Gas Chromatography Flashcards
What is GC
analytical technique for the separation of a mixture of volatile compounds
what is the stationary phase
adsorbent phase
what properties must a compound have in order to be analysed using GC
sufficient volatility and thermal stability
range for GC oven temps
50-400C
components of GC
-Carrier gas supply
-inlet/ injection system
-column oven
-detector and data system
describe gas-solid chromatography
solid stationary phaseq
describe gas-liquid chromatography
Liquid stationary phase
In which phase do analytes move down the column
mobile phase
analytes are separated according to..
-affinity for the stationary phase
-analyte volatility
if a component has high vapor pressure and low solubility for the stationary phase describe the retention time
short
if a component has low vapor pressure and high solubility for the stationary phase describe the retention time
long
if a component has high vapor pressure and high solubility for the stationary phase describe the retention time
intermediate
if a component has low vapor pressure and low solubility for the stationary phase describe the retention time
intermediate
how do analytes elute from the column according to the distribution coefficient with respect to the stationary phase
in inverse order
variable parameters in GC
Stationary phase
-column selection for seperation according to polarity
Oven temperature
-seperate according to compound vapour pressure
Carrier gas and flow -type of gas, H or He
-affects resolution and speed of analysis
types of columns
polar or non polar
types of polar columns
polyethylene glycol
free fatty acid phase
retention time of a component with high vapor pressure and low solubility for stationary phase
short
retention time of a component with high vapor pressure and high solubility for stationary phase
intermediate
retention time of a component with low vapor pressure and low solubility for stationary phase
intermediate
describe the components vapour pressure if the low and the solubility is high
long
how can the vapour pressure be controlled
oven temperature program
how can the solubility of a component to the stationary phase be controlled?
column selection
what does increasing column length do to the system? advantages
increases efficiency and resolution
what does increasing column length do to the system? disadvantages
increases band broadening and analysis time
what is meant by the statement “the increase in resolution is not proportional to length”
the amount a column is increased does not = the amount of better resolution i.e. double length = 1.3 times better resolution
what happens to column efficiency as the column diameter decreases
efficiency increases
what is band broadening
the peak of gets wider
how do different carrier gases affect a GC system?
change flow rate and velocities, therefore the speed of analysis
carrier gases from best to worst
hydrogen> helium > nitrogen
what does a GC detector do
measures the amount of a compound
example of non-specific detectors
flame ionisation detector
example of selective detectors
sulfur and nitrogen
mass selective detector (mass spec)
how does a flame ionisation detector work
column effluent passes through flame and the flammable components are burned
-this combustion increases ions which generate an electrical current
-the current is amplified and measured
advantages to a flame ionsiation detector
-signal intensity is proportional to concentration
-good linear range
-good for quantification
disadvantages to FID
no structural info
no identification of compounds
describe mass spec
the ionisation of gas phase molecules followed by the analysis of the masses of ion produced
how does fragmentation occur in mass spec
electron impact ionisation
how is the information from mass spec used
the fragmentations patterns can be compared to an MS database
application of GC
food composition
flavor analysis
environmental analysis
medical application
forensic science
petroleum
