Essentials (GC) Flashcards
Factors to consider in separation of analytes and analysis time
(1) Column properties
- column length
- column inner diameter
- SP thickness (amount)
- SP type
(2) Operation parameters
- carrier gas type
- carrier gas flow rate
- column temperature
Consequence of increasing inner diameter
Flow rate increases
Sample capacity increases
Retention time decreases
Smaller capacity factor (k’)
Consequence of increasing column length
Better resolution (Rs)
Increase retention time
Increases analysis time
Increase sample capacity
Consequence of increasing film thickness (more SP)
Increases sample capacity
Increases retention time
Increases capacity factor (k’)
Increases resolution (to a point)
Most important factor for determining retention times
Oven (column) temperature
Temp programming generally required to separate range of boiling points of interest
Effect of carrier gas on flow rate
(higher flow rate = shorter retention time)
Increase column inner diameter increases flow rate (pressure on column decreases)
Which injection methods are better for quantitative analysis
Solvent flush
Air plug
Types of injectors
Split mode
Splitless mode
Cold-on-column
Programmable temperature vap.
Splitless mode (summary)
100% sample into column
0.5 - 1 uL volume
[sample] < 50 ng
used for low analyte concentration
max sample volume is solvent dependent
Split mode (summary)
0.2-10% sample into column
10:1, 100:1, 500:1
0.5 - 1 uL volume
[sample] > 50 ng
used for high analyte concentration
not good for samples with a wide boiling point (bp) range
Programmable Temperature Vaporization (summary)
0.2-100% sample into column
0.5-100 uL volume
wide range of concentration
heating coil allows temp control
prevents non-volatile material from entering column
reduces discrimination for analytes with a wide range of bp’s
reduces breakdown of analytes
not good for analytes that elute near solvent front
Cold-on-column (summary)
100% sample in column
10-100 uL volume
[sample] < 100 ng
good for analytes close to the solvent bp (elute near solvent front)
reduces discrimination toward analytes of high bp
minimizes decomposition of sample in injector
can have non-volatile material on column and needs more maintenance
Retention gap protects column from buildup of non-volatile material (has no SP)
If have sample that do not expect any degradation of analyte with temperature, and sample concentration high such that 1 uL (standard injection volume) can be used
If bp discrimination
Narrow choices to split/splitless or PTV
(bp discrimination) PTV
(no bp discrimination) split/splitless
(low conc.) splitless
(high conc.) split
Thermal Conductivity Detector (summary)
Universal, non-destructive
Concentration sensitive
Temp. difference of heated filaments in reference vs sample give response
Want carrier gas with large difference in thermal conductance to analyte
Good for difficult to detect analytes
Not good for low concentration
Flame Ionization Detector (summary)
Universal for organics
Mass-flow sensitive, destructive
Response proportional to number of ions produced in flame
Higher sensitivity
