Infrared spectroscopy is a technique that analyzes infrared light interacting with molecules. It can be used to identify unknown substances by comparing their infrared spectra to a reference database. The technique works because molecules absorb different wavelengths of infrared light depending on their chemical bonds and structure. Common applications of infrared spectroscopy include identification of materials, detection of impurities, and studying the progress of chemical reactions.

1. Infrared Spectroscopy and its
applications
Keshav Narayan Pai
II MSc
DOS in Botany
Manasagangotri
Mysore

2. • Infrared spectroscopy (IRspectroscopy) is
the spectroscopy that deals with the infrared
region of the electromagnetic spectrum, that is
light with a longer wavelength and
lower frequency than visible light .
• Infrared Spectroscopy is the analysis of infrared
light interacting with a molecule.

3. Infrared Rays
 Infrared (IR) is invisible electromagnetic
radiation with longer wavelength than those
of visible light , extending from the
nominal red edge of the visible spectrum at
800 nm to 1 mm(0.8μm to 1000μm).
 Infrared radiation was discovered in 1800 by
astronomer Sir William Herschel.

4. They are divided into 3 regions
 Near IR Region
 Middle IR Region
 Far IR Region

5. Most of the analytical applications are
confined to the middle IR region because
absorption of organic molecules are high
in this region.

6. • When infrared 'light' orradiation hits a
molecule, the bonds in the molecule
absorb the energy of the infrared and
respond by vibrating.
IRIR
radiationradiation
vanllinvanllin
Molecular vibrationsMolecular vibrations

7.  Molecules are made up of atoms linked
by chemical bonds. The movement of
atoms and the chemical bonds look like
spring and balls (vibration).
 When internal vibrational energy
of molecule matches with
energy of externally applied IR,
quantized.

8. Symmetry streching Asymmetry streching
In plane bending
Scissoring Rocking
Out plane bending
Twisting

9. • Molecules absorb IR.
• A molecule can only absorb IRradiation
when its absorption cause a change in its
electric dipole moment.
• Molecule excited from lowerto the higher
vibrational level.
i.e. Increases the amplitude of vibration .

11. Molecularvibrations
There are 2 types of vibrations:
1.Stretching vibrations
2.Bending vibrations
1.Stretching vibrations:
 Vibration oroscillation along the line of bond.
 Change in bond length.
 Occurs at higherenergy: 4000-1250 cm-1

12. There are 2 Types of Stretching vibrations
a) Symmetrical stretching
a) Asymmetrical stretching

13. a) Symmetrical stretching:
2 bonds increase ordecrease in length
simultaneously.
H
H
C

14. b) Asymmetrical stretching
• One bond length is increased and otheris
decreased.
H
H
C

15. 2. Bending vibrations
• Vibration or oscillation not along the line
of bond.
• These are also called as deformations .
• In this, bond angle is altered.
• Occurs at low energy: 1400-666 cm-1
• 2 types:
a)In plane bending: scissoring, rocking
b)Out plane bending: wagging, twisting

16. a) In plane bending
i. Scissoring:
• Thisisan in planebending
• 2 atomsapproach each other
• Bond anglesaredecreased.
H
H
CC

17. ii. Rocking:
• Movement of atomstakeplacein thesame
direction.
H
H
CC

18. b) Out plane bending
i. Wagging:
• 2 atoms move to one side of the plane. They
move up and down the plane.
H
H
CC

19. Twisting:
• One atom moves above the plane and
anotheratom moves below the plane
H
H
CC

20. • We can also calculate an approximate value of
the stretching vibrational frequency of a bond
by treating the two atoms and theirconnecting
bond, to first approximation, as two balls
connected by a spring, acting as a simple
harmonic oscillatorforwhich the Hooke’s Law
may be applied.
• According to Hooke’s Law , The Stretching
frequency is related to the masses of the atom
and the force constant(a measure of resistance
of a bond to stretching) of a bond by the
following equation

21. Hooke’s Law

22. • There are basically two types of spectrometers
1.single beam spectrometer
2.double beam spectrometer
In a single beam spectrometerthe radiations
emitted from the source are passed through a
cell containing the sample and through the
prism which disperses the light.
• Single beam spectrometers are
simple,sensitive and versatile.

24. • The double beam spectrometers are so
constructed that the light from the source is
split into two beams of equal intensity, one
passing through the sample and the other
through the reference forcompensation.
• The two beams are recombined on to a
common axis and are alternatively focused on
to the entrance slit of the monochromator.
• Dbs are very convenient and hence used in
labs ,reserch work and in routine works

25. The main parts of IRspectrometer
are as follows:
radiation source
sample cells and sampling of
substances.
monochromators
detectors
recorder

26. IRinstruments require a source of radiant
energy which emit IRradiation which must
be:

27. Sources of IRradiations are as follows:
GLOBAR:

28. • NERNSTGLOWER:

29. • Forgas samples:
The spectrum of a gas can be obtained by
permitting the sample to expand into an
evacuated cell, also called a cuvette. Gas cell
has NaCl windows at the end.
• Forsolution sample:
Infrared solution cells consists of two windows
of pressed salt sealed. Samples that are liquid at
room temperature are usually analyzed in pure
form orin solution.

30. • Forsolid sample:
Solids reduced to small particles (less than 2 micron) can
be examined as a thin paste ormull. The mull is
formed by grinding a 2-5 milligrams of the sample in
the presence of one ortwo drops of a hydrocarbon oil
(nujol oil). The resulting mull is then examined as a
film between flat salt plates.

31. Anothertechnique is to ground a
milligram orless of the sample
with about 100 milligram
potassium bromide. The mixture
is then pressed in an evaluable
die to produce a transparent
disk.

