The document discusses Fourier-transform infrared spectroscopy (FTIR), highlighting its advantages, disadvantages, and instrumentation compared to traditional dispersive IR spectroscopy. FTIR provides rapid scans with higher sensitivity and simultaneous detection of all frequencies, making it a powerful tool for chemical analysis. Additionally, important concepts such as sample preparation, infrared wave properties, and the role of the Michelson interferometer in creating an interferogram are covered.

1. FOURIER -TRANSFORM INFRARED
SPECTROMETER [FTIR]
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2. Content
 Introduction of Infrared Spectroscopy
 Difference between Infrared and Fourier Transform Infrared
Spectroscopy
 Introduction of FTIR
 Sample preparation
 Instrumentation
 Michelson interferometer
 Interferogram
 Advantages
 Disadvantage
 References
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3. Why Infrared Spectroscopy ?
An Infrared spectrum represents a fingerprint of a sample with
absorption peaks which correspond to the frequencies of
vibrations between the bonds of the atoms making up the
material-Because each different material is a unique combination
of atoms, no two compounds produce the exact same spectrum,
therefore IR can result in a unique identification of every different
kind of material!
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4. What is Infrared?
 Infrared waves have wavelengths longer than visible but shorter than
microwaves.
 The Infrared region is divided into 3 regions :
Near IR : 0.8 to 2.5 u
Infrared region : 2.5 to 15 u
Far IR : 15u to 200 u
 Infrared radiation stimulates molecular vibrations.
 Infrared spectra are traditionally displayed as %T versus wave number.
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5. Stretching: Change in inter-atomic distance along bond axis
Bending: Change in angle between two bonds.
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6. To separate IR light, a grating is used.
Grating
Light source
Detector
Sample
Slit
To select the specified IR
light,
A slit is used.
Dispersion
Spectrometer
In order to measure an IR spectrum
the dispersion Spectrometer takes
several minutes.
Also the detector receives only
a few % of the energy of
original light source.
Fixed mirror
B.S.
Moving mirror
IR Light source
Sample
detector
An interferogram is first made
by the interferometer using IR
light.
The interferogram is calculated
and transformed
into a spectrum using a Fourier
Transform (FT).
FTIR
In order to measure an IR
spectrum,
FTIR takes only a few seconds.
Moreover, the detector receives
up to 50% of the energy of
original light source.
(much larger than the dispersion
spectrometer) .
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7. • Difference between FTIR and dispersive IR:
Sr.no. Dispersive IR Fourier Transform IR
1 There are many moving parts resulting in
mechanical slippage and wear. Only the mirror moves during an experiment
2 Calibration against reference spectra is
required to measure frequency .
Use of a laser provides high frequency (to 0.01
cm-1).
3 Slow scanning speed. Rapid scan speeds permit monitoring sample
undergoing rapid change.
4 In order to improve resolution only a small
amount of the IR beam may be allowed to
pass through the slits.
A much larger beam may be used at all times.
Data collection is easier .
5 Only radiation of a narrow frequency range
falls on the detector at any one time .
All frequencies of radiation fall on the detector
simultaniously .
6 The samples is subject to thermal effect
from the focused beam .
The sample is not subject to thermal effects.
7 Less sensitive, time consuming. High sensitivity ,precision & faster .
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8. FTIR
 FTIR is a powerful tool for identifying the type of chemical bond present in
a molecule.
 FTIR have greatly extended the capabilities of IR and applied to many areas
that are difficult to analyze by dispersive instrument.
 In dispersive IR , IR light is separated into it’s individual frequency by
dispersion using a grating monochromatic
 All frequencies and intensities can be simultaneously determined.
FTIR gives a plot of intensity v/s frequency.
 The instrument used for FTIR is Scanning Michelson interferometer.
 FTIR frequencies are allowed to intersect to produce an interference pattern
and this pattern is analyzed mathematically using “FOURIER
TRANSFORM”, to determine the individual frequencies and their
intensities.
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9. FTIR Instrumentation
The Michelson interferometer consist of following parts:
1) A moving mirror.
2) A fixed mirror.
3) A beam splitter.
4) A radiation source.
5) FTIR Detector.
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10. FTIR Instrumentation
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11. Sample Preparation
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12. •To obtain an spectrum, the sample must be
placed in a “container” or cell that is transparent
in the IR region of the spectrum.
•Sodium chloride or salt plates are a common
means of placing the sample in the light beam of
the instrument.
Sample Cell
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14. Sources
Nernst Glower
Hollow rod about 2mm in diameter & 30 mm in length,composed
of rare earth oxide.
Externally heated (1000-1800 C).
Radiation over wide wavelength range.
Globar source
Rod of sintered silicon carbide,50mm in length & 4-8mm in diameter,
Self starting.
Heated to temp.between 1300-1700 C.
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15. Tungsten Incandescent Lamp
Black body source
Used in near IR instrument.
Nichrome/Rhodium wire
Coiled , heated
Emits in Mid-IR
Black oxide layer formation.
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16. FTIR DETECTOR
A)Thermal Detector
1) Thermocouple-
Two junctions of dissimilar metals connected together.
Electricity directly propotional to energy difference between 2
connections.
2)Bolometer-
One arm of Wheastone bridge & similar strip of metal is used as balancing
arm of bridge which is not expose to radiation.
Amount of current flowing is directly prop. to amount of radiation falling
on the detector.
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17. B)Photon detector
Interaction between incident photon and semiconductor.
Energy from photon stricking electron in detector and raises them
from non-conducting to conductance state.
E.g. MCT detector.
C) Pyroelectric Detector
DTGS Detector
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18. Interferogram
The Interferogram consist of two modulated waves :
1) Consructive Interference.
2) Destructive Interference.
λλλλλλλλλλλλλλλλλλλλλλλλλ=++++λλλ
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19. δ = nλ Constructive Interference will take place for any value of δ
when the two beams are in phase
n=0,1,2,3,…..
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20. Destructive Interference
Fixed Mirror
Moving Mirror
δ = ( n + 1/2 ) λ Totally destructive interference takes place when optical path
difference is 1/2 λ or some multiple of it-beams are completely out of phase
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21. Advantages
• Fellgette advantage- High speed.
• Jacquinot advantage-High sensitivity.
• Slitless system.
• All of the energy of the source is utilized in FTIR.
• Rapid scan time with high sensitivity.
• Connes advantage.
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22. Disadvantages
FTIR instruments have a single beam, whereas dispersive
instruments usually have a double beam.
Interferograms are difficult to interpret without first
performing a Fouriertransform to produce a spectrum.
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23. References
William Kemp, Organic Spectroscopy, Published by Palgrave,
Third edition.
Gurdeep R.Chatwal, Sham K.Anand,Instrumental method of
Chemical analysis, Himalaya Publishing house,fifth edition.
Silverstein, A Spectroscopic Identification of Organic
compound,sixth edition.
Skoog wele Hollers, Analytical chemistry Introduction ,
Saunderls college publishing house ,eight edition.
B.K Sharma, Instrumental method of chemical analysis, Goel
publication, second edition.
Y.R Sharma, Elementary Organic Spectroscopy,Principles and
Chemical applications, S.Chand Publication, sixth edition.
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24. HabartH .Willard,Lynne L. Merritt,Johan A. Dean,
Frank A.Settle,Instrumental method of Analysis, CBS
Publisher and Distributors, New Delhi.
Dyer R John, Application of Absorption Spectroscopy
of unknown Organic compound, PHT, pvt. Ltd. New
Delhi.
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