CIE A Level Chemistry复习笔记8.1.3 Interpreting Rf Values in GL Chromatography

Interpreting Rf Values in GL Chromatography

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 peaks are used for further analysis

8.1-Analytical-Techniques-Labelled-GLC-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:

4.-Interpreting-Rf-Values-in-GL-Chromatography-equation

Explain Retention Times

  • 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, therefore are vapourised more readily

 

 

 

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