This document provides an overview of Fourier Transform Infrared Spectroscopy (FTIR). It defines key terms and outlines the history and development of FTIR. The basic principles of FTIR are explained, including how an interferometer splits light into two beams which undergo constructive and destructive interference. Key components of an FTIR instrument are described, such as the infrared source, beam splitter, fixed and moving mirrors, laser, and detectors. Thermal and photonic detectors are discussed. Finally, some applications of FTIR in forensics are highlighted.

1. 1

2. Fourier Transform Infrared
Spectroscopy
Submitted to – Dr. Rajvinder Submitted by – Jitesh kumar
M.sc Final (Forensic Science)
Department of Genetics
Roll no- 2901
2

3. Content
1. Terms & Definitions
2. History
3. IR Spectroscopy Region
4. Why IR Is Useful ?
5. Basic Principle Of FTIR
6. A Simplified Diagram Of An Interferometer
7. An example Of Two Light Beams Undergoing Constructive & Destructive Interference
8. Simplified Diagram Of FTIR
9. Interferometer
10. FTIR Instrumentation
11. Detector
12. Advantages & Disadvantages
13. Forensic Application
3

4. Terms & Definitions
 Spectroscopy – the study of the interaction of light with matter.
 Spectrum – a plot of measured light intensity versus some property of light such
as wavelength or wavenumber.
 Spectrometer – an instrument that measures a spectrum.
 Infrared spectrometer – an instrument that measure a infrared spectrum.
 FTIR – Fourier Transform Infrared, specific type of infrared spectrometer.
4

5. History
 Chemical IR spectroscopy was emerged as a science in 1800 by Sir William
Herschel
 Firstly most IR instrumentation was based on prism or grating monochromators
 Michelson invented interferometer in 1880s
 In 1949 Peter Fellgett obtained the first IR spectrum by using FTIR spectrometer
 In 1960s commercial FTIR spectrometers appeared
 In 1966 Cooley- Tukey developed an algorithm, which quickly does a Fourier
transform
5

6. IR Spectroscopy Region
– Infrared radiation lies between the visible and microwave portions of the
electromagnetic spectrum.
– Infrared waves have wavelengths longer than visible and shorter than
microwaves, and have frequencies which are lower than visible and higher than
microwaves.
– The Infrared region is divided into: near, mid and far infrared.
6
>1400
cm-1
Visible & UV
14,000 – 400
cm-1
Near - Infrared
4000 – 400
cm-1
Mid-Infrared
400 - 4
cm-1
Far Infrared
< 4
cm-1
Microwaves
Electronic
Transitions
Molecular
Vibrations
Molecular
Vibrations
Molecular
Vibrations
Molecular
Rotations

7. Why IR Is Useful
 Analysis of infrared spectra can tell you :-
– What molecules are present in this sample?
– Are these two samples the same?
– What are the concentrations of molecules in this sample?
– This is why infrared spectroscopy is useful. There are several types of infrared
spectrometers in the world, but the most widely used ones are FTIRs, which is
the focus here.
7

8. Basic Principle Of FTIR
– Radiation emitted from the source is split into two light beam with a
beamsplitter in the interferometer. The fixed and moving mirror reflect each of
the beam to the beamsplitter, where the two beams recombine into one and
falls on the detector. The two beams combine constructively or destructively,
varying as the optical path difference , when the moving mirror is moved. When
the combined beam is transmitted through the sample, it is detected as an
interferogram and contains all infrared information on the sample. The infrared
spectrum is obtained from the interferogram by the mathematical process of
Fourier transformation.
8

9. FTIR9

10. A Simplified Diagram Of An
Interferometer
10

11. An example Of Two Light Beams
Undergoing Constructive Interference
11

12. Two Light Beams That are ½ cycle
out of phase with each other undergo
destructive interference
12

13. Simplified Diagram Of FTIR13

14. Interferometer
– At the heart of every FTIR is an optical device called an interferometer
– The oldest and perhaps the most common type of interferometer in use today is
the Michelson interferometer. Its is named after Albert Abraham Michelson
(1852-1931) who first built his interferometer in the 1880s.
14

