User`s guide

Theory of Thermal Desorption
313
Cold Trap Flow Configuration
The ATD/TD is configured for backflush flow of the cold trap.
During tube desorption, the sample analytes enter the cold trap from
the left (flow from the heated valve to the desorb flow vent), as seen
from the front of the instrument. The analytes are subsequently
backflushed from the adsorbent bed during trap desorption to leave
through the same side of the trap (flow from the SV2 to the desorb
flow vent end of the trap and through it and the heated valve to the
transfer line and GC column).
Backflush desorption of the cold trap enables you to use two or more
adsorbents. A weak adsorbent can be used to trap the less volatile
components. It is backed up by a strong adsorbent to trap the more
volatile species. This arrangement ensures that the less volatile
components do not reach the stronger adsorbent, where they may be
irreversibly adsorbed, while any components that break through the
weaker adsorbent are trapped on the stronger adsorbent.
When backflushing is used with a single adsorbent, the sample does
not have to pass through the entire adsorbent bed. Recovery of
higher-boiling species that have a high retention volume can
therefore be improved.