There are four main techniques used to prepare solid samples for IR spectroscopy: dissolving solids in solution, solid films, mull technique, and pressed pellet technique. The mull technique involves grinding the solid sample with a mulling agent like mineral oil or nujol to form a paste between IR windows. The pressed pellet technique uses potassium bromide to form a compressed pellet, avoiding interference from a mulling agent. Liquids can be analyzed directly in liquid sample cells of appropriate thickness.

1. SAMPLE PREPARATION TECHNIQUES IN IR
Generally 4 techniques are employed for preparing solid samples:
1. Solids run in solution.
2. Solid Films.
3. Mull technique.
4. Pressed pellet technique
Solids run in solution:
• Solids may be dissolved in non-aqueous inert solvent and a
drop of this solution is placed on an alkali metal disc and
solvent is allowed to evaporate, leaving a thin film of solute
(or the entire solution is placed in a liquid sample cell)
which is then mounted in spectrometer.
• If the solution of solid can be prepared in a suitable solvent
then the solution is run in concentration of cells for liquids.
• Some solvents used are chloroform, carbon tetrachloride,
acetone, Cyclohexane etc
Demerit:
• This method can’t be used for all solids because suitable
solvents are limited in number & there is no single solvent
which is transparent throughout IR region.
Precautions:
• Solute chemical interaction with the solvent must be taken
into consideration especially for compounds having property
of H-bonding.
• The solvent should not absorb in the studied range.
Solid films:
• If a solid is polymer resins & amorphous solids, the sample
is dissolved in any reasonable volatile solvent & this
solution is poured on a rock salt plate (Nacl or KBr) &
solvent is evaporated by gentle heating.

2. SAMPLE PREPARATION TECHNIQUES IN IR
• If solid is non-crystalline, a thin homogenous film is
deposited on the plate which can be mounted and scanned
directly.
• Sometimes polymers can be “hot pressed” onto plates.
Merit and Demerit:
This method is useful for rapid qualitative analysis but becomes
useless for carrying out quantitative analysis
Mull technique:
• In this technique a small quantity of sample is thoroughly
ground in a clean mortar until the powder is very fine.
• After grinding, the mulling agent (mineral oil or Nujol) is
introduced in small quantities just sufficient to take up the
powder (mixture approximates the consistency of a
toothpaste).
• The mixture is then transferred to the mull plates & the
plates are squeezed together to adjust the thickness of the
sample between IR transmitting windows.
• This is then mounted in a path of IR beam and the
spectrum is run.
Demerit:
• Although Nujol is transparent throughout IR region, yet it
has a disadvantage that it has absorption maxima at 2915,
1462, 1376 & 719 cm-1.
• So when IR spectrum of solid sample is taken in Nujol mull,
absorption bands of solid sample that happen to coincide
with the absorption bands of the Nujol mull will be hidden
(but others will be clearly seen in IR spectrum) and then
interferes with the absorption of the sample.
• This method is good for qualitative analysis but not for
quantitative analysis

3. SAMPLE PREPARATION TECHNIQUES IN IR
Pressed pellet technique:
• In this technique a small amount of finely ground solid
sample is intimately mixed with about 100 times its weight
of powdered Potassium bromide, in a vibrating ball mill.
• This finely ground mixture is then pressed under very high
pressure (25000 p sig) in evacuable die or minipress to form
a small pellet (about 1-2 mm thick and 1cm in diameter).
• The resulting pellet is transparent to IR radiation and is run
as such.
• The powder (KBr + sample) is introduced in between the 2
bolts and the upper screw A is tightened until the powder is
compressed to a thin disc.
• After compressing the sample bolts A & A1 are removed and
a steel cylinder with pellet inside it is placed in path of the
beam of IR spectrometer and a blank KBr pellet of identical
thickness is kept in the path of reference beam
Advantages of this technique over mull technique
• The use of KBr eliminates the problem of bands which
appear in IR spectrum due to the mulling agent as in this
case no such bands appear.
• KBr pellets can be stored for longer periods of time.
• As concentration of the sample can be suitably adjusted in
pellets, it can be used for quantitative analysis.
• The resolution of spectrum in KBr is superior to that
obtained with mulls
Demerits:
• It always has a band at 3450 cm-1, from –OH group of
moisture present in the sample.
• The high pressure involved during the formation of pellets
may bring about polymorphic changes in crystallinity in the

4. SAMPLE PREPARATION TECHNIQUES IN IR
samples, (Especially inorganic complexes) which may cause
complications in IR spectrum. In some cases, even
substitution of the ligand by bromide may be possible in
inorganic complexes.
• This method is not successful for some polymers which are
difficult to grind with KBr.
Sampling for gases:
Small size particles hence the cells are large.
•10 cm to 1m long
•Multiple reflections can be used to make the effective path
length as long as 40 cm
•Lacks sensitivity
Sampling for liquids:
Liquid samples taken.
•Put it into rectangular cells of KBr, NaCl etc.
•I R spectra obtained.
•Sample thickness … such that transmittance lies between 15
– 20 % i.e., 0.015 – 0.05 mm in thickness.
•For double beam, matched cells are generally employed
•One cell contains sample while other has solvent used in
sample.
•Matched cells should be of same thickness, protect from
moisture.