15. The Michelson Interferometer
– The Michelson interferometer consists of four arms.
– The top arm contains the infrared source and a collimating mirror to collect the
light from the source and make its rays parallel.
– The bottom arm of the Michelson interferometer contains a fixed mirror, a
mirror that is in fixed position and does not move. This is in contrast to the right
arm of the interferometer, which contains a moving mirror which is capable of
moving left and right.
– The left arm of the inferometer contains the sample and detector
15

16. Cont..
– At the heart of the interferometer is an optical device called a beamsplitter.
– A beamsplitter is designed to transmit some of the light incident upon it and
some of the light reflect it. The light transmitted by the beamsplitter travels
toward the fixed mirror, and the light reflected by the beamsplitter travels
toward the moving mirror. Once the light beams reflect from these mirrors they
travel back to the beamsplitter, where they are recombined into a single beam
that leaves the interferometer, interacts with the sample, and strikes the
detector.
16

17. Beamsplitter17

18. FTIR Instrumentation
– FTIR stands for Fourier Transform Infrared. FTIR spectrometers consist of an IR source,
interferometer, sample cell or chamber, detector and a laser. A schematic of an FTIR
instrument is shown below
– IR Source
– Interferometer
– Beam Splitter
– Moving Mirror
– Fixed Mirror
– Laser
– Detector - Thermal detectors
- Photonic Detector
18

19. IR Source
1. Silicon carbide rods which are resistively heated and commonly known as a Globar. An electric
current is passed through the rod which becomes very hot (1300 – 1700 C) and emits large
amounts of IR radiation. Previously, cooling with water was required to avoid damaging
electrical components; however, advances in metal alloys have led to the production of
Globars that do not require cooling by water.
2. Nichrome and Kanthanl wire coils were once popular IR sources and did not require cooling as
they ran at lower temperatures (1400 C) than Globars, however, this also resulted in lower
amounts of IR radiation being emitted.
3. Nernst Glowers are manufactured from a mixture of refractory oxides (rare earth metal) and
are capable of reaching hotter temperatures than a Globar; however, they are not capable of
producing IR radiation above 2000 cm-1 . A current is passed through the device, heat into a
temperature between 1000 - 1800 C to result IR emission.
19

20. Interferometer
– The first interferometer was invented by Albert Abraham Michelson, who
received a Nobel Prize for his work in 1907. Without this essential piece of
optical equipment the modern day FTIR system would not exist. The
interferometer consists of a beam splitter, a fixed mirror, and a moving mirror.
20

21. Interferometer Operates21

22. Beamsplitter22

23. Beam Splitter
– The beamsplitter in most FTIRs consists of a thin film of germanium or KBr,
ZnSe sandwiched between two infrared transparent windows.
– But we use most probably germanium.
– The germanium is of the right thickness to transmit some (incident) infrared
radiation and reflects the other half. IR radiation from the source strikes the
beam splitter and is separated into two beams. One beam is transmitted
through the beam splitter to the fixed mirror while the other beam is reflected
from the beam splitter to the moving mirror. Both mirrors reflect the radiation
back to the beam splitter where the two beams interfere to produce an
interferogram.
23

24. Moving Mirror
– The moving mirror is a flat highly reflective surface mounted on air bearings
that allow for high speed movement of the mirror (movements are made once
every millisecond). The moving mirror only moves a few millimeters away from
the beam splitter.
24

25. Fixed Mirror
– The fixed mirror is a flat highly
reflective surface.
25

26. Laser
– Many instruments employ a Helium-Neon laser as an internal wavelength
calibration standard. It is imperative that the position of the moving mirror is known
at any given moment. The moving mirror moves back and forth at a precise constant
velocity that is timed using a very accurate laser wavelength.
– The intensity of the laser beam is measured at two points in the interferometer. As
the mirror moves the intensity at these two points will rise and fall due to the
enhancement and cancellation of the He-Ne beam paths, producing a sine wave of
intensity vs. mirror position. The number of “fringes” in the sine wave allows the
instrument to know exactly how far the mirror has moved, and the relative phase of
the sine wave tells the instrument in which direction the mirror is moving.
– The purpose of He-Ne laser in FTIR is to determine the IR wavelength more precise
26

27. Detector27
– There are two classes of infrared detectors - Thermal detector
- Photonic detector
– Thermal detectors use the IR radiation as heat; whereas
– quantum mechanical (photonic) detectors use the IR radiation as light which
results in a more sensitive detector.

