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QUI-1057
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3.6.3
Sample tube prepurge
With normal porous polymer or graphitised carbon sorbents, a volume of carrier gas, ~ 10 times the
capacity of the sample tube, is required to completely eliminate air. This equates to 20 ml purge volume
for glass tubes (capacity ~2 ml) and 30 ml purge volume for stainless steel tubes (capacity ~ 3 ml). A
more stringent purge (~50 times the tube volume) is required to completely eliminate air from tubes
packed with conventional or carbonised molecular sieve-type sorbents.
Control of the carrier gas flows during ambient purge
The ambient purge carrier gas flow can either pass through the cold trap to the 'desorb flow' vent or
through the split filter to the 'split flow' vent or both as it comes out of the sample tube.
Trap in or out of line
The cold trap is user selectable to be in or out of line during the ambient temperature purge. It is generally
recommended for the cold trap to be off line at this stage, to ensure that any water purged from the tube
is not re-trapped on the cold trap. Applications involving the direct desorption of volatile components from
materials such as pharmaceuticals, plastics etc. are an exception to this rule. In these cases, and
especially if the material contains ultra volatile target analytes such as acetaldehyde, it is advisable to
have the trap in line.
Split on or off
If the trap is selected to be off line during tube purge (the most usual configuration), by default the split
flow is always turned on to give a flow through the tube.
If the trap is selected to be in line the user has a choice of whether the split vent is on or off. The split
should be selected to be the same as during the tube desorb process and is therefore determined by the
sensitivity of the analysis. Further information on selecting and setting split flows is given in Section 3.4.