CIE A Level Chemistry (9701) 2019-2021

Revision Notes

8.1.1 Chromatography

Gas/Liquid Chromatography: Basics

  • Gas-Liquid Chromatography (GLC) is used for analysing:
    • Gases
    • Volatile liquids
    • Solids in their vapour form
  • The stationary phase is a column packed with small silica particles
  • The mobile phase is an inert gas (Eg. Helium or Nitrogen)
  • Sample molecules are injected into the column and carried along with the insert gas
  • Molecules separate and travel to a detector at the end of the column

Retention times

  • Once separated sample molecules reach the detector, their retention times are recorded
    • This is the time taken for a separated sample molecule to travel through the column
  • The time taken for a molecule to travel through the column depends on strength of interaction with the stationary phase
  • The retention times are recorded on a chromatogram where each peak represents a volatile compound in the analysed sample
  • Retention times are then compared with data book values to identify unknown molecules

Analytical Techniques - GLC Chromatogram, downloadable AS & A Level Chemistry revision notes

 This gas chromatogram of a volatile sample compound has six peaks. Depending on each molecule’s interaction with the stationary phase, each peak has its own retention time

Thin Layer Chromatography: Basics

  • Thin Layer Chromatography (TLC) is a technique used to analyse small samples via separation
    • For example, a dye could be separated out to determine the mixture of dyes within
  • There are 2 phases involved in TLC – stationary phase and mobile phase
  • Stationary phase
    • Adsorbs the solute molecules of the sample
    • Depending on the strength of interactions with the stationary phase, the separated components will travel particular distances through the plate
  • Mobile phase
    • Flows over the stationary phase
    • It is usually a polar or nonpolar liquid (solvent) that carries components of the compound being investigated
    • Polar solvents are water or alcohol
    • Non-polar solvents are alkanes

Rf values

  • The less polar components travel further up the TLC plate
    • Their Rf values are higher than those closer to the baseline
    • They are more soluble in the mobile phase and get carried forwards with the solvent
  • More polar components do not travel far up the plate
    • They are more attracted to the polar stationary phase
  • The extent of separating molecules in the investigated sample depends on the solubility in the mobile and stationary phases
  • Knowing the Rf values, of compounds being analysed, helps to compare the polarity of various molecules

Calculating Rf values

  • Rf values for compounds are calculated using measurements from the TLC plate

Thin Layer Chromatography Basics equation

  • These values can be used alongside other analytical data to deduce composition of mixtures

 

Analytical Techniques - Calculating Rf Values, downloadable AS & A Level Chemistry revision notes

Rf values can be calculated by taking 2 measurements from the TLC plate

Retention times for Gas/liquid Chromatography

  • The chromatogram for Gas/liquid Chromatography records the retention time for each of the sample molecules
  • Retention time is the time taken for a sample molecule to travel through the stationary phase column
  • Molecules in the gaseous mixture travels at different rates therefore giving rise to different retention time
  • 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 vapourised more readily

Exam Tip

  • Rf values are quoted as decimals and they do not have any units
  • The units used for measuring distances travelled by the component and solvent cancel each other out
  • When you are asked to explain the Rf values in TLC or retention times in Gas-liquid Chromatography, refer to the interactions between sample molecules and the stationary phase

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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