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Study Notes: GC - More About Columns
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This study note will give an overview of various aspects that have a
direct influence on the ability of a column to resolve analyte peaks.
Experience, skill and knowledge are required by the GC operator to achieve
good separations.
There are two parameters that provide a measure of resolution –
Column Efficiency and Solvent Efficiency (as defined by McNair, H. and
Bonelli, E., Basic Gas Chromatography (1969), Palo Alto, Varian
Instrument Division).
Column efficiency and solvent efficiency
As with HPLC, column efficiency is measured in theoretical plates (N)
and is a quantitative measure of the extent of peak broadening –
the narrower the peak, the more efficient will be the column thereby leading
to improved resolution.
There are various factors that affect column efficiency and so when comparing
columns these factors need to be stated:
- Stationary phase (and how thick)
- Temperature
- Carrier gas and flow rate
- Particle diameter (packed column)
- Column pressure
- Column diameter.
Whereas column efficiency concerns the narrowness of peaks, solvent efficiency
concerns the separation between peak maxima. This parameter can be expressed
quantitatively as a ratio of retention times.
The factors having most impact on solvent efficiency are the selection
of the column (in effect, the stationary phase) and temperature.
The following graphic illustrates the effect of increasing column efficiency
or solvent efficiency.
Notice the changes in resolution and position caused by
improving column efficiency or improving solvent efficiency.
The stationary phase
The properties of the stationary phase include the following.
- Low volatility - the boiling point of the liquid should be at least
100°C above the maximum operating temperature.
- Thermal stability - the stationary phase should not degrade under
normal operating conditions.
- Chemical inertness - must be non-reactive under normal operating
conditions.
- Solvent characteristics that allow good separations of analytes (analytes
will ‘dissolve’ or partition in the stationary phase to
different degrees enabling their separation).
Examples of stationary phases include:
- Polydimethyl siloxane
- Polyethylene glycol
- Poly(dicyanoallyldimethyl) siloxane.
The choice of stationary phase is the key element for achieving good
separations.
Liquid phase percentage
For packed columns, the liquid phase used should be enough to coat the
particles of the column with a thin uniform layer. Too much (>
30%) will pool between the particles with a decrease in efficiency.
The retention time is also proportional to the amount of liquid phase
present so modern trends are to use low % liquid phases to produce fast
analyses.
Notice that as % liquid phase decreases, band broadening, resolution
and retention time is also reduced.
Column temperature
An increased temperature means reduced migration time (ie shorter retention
time) but decreased resolution. Generally an increase of 30°C will
double the rate of migration and significantly decrease resolution. Generally
resolution can be improved by lowering the temperature but of course this
will increase the run time.
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