Infrared Spectroscopy

1. INFRARED
SPECTROSCOPY
Prepared By :- Parijat Sunil
Suryawanshi
(First Year
M.Pharmacy)
Department :-
Pharmaceutical Quality
Assurance

2. INTRODUCTI
ON
• Infrared Spectroscopy is one of the most important analytical techniques
among the spectroscopic techniques. IR provides a valuable tool for
probing the structure of organic molecules.
• When infrared light is passed through a sample of an organic compound,
some of the frequencies are absorbed, while other frequencies are
transmitted through the sample without being absorbed. If we plot
absorbance or transmittance against frequency, the result is an infrared
spectrum
• Infrared (IR) Spectroscopy uses a beam of infrared light to analyze the
structure of organic compounds. IR analyzes the bonds present , produces
broad absorptions which may frequently overlap.
• IR spectroscopy gives sufficient information about structure of compound
and also determine the functional group.

3. THEORY
• IR spectra are commonly measured in units of wavenumbers (units of cm-
1).
• Wave Number = 1/Wavelength (in Centimeter)
• The infrared part of the electromagnetic spectrum covers the range from
roughly 300 GHz to 400 THz (1 mm – 750 nm). It can be divided into three
parts: Wavelength range
• Far-infrared, from 1 mm – 10 μm
• Mid-infrared, from 10–2.5 μm
• Near-infrared, from 2,500–750 nm
Region Wavelength range (µm) Wave number range
(cm-1)
Near 0.8-2.5 12500-4000
Middle 2.5-25 4000-400
Far 25-1000 400-10

4. IR ABSORPTION
PROCESS
• Molecules are excited to higher energy when they absorb IR radiation.
• Absorption of IR radiation is Quantized Process – Molecules absorbs only selected
frequencies of IR radiation.
• IR radiation absorbtion corresponds to energy change on order of 8 to 40 KJ/mole
• Radiation in this energy range corresponds to the stretching and bending vibrational
frequencies of the bonds in most covalent molecules.
• In the absorption process, those frequencies of infrared radiation which match the
natural vibrational frequencies of the molecule, serves to increase the amplitude of the
vibrational motions of the bonds in the molecule.
• Only those bonds that have a dipole moment that changes as a function of time are
capable of absorbing infrared radiation. To absorb light a molecule must have a bond
within its structure that can exhibit what is referred to as a ‘dipole moment’ which
means electrons in bond are not shared equally.
• Symmetric bonds, such as those of H2 or Cl., do not absorb infrared radiation. A bond
must present an electrical dipole that is chang ing at the same frequency as the
incoming radiation in order for energy to be transferred. The changing electrical dipole
of the bond can then couple with the sinusoidally changing electromagnetic field of the

5. MOLECULAR
VIBRATIONS
• Types :-
1) Stretching Vibration : that changes the Bond Length
2) Bending Vibration : that changes the Bond Angle
1) STRETCHING VIBRATION :-
• Change in inter-atomic distance along bond axis
• Types :-
a) Symmetric Stretching :- Change in inter-atomic distance in equal manner. Two
or more bonds vibrate in and out together. The atom of a molecule
either move away or towards the central atom, but in the
same direction.

6. b) Asymmetric Stretching :- One atom approach towards the central atom
while other
departs from it. It generally occurs at higher frequencies than that of
symmetric
stretching.
2) Bending Vibrations :-
• Change in angle between two bond. Also called as Deformation
Vibrations.
• Types :-

7. i) Scissoring :- There will be vibrations of atoms in the plane of
molecule towards each other like the movement of a scissor.
ii) Rocking :- The atoms are vibrated in the plane of molecules towards
one direction. The angle of the bond changes towards the same
direction.
b) Out-of-plane Bending :-
i) Wagging :- The vibrations in which atoms vibrates near to the
attached atom at
either side away from the plane.
ii) Twisting :- The Vibrations in which the atoms vibrate in opposite
directions away
the plane in twisted manner.

