Interpreting Rf Values in GL Chromatography (CIE A Level Chemistry)

Features of a gas-liquid chromatogram

• Peaks represent different molecules from the sample - each roughly taking the shape of a triangle
• The area under each peak is the relative concentration of each component (the peak integration value)

Area under the peak = ½ x base x height

• If the area under each peak is very small or too difficult to decipher, the height of the peaks are used for further analysis

Calculating areas under curves on a gas chromatogram

To find the area under each peak, treat each peak as a triangle - see the examples shown using blue triangles in the diagram

Percentage composition of a mixture

• We can calculate the amount of a particular molecule in a sample by using an expression
• If a chromatogram shows peaks for alcohols A, B, C and D, to calculate the % composition of alcohol C, use this expression:

• Retention time is the time taken for a sample molecule to travel through the column, from the time it is inserted into the machine to the time it is detected
• Molecules in the gaseous mixture travel at different rates, therefore giving rise to different retention times
• Longer retention times are associated with:
  • Non-polar components in the mixture
  • They are more attracted to the non-polar liquid in the stationary phase
  • So non-polar molecules travel slower through the column
• Shorter retention times are associated with:
  • Polar components in the mixture that prefer to interact with the carrier gas
  • They are less attracted to the non-polar liquid in the stationary phase
  • So polar molecules travel faster through the column
  • These molecules may have lower boiling points and, therefore, are vapourised more readily