Chem. 231 – 2/25 Lecture

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

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

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

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

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

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