METHODS OF CHARACTERIZATION OF ADSORBATES-A REVIEW IV

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METHODS OF CHARACTERIZATION OF ADSORBATES-A REVIEW IV

chemonaire
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2.0 LITERATURE REVIEW

2.1 Fourier Transformed Infrared Spectroscopy (FT-IR)

Fourier Transform Infrared spectroscopy (FTIR) is a technique based on the vibrations of the atoms within a molecule. It becomes prominent at the beginning of 20th century and then developed due to the rapid development of technologies on computer and Fourier transform. It has many advantages such as short time of measurement, high sensitivity and resolution and broad range of measurement spectrum.

 An infrared (IR) spectrum, from which the details of the functional groups present on the adsorbent can be determined, is obtained by passing IR radiation through a sample and determining what fraction of the incident radiation is absorbed at a particular energy.  The energy at which any peak in an absorption spectrum appears corresponds to the frequency of a vibration of a part of a sample molecule.  Moreover, chemical bonds in different environments will absorb varying intensities and at varying frequencies.  

The FTIR sample handling is to grind the adsorbent finely with a specially purified salt (usually potassium bromide) to remove scattering effects from large crystals.  This powder mixture is then crushed in a mechanical die press to form a translucent pellet through which the beam of the spectrometer can pass.

Therefore IR spectroscopy involves collecting absorption information and analyzing it in the form of a spectrum.  Since each interatomic bond may vibrate in several different motions (stretching or bending), individual bonds may absorb at more than one IR frequency.  Stretching absorptions usually produce stronger peaks than bending, however the weaker bending absorptions can be useful in differentiating similar types of bonds (e.g. aromatic substitution).

The basic components of an FTIR spectrometer are shown schematically in the figure below.  The radiation emerging from the source is passed to the sample through an interferometer before reaching a detector. Amplification of the signal converts the data to a digital form by an analog-to-digital converter and then transferred to the computer for Fourier transformation to be carried out.

 One of the great advantages of infrared spectroscopy is that virtually any sample in nearly any state can be studied.  Liquids, solutions, pastes, powders, films, fibres, gases and surfaces can all be examined by a judicious choice of sampling technique. FTIR spectroscopy is carried out to study the potential existence of C-N, C=C and C≡N bonds among others in the adsorbent.

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