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Basic introduction of FTIR

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jitesh yadav
jitesh yadav

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.

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Fourier Transform Infrared Spectroscopy Submitted to – Dr. Rajvinder Submitted by – Jitesh kumar M.sc Final (Forensic Science) Department of Genetics Roll no- 2901 2
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
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
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
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
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
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
FTIR9
A Simplified Diagram Of An Interferometer 10
An example Of Two Light Beams Undergoing Constructive Interference 11
Two Light Beams That are ½ cycle out of phase with each other undergo destructive interference 12
Simplified Diagram Of FTIR13
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
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
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
Beamsplitter17
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
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
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
Interferometer Operates21
Beamsplitter22
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
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
Fixed Mirror – The fixed mirror is a flat highly reflective surface. 25
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
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.
Types Of Detector Detectors Quantum Photoconductor Photonic Thermal Thermocouple Bolometer Golay cell 28
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
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
Thermocouple Diagram31
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
Golay Cell33
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
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
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
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

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Basic introduction of FTIR

  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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