Ir spectroscopy instrumentation, b y -dr. umesh kumar sharma and arathy s a

INFRA – RED
SPECTROSCOPY
INSTRUMENTATION
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By: Dr. Umesh Kumar Sharma
& Arathy .S .A
Department Of Pharmaceutics,
Mar Dioscorus College Of Pharmacy,
Alathara, Sreekaryam, Thiruvananthapuram

Contents
 Instrumentation of Dispersive and Fourier Transform Infra-
Red Spectrophotometer.
 Factors affecting Vibrational Frequencies.
 Application of Infra-Red Spectrophotometer.
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INSTRUMENTATION
 The usual optical materials, glass or quartz absorb strongly in
the infrared region.
 The apparatus for measuring infrared spectra is appreciably
different from that for the visible and ultraviolet regions.
The main parts of an IR spectrometer are:
 IR radiation sources,
 Monochromators,
 Sample cells and sampling of substance,
 Detectors,
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The sources of IR radiation are :
a. Incandescent lamp
 In the near IR instruments an
ordinary incandescent lamp is
generally used.
 This fails in the far IR because
it is glass enclosed and has a
low spectral emissivity. Incandescent Lamp
IR radiation sources :
Requirements:
 A source of radiant energy which provides a means for
isolating narrow, frequency bands.
 Intense enough for detection.
 Steady.
 Extend over the desired wavelengths.

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b. Nernst glower
•It consists of a hollow rod,
composed of rare earth oxides such
as zirconia, yttria and thoria.
•It is non conducting at room
temperature and must be heated by
external means to bring it to a
conducting state.
•Glower is generally heated to a
temperature between 1000 to
18000c.
• Maximum radiation is about
7100cm-1 .
•Disadvantage of Nernst glower is
its frequent mechanical failure.
Nernst Glower

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c. Globar source
•It is a rod of sintered silicon
carbide.
•It is heated to a temperature
between 1300 and 17000 c.
•It emits maximum radiation at
5200 cm-1 .
•The main disadvantage is, it is a
less intense source than the Nernst
glower. Globar Source

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d. Mercury arc lamp
 In the far IR region the sources described above lose
their effectiveness and special high pressure mercury arc
lamps are used.
Mercury Arc
 This device consists of a quartz-jacketed tube containing
mercury vapor at a pressure greater than 1 atmosphere.

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Monochromators:
 The radiation source emits radiation of various
frequencies.
 The sample in IR spectroscopy absorbs only at certain
frequencies, it is therefore becomes necessary to select
desired frequencies from the radiation source and reject
the radiation of other frequencies.
 This selection has been achieved by means of
monochromators which are mainly of two types-
1. Prism monochromators
2. Grating monochromators

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Prism monochromator
Single pass monochromator:

Sample cells and sampling of substances:
 IR spectroscopy is used for characterization of soild,
liquid and gas samples.
 The only common point to the sampling of different
phases have to be treated differently.
 This condition restricts selection to only certain salts like
Nacl or KBr.
Detectors
The various types of detectors used in IR spectroscopy are
A. Bolometers
B. Thermocouple
C. Thermistors
D. Golay cell
E. Photoconductivity cell
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Bolometers:
 It consist of a thin metal conductor.
 When radiation falls on this conductor, its temperature
changes, as the resistance of a metallic conductor changes
with temperature.
 The degree of change in resistance is regarded as a
measure of the amount of radiation that has fallen on the
bolometer.
 It is made one arm of the Wheatstone bridge.
 When no radiation falls on the bolometers, the bridge
remains balanced.
 If radiation falls on the bolometers, the bridge become
unbalanced due to changes in the electrical resistance
which causes a current to flow through the galvanometer.
13BOLOMETER

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Thermocouple:
 It is based upon the fact that an electric current will flow
when two dissimilar metal wires are connected together at
both ends and a temperature differential exits between the
two ends. The end exposed to the IR is called the hot
junction.
 In order to increase the energy gathering efficiency it is
usually a black body.
 The other connection the cold junction is thermally
insulated and carefully screened from stray light.
 The electricity which flows is directly proportional to the
energy differential between the two connections.
THERMOCOUPLE

Thermistors:
 A thermistor is made of a fused mixture of metal oxides.
 As the temperature of the mixture increases, its electrical
resistance decreases.
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THERMISTORS

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Golay cell:
 It consists of a small metal cylinder which is closed by a
blackened metal plate at one end and by a flexible
metalized diaphragm at the other.
 After filling the cylinder with xenon, it is sealed.
 When IR radiation is allowed to fall on the blackened
metal plate, it heats the gas which causes it to expand.
 The signal seen by the phototube is modulated in
accordance with the power of the radiant beam incident
on the gas cell.
Golay cell:

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Photoconductivity cell:
 This is a nonthermal detector of greater sensitivity.
 It consists of a thin layer of lead sulphide supported on
glass and enclosed into an evacuated glass envelope.
 When IR radiation focussed on lead sulphide, its
conductance increases and causes more current to flow.
PHOTOCONDUCTIVITY CELL

FOURIER TRANSFORM SPECTROMETERS
 In the FT spectrometers, coding is accomplished by splitting the
source into two beams.
 It is used for data processin.g
 FT spectrometers can be single beam or double beam instruments.
 This type of instrument is to first obtain a reference interferogram
by scanning a reference, coding the data and storing the results in
the memory of this instrument computer.
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DISPERSIVE INSTRUMENTS
 It is double beam recording instrument.
 The double beam design is less demanding with respect
to the performance of sources and detectors.
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FACTORS AFFECTING
VIBRATIONAL FREQUENCIES
1. Coupled vibrations and Fermi resonance,
 There is one stretching absorption frequency for an
isolated C-H bond.
 In the case of methylene (-CH2-) group, two absorption
occur which correspond to symmetric & asymmetric
vibrations.
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H H H H
 Asymmetric vibrations always takes place at higher
wavelength compared with the symmetric vibrations.
These are known as coupled vibrations.

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 In IR spectrum absorption bands are spread over wide
of frequencies.
 It may happen that the energy of an overtone level
chances to coincide with the fundamental mode of
different vibrations
 A type of resonance takes place as in the case of
coupled pendulums. This type of resonance is known
as Fermi resonance
2. Electronic Effects
 The frequency shifts from the normal position of
absorption occurs because of electronic effects which
include inductive effect, mesomeric effect, field effect
etc.
 Under the influence of these effects, the force constant
or the bond strength changes and its absorption
frequency shifts from the normal value.

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3. Hydrogen Bonding:
 It gives rise to downward frequency shifts.
 Stronger the hydrogen bonding greater the absorption shift
towards lower wave number from the normal value.
APPLICATIONS OF IR
 Identification of functional group and structure elucidation.
 Identification of substances.
 Studying the progress of the reaction.
 Detection of impurities.
 Quantitative analysis.

REFERENCE:
 Instrumental analysis, Skoog, page no. 488-505.
 Organic spectroscopy,William Kemp
 Instrumental methods of chemical analysis, Gurdeep R.
Chatwal.
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