28. Types Of Detector
Detectors
Quantum
Photoconductor Photonic
Thermal
Thermocouple Bolometer Golay cell
28

29. Thermal detectors
– Thermal detectors: detect changes in temperature of an absorbing material lithium
tantalate (LiTaO3), lead selenide (PbSe), germanium etc. Many temperature
dependent phenomena can be followed to measure the effects of the incident IR
radiation.
– The output obtained from them may be in the form of:-
– Electromotive form (thermocouples).
– A change in resistance of an conductor (bolometer) or semiconductor (thermistor
bolometer)
– The movement of diaphragm caused by expansion of gas (neumatic detector)
– Which may lead to change in illumination of subsidIary photocell (Golay detector).
29

30. Thermocouple
– THERMOCOUPLE : Two dissimilar metals like bismuth and antimony.
– Two ends are called Hot junction, Cold junction.
– The surface at the junction of the wires is coated with black metallic oxide.
– IR radiation falls on hot junction By heat source change in temperature at the
junction between the metallic wires causes an electric potential to develop between
the wires.
– The potential difference between the unjoined ends of the wires is amplified and
measured.
– Cold junction is not exposed to IR
– Response time is 60 m/sec
30

31. Thermocouple Diagram31

32. Golay Cell
– Golay cell consists of a small metal cylindrical closed by a rigid blackened metal plate.
– Pneumatic chamber is filled with xenon gas.
– At one end of cylinder a flexible silvered diaphragm and at the other end Infra red transmitting
window is present.
– When infra red radiation is passed through infrared transmitting window the blackened plate
absorbs the heat. By this heat the xenon gas causes expand
– The resulting pressure of gas will cause deformation of diaphragm. This motion of the
diaphragm detects how much IR radiation falls on metal plate.
– Light is made to fall on diaphragm which reflects light on photocell
– Response time is 20 m/sec 1
32

33. Golay Cell33

34. Photonic Detector
– Photonic Detector: exhibit faster response times and higher sensitivity in
comparison to their thermal counterparts, therefore, they are much more
prolific in FTIR instruments. The materials used in these detectors are
semiconductors with narrow band gaps. The incident IR radiation causes
electronic excitations between the ground and first excited states, which in
photoconductive detectors result in a change in resistivity which is monitored
34

35. Advantages & Disadvantages
ADVANTAGES DISADVANTAGES
Almost universal Can’t detect some molecules
Spectra are information rich Mixtures
Relatively fast and easy Water
Relatively inexpensive
Sensitivity
35

36. Forensic Application
– Seized drugs :- controlled substances and cutting agents
– Clandestine labs :- chemical evaluation
– Hit and run :- paint and materials
– Textile identification :- fibers, coatings, and residues
– In Pharmaceuticals
– A non-destructive confirmation of the identity of a sample, which could be a
powder, fiber, liquid or paint chip, is required.
36

37. References
1. Brian C. Smith. “Fundamentals of Fourier Transform Infrared Spectroscopy”
Chapters 1 to 3. CRC Press. United States of America. Taylor and Francis Group.
2011.
2. Instrumental methods of chemical analysis-by Robert D. Braun 6th edition page
No. 360-364.
3. https://www.chemguide.co.uk/analysis/ir/fingerprint.html
4. https://www.slideshare.net/kamarapusudheerkumar/ir-spectroscopy-sud-
mpharm-pdf
5. Peter R. Griffiths, James A. de Haseth,1942.”Fourier Transform Infrared
Spectrometery” Chemical analysis volume 83, chapter 1 & 5 page no. 209- 213
37