1. The document discusses various sample handling techniques for infrared spectroscopy, including techniques for solids, liquids, and gases. 2. For solids, it describes techniques like pelleting, mulling, casting thin films, and reflectance methods. For liquids, it discusses analyzing neat liquids, solutions, and reflectance techniques. For gases, it mentions using an absorbance cell or photoacoustic spectroscopy. 3. The document provides details on specific reflectance techniques like photoacoustic spectroscopy, specular and diffuse reflectance, and attenuated total reflectance. It also discusses how to prepare and analyze samples using these various infrared spectroscopy techniques.

1. Sample Handling Techniques in
INFRARED SPECTROSCOPY
Dr. Tambe V.S.
PES Modern College of Pharmacy (For Ladies)

2. The desirable characteristics of cell materials are:
• Inertness towards the sample
• High transmission in the wavelength range to be scanned
• Should be available in pure state that is suitable for spectroscopic
studies (Silver salts: have limited water solubility, but are
photosensitive and are easily darkened over prolonged use.)
• Should be safe to handle ,i.e, non-toxic
• For transmission mode: Should have low refractive index in the
wavelength region of interest so that reflection loss is minimal and
there are few interference fringes
• For reflectance mode: Should have high refractive index
• Should not be hygroscopic as this can lead to fogging of windows
• Should not be too hard or too soft for convenience of polishing
• Cost

3. Thallium
bromide-iodide
(KRS 5)
Zinc Sulphide
Silver bromide
Polyethylene

4. SOLIDS
• Pelleting technique
• Mulling technique
• Solid run in solution thin films or cast films
• Reflectance technique (specular, diffuse, total internal reflection, attenuated
total reflectance)
• Photoacoustic spectroscopy
• FT-IR microscopy
LIQUIDS
• Neat liquids
• Solutions
• Reflectance technique (specular, diffuse, total internal reflection, attenuated
total reflectance)
• Photoacoustic spectroscopy
GASES
• Absorbance in multiple pass cell
• Photoacoustic spectroscopy

5. Photoacoustic Spectroscopy (PAS)
It is a non-invasive reflectance technique with penetration depths in the range from
microns down to several molecular monolayers.
Gaseous, liquid or solid samples can be measured in particular for highly absorbing
samples.
The photoacoustic effect occurs when intensity-modulated light is absorbed by the
surface of a sample located in an acoustically isolated chamber filled with an inert gas. A
spectrum is obtained by measuring the heat generated from the sample due to a re-
absorption process. The sample absorbs photons of the modulated radiation, which have
energies corresponding to the vibrational states of the molecules.
The absorbed energy is released in
the form of heat generated by the
sample, which causes temperature
fluctuations and, subsequently,
periodic acoustic waves. A
microphone detects the resulting
pressure changes, which are then
converted to electrical signals.
Fourier-transformation of the
resulting signal produces a
characteristic infrared spectrum.

6. PAS (Solids, Liquids, Gases)
Modulated light
Sent to sample
surrounded by inert
gas like Helium,
Nitrogen with good
thermal conductivity
Sample absorb energy
and is vibrationally
excited and it releases
the energy
Released energy is
reabsorbed by gas
and is heated
Pressure fluctuation
in gas
Sensed by a sensitive
microphone

7. Reflectance Technique (Solids, Liquids)
Reflectance techniques may be used for samples that are
difficult to analyse by the conventional transmittance
methods.
1. Specular reflectance
2. Diffuse reflectance
3. Attenuated total reflectance/ Multiple internal
reflectance

8. Specular Reflectance
• Seen when reflecting medium is a smooth
polished surface.
• The angle of reflection is identical to the
incident angle of the radiation.
• If the surface is made up of an IR
absorber, the relative intensity of
reflection is less for wavelengths that are
absorbed than for wavelengths that are
not.
• Thus, a plot of reflectance R, which is the
fraction of the incident radiant energy
reflected, versus wavelength or
wavenumber provides a spectrum for a
compound similar in general appearance
to a transmission spectrum for the
species.

