallography.

• IR spectroscopy is notoriously sensitive to the absorption of water, and it has the tendency to overwhelm all of the other peaks. If there is significant moisture in the sample the penetration distance of the light decreases. It may be advantageous to go with Raman in place of IR in the case of excess moisture.

• Spectra in the frequency domain can never be eyeballed conclusively. They are always subject to some sort of manipulation, leading some to believe that the data can say whatever the experimenter wants it to say depending on how it is manipulated.

Advantages and Disadvantages(contd.)

• Much higher signal to noise ratio.Felgett’s advantage=

• Greater throughput of power.• Hard to do samples having low transmission and

weak spectra can be done with FTIR.• Greater wavenumber accuracy. Most FT

instruments have an accuracy of +/- 0.01 cm-1.