MODERN PHARMACEUTICAL ANALYSIS-FTIR

2. CONTENTS
• Introduction
• Theory of FTIR
• Instrumentation of FTIR
• Advantages
• Disadvantages
• Applications
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3. INTRODUCTION
• Infrared spectrum is an important record which gives sufficient
information about the functional groups of a compound.
• The region from o.8 µ to 2.5µ is called Near IR, from 2.5µ to 15µ
is called Mid IR or Ordinary IR and that from 15µ to 200µ is
called Far IR.
• FTIR stands for Fourier Transform Infra Red
Spectrophotometer — the preferred method of infrared
spectroscopy.
• A method for measuring all of the infrared frequencies
simultaneously, rather than individually as with dispersive
instruments.
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4. FTIR spectroscopy is preferred over dispersive method of IR
spectral analysis for several reasons:
 It is a non-destructive technique.
 It provides a precise measurement method which
requires no external calibration.
 It can increase scan speed, collecting a scan every
second.
 It is mechanically simple with only one moving part.
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5. Theory of FTIR
 Fourier Transform Infrared (FT-IR) spectrophotometry was
developed in order to overcome the limitations encountered
with dispersive instruments.
 The main difficulty was the slow scanning process. A method
for measuring all of the infrared frequencies simultaneously,
rather than individually, was needed.
 A solution was developed which employed a very simple
optical device called an interferometer. The interferometer
produces a unique type of signal which has all of the infrared
frequencies “encoded” into it.
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6.  Most interferometers employ a beam-splitter which takes the
incoming infrared beam and divides it into two optical
beams.
 One beam reflects off a flat mirror which is fixed in place.
The other beam reflects off a flat mirror which is on a
mechanism which allows this mirror to move a very short
distance (typically a few millimeters) away from the beam-
splitter.
 The two beams reflect off of their respective mirrors and are
recombined when they meet back at the beam-splitter.
 Because the path that one beam travels is a fixed length and
the other is constantly changing as its mirror moves, the
signal which exits the interferometer is the result of these
two beams “interfering” with each other. 6
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7.  The resulting signal is called an interferogram which has the
unique property that every data point (a function of the
moving mirror position) which makes up the signal has
information about every infrared frequency which comes from
the source.
 Because the analyst requires a frequency spectrum (a plot of
the intensity at each individual frequency) in order to make
an identification, the measured interferogram signal can not
be interpreted directly.
 Hence Fourier transformation is performed by the computer
which then presents the user with the desired spectral
information for analysis.
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8. Spectrometer components
 Three basic components of FT system are
1. Radiation source
2. Interferometer
3. Detector
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9. Radiation sources
 Incandecent lamp
 Nernst glower
 Globar source
 High pressure mercury arc
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10. Interferometer
• The heart of the FTIR is a Michelson Interferometer.
• The monochromator is replaced by an interferometer,
which divides radiant beam, then recombines them in
order to produce repetitive interference signals measured
as a function of optical path difference as the name implies,
the interferometer produces interference signals, which
contain IR spectral information generated after passing
through a sample.
• Consists of three active components-
• A moving mirror
• A fixed mirror
• A beam splitter
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11. 1 .Mirrors in an FTIR are generally made of metal. The
mirrors are polished on the front surface and may be gold-
coated to improve corrosion resistance.
- commercial FTIRs use a variety of flat and curved mirrors
to move light within the spectrometer, to focus the source
onto the beam splitter, ant to focus light from the sample
onto the detector.
2.The beam splitter can be constructed of a material such
as Si or Ge deposited in a thin coating onto an IR-
transparent substance. The germanium or silicon is coated
onto the highly polished substrate by vapour deposition.
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- A common beam splitter material for the mid-IR region is
germanium and the most common substrate for this region is
KBr. Both the substrate and the coating must be optically
flat. KBr is an excellent substrate for the mid-IR region.

13. Schematic diagram of Michelson Interferometer.
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14. Construction: The two mirrors are perpendicular to each other.
The beam-splitter is a semi-reflecting device and is often made by
depositing a thin film of Germanium onto a flat KBr substrate.
Working: Radiation from the broadband IR source is collimated
and directed into the interferometer, and impinges on the beam-
splitter.
• At the beam-splitter half of the IR beam is transmitted to the
fixed mirror and the remaining half is reflected to the moving
mirror.
• After the divided beams are reflected from the two mirrors,
they are recombined at the beam-splitter.
• Due to changes in the relative position of the moving mirror,
an interference pattern is generated. The resulting beam then
passes through the sample and is eventually focused on the
detector.
14
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15. Fixed mirror
Moving mirror
positions
Michelson
interferometer
sampleTo
detector
IR
Source
Schematic diagram of FTIR spectrometer
ZPD
¼ λ ¼ λ
• NERNST GLOWER
• GLOBAR SOURCE
• HIGH PRESSURE MERCURY
ARC LAMP
• Dueterated triglycine sulphate (DTGS)
• Mercury cadmium telluride
(MCT)
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16. FTIR spectrophotometer
 FTIR spectrophotometer obtains an infrared spectra by
collecting an interferogram of a sample signal using an
interferometer and then performs a Fourier transform
on the interferogram to obtain a spectrum.
 Fourier transform defines a relationship between
signal in time domain and its representation in
frequency domain.
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18. • The sampling rate is controlled by an internal,
independent reference, a modulated monochromatic
beam from a Helium-Neon Laser focused on a
separate detector.
Source interferometer sample detector
interferogram (encoded data of sample at each frequency)
decoding by FT IR Spectra
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19. 191/31/2016
In phase
Out of
phase

