Fourier transform infrared spectroscopy (FTIR): Introduction, Principle, Instrumentation, Applications


FT-IR is the preferred method of infrared spectroscopy. In infrared spectroscopy, IR radiation is passed through a sample in which some of the infrared radiation is absorbed by the sample while other are passed through transmitted.


  • FT-IR always relies on the fact that the most molecules absorb light in the infra-red region of the electromagnetic spectrum.
  • This absorption corresponds specifically to the binds present in the molecule of the analyte. The frequency range is commonly measured using wave numbers ranging from 4000-600cm-1.
  • Firstly, the background emission spectrum of the IR source is recorded, followed by the emission spectrum of the IR source with the sample in place.
  • The ratio of the sample spectrum to the background spectrum is proportional to the sample’s spectrum absorption.
  • The resultant absorption spectra from the bond natural vibration frequencies indicates that the sample has a variety of chemical bonds and functional groups.
  • Because of the variety of functional groups, side chains, and cross-links involved, FTIR is especially useful for identifying organic molecular groups and compounds, all of which have distinct infrared vibrational frequencies.


The source

Glowing black body is mainly source for the emission of infrared energy. This beam passes through an aperture which control the amount of energy presented to the sample.

The interferometer

The beam will enter the interferometer where the “spectral encoding” tube is placed. The resulting interferogram signal then exits the interferometer.

Instrumentation of Fourier transform infrared spectroscopy (FTIR)

Fig: Instrumentation of Fourier transform infrared spectroscopy (FTIR)

The sample

The beam enters the sample compartment where it is transmitted through or reflected on the surface of the sample depending on the type of analytes being accomplished. This is where specific frequencies of energy which are uniquely characteristics of the sample are absorbed.


The beam is finally measure after passing through the detector. The detectors used are specifically designed to measure the specific interferogram signal.


The measured signal is digitalized and forwarded to the computer where the Fourier transformation takes place. The final infrared spectrum is them presented to the use for the interpretation and further investigation.


  • Opaque or cloudy samples
  • High resolution experiments
  • Identify and characterize of unknown materials such as powder, films, solids, or liquids
  • Trace analysis of raw materials of finished products
  • Depth profiling and microscopic mapping of samples
  • Kinetics reactions on the microsecond time-scale
  • Analysis of chromatographic and thermogravimetric sample fractions

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