Sampling techniques of Infrared Spectroscopy

SAMPLING TECHNIQUES IN
INFRA RED SPECTROSCOPY
UNDER THE GUIDANCE OF
MR.M.M. ESWARUDU M.PHARM
ASST.PROFESSOR
DEPT OF PHARMACUTICALANALYSIS
VIGNAN PHARMACY COLLEGE
VADLAMUDI CHEBROLU
A.P 522213
SUBMITTED BY
P. JAGADEESH
15AB1R0054
VIGNAN PHARMACY COLLEGE, VADLAMUDI 1

Contents :
1. Introduction
2. IR Region
3. Principle
4. Sample Handling Techniques In IR
5. Sample Cell
6. Sampling Of Solids
7. Sampling Of Liquids
8. Sampling Of Gases
9. Instrumentation
10. Conclusion

INTRODUCTION
Spectroscopy :
Spectroscopy is the study of the interaction between matter and radiated
energy or radiation.
Infrared Spectroscopy : (IR spectroscopy) is the spectroscopy that
deals with the interaction of only infrared region of the electromagnetic
spectrum with the matter.
Spectroscopy : Qualitative
Spectrometry : Quantitative
Valuable tool for the detemination of organic, and to a lesser extent,
inorganic structure.

This region is divided into different sections:
Region Energy
(kj/mole)
Wave number
(cm-1 )
Wave
length(µm)
Near IR 150-50 12,800-4000 0.78-2.5
Mid IR 50-2.5 4000-200 2.5-50
Far IR 2.5-0.1 200-10 50-1000
IR REGION
IR spectroscopy refers broadly to that region of electromagnetic
spectrum which lies between visible and microwave regions.

Principle:
• A chemical substance shows marked selective absorption in IR region.
• After absorption of IR radiation the molecules of chemical substance
vibrate at many rates of vibration giving rise to closely packed
absorption spectrum called as IR absorption spectrum.
• Various bands present in the IR spectrum corresponds to characteristic
functional group and bonds present in the chemical substance.
• Thus IR spectrum of a chemical substance is fingerprint for its
identification.

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.
• However, a final choice of salt will depend on the wavelength range to
be studied.
• Miller (1965) developed appropriate methods to handle samples in the
gas, liquid and solid phase.

SAMPLE CELL:
• Sample handling presents a number of problems in IR region.
• There is rugged window material for cuvette that is transparent and
also inert over this region.
• The alkali halides are widely used, particularly NaCl, KBr and 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, but Teflon
shows bands due to C-C & C-F.
• For frequencies less than 600 cm-1, a polyethylene cell is used.
•Sample concentrations required is from 0.1-10%

Precautions:
• Cell windows are easily fogged by exposure to moisture and require
frequent polishing with buffer powder which returns them to their
original condition.
• Since alkali metal halides, with which cell is made up of are
hygroscopic so must be protected from moisture by working at a
suitable temperature.

SAMPLING OF SOLIDS:
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 solvent.
 No chemical interaction between solute and solvent and also solvent
not absorb in the studied range.
 A drop of the solution is placed on an alkali metal disc and the solvent
allowed to evaporate, leaving a thin film of the solute or the entire
solution placed in liquid sample cell.
 If the solution can be prepared in a suitable solvent then the solution is
run in one of cells for liquid.
 But this method is not for all solids as suitable solvents are limited and
no single solvent is transparent through the IR region.

Solvents:
• Solvent must be dry and transparent in the region of interest.
• A common pair of solvents is CCl4 and CS2.
• CCl4 is relatively free of absorption at frequencies above 1333 cm-1
where as CS2 shows little absorption below 1333cm-1.
• Solvent and solute combinations that must be avoided.
• For example, CS2 cannot be used as a solvent for primary or
secondary amines. Amino alcohols react slowly with CS2 CCl4.
• To obtain the spectra of polar materials that are insoluble in CCl4 and
CS2, chloroform, methylene chloride, acetonitrile and acetone are
useful solvents.

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:
• Use only when the material can be deposited from the solution or
cooled from a melt as micro crystals or as glassy film.
• Care must be taken to free the sample of solvent.
• Films of crystalline solid generally lead to excessive light scattering.
• This technique is used for qualitative analysis but not for quantitative
analysis
Evaporate solvent by gentle heating
Dissolve them in reasonable volatile solvent and solution is formed.
Polymers, resins and amorphous solids

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.

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.
• This method is good for qualitative analysis but not for quantitative
analysis.

