CdThe Chrom
Doctor
How to understand and deal with
overloading in GC
Overloading in gas-liquid vs gas-solid
chromatography
In gas-liquid chromatography (GLC), in which
we use polysiloxane (liquid)-type stationary
phases an overloaded peak will show itself
with a slow-raise and a sharp end. This is also
known as fronting.
In gas-solid chromatography (GSC),
in which the stationary phases used are
adsorbents such as alumina, molecular
sieves and porous polymers, an overloaded
component will show a fast rise but slow tail.
This is also known as tailing (Figure 1).
To deal with these phenomena, it is vital to
know whether we are dealing with GLC or
GSC.
Gas-liquid chromatography
Ideally, a component that is separated in a
capillary should elute with a symmetrical,
Gaussian peak shape. However, this will
only occur if with sufficient loadability. The
maximum amount that can be injected onto
a particular capillary column depends mainly
on:
When operating capillary columns, we must run at optimal conditions of linear gas velocity to maximize
separation power. However, when the amount injected into the column is too high, peaks become non-
symmetrical � this effect is referred to as overloading. An overloaded peak is generally not a problem for
quantification, but when it starts to affect the separation of a neighbouring component, corrective action is
required. In addition, overloading can change the retention time of the component. As the stationary phase
is saturated, the component itself will also act as a stationary phase resulting in`strange'chromatograms.
Overloading mainly occurs in stationary phases for which there are several parameters for manipulation. For
appropriate corrective action, it is important is to understand the type of stationary phase being used; that is, a solid
or a liquid.
Figure 1
b
(s
g l g s
Figure 1: Peak shape of overloaded peak in gas-solid and gas-liquid chromatography.
1. The polarity of the stationary phase:
Polar components will dissolve better in a
polar stationary phase than in a non-polar
stationary phase. Figure 2 shows the elution
of an acidic component, 2-ethylhexanoic
acid from two different columns. On the
non-polar column (a) the peak is strongly
overloaded, while on the polar column (b),
the peak is high and symmetrical. Also on
this phase, the acid peak elutes after the
methyl dodecanoate peak.
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