Gas Chromatography Flashcards
What is gas chromatography
Gas chromatography is specifically gas liquid chromatography
A gaseous mobile phase flows under pressure through a heated column either coated with a liquid stationary phase or packed with liquid stationary phase coated on a solid support
What is gc used to separate
Volatile organic molecules
Instrumentation
Look at notes
Commonly used carrier gases
Nitrogen
Helium
Hydrogen
Argon
Carbon dioxide
Requirements of a carrier gas
Inert
Suitable detector
Easily available
Cheap
Less risk risk of explosion /hazard
Should give best column performance incl speed of analysis
How is flow rate of a Carrier gas regulated
By a two stage regulator on the gas cylinder and additional controls in the system.
If the inlet pressure remains constant , flow rates will be constant
Why is the injection port important
Column efficiency depends on this system.
Column efficiency requires sample of suitable size and has to be introduced as a plug of vapour
What causes band spreading and poor resolution
Slow injection of over sized samples
Names of methods of sample injection
Micro flash vaporiser direct injector (commmonly used )
Rotatory sample valve (reproducible results )
Injection port method
Sample is injected using a syringe into a flow stream of hot mobile phase.
Hugh temperature (at least 50 degrees above boiling point of sample ) causes the sample to vapourise (pre Heaters )
Introduced a narrow plug of sample vapour into the column
Why does oven temperature have to be thermostatically controlled
Partition coefficient of the solute is dependent on the temp
