Sampling techniques in infra red spectroscopy

2. Sampling techniques in infra
red spectroscopy
By
Ssb.saikanth
Dept of pharmaceutical analysis and
quality control

3. SAMPLE HANDLING TECHNIQUES IN IR
SPECTROSCOPY
• Samples of different phases have to be treated
differently
• The only common point to the sampling of
different phases is that the material
containing the sample must be transparent to
IR radiation
• This condition restricts our selection to only
certain salts like NaCl or KBr

4. SAMPLE CELLS
• The alkali halides are widely used, particularly
NaCl, KBr and ThBr which is transparent at
wavelength as long as 625 cm -1.
• AgCl cells are used for aqueous and moist samples
but it is soft and easily gets deformed and darkens
on exposure to visible light.
• Cells made up of Teflon and polyethylene can be
used.

5. SAMPLE CELLS

6. DIFFERENT KINDS OF SAMPLING
1.SAMPLING OF SOLIDS
Generally 4 techniques are employed for preparing solid samples .
• Solids run in solution.
• Solid Films.
• Mull technique.
• Pressed pellet technique.
2.SAMPLING OF LIQUIDS
3.SAMPLING OF GASES

7. SAMPLING OF SOLIDS
• 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 which is then mounted in
spectrometer.
• Some solvents used are chloroform, carbon tetrachloride,
acetone, Cyclohexane etc
• 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.
• This method is useful for rapid qualitative analysis but becomes
useless for carrying out quantitative analysis.

8. • 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
• 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.
• Although Nujol is transparent throughout IR region, yet it has a
disadvantage that it has absorption maxima at 2915, 1462, 1376 & 719 cm-
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 interference can be avoided by using Hexa chlorobutadiene in
combination with nujol

9. MULL TECHNIQUE

10. • 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.

11. 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

12. Sampling of Gases:
The gas sample cell is similar to cell for liquid samples and made of KBr, NaCl &
so on. To compensate for the small number of molecules of a sample that is
contained in a gas, however, the cells are larger; usually they are about 10cm
long, but they may be up to 1m long. Multiple reflections can be used to make
effective path length as long as 40m, so that constituents of the gas can be
determined.

13. • Sampling of Liquids:
• Liquids at room temperature Rectangular cells
made of NaCl , KBr or ThBr Sample thickness should
be 0.01-0.05mm to give transmittance between
15% and 70%.
• If a cell possesses good quality windows, flat and
parallel, its thickness, t, in cm can be calculated
from the following equation:
• (2t=N/w 1 -w 2)
• Where, N is the number of fringes between wave
numbers w 1 & w 2

14. THANK YOU