9. Diffuse-reflectance IR Fourier
transform spectrometry (DRIFTS)
Effective way of directly obtaining IR spectra
on powdered samples with a minimum of
sample preparation.
Uses FTIR instruments offer adapters that fit in
cell compartments and permit diffuse-
reflectance measurements.
The collimated beam from the interferometer
is directed to an ellipsoidal mirror and then to
the sample.
The sample is usually ground and mixed with
KBr or KCI as a diluent. The mixture is then
placed in a sample cup 3-4 mm deep and
about 10-15 mm in diameter.
A complex combination of reflection,
absorption, and scattering occurs before the
beam is directed to the detector.

10. Attenuated Total Reflectance (ATR)
Internal-reflection spectroscopy is a technique
for obtaining IR spectra of samples that are
difficult to deal with, such as solids of limited
solubility, films, threads, pastes, adhesives, and
powders

11. When a beam of radiation passes from a more dense to a less
dense medium, reflection occurs. The fraction of the incident
beam reflected increases as the angle of incidence becomes
larger; beyond a certain critical angle, reflection is complete.
During the reflection process, the beam penetrates a small
distance into the less dense medium before reflection occurs.
The depth of penetration depends on –
the wavelength,
the index of refraction of the two materials, and
the angle of the beam with respect to the interface.
The penetrating radiation is called the evanescent wave. At
wavelengths where the less dense medium absorbs the
evanescent radiation, attenuation of the beam occurs, which is
known as attenuated total reflectance.

12. • The sample is placed on opposite sides of a
transparent crystalline material of high refractive
index.
• By proper adjustment of the incident angle, the
radiation undergoes multiple internal reflections
before passing from the crystal to the detector.
• Absorption and attenuation take place at each of
these reflections. This adapter fits into the cell
area of most IR spectrometers and permits ATR
measurements. Cells for liquid samples are also
available.

13. Multiple internal reflectance (MIR)
• Similar to ATR
• but MIR produces more intense spectra from multiple
reflections.
• While a prism is usually used in ATR work, MIR uses specially
shaped crystals which cause many internal reflections,
typically 25 or more.
•

14. • Solids
Most organic compounds exhibit numerous
absorption bands throughout the mid-IR region,
and finding a solvent that does not have
overlapping peaks is often impossible. Because of
this, spectra are often obtained on dispersions of
the solid in a liquid or solid matrix. Generally. in
these techniques. the solid sample must be ground
until its particle size is less than the wavelength of
the radiation to avoid the effects of scattered
radiation.

15. 3. Pelleting:
A milligram or less of the finely ground sample is intimately mixed with about 100 mg
of dried potassium bromide powder. (alkali-metal halides)
Halide salts have the property of cold flow, in which they have glasslike transparent or
translucent properties when sufficient pressure is applied to the finely powdered
materials. Being ionic, KBr transmits throughout most of the lR region with a lower
cutoff of about 400 cm-1.
Mixing is carried out with a mortar and pestle or, better. in a small ball mill. The
mixture is then pressed in a special die at 10000 to 15000 pounds per square inch to
yield a transparent disk. Best results are obtained if the disk is formed in a vacuum to
eliminate occluded air. The resulting spectra may exhibit bands at 3450 and 1640 cm -I
(2.9 and 6.1 /lm) due to absorbed moisture.
Ion exchange can occur with some samples such as amine hydrochlorides or inorganic
salts. With the former, bands of the amine hydrobromide are often found.
Polymorphism can also occur because of the forces involved in grinding and pressing
the pellet. These can convert one polymorph into another.
CsI and CsBr are sometimes used for pelleting. Cesium iodide has greater transparency
at low frequencies than KBr.

