1. Chem. 231 – 2/25 Lecture

2. Announcements Quiz 2 – Today New Homework – Pass out Set 1 Labs
should be finishing by next week
Today’s Lecture
Practical Aspects of GC

3. Practical Aspects of GC Sample 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

4. Practical Aspects of GC Sample 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

5. Practical Aspects of GC Sample 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

6. Practical Aspects of GC Sample 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

7. Practical Aspects of GC Selection 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