This document discusses the theory, instrumentation, and applications of dispersive and Fourier transform infrared (FTIR) spectroscopy. It begins with an introduction to IR spectroscopy and the IR region. It then covers dispersive IR instrumentation, which uses prism or grating monochromators to separate wavelengths, and has limitations like slow scan speeds and limited resolution. The document introduces FTIR instrumentation, which uses an interferometer to simultaneously measure all wavelengths and overcomes the limitations of dispersive IR. It concludes that FTIR provides faster, more accurate and sensitive analysis compared to dispersive IR.

1. THEORY, INSTRUMENTATION
AND
APPLICATIONS OF
DISPERSIVE & FTIR
Presented By
Bhavana Vedantam,
Dept. Of Pharmaceutical Analysis

2. Contents
It Contains…
Introduction to IR Spectroscopy
Dispersive IR Spectroscopy
FT - IR Spectroscopy
Conclusion
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3. INTRODUCTION
Spectroscopy is the branch of science dealing with
the study of interaction of electromagnetic radiation with
matter.
IR spectroscopy is Absorption spectroscopy in
which molecular vibrations observed due to absorption of IR
radiation.
Infrared radiation was discovered in 1800
by William Herschel.
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4. The range of EMR between the visible and microwaves
region is called INFRARED region(14000-40 cm-1 ).
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5. IR REGION
From application and instrumentation point of view
infrared region is subdivided into
Wave length (m)
Wave number(cm-1 )
Near infrared
0.75-2.5
14000-4000
Mid infrared
2.5-50
4000-400
Far infrared
50-300
400-40
Region
Mid IR
(4000-40 cm-1)
Functional group
region
Finger Print/Single
bond region
(4000-1400 cm-1)
(1400-40cm-1)
 Stretching vibrations occurs in
F.G. region
 Bending vibrations occurs in F.P.
region
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6. General Purpose Of
IR Radiation
Infrared light is used in industrial, scientific, and
medical applications.
• In Telescopes to detect planets
• Finding heat leaks from houses
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7. Contd...
• Infrared thermal-imaging cameras are used to detect heat loss
in insulated systems, to observe changing blood flow in the
skin, and to detect overheating of electrical apparatus.
• Night-vision devices
• Remote temperature sensing, short-ranged wireless
communication, spectroscopy, and weather forecasting.
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8. Pharmaceutical Applications




