ftir

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FOURIER TRANSFORM OF INFRARED SPECTROSCOPHY

• Shivaram
• B.PHARMACY
• Shiva.pharmacist_at_gmail.com

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DEFINITION OF INFRARED SPECTROSCOPHY

• The absorption of light, as it passes through a
  medium, varies linearly with the distance the
  light travels and with concentration of the
  absorbing medium. Where a is the absorbance, the
  Greek lower-case letter epsilon is a
  characteristic constant for each material at a
  given wavelength (known as the extinction
  coefficient or absorption coefficient), c is
  concentration, and l is the length of the light
  path, the absorption of light may be expressed by
  the simple equation a epsilon times c times l.

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INFRARED SPECTROSCOPHY

• Infrared spectroscopy is the measurement of the
  wavelength and intensity of the absorption of
  mid-infrared light by a sample. Mid-infrared is
  energetic enough to excite molecular vibrations
  to higher energy levels.
• The wavelength of infrared absorption bands is
  characteristic of specific types of chemical
  bonds, and infrared spectroscopy finds its
  greatest utility for identification of organic
  and organometallic molecules. The high
  selectivity of the method makes the estimation of
  an analyte in a complex matrix possible

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EXAMPLE OF IR
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THEORY OF INFRARED ABSORPTION SPECTROSCOPHY

• For a molecule to absorb IR, the vibrations or
  rotations within a molecule must cause a net
  change in the dipole moment of the molecule. The
  alternating electrical field of the radiation
  (remember that electromagnetic radiation consists
  of an oscillating electrical field and an
  oscillating magnetic field, perpendicular to each
  other) interacts with fluctuations in the dipole
  moment of the molecule.
• If the frequency of the radiation matches the
  vibrational frequency of the molecule then
  radiation will be absorbed, causing a change in
  the amplitude of molecular vibration

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MOLECULAR ROTATIONS

• Rotational transitions are of little use to the
  spectroscopist. Rotational levels are quantized,
  and absorption of IR by gases yields line
  spectra.
• However, in liquids or solids, these lines
  broaden into a continuum due to molecular
  collisions and other interactions

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INSTRUMENT OF IR
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• INTRODUCTION TO FOURIER TRANSFORM OF INFRARED
  SPECTROSCOPHY

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WHAT IS FTIR SPECTROMETER

• A spectrometer is an optical instrument used to
  measure properties of light over a specific
  portion of the electromagnetic spectrum, 5
  microns to 20 microns.
• FTIR (Fourier Transform InfraRed) spectrometer is
  a obtains an infrared spectra by first collecting
  an interferogram of a sample signal using an
  interferometer, then performs a Fourier Transform
  on the interferogram to obtain the spectrum.
• An interferometer is an instrument that uses the
  technique of superimposing (interfering) two or
  more waves, to detect differences between them.
  The FTIR spectrometer uses a Michelson
  interferometer.

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FOURIER TRANSFORMS

• Fourier transform defines a relationship between
  a signal in time domain and its representation in
  frequency domain.
• Being a transform, no information is created or
  lost in the process, so the original signal can
  be recovered from the Fourier transform and vice
  versa.
• The Fourier transform of a signal is a continuous
  complex valued signal capable of representing
  real valued or complex valued continuous time
  signals

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SAMPLE ANALYSIS PROCESS

• 1. The Source Infrared energy is emitted from a
  glowing black-body source. This beam passes
  through an aperture which controls the amount of
  energy presented to the sample (and, ultimately,
  to the detector).
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• 2. The Interferometer The beam enters the
  interferometer where the spectral encoding
  takes place. The resulting interferogram signal
  then exits the interferometer.
•  
• 3. The Sample The beam enters the sample
  compartment where it is transmitted through or
  reflected off of the surface of the sample,
  depending on the type of analysis being
  accomplished. This is where specific frequencies
  of energy, which are uniquely characteristic of
  the sample, are absorbed.
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• 4. The Detector The beam finally passes to the
  detector for final measurement. The detectors
  used are specially designed to measure the
  special interferogram signal.
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• 5. The Computer The measured signal is digitized
  and sent to the computer where the Fourier
  transformation takes place. The final infrared
  spectrum is then presented to the user for
  interpretation and any further manipulation.
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FTIR THEORY

• The spectrometer described here is a modified
  Bomem MB-100 FTIR.
• The heart of the FTIR is a Michelson
  interferometer .
• The mirror moves at a fixed rate. Its position is
  determined accurately by counting the
  interference fringes of a collocated Helium-Neon
  laser.
• The Michelson interferometer splits a beam of
  radiation into two paths having different
  lengths, and then recombines them.
• A detector measures the intensity variations of
  the exit beam as a function of path difference.
• A monochromatic source would show a simple sine
  wave of intensity at the detector due to
  constructive and destructive interference as the
  path length changes.

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• In the general case, a superposition of
  wavelengths enter spectrometer, and the detector
  indicates the sum of the sine waves added
  together.
• shows some idealized light sources, and the
  interferograms that they would theoretically
  produce.
• The difference in path length for the radiation
  is known as the retardation d (OM OF d) .
• When the retardation is zero, the detector sees
  a maximum because all wavenumbers of radiation
  add constructively.
• When the retardation is l/2, the detector sees a
  minimum for the wavelength l. An interferogram is
  the sum of all of the wavenumber intensities

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FTIR BASICS
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SCHEMATIC OF MICHELSON INTERFEROMETER
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SAMPLE INTERFEROGRAMS AND THEORETICAL SOURCE
INTENSITY
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FTIR INSTUMENTATION

• In a conventional IR spectrophotomer, a sample IR
  beam is directed through the sample chamber and
  measured against a reference beam at each
  wavelength of the spectrum. The entire spectral
  region must be scanned slowly to produce good
  quality spectrum. In 5.32, we will be using a
  Nicolet FTIR Spectrophotometer (Nicolet was
  heavily involved in the design of the Hubble
  telescope!). IR spectroscopy has been
  dramatically improved by the development of the
  Fourier Transform method in much the same way as
  NMR has been revolutionized by this method.

