Elements, Compounds and Mixtures

• A chromatogram is read by comparing the spots of an unknown sample against spots from known reference substances
• Pure substance → a single spot in its lane
• Impure substance → two or more spots in the same lane
• Two samples of the same substance → produce identical chromatograms (same number of spots at the same heights)
• A mixture separates on the paper, with each of its constituent substances showing up as a distinct spot in the lane
• Standard practice is to run known reference samples alongside the unknown in adjacent lanes
  • The references give you fixed positions to compare against
  • Any spot in the unknown lane that sits at the same height as a reference spot is the same substance

Rf values

• R_f (retention factor) is used to identify a component quantitatively
• Defined by:

$R_f = \frac{\text{distance moved by the substance}}{\text{distance moved by the solvent}}$

• Distance moved by the substance is measured from the baseline up to the centre of the spot
• Distance moved by the solvent is measured from the baseline up to the solvent front
• Properties:
  • Has no units (it's a ratio)
  • Always less than 1 (a spot can't travel further than the solvent that carries it)
  • Constant for a given solute, solvent and paper at a given temperature
  • Changes if you change the solvent or the paper