7.3: Chromatography

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  • Chromatography depends on the equilibrium set up when a compound distributes itself between two phases - one phase stands still (the stationary phase) while the other moves over it (the mobile phase).
  • Different compounds distribute themselves between the two phases to different extents, and so move along with the mobile phase at different speeds.
  • The distance a particular substance travels in paper or thin layer chromatography depends on the nature of the substance, the total distance travelled by the solvent front and the conditions under which chromatography is carried out - such as temperature, type of paper etc.
  • The Rf value for a substance is the distance that substance travels relative to the solvent front.
  • In gas-liquid chromatography, the mobile phase is an unreactive gas (carrier gas) and the stationary phase is a small amount of a high boiling point liquid held on a finely divided inert solid support.
  • The more volatile compounds usually emerge first.
  • Signals from a detector are plotted by a recorder on a chromatogram. 
  • The time that a compound is held on a column under given conditions is characteristic of the compound and is called its retention time.
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