This document discusses various methods for preparing samples for infrared spectroscopy analysis in different phases. It begins by outlining some common challenges in sampling solids, liquids, and gases for IR. It then describes several techniques for preparing solid samples, including dissolving solids in solution, making solid films, using KBr pellet or mull techniques. For liquids, it recommends using cells of fixed pathlength and adjusting parameters like concentration and path length. Gases can be analyzed using long pathlength gas cells. Problems that may occur in spectra are also outlined.

1. Submitted By:
HEMA BANSAL
M.Pharma(SEM-1)
Section-B

2. Introduction
IR transmitting material
Sampling of solids
Sampling of gases
Sampling of liquids
Problems

3. In 1965 Miller has developed appropriate method to handle
sample in the gas, liquid and solid phase. However a no. of
problems exit with regard to sample handling.
The main problems-
 Is that almost all substances absorb IR radiation.
 This restricted choice of the materials that may be used for
construction of the sample cell.
IR spectroscopy used for the characterization of:
 Solid
 Liquid
 Gas

4. Sample must be transparent to IR radiation.
Conditions restricts selection of salts like NaCl or KBr.
Final choice of salt depends on the wavelength range.
INFRARED-TRANSMITTING MATERIALS
Materials Wavelength Refractive index
range(um) at 2um
NaCl 0.25-17 1.52
KBr 0.25-25 1.53
KCl 0.30-20 1.5
AgBr 0.40-23 2.2
CaF2 0.15-9 1.40
BaF2 0.20-11.5 1.46
ZnS 0.57-14.7 2.26
Germanium 0.50-16.7 4.0
Silicon 0.20-6.2 3.5
Polyethylene 16-300 1.54

5. Various techniques for preparing solid sample:
 Solids Run in Solution
 Solid Films
 KBr Pellete Technique
 Mull Technique
 Cast Film Technique

6. Solids may also be dissolved in a non-aqueous solvent.
A drop of the solution is placed on a alkali metal disk.
The solvent allowed to evaporate leaving a thin film of the
solute.
The entire solution is placed in a sample cell.
Limitation:
 Cannot be used for all solids (limited number of suitable
solvents)
 No single solvent which is transparent throughout IR region.

7. Solid films
For amorphous solids
Sample is deposited on the surface of a KBr or NaCl
cell evaporation of a solution of the solid.
Useful for rapid qualitative analysis.
Not carried out for quantitative analysis.

8. KBr Pellet Technique:
A sample in KBr powder can be compressed under pressure with or without
vacuum to form transparent disks.
A solid sample of approximately 2-3 mg is allowed to mix with about 0.5- 1 g of
KBr.
Thoroughly grind the mixture in a mortar, then press the mixture in a pellet die
under a pressure of about 6000-10000 psi to obtain a transparent disk.
This powder mixture is then pressed in a mechanical press to form a translucent
pellet through which the beam of the spectrometer can pass.
Advantages:
KBr pellets stored for long periods of time.
Resolution of the spectrum is superior.
Disadvantages:
High pressure causes polymorphic changes
Not successful for some polymer(difficult to grind)

9.  Substances that transmit a wide range of IR frequency
 Help to minimize scattering by surrounding the analyte
with a medium whose refractive index more closely
matches to that of the sample than does air.
 Examples:
NUJOL
Fluorolube mulls
Refined mineral oil
Perfluro kerosene
Hexafluro butane

10. Mull technique
The mull technique is probably one of the most common and easiest ways to
prepare a solid sample.
The sample is prepared by first grinding about 5 mg of a solid sample in a
mortar.
Adding 2 drops of the mulling oil (Nujol or Fluorolube mulls) to the sample
Grinding the mixture thoroughly.
The suspended particles obtained must be less than 2m in order to avoid
excessive scattering of radiation.
 Paste the suspended particles on one of the IR salt plates, and cover with the
matching salt plate.
Mounted in a path of infrared beam and spectrum is run.

11. Cast Film Technique
 It is used mainly for polymeric materials.
The sample is first dissolved in a suitable, non hygroscopic solvent.
A drop of this solution is deposited on surface of KBr or NaCl cell.
The solution is then evaporated to dryness and the film formed on the cell is
analysed directly.
Care is important to ensure that the film is not too thick otherwise light cannot pass
through.
This technique is suitable for qualitative analysis.
The final method is to use microtomy to cut a thin (20–100 µm) film from a solid
sample.
This is one of the most important ways of analysing failed plastic products for
example because the integrity of the solid is preserved.

12. GASES
Gas sample cell –
Cylindrical glass body
Surfaces in the light path are made of KBr,NaCl.
Cell length-10cm
To end of the cell are attached disk of appropriate window
material with wax epoxy cement or pressure plate.
The beam is directed to 90degree angle from its normal direction
and thus longer path length is provide.
The use of longer cell even with effective path length as long as
1km.
For some special application-
Example: Corrosive gases or vapors metals such as stainless
steel, nickel are used for cell bodies.

13. The gas sample is carried from the vessel to the detector site, where
it flows through a tube only to return, unchanged, to the sterilizer.
The IR source is placed at one end of the tube and the IR detector
at the other.
The IR beam passes through the gas sample, and the detector
collects the spectral reading and transmits the data to a
microprocessor .
Gas can be conducted through the beam path, or the beam can be
reflected through a window in the sterilizer so that it crosses the
headspace gas and returns to the detector.

14. Liquid samples are used at room temperature.
There IR spectra are obtained directly
To obtain the transmittance in the range of 15%-70% ,select-
 Sample concentration
 Path length should be selected
Parameters:
Rectangular cell(NaCl,KBr)
Cell thickness-0.02mm in the case of most neat liquids
Concentration-10%
Cell length-0.1 mm
 The solvent selected must be transparent
Solvents used:
Acetone Methane
Toluene Hexane
Benzene Dioxene
CS2 Ether
CCl2 Tetrachloro ethylene

15. Neat liquids can be analyzed between salt plates made of NaCl or KBr.
Non- or low volatility liquids can be analyzed by placing a drop of the sample onto
specially prepared thin polyethylene polymer substrates.
Use a fixed pathlength cell.
Determine pathlength when empty by counting interference fringes

16. Cells for liquids
Fixed pathlength

17. Some problems with spectra
Asymmetric,
sloping bands.
Badly ground.
Sample does not
cover beam.
Sample (mull) too thick
Also for air bubble in liquid cell;
polymer film with hole or crack

18. Liquid evaporated between
KBr plates
Wet sample. Sloping to high
energy. Water bands.
Sample too thin