8. TYPICAL IR ABSORPTION
FREQUENCIES

10. SAMPLE HANDLING
1) Sampling for Solids :-
I. Mull technique : In this technique, the finely crushed sample is mixed with
Nujol (mulling agent) in n a marble or agate mortar, with a pestle to make a
thick paste. A thin film is applied onto the salt plates. This is then mounted in
a path of IR beam and the spectrum is recorded.
II. KBr disks : The solids are dissolved in suitable solvents to prepare a
concentrated solution and it is used directly in the IR plates or else the sample
is placed in the IR plates and a drop of solvent is placed on it. The standard
method to prepare solid sample for FTIR spectrometer is to use KBr. About 2
mg of sample and 200 mg KBr are dried and ground. The particle size should
be unified and less than two micrometers. Then, the mixture is squeezed to
form transparent pellets which can be measured directly.
III. Solid films : It can be deposited onto NaCI plates by allowing a solution in a
volatile solvent to evaporate drop by drop on the surface of the flat. Polymers

11. 2) Sampling for Liquids :
Liquid sample cells can be sandwiched using liquid sample cells of highly
purified alkali halides, normally NaCl. Other salts such as KBr and CaF2 can
also be used. Aqueous solvents cannot be used because they cannot
dissolve alkali halides. Organic solvents like chloroform can be used. The
sample thickness should be selected so that the transmittance lies between
15-20%. For most liquids, the sample cell thickness is 0.01-0.05 mm.
Some salt plates are highly soluble in water, so the sample and washing
reagents must be anhydrous
3) Sampling for Gases :
The gas sample is introduced into a gas cell. The sample cell is made up of
NaCl, KBr etc. and it is similar to the liquid sample cell. A sample cell with a
long path length (5 – 10 cm) is needed because the gases show relatively
weak absorbance.

12. INSTRUMENTATI
ON
There are two basic types of infrared spectrophotometer characterized by
the manner in which the infrared frequencies are handled.
1) Dispersive Instrument :-
In this type the infrared light is separated into its individual
frequencies by dispersion, using a grating monochromator
2) Fourier Transform Infrared Instrument (FTIR):-
In this type the infrared frequencies are allowed to interact to produce
an interference pattern, and this pattern is then analyzed mathematically,
using Fourier Transforms, to determine the individual frequencies and their
intensities.
Both types of instrument require, in addition, a suitable source of infrared
light and a detector to measure light intensity.

13. A) INFRARED
SOURCES
• Common sources are rods of the following, electrically heate1d to near
1800°C
1) Nernst glower or Nernst filament :- (L :- 22mm ; D :- 1-2mm)
• Cylinder made up of Sintered mixtures of the oxides of Zr, Th , Ce, Y, Er,
etc .
• At the end of cylinder platinum wire is attached and current is passed
through the cylinder.
• Temperature reach upto 2200K
2) Globar (silicon carbide) :- (L-50mm ; D-5mm)
• Temperature reach upto 1500K
3) Incandescent wire Source :-
• It is tightly wound coil of Nichrome wire , electrically heated upto 1100K ,
produces low intensity.

14. C) DETECTORS
1. Thermopile detectors :- These consist of several thermocouples
connected in series so that their outputs are added together for greater
sensitivity. A thermocouple works on the principle that if wires of two
dissimilar metals (such as bismuth and antimony) are joined head to tail,
then a difference in temperature between head and tail causes a current
to flow in the wires
2. Thermal detectors :- Based on pyroelectric materials or on solid state
semiconductor devices based on photovoltaic or photoconductive
principles.
3. Photovoltaic detector :- This Is an extremely sensitive detectors, uses a
diode-type device based on doped silicon or indium antimonide (InSb)
or indium-gallium arsenide (InGaAs)
4. Photoconductive detectors :- It based on the photoconductive properties
of mercury cadmium telluride (MCT). This is a homogeneous (undoped)

15. 5. Pyroelectric detectors :- Substances such as deuteriated triglycine
sulphate (DTGS) or lithium tantalate (LiTaO3), which are used
at room temperature. Crystals of these highly polar materials exhibit an
electric polarization along certain axes. This detector is also used in the
more expensive 'ratio recording' dispersive instruments.

