This document discusses the principles, instrumentation, and applications of a dispersive infrared spectrophotometer. It describes how this type of IR spectrometer works by using radiation sources like globars or Nernst glowers, monochromators to separate wavelengths, and detectors like photo detectors or thermal detectors to analyze the absorbed infrared wavelengths. Key applications of dispersive IR spectrophotometers include identifying organic and inorganic compounds by their functional groups and determining molecular structure and orientation. However, it also notes some disadvantages like slower scan speeds and less sensitivity compared to Fourier transform IR spectrometers.

1. Presented by:
Himani peshavaria
19-PCH-055
Msc analytical chemistry sem 3.
Dispersive IR Spectrophotometer

2. Contents
 Principle
 Instrumentation
 Working
 Applications
 Disadvantages

3. Principle
 IR Spectroscopy detects frequencies of infrared
light that are absorbed by a molecule.
 Molecules tend to absorb these specific
frequencies of light since they correspond to the
frequency of the vibration of bonds in the
molecule.
 The energy required to excite the bonds
belonging to a molecule, and to make them
vibrate with more amplitude, occurs in the
Infrared region.
 A bond will only interact with the electromagnetic
infrared radiation, if it is polar.

4. Instrumentation
 An IR spectrometer consists of three basic
components:
1. Radiation source.
2. Monochromator.
3. Detector.

5. 1) Radiation source
 The common radiation source for the IR
spectrometer is an inert solid heated electrically
to 1000 to 1800 °C.
 Three popular types of sources are :
a) Nernst glower (constructed of rare-earth
oxides),
b) Globar(constructed of silicon carbide),
c) and Nichrome coil.

6. a) Nernst glower
 It is composed of rare earth oxides e.g.,
zirconium oxide , yttrium oxide.
 It consists of a hollow rod with a diameter of 1 to
2 mm and a length of perhaps 20 mm.
 Platinum ends leads to passage of electricity.

7.  It is non conducting at room temperature and
must be heated by external means to bring it to a
conducting state.
 Advantage is that it produces intense IR radiation
and IR range is also wide.
 Disadvantage of Nernst glower is its frequent
mechanical failure.

8. b) Globar
 It is a rod of silicon carbide, usually about 5 cm in
length and 0.5 cm in diameter.
 It is heated to a temperature between 1300 and
1700 °C.

9. c) Nichrome coil.
 It produces a lower intensity of radiation than the
Nernst or Globar sources, but has a longer
working life.
 The wire source is a tightly wound coil of
nichrome wire.
 Electrically heated to 1100 °C.

10. 2) Monochromators
 Monochromator splits polychromatic light into
monochromatic light.
 They are mainly of two types:
a) Prism monochromators.
b) Grating monochromators.

11. a) Prism
 Several materials are used for the construction of
prism.
 Quartz is employed for near IR region.
 Crystalline sodium chloride is the most common
prism material.
 Its dispersion is high between 5-15 μm.
 Crystalline potassium bromide and
cesium bromide provide prism
materials for far IR.
 Lithium fluoride is useful
for near ir region.

12. b) Grating
 A grating is a device that consists of a series of
identically shaped, angled grooves.
 Gratings for the infrared region have a much
wider spacing between the grooves.

13. 3) Detectors
 Most detectors used in dispersive IR
spectrometers can be categorized into two
classes:
a) photo detectors : they use the quantum effect of
the infrared radiation to change the electrical
properties of a semiconductor.
b) Thermal detectors : IR radiation produces a
heating effect that alters some physical property
of the detector.

14. a) Photo detectors
 They are constructed from semi conductor
crystals such as lead sulfide, lead selenide and
germanium.
 Absorption of radiation by these materials results
in excitation of non conducting electrons to an
excited conducting state.
 The increase in conduction or decrease in
resistance can be readily measured and is
directly related to the number of photons reaching
the semiconductor surface.

15. b) Thermal detectors
 Thermal detectors are mainly of three types :
1. Thermocouple
2. Bolometer
3. Golay cell

16. 1) Thermocouple
 Thermocouple consists of a pair of junction of different
metal wires.
 The potential difference (voltage) between the junctions
changes according to the difference in temperature
between the junction.
 Hence, current flow which is altered is recorded by the
recorder.

17. 2) Bolometer
 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.
 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

18. 3) 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 device is
modulated in accordance with
the power of the radiant beam

19. Working
 Two equivalent beams from the same source
pass through the sample and reference chambers
respectively.
 Using an optical chopper, the reference and
sample beams are alternately focused on the
detector.
 Commonly, the change of IR radiation intensity
due to absorption by the sample is detected as a
signal that is translated into the recorder
response through the actions of synchronous
motors.

21. Applications
 Identification of all types of organic and many
types of inorganic compounds.
 Determination of functional groups in organic
materials.
 Determination of molecular orientation .
 Determination of molecular conformation
(structural isomers) and stereochemistry (
geometrical isomers ).

22. Disadvantages
 Slow scan speed make dispersive instruments
too slow for monitoring systems undergoing rapid
changes .
 Less sensitivity .
 Many moving parts may results in mechanical
slippage .
 Accuracy is less compared to Fourier Transform
IR(FTIR) .

23. References
 Introduction to Spectroscopy Book by Donald L.
Pavia, Gary M. Lampman, and George S. Kriz
 Handbook of Instrumental Techniques for Analytical
Chemistry Book by Frank A. Settle.
 Principles of Instrumental Analysis 6th Edition
by Douglas A. Skoog , F. James Holler , Stanley R.
Crouch .
 http://www.authorstream.com/Presentation/vvkvjoshi
-1948442-dispersive-ir-spectrophotometer/
 http://delloyd.50megs.com/MOBILE/infrared_spectro
scopy.html
 spectroscopyinstrumentationby-dr-
190618085453.pdf