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PRESENTED BY :-
M.S. PHARM ,NIPER ,KOLKATA
Infrared (IR) Spectroscopy
IR deals with the interaction of infrared radiation with matter.
The IR spectrum of a compound can provide important
information about its chemical nature and molecular structure.
Most commonly, the spectrum is obtained by measuring the
absorption of IR radiation, although infrared emission and
reflection are also used.
Widely applied in the analysis of organic materials, also useful
for polyatomic inorganic molecules and for organometallic
NEAR INFRARED or OVERTONE: 1.2 -2.5 m,
MID INFRARED or VIBRATION: 2.5 - 25 m,
FAR INFRARED or ROTATION: 25 - 400 m,
X-RAY ULTRAVIOLET INFRARED MICRO-
200 nm 400 nm 800 nm
2.5 m 25 m 1 m 5 m
ORGANIC STRUCTURE DETERMINATION
How do we know
How atoms are connected together?
Which bonds are single, double, or triple?
What functional groups exist in the molecule?
If we have a specific stereoisomer?
1. IR radiation source
Be continuous over the wavelength range used.
Cover a wide wavelength range.
Be constant over long periods of time.
1. The Nernst Glower
They electrically heated to about 2000 °C.
It composed of a mixture of rare earth oxides such as zirconium
oxide (ZrO2), yttrium oxide (Y2O3) and thoria.
sealed by a platinum leads to the ends to permit electrical
Consist of cylindrical hollow rod or tube having a diameter of 1-
2 mm and length of 30 mm.
its lifetime depends on the operating temperature and the care
taken in handling it
It provide maximum radiation about 7100cm-1 (1.4 )
2. The Globar source
The power consumption is normally higher than that of the
It provide maximum radiation about 5200cm-1 .
Heated to about 2000 °C.
It’s sintered silicon carbide rod, usually about 50 mm in length
and 5 mm in diameter
Less convenient to use, more expensive & less intense than
Water cooling is needed to cool the metallic electrodes attached
to the rod
4. The Mercury Arc.
In the UV and visible regions, this lamp emits atomic
Hg emission lines that are very narrow and discrete, but
emits an intense continuum in the far-IR region.
When current passes through the lamp, mercury is
vaporized, excited, and ionized, forming a plasma
discharge at high pressure.
It is a high pressure mercury arc which consist of a
quartz - jacketed tube containing Hg vapor at P > 1 atm.
Used for Far IR region (wave no<200cm-1).
3. Incandescent Wire Source
Incandescent wire sources are longer lasting but
of lower intensity than the glower or globar.
A similar source is a rhodium wire heater sealed
in a ceramic cylinder.
A tightly wound spiral of nichrome wire heated
to about 1100 k by an electric current.
Lower intensity IR source but longer life than the
Globar or Nernst glower.
• Used as dispersive element.
• Constructed of various metal halide salts
• Sodium chloride is most commonly prism salt used.
• These salts are subjected to mechanical & thermal
instability or water solubility.
• Protection against damage must be continuously
• Gratings are nothing but rulings made on some
materials like glass, quartz or alkylhalides depending
upon the instrument, The mechanism is that
diffraction produces reinforcement. The rays which
are incident upon the gratings gets reinforced with the
Advantages over Prism:-
Made with materials like aluminium
which are not attacked by moisture.
Used over cosiderable wavelength range.
3. Sample cell & Sampling of
Infrared spectra may be obtained for gases, liquids or
solids (neat or in solution)
• Material containing sample must be transparent to
the IR radiation. So, the salts like NaCl, KBr are only
A. Sampling of solids
Various techniques used for preparing solid samples are as
1. Solid run in Solution –
• In this technique, solid sample may be dissolved in a non-
• Provided that there is no chemical interaction with the
solvent and the solvent is not absorbed in the range to be
• A drop of solution is placed on the surface of alkali metal disc
solvent is to dryness leaving a thin film of the solute.
2. Solid film technique
• If the solid is amorphous in nature.
• Then the sample is deposited on the surface of a KBr
or NaCl cell by evaporation of a solution of the solid.
• Ensured that the film is not too thick to pass the
3. Pressed pellet technique
• In this technique, a small amount of finely ground solid
sample is mixed with 100 times its weight of potassium
bromide and compressed.
• To form thin transparent pellet(1-2mm thick & 1cm in
diameter) using a hydraulic press.
• These pellets are transparent to IR radiation and it is used for
• Kbr pellets stored for long period of time.
• Resolution of spectrum is superior.
• Always have a band at 3450cm-1 due to OH group of
• Due to high pressure(~25000 psig) polymorphic
• Not successful for polymers. (diffcult to grind with
4. Mull technique
This is then mounted in a path of IR beam and the spectrum is
A thin film is applied onto the salt plates.
In this technique, the finely crushed sample is mixed with Nujol
(mineral oil) in a marble or agate mortar, with a pestle to make
a thick paste.
2. Sampling of liquids
Some salt plates are highly soluble in water, so the sample and washing reagents must
For most liquids, the sample cell thickness is 0.01-0.05 mm.
The sample thickness should be selected so that the transmittance lies between 15-
Aqueous solvents cannot be used because they cannot dissolve alkali halides. Organic
solvents like chloroform can be used.
Liquid sample cells can be sandwiched using liquid sample cells of highly purified
alkali halides, normally NaCl. & KBr can also be used.
3. Sampling of gases
Gas analysis are not commonly used because of lack of
A sample cell with a long path length (5 – 10 cm) is needed
because the gases show relatively weak absorbance.
It is similar to the liquid sample cell.
The sample cell is made up of NaCl, KBr etc
• The detectors can be classified into three
1. Thermal detectors:- Their responses depend upon
the heating effect of radiation .
2. Pyroelectric detectors:- Pyroelectric effect depends
on the rate of change of the detector temperature rather than
on the temperature itself.
3. Photoconducting detectors:- Most sensitive .
If one welded joint (called the hot junction) becomes hotter than the other
joint (the cold junction), a small electrical potential develops between the
A thermocouple is made by welding together at each end two wires made
from different metals.
Response time ~60m sec.
The potential difference generated in the wires is a function of the
temperature difference between the junctions and, therefore, of the intensity
of IR radiation falling on the hot junction.
The hot junction is xposed to the IR radiation, which increases the
temperature of the junction.
In IR spectroscopy, the cold junction is carefully screened in a protective box
and kept at a constant temperature.
To enhance sensitivity several thermocouples connected in series
are called a “thermopile”
Bridge remain balance.
When no radiation fall on bolometer.
A similar strip of metal used as balancing arm of bridge.
A bolometer is made one arm of Wheatstone Bridge.
It consist of thin metal conductor.
A bolometer is a type of resistance thermometer.
Response time ~4m sec.
Measure the intensity of radiation.
Amount of current flow.
Current flow through Galvanometer
Unbalanced, due to change in resistance.
When IR radiation fall.
Made of fused mixture of metal oxides.
As the temp. of mixture increases, it’s electrical
resistance decreases. (opposed to Bolometer)
Response time very slow...