16. MODE OF OPERATION - DISPERSIVE INSTRUMENT

18. SCHEMATIC DIAGRAM OF DISPERSIVE IR
SPECTROMETER

19. FOURIER TRANSFORM INFRARED
SPECTROMETER
• Infrared light from a suitable source passes through a scanning
Michelson interferometer, and Fourier Transformation gives a plot of
intensity versus frequency. When a sample compound is placed in the
beam (either before or after the interferometer), it absorbs particular
frequencies, so that their intensities are reduced in the interferogram
and the ensuing Fourier Transform is the infrared absorption spectrum
of the sample.
• The scan time for the moving mirror dictates the speed with which the
infrared spectrum can be recorded; digitization of the data and
calculation of the Fourier Transform take a few seconds more, but the
information which constitutes the spectrum can be acquired in
exceedingly short times, even in a few milliseconds.

20. INTERFEROMETER
• The interferometer is a basically different component than a
monochromator. The interferometer consists of two mirrors, an infrared
light source, an infrared detector, and a beam splitter.
The light passes through a beam splitter, which splits the light in two
directions at right angles. One beam goes to a stationary mirror then
back to the beam splitter. The other goes to a moving mirror. The
motion of the mirror makes the total path length variable versus that
taken by the stationary-mirror beam. When the two meet up again at
the beam splitter, they recombine, but the difference in path lengths
creates constructive and destructive interference thus an interferogram.
and this subtracts specific wavelengths from the interferogram.
• The detector reports variation in energy versus time for all wavelengths.
The recombined beam passes through the sample. The sample absorbs
all the different wavelengths characteristic of its spectrum,
simultaneously. A laser beam is superimposed to provide a reference for

21. MICHELSON INTERFEROMETER

22. ADVANTAGES OF FTIR
i. The entire energy from the source gets to the sample, thus signal-
to-noise ratio is improved.
ii. Resolution is limited by use of interferometer.
iii. The accuracy of wave number is high.
iv. Scan time is short.
v. The resolution is extremely high.
vi. Scan range is wide.
vii.The interference from stray light is reduced.

23. READ OUT DEVICES
• The read out devices is the computing system.
• Now a days a sophisticated software is involved for readout.

24. APPLICATION
• Infrared spectroscopy is widely used in industry as well as in research.
It is a simple and reliable technique for measurement, quality control
and dynamic measurement. It is also employed in forensic analysis in
civil and criminal analysis
 Identification of functional group and structure elucidation.
• In group frequency region, the peaks corresponding to different
functional groups can be observed. According to corresponding peaks,
functional group can be determined.
• Each atom of the molecule is connected by bond and each bond
requires different IR region so characteristic peaks are observed. This
region of IR spectrum is called as finger print region of the molecule. It
can be determined by characteristic peaks.
 Identification of substances
R spectroscopy is used to establish whether a given sample of an

25.  Studying the progress of the reaction
• Progress of chemical reaction can be determined by examining the small
portion of the reaction mixure withdrawn from time to time. The rate of
disappearance of a characteristic absorption band of the reactant group
and/or the rate of appearance of the characteristic absorption band of the
product group due to formation of product is observed.
 Detection of impurities
IR spectrum of the test sample to be determined is compared with the
standard compound. If any additional peaks are observed in the IR spectrum,
then it is due to impurities present in the compound.
 Quantitative analysis
The quantity of the substance can be determined either in pure form or as a
mixure of two or more compounds. In this, characteristic peak corresponding
to the drug substance is chosen and log I0/It of peaks for standard and test
sample is compared. This is called base line technique to determine the

26. CASE STUDY

30. REFERENCE
1. Donald L Pavia, Gary M Lampman, George S Kriz, “ Introduction to
Spectroscopy”, Thomson Learning Academic Resource Center ,
Third edition, pg no.13-101
2. William Kemp, “ Organic Spectroscopy” Published by PAlGRAVE ,
3rd edition, pg no. 19-38
3. https://www.pharmatutor.org/pharma-analysis/analytical-
aspects-of-infra-red-spectroscopy-ir/application-ir-
spectrophotometry/
4. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical
_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_
Theoretical_Chemistry)/Spectroscopy/Vibrational_Spectroscopy/Inf
rared_Spectroscopy/How_an_FTIR_Spectrometer_Operates