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 and then interferes with the absorption of the
sample.
• This interference can be avoided by using Hexachlorobutadiene in
combination with nujol which absorbs in regions 1630-1510 cm -1, 1200-
1140 cm-1, 1010-760 cm-1 and thus permits the recording of IR spectra of
only the sample.

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.

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 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 OF LIQUIDS:
• Liquids at room temperature.
• Rectangular cells made of NaCl, KBr and 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.
• Where ,
N is the number of fringes
w1–w2 are wave numbers
2t = N/w1–w2

• For double beam work “matched cell” are generelly employed.
• One cell will contain the sample while the other will have solvent
used in the sample.
• Matched cells must have same thickness.
• All cells should be protected from moisture because they dissolve in
water.
• For similar reasons organic liquid samples must be dried before
pouring into cells.

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 if 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 40cm, so that constituents of the can be determined.
•Gas must not react with the cell windows or the reflecting surfaces.

• Gas analysis are performed with IR but the method is not commonly
used because of its lack of sensitivity.
• Moisture can be avoided.
•It is strong absorption bands at 3701cm-1 and 1625cm-1 may interfere
in the analysis .
• IN addition, the windows and other instrument components which are
constructed of soluble salts may be damaged.

INSTRUMENTATION
Recorder
Amplifier
Detector
Monochromator
Sampling area
Radiation source condensing optics and Beam chopper
Block Diagram of the major components of a dispersive
infrared spectrophotometer

Source:
Infrared radiation is produced by electrically heating a source, usually
a Nernst filament or a Globar to 1000-1800°C.
The Nernst filament is fabricated from a binder and oxides of
thorium, cerium, zirconium and yttrium.
The Globar is a small rod of silicon carbide usually 5 cm in length
and 0.5 cm in diameter.
 The maximum radiation for the Globar occurs in the 5500-5000cm-1
region.
Nichrome wire, carbon arc, rhodium wire and tungsten filament lamp
are also used as light source.

Monochromator:
A monochromator is a means of separating wavelengths of the source
radiation.
The monochromator is used to separate polychromatic radiation into
a suitable monochromatic form [105].
 This is achieved by means of prisms or diffraction grating.
 A monochromator thus carries out three functions:
1 • It disperses the radiation according to its wave number
components.
2
• It restricts the radiation falling on the detector into a narrow wave
number range.
3
• It maintains the energy incident on the detector to an
approximately constant level.

Detector:
Detectors used in infrared spectrophotometers usually convert the
thermal radiant energy into electrical energy, which can subsequently be
plotted on a chart recorder.
Two Types of the detectors are commonly used:
1. Thermal Detectors, in which the infrared radiation produces a heating
effect that alters some physical property of the detector.
2. Photon detectors, which use the quantum effects of the infrared
radiation to change the electrical properties of a semiconductor

Amplifiers and recorders :
The radiant energy received by the detector is converted into
measurable electrical signal and is amplified by the amplifiers.
The amplified signal is registered by a recorder or a plotter.
The recorder is driven with a speed which is synchronized with that of
a monochromator, so that, the pen moving across the chart, records the
transmittance of the sample as a function of the wavenumber.

REFERENCES:
1. Instrumental methods of chemical analysis, 5th edition,
Gurdeep.R.Chatwal & Sham.K.Anand, Pg.No.2.29.
2. Instrumental methods of analysis, 7th edition, Willard, Merritt, Dean
& Settle, Pg. no. 305-310.
3. Instrumental methods of chemical analysis, 26th edition,
B.K.Sharma, Pg.No.262-264.
4. Organic spectroscopy, third edition, William Kemp, pg. no. 51.
5. Elementary organic spectroscopy, Y. R. Sharma, pg. no. 91.
6. Spectrometric identification of organic compounds,
R. M. Silverstein& F.X. Webster, pg. no. 77.
7. www.redpoll.pharmacy.ualberta.ca/.../slideoo31.htm

ACKNOWLEDGEMENT
 I would like to express my gratitude to all those who gave me
the possibility to complete this seminar.
 I express my sincere gratitude to my guide M.M. Eswarudu,
who suggested with great patience.
 A special thanks to the principal sir , Dr. P. Srinivasa Babu
garu and special thanks to Sowjanya madam and seminar
committee whose encouragement contributed immensely to
complete my seminar.
VIGNAN PHARMACY COLLEGE, VADLAMU
37

Sampling techniques of Infrared Spectroscopy

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