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• The heart of an FTIR Spectrophotometer is a
  Michelson Interferometer built around the sample
  chamber. Radiation from an IR source is directed
  through the sample cell to a beam splitter. Half
  of the radiation is reflected from a fixed mirror
  while the other half is reflected from a mirror
  which moved continuously over a distance of about
  2.5 micrometers. When the two beams are
  recombined at the detector, an interference
  pattern is produced. A single scan of the entire
  distance takes about 2 seconds and is stored in
  the computer. In order that several scans may be
  added, they must coincide exactly. Obviously,
  this would be impossible
• considering the thermal fluctuations and
  vibrations in the laboratory. In order to solve
  this problem, a helium-neon laser is
  simultaneously directed through the Michelson
  Interferometer and the interference pattern of
  the laser is used as a frequency reference. The
  performance of an FTIR is dramatically superior
  to that of conventional instruments. Generally,
  only a small amount of sample will produce an
  excellent spectrum in a fraction of the time.

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PREPARATION OF SAMPLE

• Due to the sensitivity of the FTIR instrument,
  the most convenient and satisfactory method
  involves simple evaporation of a solution of the
  sample (chloroform, ether, dichloromethane or
  even a CDCl3 NMR sample may be used) onto a KBr
  salt plate and acquisition of the spectrum from
  the thin film remaining. This method provides
  excellent spectra with flat baseline unless the
  thin film is too powdery in which case
• excessive scattering of the light leads to an
  irregular baseline. The sample may alternatively
  be prepared as a nujol mull (mull accessories
  agate mortar and pestle, nujol and NaCl discs may
  be obtained from LS).

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PREPARATION OF INSTRUMENT

• If the instrument has just been turned on, then
  it is necessary to runa TEST ( F10 ) to be sure
  that all components are ON. If the instrument is
  not turned on or does not check out when the TEST
  is performed, then ask the instrument TA for
  help. In addition, it is important that N2 is
  flowing through the chamber so that most of the
  CO2 and
• H2O are flushed from the chamber and from inside
  the instrument. F4 SCAN BACKGROUND is performed
  with a blank IR plate in the chamber. F8 then F4
  DISPLAY BACKGROUND will show the spectrum of CO2
  and H2O that remain in the chamber. If the
  background shows excessive CO2 and H2O, then be
  sure the N2 is flowing briskly, wait a minute or
  two and try again. Once a good background has
  been obtained, several students in succession can
  use the same background.

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SCANNING OF SAMPLE

• Place the sample plate in the FTIR and wait for
  N2 to purge out the air.
• F5 SCAN SAMPLE. Wait until the scan and Fourier
  transform are completed.
• F8 then F1 DISPLAY SPECTRUM will automatically
  subtract the stored background and display the
  spectrum.
• F7 PRINT. Important Make sure that the printer
  is on-line before pressing F7.
• Type PEAKPICK S 4000 600 to find the peaks in the
  spectrum. This data is printed by pressing F7 .
  If no one else is using the instrument next,
  please turn off the nitrogen purge

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FTIR METHODS

• EPA Method 318 - Extractive FTIR Method for
  Measurement of Emissions from Mineral Wool and
  Wool Fiberglass Industries
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• EPA Performance Specification 15 for Extractive
  FTIR CEMS in Stationary Sources
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• EPA Method 320 -Vapor Phase Organic and
  Inorganic Emissions by FTIR (extractive)
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• EPA Method 321 - Determination of HCl for
  Portland Cement Industries
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• EPA Protocol for Extractive FTIR for Analysis
  of Gas Emissions
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• NIOSH Method 3800 - Organic and Inorganic gases
  by Extractive FTIR Spectrometry
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FTIR BENEFITS

• Real-time measurement results.
•  
• Simultaneous analysis of multiple gaseous
  compounds.
•  
• Measures a wide variety of volatile compounds
  (Inorganic and Organic).
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• Sensitivity from very low parts per million to
  high percent levels.
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• Provides a precise measurement method which
  requires no rigorous external calibration.
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• Speed. Measurements take only seconds

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ADVANTAGES OF FTIR

• Some of the major advantages of FT-IR over the
  dispersive technique include
•  
• Speed Because all of the frequencies are
  measured simultaneously, most measurements by
  FT-IR are made in a matter of seconds rather than
  several minutes. This is sometimes referred to as
  the
• Felgett Advantage.
•  
• Sensitivity Sensitivity is dramatically
  improved with FT-IR for many reasons. The
  detectors employed are much more sensitive, the
  optical throughput is much higher (referred to as
  the Jacquinot Advantage) which results in much
  lower noise levels, and the fast scans enable the
  coaddition of several scans in order to reduce
  the random measurement noise to any desired level
  (referred to as signal averaging).
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• Mechanical Simplicity The moving mirror in the
  interferometer is the only continuously
• moving part in the instrument. Thus, there is
  very little possibility of mechanical breakdown

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APPLICATIONS OF FTIR

• Identification of inorganic compounds and organic
  compounds
• Identification of components of an unknown
  mixture
• Analysis of solids, liquids, and gasses
• In remote sensing
• In measurement and analysis of Atmospheric
  Spectra
• - Solar irradiance at any point on earth
• - Longwave/terrestrial radiation spectra
• Can also be used on satellites to probe the space
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• thanQ