Working principle and instrumentation

An FT-IR instrument uses a system called an interferometer to collect a spectrum. The interferometer consists of source, beam-splitter, moving mirror, fixed mirror and detector.The energy goes from the source to the beam-splitter which splits the beam into two parts. While one half is transmitted to a moving mirror, the other half is reflected to a fixed mirror. The moving mirror moves back and forth at a constant velocity. The two beams are reflected back from the mirrors and recombined at the beam-splitter. The beam from the moving mirror travels a different distance than the beam from the fixed mirror.

Fig. 3. Schematic diagram of FT-IR interferometer

Consequently an optical path difference (δ) is introduced by change in position of moving mirror. When the reflected beams are combined, some of the wavelengths recombine constructively and some destructively creating an interference pattern. Two split beams will show constructive and destructive interference periodically, with continuous change of δ value. Completely constructive interference occurs when δ = nλ, but completely destructive interference occurs when δ = ( 1/2+ n)λ. A plot of light interference intensity as function of optical path difference is called an interferogram. This interferogram then goes from the beam-splitter to the sample, where some energy is absorbed and some is transmitted. The transmitted portion reaches the detector. The interferogram received is converted into a spectrum by using an algorithm called a Fourier transform using a computer. Fourier transform algorithm transfers information between a function in the time (t) domain and its corresponding one in the frequency (ω) domain.

Optical path difference can be considered to be in the time domain because it is obtained by multiplying time with the speed of a moving mirror. Wave-number can be considered in the frequency domain because it is equal to frequency divided by the light speed. Thus Fourier transformation convert an interferogram (intensity vs optical path difference) into an infrared spectrum (intensity versus wavenumber) plot. A reference or “background” single beam is also collected without a sample and the ratio of sample single beam to the background single beam is expressed as a transmittance spectrum. This transmittance spectrum can be converted to absorbance by using the relation A = −log T. The x-axis of the FT-IR spectrum is typically displayed in “wave-numbers” or cm^-1.

Application

In heterogeneous catalysis IR spectroscopy is used to:

• Study the molecular structure of catalysts
• Identify adsorbed species or adsorbed reaction intermediates and their structures on catalysts
• Provide informations on the nature of acid – basic sites present on catalyst surface during preparation and reactions