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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:
This is the end of the topic! Well done!
Drafted by Cherry (Chemistry)