Chromatography

• What are the basic principles of all kinds of chromatography

  • A family of separation techniques
  • Depends on the principle that a mixture is separated if it is dissolved in a solvent
  • This mobile phase is then passed over a solid (the stationary phase)
• What is the mobile phase

  Carries the soluble components of the mixture
• What relationship between a sample and the mobile phase makes the sample move faster

  • More soluble components/components with more affinity to the solvent MOVE FASTER
• What does the stationary phase do

  Holds back components of the mixture that are attracted to it
• What is the relationship between a sample and the stationary phase that make the sample move slower

  • More affinity for the stationary phase means that a component moves slower
  • They are often attracted by HYDROGEN BONDING
• How are substances separated by chromatography

  • If suitable stationary/mobile phases are chosen, the balance between affinity for the mobile phase and affinity for the stationary phase is different for each component of the mixture
  • Thus, they move at different rates and are separated over time
• Why will different substances show different Rf values?

  • They are bonded differently and have different polarities -> more polar bonds mean longer retention time or smaller Rf
  • This is due to the HYDROGEN bonding and DIPOLES are attracted more strongly to the stationary phase
• What does TLC stand for

  Thin Layer Chromatography
• What is the stationary phase in TLC

  • Plastic
  • Glass
  • Metal sheet
  • COATED IN SILICA (SiO2) or ALUMINA (Al2O3)
• What are the advantages of TLC over paper chromatography

  • Runs faster
  • Smaller amounts of a mixture can be separated
  • TLC plates are more robust that paper
• How can you observe colourless spots on TLC

  Shine UV light on them
• How do you calculate the Rf value

  • Measure the distance from the initial line (that the mixture was spotted onto) to the solvent front, and the distance from the initial line to the spot
  • Calculate Rf
  • Rf = distance moved by spot / distance moved by solvent front
• What does Rf value stand for

  • RETENTION FACTOR
  • A measure of the rate of movement of a component through the chromatography apparatus
  • A ratio between the rate movement of the solvent and that component
• How could you confirm the identity of a substance from its Rf value

  • Compare your Rf value to accepted values
  • Rf for that substance should be run in the same solvent and set-up
  • If they match, then the identity is confirmed
• What is COLUMN chromatography

  • A column packed with silica, alumina or resin
  • It has a solvent run through it downwards
• What is the stationary phase in column chromatography

  Silica, alumina or resin packed into a column
• What is the mobile phase in the column chromatography

  • The solvent is added at the top and runs down the column
  • It is called ELUENT
• Draw a diagram of column chromatography

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• What are the ADVANTAGES of column chromatography

  • More than one eluent can be used, which leads to BETTER separation
  • Fairly large amounts can be separated and collected after separation
• Draw a diagram of gas-liquid chromatography

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• What is the stationary phase in gas-liquid chromatography

  • Powder coated with oil
  • Packed into a long, thin capillary tube
  • Coiled and placed in an oven, the temperature of which can be varied
• What is the mobile phase in gas-liquid chromatography

  • Carrier gas
  • Inert
  • E.g N2 or He
• What do you measure in gas-liquid chromatography

  Retention time; different compounds of the mixture take different amounts of time to move through
• What are the ADVANTAGES of gas-liquid chromatography

  • Very sensitive
  • GC can detect minute traces of substances in foodstuffs, and link oil pollution on beaches to the specific tanker the oil came from
• What are Gas-liquid chromatographies used for

  • Test atheletes and horses blood and urine for blood
• How can you use GC or GCMS to identify substances

  • Match the gas chromatograph to that of a KNOWN substance under the SAME conditions
  • The retention times should be an exact match
  • The substances identity can be confirmed by mass spectrometry, NMR or infrared spectroscopy
• How does GCMS

  • Gas chromatography is run
  • The retention time is recorded
  • The mixture is run through a mass spectrometer
  • Fragmentation patter/molecular ion peak confirms the identity
• Will an alcohol or an aldehyde have a shortest retention time by column chromatography

  • Aldehyde has the SHORTEST retention time
  • Since it has a LESS POLAR bond than an ALCOHOL
  • It therefore adsorbs LESS STRONGLY to the stationary phase, so it moves down the column at a quicker rate
  • The force of attraction between the stationary phase and aldehyde is LESS