Chem. 231

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Chem. 231 2/25 Lecture
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Announcements

• Quiz 2 Today
• New Homework Pass out
• Set 1 Labs
• should be finishing by next week
• Todays Lecture
• Practical Aspects of GC

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Practical Aspects of GCSample and Analyte Types

• Because GC performance generally exceeds HPLC in
  most categories, GC is often considered even if
  not ideal
• Main Requirements
• Analyte must be capable of moving through column
  and being detected
• Rest of sample should not interfere with analysis
  or lead to degradation of method

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Practical Aspects of GCSample and Analyte Types

• Analyte Considerations
• Based on volatility
• permanent gases
• semi-volatile compounds
• non-volatile compounds
• Other requirements
• stability in column
• lack of secondary retention (e.g. carboxylic
  acids often interact with small percent of column
  uncoated by stationary phase)
• Based on detectability
• must be detectable
• in complex samples, may need specific detector

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Practical Aspects of GCSample and Analyte Types

• Volatility
• Permanent gases
• often require cyrogenic cooling of column oven
• may require specific injection techniques
• Semi-volatile compounds
• most common, particularly as solute
• direct injection as gases is difficult (due to
  partitioning to other phases)
• Non-volatile compounds
• can be derivatized (usually to less polar
  derivatives such as conversion from carboxylic
  acid to ester)
• pyrolysis methods

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Practical Aspects of GCSample and Analyte Types

• Sample Interferences (must be minimized)
• Non-volatile compounds
• will be permanently retained on column,
  eventually changing column behavior
• Problematic compounds
• polar compounds (e.g. water) often damage column
  stationary phase
• thermally labile compounds can disintegrate upon
  heating
• some compounds interfere with normal detector
  response (O2 in ECDs)
• Compounds that overlap with analyte

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Practical Aspects of GCSelection of
Instrumentation

• Components should match needs from analyte and
  sample
• Injectors
• split/splitless is most common for liquids
• split vs. splitless choice depends on 1)
  concentrations, 2) resolution needed, 3)
  sensitivity needed, 4) how volatile compound is
  vs. solvent
• More on this next time