Chem. 231 – 2/25 Lecture

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 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

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

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

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

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