Upload
mead
View
32
Download
0
Embed Size (px)
DESCRIPTION
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. - PowerPoint PPT Presentation
Citation preview
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