Basics of FTIR

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FTIR stands for Fourier Transform Infrared Spectroscopy.

It is the most widely used technique in organic chemistry for the elucidation of structure of unknown compound by producing its IR spectrum or also called as emission spectrum.

In FTIR spectrophotometer the IR radiation passes through the sample, some radiation is absorbed by the sample and some radiations passes through the sample, which produces the IR spectrum of the sample represents fingerprint of the sample because like fingerprint no two unique molecule produces the same IR spectrum.

Principle of IR Spectroscopy

IR spectroscopy is based on the absorption principle, it is the interaction between IR radiations with molecule. When IR radiation absorbed by the molecule it causes excitation of a molecule to higher vibration or rotation level and generate the IR spectra of a molecule which contains functional group region and fingerprint region.

IR spectrum (graph) is plot between intensity (% transmittance) on vertical axis and wavenumber on horizontal axis. Wavenumber is a reciprocal of wavelength, hence its unit is cm-1.

Regions of Infrared Spectrum

  • Near Infrared region (overtone region): It ranges from 0.8 µm to 2.5 µm (14000 to 4000 cm-1)
  • Mid Infrared region (vibration-rotation region) : It ranges from 2.5 µm to 25 µm (4000 to 400 cm-1).
  • Far Infrared region (rotation region) : It ranges from 25 µm to 1000 µm (400 to 10 cm-1).

 

Components of FTIR

  • IR radiation source: IR radiations produced by heating the source electrically. Light source for the mid IR region is Nernst glower or a globar. Nernst glower is fabricated with binder and oxides of thorium, cerium, zirconium and yttrium it is less common used as a light source because at ambient temperature it is non conducting. Globar is a small rod of silicon carbide.High pressure mercury lamp and tungsten halogen lamp are also used as light source.

 

  • Monochromator or Optical material: Monochromator is optical device consist of prism and gratings. Monochromator disperse the radiation according to its wave number and selects the radiation falling to detector into a narrow wave number range. Grating disperse the radiations and separate the undesirable wavelength by high quality filters.

 

  • Interferometer : It is the most important component of FTIR. It splits the light into two path and then recombine the two beam of light. Interferometer consist of two mirrors one is fixed mirror and other is movable mirror and a beam splitter. Beam splitter separate the light in two parts half light is transmitted and half light is reflected. These transmitted and reflected light reaches the fixed mirror and then to movable mirror and then reflected by mirror , two beam recombines with each other at beam splitter.

 

  • Sample Compartment : Sample to be examined is placed in the sample compartment. Generally ATR (Attenuated total reflectance ) assembly is employed in the FTIR , ATR crystal is consist of germanium, zinc selenide and silicon and it has property of total internal reflection.

 

  • Detector :Detector is a device which convert the thermal radiant energy into electrical energy. Two types of detector are thermal and photonic detector. Commonly used detector in FTIR is :
    1. Thermocouple detector
    2. Bolometer
    3. Pneumatic detector

Recent detectors are fabricated with crystals such as lithium, niobate, barium titnate, MCT (mercury, cadmium, telluride). These crystals are known as pyroelectrics due to these crystals radiation can de chopped at higher rate and take less time then other detectors, However these detectors are more expensive so not widely employed.

  • Amplifier : It amplify the signals received from detector . The radiation beam is chopped at a suitable frequency to provide a AC signal to the detector then the signal is rectified and used to derive the recorder. Software employed in the computer stores the data and generate the IR spectra of a sample after scanning.

Application of FTIR

  • Qualitative Application: Identification of functional group of a compound. Structure elucidation of a compound
  • Quantitative Application :
  1. Determination of aspirin, phenacetin and caffeine in analgesic tablets
  2. Determination of meprobamate in tablet
  3. Determination of codeine phosphate in tablet
  4. Distinguish between primary, secondary and tertiary amine salts.
  5. Determine the assay of various pharmaceutical dosage form.

 

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