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 In the first three issues of Peak Performance we looked at:
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eliminating tailing on peaks by correct cutting of columns
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aiding reproducibility through the use of a quartz wool
plug in the liner
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the relationship between flow rate and resolution.
This month I am exploring the importance of getting your
sample into the column in the narrowest band possible and
why decreasing the split ratio or injecting more sample may
not be the answer.
Split ratio is the ratio of the carrier gas flow in the
column and out of the split vent, with values typically
ranging from 1:100 to 1:1000. A higher split ratio (e.g.
1:100 or 1%) introduces less sample into the column than a
lower value (1:50 or 2%) but the actual amount of each
sample compound entering the column varies and depends on
many injector and carrier gas variables. The most important
concern should be to obtain a consistent and reproducible
split by calculating, measuring and setting the split ratio
in the same manner for any given analysis.
Two things to note:
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The split ratio will increase at higher oven temperatures.
This is because gases become viscous at higher temperatures
and so, for a given inlet pressure, the flow will decrease.
A lower column flow means more split vent flow and, hence, a
higher split ratio.
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If there is a faulty liner seal then vapour can come back
into the liner and circulate around the injector. This will
cause an excessive tailing solvent peak and is symptomatic
of a poor inlet liner seal.
Reducing the split ratio between 200:1 and 25:1 will have a
linear effect on sample concentration. However, below 25:1 a
linear increase in sample concentration does not result. For
instance, going from 20:1 to 10:1 will not double the amount
of sample on the column. As you can see on the
graph, a
ratio of 12:1 is more like 20:1. So if you want to
accurately change the concentration of the sample on column
keep the split ratio above 25:1.
In splitless mode, increasing the volume of the injection
increases the amount of sample on the column. But once
again, the increase is not linear. If you double the
injection volume you will not double the amount of sample on
column.
The main reason why this happens is that the liner cannot
hold the entire volume of vaporised sample. Remember, the
liner does not have a lid. So injecting more sample will
just make more sample flow out the top of the liner. This
always happens, even with small injection volumes, but the
loss of sample becomes exaggerated when large volumes are
used.
Not all solvents will expand to the same volume under a
particular set of conditions. The volume of the expanded gas
from a liquid of known volume and density can be calculated
from the ideal gas law. It is important to remember that the
volume of gas will increase as the molar mass decreases.
This is one of the reasons why water is a very difficult
solvent as it expands to 3 or 4 times the volume compared
with methylene chloride.
Put simply, if the volume of the liner is not sufficient to
handle the expanded gas volume, liner overload or Flashback
will occur. Flashback can cause severe problems especially
in quantitation. If semi volatile solutes enter the inlet
lines, this can cause ongoing system contamination in the
form of ghost peaks. A really bad case can result in the
need to rip out the plumbing and replace the lines.
Flashback, however, can also be used to your advantage. If
the injection system has become contaminated, water can be
injected into the liner at high temperature and pressure
deliberately to cause flashback and flush out contamination
through steam distillation.
As you can see, it is not always possible to increase
sensitivity by decreasing the split ratio (the peaks may get
broader, costing you resolution, and making small peaks hard
to see above the baseline), or by injecting more (because
you may get flashback). If you need more sensitivity, are
already using splitless injection or have the split ratio as
low as you go and are injecting as much as you can, then the
AirSharp may be just the answer.
If you have further questions, please send me an email or
phone me on 01908 568844
Happy Chromatographing!
Anthony Addinall
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