17. 4. Mulls
IR spectra of solids that are not soluble in an IR-transparent
solvent or are not conveniently pelleted in KBr are often
obtained by dispersing the anaIyte in a mineral oil or a
fluorinated hydrocarbon mull.
Mulls are formed by grinding 2 to 5 mg of the finely powdered
sample (particle size <2 micrometer) in the presence of one or
two drops of a heavy hydrocarbon oil (Nujol). If hydrocarbon
bands are likely to interfere, Fluorolube, a halogenated
polymer can be used. Mull is then examined as a film between
flat salt plates.

18. Limitations:
• Too little sample and there will be no sign of the sample in the
spectrum.
• Too much sample and a thick paste will be produced and no
radiation will be transmitted.
• If crystal size of the sample is too large, it leads to a scattering of
radiation which gets worse at the high-frequency end of the
spectrum.
• If the mull is not spread over the whole plate area, part of the
beam of radiation passes through the mull and consequently only
part through the plate, thus producing a distorted spectrum.
• Too little mull leads to a very weak spectrum showing only the
strongest absorption bands. Too much mull leads to poor
transmission of radiation so that the base.

19. 5. Cast film
used mainly for polymeric materials.
Solvent Casting: 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.

20. • You need to choose a solvent which not only
dissolves the sample, but will also produce a
uniform film. The solution is poured on to a
leveled glass plate (such as a microscope slide)
or a metal plate and spread to a uniform
thickness. The solvent is then evaporated in an
oven and, once dry, the film can be stripped
from the plate. Alternatively, it is possible to
cast a film straight on to the infrared window
to be used.

21. Melt casting:
Solid samples which melt at relatively low
temperatures without decomposition can be
prepared by melt caşting. A film is prepared by
hot-pressing the sample in a hydraulic press
between heated metal plates.

22. Microtomy
• cut a thin (20–100 μm) film from a solid
sample.
• This is one of the most important ways of
analysing plastic products as the integrity of
the solid is preserved.

23. Liquids
A common problem encountered in obtaining good quality spectra
from liquid films is sample volatility. When the spectrum of a volatile
sample is recorded it progressively becomes weaker because
evaporation takes place during the recording period. Liquids with
boiling points below 100°C should be recorded in solution or in a
short-pathlength sealed cell, but cannot be taken apart for cleaning.
Before producing an infrared sample in solution, a suitable solvent
must be chosen. In choosing a solvent for your sample you need to
consider the following points:
(a) it has to dissolve the compound;
(b) it should be as non-polar as possible to minimise solute-solvent
interactions;
(c) it should not strongly absorb infrared radiation.

24. Semi-permanent cells
demountable so that the windows can be cleaned.
The spacer is usually made of polytetrafluoroethylene (PTFE) and is
available in a variety of thicknesses, allowing one cell to be used for
various pathlengths.
All these cell types are filled by using a syringe.
An important consideration in the choice of infrared cells is the type
of window material. The material must be transparent to the incident
infrared radiation and therefore alkali halides are normally used in
transmission methods. The cheapest material is sodium chloride
(NaCl).

25. Gaseous Sample

26. • Used for Low boiling point liquid and gases
• Cylindrical cells has reflecting internal surface which achieves pathlength
of few cm to 10 m
• For adequate measurement sensitivity, closed-path optical instruments that
sense trace gases need long optical path lengths.
• for large instruments, multipass gas cells are used which are bulky.
• For smaller instruments, a small multipass cell with a small, circular mirror
that folds the sensing laser beam into a dense star pattern with an optical
path length of up to 4 m in a volume of only 30 ml is used.

27. (GC-IR) allows the identification of the components eluting from a gas
chromatograph. GC-IR can easily distinguish structural isomers. This
technique is also particularly useful for pollution studies or gas purity
determinations.
Thermogravimetric analysis (TGA) is a technique which involves
measuring the change of mass of a sample when it is heated.
Identification of the decomposition products is done with IR. TGA and
infrared spectroscopy have been combined to provide a complete
qualitative and quantitative characterization of thermal decomposition
processes.

28. Thank You….