Qualitative determination of substances
Structural elucidation by determining the functional groups.
Detection of impurities
Identification of geometrical isomers for both organic &
inorganic samples
 Detection of presence of water in sample
 Quantitative determination of sample by using Beer’s-Lamberts
law
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9. PRINCIPLE
 In any molecule , atoms or groups of atoms are connected by
bonds which are similar to springs and not rigid in nature.
 Bonded atoms having different strengths due to different
masses.
 Absorption of IR energy will cause vibrational changes in
molecule and a peak will be observed, when
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10. Contd…
Conditions to obtain IR spectrum
1) Selection rule for IR spectroscopy: Dipole moment of bonds
should change during vibration.
2) When Applied infrared frequency = Natural frequency of
vibration
3) When covalent bonds are polar in nature
When sample obeys these conditions, then it gets vibrated
by absorbing radiation and gives IR spectrum.
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11. Hooke’s law
Hooke's law
of elasticity is an
approximation
which states that the
extension of a spring
is in direct
proportion with the
load added to it as
long as this load
does not exceed the
elastic limit.
K = force constant
(in dynes/cm)
m = atomic masses of atom 1 & 2
Used to calculate approximate position of band
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12. TYPES OF VIBRATIONS
Stretching Mode
In plane bending vibrations
Scissoring
Rocking
Out plane bending vibrations
Symmetric
Asymmetric
Wagging
Twisting
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13. Radiation sources
They must produce intense & steady radiation.
Nernst Glower
heated rare earth oxide rod
1-50 µm
(zirconium, yttria, thoria) (~1500 (mid- to far-IR)
K)
Globar
heated Silicon Carbide rod
(~1500 K)
1-50 µm
(mid- to far-IR)
W filament lamp
1100 K
0.78-2.5 µm
(Near-IR)
Hg arc lamp
Hg plasma
50 - 300 µm
(far-IR)
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14. Monochromators
 Filtration of desired frequency of radiation
Monochromators are 2 types
1) Prismatic Monochromator
2) Grating Monochromator
Prismatic Monochromators:
 Composed of glass or quartz and coated by alkyl halides (NaCl)
 These are 2 types
 Mono pass Prismatic Monochromator: radiation will pass once
through the prism
 Double pass Prismatic Monochromator: radiation will pass
twice through the prism
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15. Grating monochromators introduced in 1950’s
 High dispersion and resolution than prismatic
monochromators
 These are 2 types
 Reflection Grating Monochromator are common
than Transmittance Gratings
Gratings are linear grooves or lines which are
made up of Aluminium.
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16. Detectors/Transducers
Thermocouple
Thermoelectric effect dissimilar metal junction
cheap, slow,
insensitive
Bolometer
Ni, Pt resistance
thermometer (thermistor)
Highly sensitive
<400 cm-1
Golay cell
Metal cylinder with Xe gas
Faster than others &
having wide
wavelength range
Pyro electric
Tri glycine sulfate
piezoelectric material
fast and sensitive
(mid IR)
Photoconductive PbS, CdS, Pb Se light sensitive fast and sensitive
(non-thermal)
cells
(near IR)
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17. TYPES OF IR
INSTRUMENTS
NonDispersive
Dispersive
FTIR
systems
• Filters used for wavelength selection
& having sample specific Detector
• Sequential scanning of each wave
number takes place
• Widely applied and quite popular in
the far-IR and mid-IR spectrometry.
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18. Dispersive IR
Instrument
Dispersive IR
instruments are introduced in
1940’s.
Double-beam
instruments are mostly used
than Single beam instrument.
In dispersive IR
sequential scanning of wave
numbers of light takes place.
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19. In double beam spectrometer , beam separates into two
and passes to sample & reference.
Prismatic monochromators have been replaced with
Grating monochromator.
Dispersive IR failed due to monochromator containing
narrow slits which limit the wave number of radiation.
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20. (X) It containing all movable parts which causes mechanical
slippage
(X) Slow scan speed
(X) Less resolution, accuracy and sensitivity
(X) Only narrow frequency range can be studied
(X) Involvement of stray light
(X) Atmospheric absorptions by CO,
water also takes place.
To overcome all these problems FTIR has been developed
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21. Fourier Transform
IR Instrument
FTIR collects all wavelengths simultaneously and scans at
once.
FTIR works based on Michelson Interferometer which
having
• Beam splitter
• Fixed mirror
• Movable mirror
21

22. FTIR Instrumentation
Light
source
He-Ne gas laser
Beam splitter
Movable mirror
Sample chamber
Fixed mirror
Detector
Interferometer
22

23. When the beams are combined an
interference pattern is created
Combined beam reaches detector by
passing through sample
Obtained spectrum is referred as
Interferogram
This will be amplified and translated
into IR spectrum by FTIR
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24. Advantages
 Fast & sensitive
 All frequencies can be modulated at once
 Simple mechanical design with only one
moving part
 No stray light is involved
 When using He-Ne laser as internal
standard, no need of external calibration
 Availability of easy sampling accessories
 Air pollutants like CO, ethylene oxide
etc. can be analysed
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25. FTIR having significant advantages over
Dispersive IR due to its fast and accurate analysis.
25

26. References
Instrumental Methods Of Chemical Analysis; By Gurudeep
R. Chatwal, pg No. 2.29-2.82
Infra Red Spectroscopy: Fundamentals And Applications;
By Barbara Stuart, pg No. 16-23
Introduction to Spectroscopy, 4th edition, By Pavia,
Lampman, Kriz.
Elementary organic chemistry, By Y.R.Sharma; 2007,Pg
No.69-137
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