Chromatography

• Chromatography is an analytical technique that separates components in a mixture
• During chromatography a mobile phase sweeps a mixture over a stationary phase
  • Stationary phase (S.P) is fixed in place and does not move
  • Mobile phase (M.P) moves in a definite direction
• Different components have different affinities for a stationary and mobile phase
  • S.P interacts with components in the mixture and slows them down.
  • Greater the interaction, the more components are slowed down - so components move over at different speeds and are separated

Thin Layer Chromatography is the most common chromatography technique seen in AS/A-level Chemistry! You will have to do this experiment in labs! 🧪

Thin Layer Chromatography

• S.P is a solid: thin layer of an adsorbent eg. silica gel or alumina
• M.P is a liquid solvent

Producing chromatogram:

• As the solvent rises it meets the spot of sample and the components in mix are swept along with the solvent.
• Separation is achieved by adsorption - some components bind strongly, some weakly and the result is different components are separated and travel different distances up plate.
• Each separated component is seen as a spot on the TLC plate. Use colours or UV radiation to show up by fluorescence.

TLC is interpreted by Rf values:

Rf = distance moved by component divided by distance moved by solvent front

Limitations:

• Similar compounds often have similar Rf values
• Unknown compounds have no reference for Rf comparison
• Difficult to find a solvent that separates all components in mixture

Gas Chromatography

Separates volatile components in a mixture, useful when compounds have a low BP and evaporate easily.

• S.P: thin layer of liquid or solid coated on tubing (acts as a support). Liquid is often a long chain alkane with a high BP. Solid can be silicon polymers.
• M.P: inert, unreactive carrier gas eg helium or nitrogen

Producing chromatogram

• As mix moves through column, components slow down as the interact with S.P lining tube.
  • if liquid: components dissolve
  • if solid: components become adsorbed to the surface - greater the absorption, greater they are slowed down. Slow down different amounts so separate.
  • Separation can be improved by different temperatures or different gas flow rates
• Each component leaves the column at different times and is detected as it leaves. Computer processes timings and produces chromatogram.

• Retention time: time for a component to pass from the column inlet to the detector:
  • Different compounds have different retention times
  • Can compare retention times with known compounds to identify unknown compounds
  • Area under each peak is proportional to the amount of compound in a sample

Limitations:

• Thousands of chemicals may have the same retention time, peak shape and detector response
• Not all substances will be separated and detected.
• Unknown compounds have no reference retention time for comparison - need to know which compounds are expected.

Gas chromatography-mass spectrometry (GC-MS)

• Combine two techniques because:
  • GC can separate components but cannot identify
  • MS cab provide detailed structural info but cannot separate components

• Components are first separated using GC, retention time provides a preliminary identification
• Separated components are directed into an MS where they are detected. MS can be analysed and provides a positive identification for each component in mix as MS are unique and definitive.

• Uses:
  • Forensics to identify particles from crime scenes
  • Environmental analysis to monitor and analyse atmospheric pollutants
  • Airport security to detect explosives
  • Space Probes to collect and analyse material

Reference:

• Chromatography | CIE A Level Chemistry 2019-21 Notes (savemyexams.co.uk)

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