32. FORE OPTICS
•Consists of Source, Mirrors,
M1,M2 and a Rotating
mirrors
•M1,M2 divides the beam.
•M alternately allows the
sample beam and reference
beam to pass through.

33. MONOCHROMATOR
• Splits the polychromatic radiation to
component wavelengths.
• Make use of prisms orgrating orboth.
• Resolution depends on slit width and quality
of mirrors.
• Rock salt prism is generally used in the
range of 650-4000cm-1

35. Detector
Thermal Non-thermal

37. There are fourtypes of thermal detector.
Bolometers
Thermocouple and thermopile
Pyro electric detector
Golay cell

40. THE RECORDER
•The amplified signal is recorded by an Pen Recorder.
•This instrument optically balances out differential
between 2 beams. This kind of Instrument is called
Optical null recording Spectrometer.
•More sophisticated Instruments are called Ratio-
recording Instruments. In these instruments the
intensities of both sample and reference beams are
measured and ratioed.

42. FT-IR
• FT-IRstands forFourierTransform Infrared
Spectrometer, the preferred method of infrared
spectroscopy.
• Dispersive infrared spectrometers sufferfrom several
disadvantages in sensitivity, speed and wavelength
accuracy.
• An entirely different principle is involved in Fourier
Transform infrared spectroscopy, which centres on a
Michelson interferometer, so that the method can also
be called Interferometric infrared spectroscopy.

44. Dispersive Spectrometer FTIR
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.
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.)

45. • 1. Identification of Substances1. Identification of Substances
To compare spectrums.
No two samples will have identical IR spectrum.
Criteria: Sample and reference must be tested in
identical conditions, like physical state, temperature,
solvent, etc.

46. The “Fingerprint” Region (1200 to 700 cm-
1
) :
• Small differences in structure &
constitution of molecule  result in
significant changes in the peaks in this
region.
• Hence this region helps to identify an
unknown compound.

47. Structural/Functional ComponentsStructural/Functional Components

49. Alkene
Alkane

50. • IR Spectrum in Alcohol

51. Computer Search Systems
• Virtually all infrared instrument manufactures
now offercomputersearch systems to assist
chemist in identifying compounds from stored
infrared spectral data.
• The position and relative magnitudes of peaks
in the spectrum of the analyte are determined
and stored in memory to give a peak profile,
which can then be compared with profiles of
pure compounds stored.

53. Studying Progress of ReactionsStudying Progress of Reactions
• Observing rate of disappearance of
characteristic absorption band in
reactants; or
• Rate of increasing absorption bands in
products of a particularproduct.
• Eg.: O—H= 3600-3650 cm-1
C=O = 1680-1760 cm-1

54. Detection of ImpuritiesDetection of Impurities
• Determined by comparing sample
spectrum with the spectrum of pure
reference compound.
• Eg.: ketone impurity in alcohols.

55. Measurement of Paints &Measurement of Paints &
VarnishesVarnishes
• Measured by ‘reflectance
analysis’.
• Advt: Measure IR
absorbance of paints on
appliances orautomobiles
without destroying the
surface..

56. Examination of Old Paintings &Examination of Old Paintings &
ArtifactsArtifacts
• Help to determine fake
“masterpieces”.
• Varnish &paints from
old items (statues,
canvas, etc.) are
analysed by IR
spectroscopy.
• Presence of new paint
traces implies the
“masterpiece” is fake.

57. In IndustryIn Industry
1. Determine impurities in raw materials (to
ensure quality products).
2. ForQuality Control checks; to determine the %
of required product.
3. Identification of materials made in industrial
research labs,
ormaterials of competitors.
E.g.: Impurity in bees wax (with petroleum
wax)

58. Medical applications
Hair:
• Narcotics in hair
• Effects of bleaching
• Effects of UV-radiation
Skin:
• Moisture content
• Lipid content
• Effects of cosmetics

59. SKIN

60. Difference IRspectrum of two healthy
patients is very small
0
.2
.4
.6
.8
1
1.2
1.4
Absorbance
1750 1700 1650 1600 1550 1500 1450
Wavenumber (cm
-1
)
difference

61. Spectral deviations between
“healthy” and “ill” patients,
exhibiting different RIARvalues
-.15
-.1
-.05
0
.05
.1
.15
.2
Arbitrary
1700 1600 1500 1400 1300 1200 1100 1000 900
Wavenumber(cm
-1
)
1550 RIAR
720 RIAR
20 RIAR
-.15
-.1
-.05
0
.05
.1
.15
.2
1600 1500 1400 1300 1200 1100 1000 900

62. HAIR
Infrared spectra of untreated (lower trace) and bleached (upper trace) female hair sample. The
difference spectrum is given below (dashed line, with 5x ordinate expansion)

63. ADVANTAGES
• Detection (health condition)
• Prevention (early diagnosis)
• Monitoring
• Diagnosis (underinvestigation)

64. • FT-IRSpectroscopy can be applied forthe
determination of a biochemical
metabolite in biological fluids.
• FT–IRspectroscopy has been used forthe
determination of glucose, total protein,
urea, triglyceride, cholesterol, and very
low density lipoproteins in plasma and
serum.

66. Reference
• Stuart.B.2004.Infrared Spectroscopy: Fundamentals and
Applications, John Wiley and Sons Inc,New York, Pp:
223-240.
• Upadhyay.A,Upadhyay.K, and Nath.N. 2012. Biophysical
Chemistry(Principles and Techniques). Himalaya
Publishing House. Pvt. Ltd, Mumbai. Pp.175-186.
• Holme.D.J& Peck.H.1998.Analytical Biochemistry,Third
Edition,Pearson Education Limited.Pp:37-39.
• http://www.chem.ucla.edu/~webspectra/irtable.html