User`s guide

Theory of Thermal Desorption
310
The number of retries can be set to between zero and two (0-2); the
default value is two (with two retries, a total of three tubes are leak-
tested).
Carrier Gas Purge of the Sample Tube
When the leak test has been completed, the ATD/TD again checks
that all heated zones are at their set temperatures and that the GC is
ready. SV1 and SV3 are then opened and air is purged from the
sample tube prior to heating it for desorption. If the inlet split has
been set in the method, SV5 will also open. The flow of gas through
SV3 and SV5 is controlled by their respective needle valves.
To improve accuracy, set (measure) the inlet and outlet split flows
during actual desorption.
The sample tube is purged with carrier gas for the duration of the
purge time, which is set in the ATD/TD method. When purging a
tube prior to primary desorption, the purge gas flows through the
cold trap. See Figure 88. Carrier gas purge of the tube (forward
flow), is intended to remove air, especially oxygen from the tube
prior to desorption to avoid oxidation of adsorbent and analytes.
Everything purged from the tube will go through the trap, which
means that no volatile analytes are lost.