20.  When the mirror is moved at a constant velocity, the
intensity of radiation reaching the detector varies in a
sinusoidal manner to produce the interferogram. The
interferogram (which has every infrared frequency encoded
into it) is the record of the interference signal. It is a time
domain spectrum and records the detector response
changes versus time within the mirror scan.
 If the sample happens to absorb at this frequency, the
amplitude of the sinusoidal wave is reduced by an amount
proportional to the amount of sample. The IR signal after
interaction with the sample is uniquely characteristics of
the sample. The beam finally arrives at the detector and is
measurde by the detector.
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21. Cont...
 The detected interferogram can not be directly interpreted.
It has to be decoded with a well-known mathematical
technique in term of Fourier Transformation.
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22. Detectors in FTIR
 Two most popular detectors for FTIR spectrometer
are
1. Pyroelectric Detector: Deuterated triglycine
sulphate (DTGS).
2. Photon-sensitive semiconducting Detectors:
Mercury cadmium teluride (MCT)
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23. Pyroelectric Detector
 Pyroelectric materials change their electric polarization as
a function of temperature. These materials may be
insulators, ferroelectric materials or semiconductors.
 A dielectric placed in an electrostatic field becomes
polarized with the magnitude of the induced polarization
depending on the dielectric constant. The induced
polarization generally disappears when the field is
removed.
 Pyroelectric materials, however, stay polarized and the
polarization is temperature dependent.
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24. Cont...
 It consists of a thin single crystal of pyroelectric material
placed between two electrodes. Upon exposure to IR
radiation, the temperature and the polarization of the
crystal changes. The change in the polarization is detected
as a current in the circuit connecting the electrodes.
 The signal depends on the rate of change of polarization
with temperature and the surface area of the crystal.These
crystals are small: they vary in size from about 0.25 to 12.0
mm².
 Pyroelectric material used as an IR detector is Deuterated
triglycine sulfate(DGTS), Lithium tantalite(LiTaO3) and
Strontium barium niobate.
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25. Photon Detectors
 Here semiconductors are used. Semiconductors are
materials that are insulators when no radiation falls on
them but become conductors when radiation falls on them.
 Exposure to radiation causes very rapid change in their
electrical resistance and therefore a very rapid response to
the IR signal.
 The response time of a semiconductor detector is the time
required to change the semiconductor from an insulator to
a conductor.
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26. Cont...
 It consists of a thin film of suitable semiconductors like
lead telluride or germanium which are non conducting at
lower energy state.
 When the radiation falls on these, they are raised to higher
level which can conduct and produce a signal which is
proportional to amount of radiations.
 In this, there is drop of electrical resistance and if small
voltage is applied there is large increase in current which
can be applied and indicated on meter or recorder.
 Materials such as lead selenide(PbSe), indium gallium
arsenide(InGaAs), indium antimonide(InSb) and Mercury
cadmium telluride(HgCdTe, also called MCT) are intrinsic
semiconductors commonly used.
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27. Advantages of FTIR
FTIR instruments have distinct advantage over dispersive
spectrometers.
1. Simpler mechanical design.
2. Elimination of Stray light and emission contributions.
3. Powerful data station.
4. Majority of molecules in the universe absorb mid-infrared
light, making it a highly useful tool.
5. Universal technique.
6. Sensitive, fast and easy.
7. Relatively inexpensive and provides rich information.
8. Sensitive to “molecules”-anything that contains chemical
bonds.
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28. Disadvantages of FT-IR
• Cannot detect atoms or monoatomic ions - single
atomic entities contain no chemical bonds.
• Cannot detect molecules comprised of two identical
atoms symmetric-such as N2 or O2.
• Aqueous solutions are very difficult to analyze - water
is a strong IR absorber.
• Complex mixtures - samples give rise to complex
spectra.
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29. Applications
1. Identification of an organic compound.
2. Structure determination.
3. Study of chemical reaction.
4. Study of Keto-Enol tautomerism.
5. Study of complex molecules.
6. Detection of impurities in a compound.
7. Conformational analysis.
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30. Reference
1. Instrumental analysis. Skoog, Holler, Crouch. Pg
No:488-493.
2. Fundamentals of Analytical chemistry. F James Holler,
Stanley R. Crouch. Pg No:749-750.
3. Introduction to Instrumental Analysis. Robert D. Braun.
Pg No:372-373.
4. Instrumental methods of Chemical Analysis. Gurudeep
R. Chatwal, Sham K.Anand. Pg No:2.49-2.59.
5. Text book of Quantitative Chemical Analysis. Vogel’s et
al.,Pg No:680-681.
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31. 6. Elementary organic spectroscopy. Y. R. Sharma. Pg No:137-
140.
7. Instrumental methods of Analysis. Willard, Merritt. Pg
No:302-305.
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