Chapter 29 CHROMATOGRAPHY & SPECTROSCOPY

• What is thin layer chromatography used for?

  Separation and identification of compounds
• What is the stationary phase in TLC?

  Silica gel mounted on a glass plate
• What is the mobile phase in TLC?

  Solvent
• How are compounds separated in TLC?

  Separation is dependent on compounds‘ solubility in solvent and their adsorption to the stationary phase
• What can be said about compound that has travelled far up the TLC plate?

  The compound is soluble in the solvent and less attracted to the stationary phase
• What causes Rf values to change?

  If temperature, the solvent or the makeup of the TLC plate changes
• How can colourless compounds be identified on a TLC chromatogram?

  Using a UV lamp or placing it in iodine
• What are the limitations of TLC?

  • Some compounds may have the same Rf value
  • New compounds may have unknown Rf values
• What is gas chromatography used for?

  Separation of a mixture of liquids that are volatile
• How does gas chromatography work?

  Column inside GC is lined with solid or viscous liquid which is the stationary phase. Sample is injected into machine and carried by an inert gas (usually nitrogen) which is the mobile phase.
• What is retention time?

  The time taken for each sample to hit the detector
• How are compounds separated in GC?

  Separation is dependent on compounds’ solubility in the stationary phase
• What can be said about a compound with a large retention time?

  It is soluble in the stationary phase therefore takes longer to reach the detector
• What does a GC spectra show?

  Peaks with the area under each representing the relative amount of the substance present in the sample
• What is GC used to detect?

  Alcohol level in urine and blood which can be used as evidence in court
• What are calibration curves and how do they work?

  They’re used to measure the concentration of a substance.

  Known concentrations of the substance are plotted on a graph. The sample of unknown concentration is run through GC and its area under the peak is found and then concentration can be found.
• What is retention time affected by?

  Solubility, temperature of the GC machine and boiling point of the substance