Gas Chromatography GC - NEW

• Retention times and Rf values are used to identify different substances.
• • Each substance takes a different amount of time to travel through the column and reach the detector
  • Length of time it takes is called the retention time
• • gas chromatography (GC) – a column is packed with a solid or with a solid coated by a liquid, and a INERT gas is passed through the column
  • under pressure at high temperature
• • Gas spectrometry show peaks of varying sizes and appearing at different times
  • Compare retention times with known retention times to help identify substance in the mixture
• • Area under the peaks tell us the amount of each substance
• The mixture of tripeptides can be analysed by using gas chromatography, coupled with mass spectrometry. Summarise how each method contributes to the analysis.

  • gas/liquid chromatograph separates the tripeptides (1)
  • mass spectrometer produces a distinctive fragmentation pattern (1)
  • identification by computer using a spectral database (1)
• Define the following terms used in chromatography

  Retention time

  • Retention time is the time between injection and or detection of a component
• Explain why different compounds will have different retention times in the same column under the same conditions

  • Retention time depends on the POLARITY or attraction / affinity / solubility of the component for the STATIONARY PHASE
  • The greater attraction to stationary phase, the greater the retention time
  • IGNORE attraction TO MOBILE PHASE
• Sample of tripeptide was hydrolysed and then placed on TLC plate. Only two spots were seen.

  Give a reason for the lack of a third spot.

  • One amino acid is present twice in the tripeptide
  • OR another amino acid has the same Rf value as (the two spots seen)