Comparison of different Solid Sampling Techniques for FT- IR Spectroscopy

Comparison of different Solid Sampling Techniques for FT- IR Spectroscopy
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Apart from liquids solids constitute a major segment of samples characterized using FT-IR spectroscopy. The following techniques are commonly adopted for solid samples

  • Solids dissolved in solution
  • Solid films
  • Mull Technique
  • KBr Pellet technique

The merits and de-merits of each technique are discussed in this article

Solids in Solution

The solid is dissolved in a suitable non-aqueous solvent and a drop is allowed to evaporate on alkali metal plate. On evaporation it leaves a deposit in the form of a thin film of the solid on the plate. Solvents commonly used are moisture free chloroform, carbon tetrachloride, cyclohexane, acetone,etc.

Demerits

  • Solid sample should be soluble in the chosen solvent and the solvent should have no absorbance in the selected wavelength range
  • Solute should have no chemical interaction with the chosen solvent
  • Selected solvent should be non-toxic

Films

A transparent polymer films can be mounted in the sample compartment using a film holder accessory. Alternately polymer resins are dissolved in a compatible solvent. The solution is poured on the appropriate salt plate and solvent allowed to evaporate. Polymers can also be hot pressed on the plates.

The method is convenient for rapid qualitative analysis but has limitations for quantification

Mull Technique

Small quantity of powder is ground to a fine state in a mortar and a thick paste is made by addition of Nujol in small quantities. A portion of the paste is transferred to a salt plate and another plate is pressed against it to form a uniform film of desired thickness before taking the spectrum. Nujol exhibits absorption bands in the IR range and can result in interferences. The problem can be overcome to some extent by mixing hexachlorobutadiene to nujol. for making the mull.

KBr Pellet Technique

The KBr pellet is prepared using commonly available hydraulic press. The pellet is a transparent button sized disk which can be mounted in the IR beam path for recording the spectra of the compound

Merits

  • KBr is transparent over the mid IR region and does not contribute to any interfering absorbance bands as in the case of nujol
  • It is possible to store pellets under moisture free conditions for recording spectra at a later stage.
  • Resolution of absorption peaks can be improved by varying solute concentration in the KBr powder mixture.

De-merits

  • KBr pellets if not stored under moisture free conditions tend to fog due to hygroscopic nature of KBr
  • Technique cannot be applied to materials such as polymer granules which cannot be ground with KBr powder
  • Pellet making is time consuming and can take up to 5 min per sample
  • Pellets require careful handling as they can crack or break due to their brittleness
  • Polymorphic changes in crystallinity of samples can take place during pellet making due to high pressures involved.

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