Cd How to understand and deal with overloading in GC The Chrom

Cd
The Chrom
Doctor
How to understand and deal with
overloading in GC
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 nonsymmetrical – 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.
Overloading in gas-liquid vs gas-solid
1. The polarity of the stationary phase:
chromatography
Polar components will dissolve better in a
In gas-liquid chromatography (GLC), in which
polar stationary phase than in a non-polar
we use polysiloxane (liquid)-type stationary
stationary phase. Figure 2 shows the elution
phases an overloaded peak will show itself
of an acidic component, 2-ethylhexanoic
with a slow-raise and a sharp end. This is also
acid from two different columns. On the
known as fronting.
non-polar column (a) the peak is strongly
In gas-solid chromatography (GSC),
overloaded, while on the polar column (b),
in which the stationary phases used are
the peak is high and symmetrical. Also on
adsorbents such as alumina, molecular
this phase, the acid peak elutes after the
sieves and porous polymers, an overloaded
methyl dodecanoate peak.
component will show a fast rise but slow tail.
This is also known as tailing (Figure 1).
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
g
l
g
s
b
(s
maximum amount that can be injected onto
a particular capillary column depends mainly
on:
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Figure 1: Peak shape of overloaded peak in gas-solid and gas-liquid chromatography.
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2. The amount of stationary phase present
identification. Injecting less sample, as
within the column: The main parameter here
shown in Figure 4(b), improves the peak
is film thickness. Thicker films allow more
shape considerably.
sample to be while keeping symmetrical
If injection of less sample is not possible,
peaks. Also, if we use wider diameter,
because of GC setup restrictions (for
longer columns, we will benefit from
example, when a fixed sample loop is used
higher loadability. However, be aware that
with direct injection), or for detecting a trace
with increased film thickness and column
analyte, consideration should be given to a
length, retention times and column bleed
0.53 mm type PLOT column, perhaps even a
will increase proportionally. Figure 3 shows
longer one.
the impact of increased film thickness on
Another approach is running at a higher
loadability. For comparison reasons, the test
temperature as this will improve peak shape
mixture on the thicker film was analysed at a
significantly using PLOT columns. Figure 5
higher temperature for which the retention
factors are similar.
Figure 2
3. The retention (factor) of the component
on that particular column: Components
(a)
with a higher retention will often show
(b)
quicker overloading phenomena than early
eluting components. Note that we can
influence retention factor with the oven
temperature. If a late-eluting peak shows
sign of overloading, run the analysis at a
higher temperature and/or increase the
temperature program rate.
Gas-solid chromatography
In gas-solid chromatography there is
Time (min)
Time (min)
Figure 2: Effect of solubility on loadability. Columns: (a) Rtx-1, (b) Stabilwax-DA; dimensions = both 30 m x
0.53 mm with 0.5 μm film.
generally little flexibility in the amount
of stationary phase in porous layer open
tubular (PLOT) columns – the layers are
Figure 3
already very thick to generate maximum
retention for volatiles. Overloading in gassolid chromatography is visualized by a
strong tailing of the component. As the
capacity of adsorbents is usually lower than
liquids, the overloading phenomena is
observed much faster.
When peaks start to tail using a PLOT
column, try to inject less sample. Figure 4(a)
shows hydrocarbon overload on a PLOT
column, in which the polar hydrocarbons,
methyl-acetylene and 1,3 butadiene, tail
Time (min)
Time (min)
significantly. The overloaded hydrocarbons
also elute earlier, which can result in a false
separation science — volume 1 issue 4
Figure 3: Effect of liquid stationary phase film thickness on loadability. Column: Rtx-1, 30 m x 0.25 mm i.d.
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39
Figure 4
(a)
(b)
Time (min)
Figure 4: Impact of absolute sample load on solid stationary phase/PLOT column. Column: 30 m x 0.32 mm Rt-Alumina BOND/KCl: (a) overloaded injecting 50 ng
per component, (b) injecting 5 ng/component. Peaks: 1 = pentane; 2 = methylacetyene; 3 = pentane; 4 = 1,3-butadiene.
Figure 5
(polar) hydrocarbons will elute faster relative
to saturated (non-polar) hydrocarbons. This is
why, in the example, both peaks elute closer
to the pentane peaks.
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In practice
100
To solve an overloading issue that impacts
50
0
0
1
2
3
4
5
6
7
on quantification a new column with more
8
capacity or solubility can always be applied.
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Initially, it is preferable to overcome the
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challenge using the existing column by
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reducing the absolute amount of sample
50
25
0
0.0
compound injected on to the column. This
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Figure 5: Impact of temperature on peak shape of 1,3-butadiene and methyl acetylene. Column: 30 m x 0.53 mm
Rt-Alumina BOND/Na2SO4. Peaks: 1 = 1,3-butadiene, 2 = methyl acetylene.
can be done via:
• Injectingless,smallersamplevolume,
higher split ratio (Figure 6).
• Dilutingofthesamplepriortoinjection.
shows the separation of C1-C5 hydrocarbons
using Al2O3, but operated at different
By doing this a higher sensitivity setting of
temperatures. The peak shapes for both
the detector must be used. However, if this
1,3-butadiene and methyl acetylene improve
does not solve the problem, a column with
at higher temperatures. It should also be
more capacity/solubility is required.
noted that changing temperature will initiate
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chrom doctor
another effect – the alumina will become
Acknowledgements
less ‘polar.’ The result being that unsaturated
SpecialthankstoBillBromps,RestekR&D
www.sepscience.com
Figure 6
20 ng/component
on the column
Split ratio was increased by
a factor of 2
10 ng/component
on the column
Figure 6: Example of practically improving the separation by decreasing the injected amount on
the column by a factor 2. Column: 30 m x 0.25 mm Rxi-5Sil MS, film = 0.25 μm.
and Tom Vezza, Restek PLOT specialist for supplying
chromatograms.
Jaap de Zeeuw is a specialist in gas chromatography
working for Restek Corp.
21 - 24 September 2009
Grand Hotel Malahide
Dublin, Ireland
CASSS
AN INTERNATIONAL SEPARATION SCIENCE SOCIETY
separation science — volume 1 issue 4
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