7
Chem. 231 – 2/25 Lecture

Chem. 231 – 2/25 Lecture

  • 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

Page 1: Chem. 231 – 2/25 Lecture

Chem. 231 – 2/25 Lecture

Page 2: 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

Page 3: Chem. 231 – 2/25 Lecture

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

Page 4: Chem. 231 – 2/25 Lecture

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

Page 5: Chem. 231 – 2/25 Lecture

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

Page 6: Chem. 231 – 2/25 Lecture

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

Page 7: Chem. 231 – 2/25 Lecture

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