3.3.16 - Chromatography

• State in general terms what determines the distance travelled by a spot in TLC. [1] Balance between solubility in moving phase and retention by stationary phase
• What is the use of chromatography? To separate and identify the components in a mixture"
• What are the two phases in a chromatography set up? What do they normally consist of?  Stationary phase - solid/liquid held in a solidMobile phase - liquid or gas"
• What does the movement of a substance in chromatography depend on? A balance between:Solubility in solventRetention in the stationary phase
• Column Chromatography: Used to separate an organic product from impurities. 
• Thin Layer Chromatography. Used to separate and identify components in a mixture
• Gas Chromatography: Used to separate volatile liquids
• What two properties can be measured to identify substances in chromatography?

  1. Retention times: how long each component takes from being injected to recorded
  2. Rf values: Distance moved by sample / distance moved by solventIn both instances, they are compared with standards from a data book
• How to carry out TLC:

  1. Draw pencil line
  2. Use capillary tube to add a tiny drop of each
  3. Add solvent to a chamber
  4. Place TLC into chamber; ensuring solvent is below pencil line
  5. Leave for time; then remove and mark solvent front with pencil
  6. Dry in a fume cupboard
• TLC: Why do we wear gloves? prevents contamination from hands to the plate
• TLC: Why do we use a lid? prevents evaporation of toxic solvent
• TLC: Why do we care about the depth of solvent? if solvent is too deep, it dissolves the sample spots from the plate
• TLC: Why do we dry in a fume cupboard? solvent may be toxic
• How can you suggest which compounds move more quickly through the column? If moving phase is non-polar e.g hexane: non-polar compounds pass through more quicklyIf moving phase is polar e.g ethanol: polar compounds pass through quicklyIt's all about solubility and IMF.
• Suggest why it was necessary to use two different solvents. [1] Some of the amino acids didn't dissolve with the first solvent
• Thin layer chromatography:

  • Stationary Phase: Plate coated with a solid
  • Mobile Phase: Solvent moving up the plate
• Column Chomatography:

  • Column is packed with a solid (Al2O3) and a solvent moves down the column
  • More soluble components move down column faster.