Transcript Fourier Transform IR Spectroscopy

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
I ( )
• Optical path difference is 
• Intensity of the detector I ( ) has maxima at
  n , n  0,1,2
and minima at   ( n  1 / 2)
Mathematics (contd.)
• The resulting interferogram is described as an infinitely long
cosine wave

•
B ( ) cos( 2 ) where B ( ) =intensity as F(v)

• For non-monochromatic source treat each frequency as if it
resulted in a separate cosine train.
Mathematics (contd.)
• An infrared source is typically approximated as a black body
radiator and the summation can be replaced by an integral.

I ( )   B( ) cos( 2)d
• At

0
= 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.
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
B( ) 

 I ( ) cos(2 )d

For mathematical validity this integration must be
carried out over all possible values of delta i.e +  to
–
Measurement Techniques
Though 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.
Applications
Applications 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.