FT-IR Instrument

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FT-IR Instrument
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Most commercial instruments separate and measure
IR radiation using

• Dispersive spectrometers or
• Fourier transform spectrometers.

Dispersive Spectrometers Dispersive
spectrometers, introduced in the mid-1940s and
widely used since, provided the robust
instrumentation required for the extensive
application of this technique.
This consists of three basic components
radiation source, monochromator, and detector
Radiation source
The common radiation source for the IR
spectrometer is an inert solid, heated
electrically to 1000 to 1800 C. Three popular
types of sources are Nernst glower (constructed
of rare-earth oxides), Globar (constructed of
silicon carbide), and Nichrome coil. They all
produce continuous radiations, but with different
radiation energy profiles.
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Monochromator

• The monochromator is a device used to disperse a
  broad spectrum of radiation and provide a
  continuous calibrated series of electromagnetic
  energy bands of determinable wavelength or
  frequency range.
• Prisms or gratings are the dispersive components
  used in conjunction with variable-slit
  mechanisms, mirrors, and filters.

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Detectors

• Most detectors used in dispersive IR
  spectrometers can be categorized into two
  classes
• Thermal detectors and
• Photon detectors.
• Thermal detectors include thermocouples,
  thermistors, and pneumatic devices (Golay
  detectors).
• They measure the heating effect produced by
  infrared radiation. A variety of
• physical property changes are quantitatively
  determined expansion of a nonabsorbing gas
  (Golay detector), electrical resistance
  (thermistor), and voltage at junction of
  dissimilar metals (thermocouple).

Photon detectors rely on the interaction of IR
radiation and a semiconductor material.
Nonconducting electrons are excited to a
conducting state. Thus, a small current or
voltage can be generated. Thermal detectors
provide a linear response over a wide range of
frequencies but exhibit slower response times and
lower sensitivities than photon detectors.
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General Concepts Interferometry

• Optical Interfrometry is an optical measurement
  technique that provides extreme precise
  measurements of distance, displacement or shape
  and surface of objects.
• It exploits the phenomenon of light waves
  interference .
• Where under certain conditions a pattern of
  dark and light bars called interference fringes
  can be produced. Fringes can be analyzed to
  present accurate measurements in the range of
  nanometer.
• The recent developments in laser, fiber optics
  and digital processing techniques have supported
  optical interferometry .
• Applications ranging from the measurement of a
  molecule size to the diameters of stars.

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Interference

• Interference is a light phenomenon .It can be
  seen in everyday life. e.g.. colures of oil film
  floating on water.
• In electromagnetic waves , interference between
  two or more waves is just an addition or
  superposition process. It results in a new wave
  pattern .

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Superposition of two waves

• When two waves with an equal amplitudes are
  superposed the output wave depends on the phase
  between the input waves.
• Y y1 y2
• Where y1A1 sin (wt ?1 )
• y2A2 sin (wt ?2)
• Since the energy in the light wave is intensity I
  ,which is proportional to the sum of square
  amplitudes A2
• where AA12A222A1A2 cos (?1 ?2)
• If A1A2A then
• A2A22A2 cos (?1 ?2)
• If y1y2 in phase ,cos(0)1 hence,
• Y 4A2 ,it gives a bright
  fringe.
• If y1y2 out of phase by (p)
  ,cos (p)-1 hence,
• Y 0 ,it gives a
  dark fringe

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Optical Path Length OPL

• When light beam travels in space from one point
  to another, the path length is the geometric
  length d multiplied by n (the air refractive
  index) which is one
• OPL d
• Light beam travels in different mediums will
  have different optical path, depending on the
  refractive index (n)of the medium or mediums.
• OPL n d

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Components
Fourier transform spectrometers

• Superior speed and sensitivity
• Instead of viewing each component frequency
  sequentially, as in a
• Dispersive IR spectrometer, all frequencies are
  examined simultaneously in Fourier transform
  infrared (FTIR) spectroscopy.

Michelson Interferometer

• Source
• Michelson Interferometer
• Sample
• Detector

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Sources

• Black body radiators
• Inert solids resistively heated to 1500-2200 K
• Max radiation between 5000-5900 cm-1 (2-1.7 mm),
  falls off to about 1 max at 670 cm-1 (15 mm)
• Nernst Glower cylinder made of rear earth
  elements
• Globar- SiC rod
• CO2 laser
• Hg arc (Far IR), Tungsten filament (Near IR)

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Michelson interferometer

• Configuration
• Michelson interferometer consists of a
  coherent
• light source, a beam splitter (BS), a reference
  mirror ,a movable mirror and a screen .
• Applications
• There are many measurements that
  Michelson interferometer can be used for,
  absolute distance measurements, optical testing
  and measure gases refractive index.
• Work method
• The BS divides the incident beam into two
  parts one travel to the reference mirror and the
  other to the movable mirror .both parts are
  reflected back to BS recombined to form the
  interference fringes on the screen.

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Sample

• Sample holder must be transparent to IR- salts
• Liquids
• Salt Plates
• Neat, 1 drop
• Samples dissolved in volatile solvents- 0.1-10
• Solids
• KBr pellets
• Mulling (warm)(dispersions)
• Quantitative analysis-sealed cell with
  NaCl/NaBr/KBr windows

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Detector

• The two most popular detectors for a FTIR
  spectrometer are
• deuterated triglycine sulfate (DTGS) and
• mercury cadmium telluride (MCT).
• The response times of many detectors (for
  example, thermocouple and thermistor) used in
  dispersive IR instruments are too slow for the
  rapid scan times (1 sec or less) of the
  interferometer.
• The DTGS detector is a pyroelectric detector that
  delivers rapid responses Because it measures the
  changes in temperature rather than the value of
  temperature.
• The MCT detector Is a photon (or quantum)
  detector that depends on the quantum nature of
  radiation and also exhibits very Fast responses.
  Whereas DTGS detectors operate at room
  temperature, MCT detectors must be maintained at
  liquid nitrogen temperature (77 K) to be
  effective. In general, the MCT detector is faster
  and more sensitive than the DTGS detector.

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FTIR Advantages

• Better speed and sensitivity (Felgett
  advantage). A complete spectrum can be obtained
  during a single scan of the moving mirror, while
  the detector observes all frequencies
  simultaneously.
• Increased optical throughput (Jaquinot
  advantage). Energy-wasting slits are not required
  in the interferometer because dispersion or
  filtering is not needed.
• Internal laser reference (Connes advantage). The
  use of a helium neon laser as the internal
  reference in many FTIR systems provides an
  automatic calibration in an accuracy of better
  than 0.01 Cm 1 . This eliminates the need for
  external calibrations.
• Simpler mechanical design. There is only one
  moving part, the moving mirror, resulting in less
  wear and better reliability.
• Elimination of stray light and emission
  contributions. The interferometer in FTIR
  modulates all the frequencies. The unmodulated
  stray light and sample emissions (if any) are not
  detected.
• Powerful data station. Modern FTIR spectrometers
  are usually equipped with a powerful, com-
  puterized data system. It can perform a wide
  variety of data processing tasks such as Fourier
  transformation, interactive spectral subtraction,
  baseline correction, smoothing, integration, and
  library searching.