dispersive ir spectrometer

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