Figure 7.
The flow paths for purging and desorbing are determined by the valve position.
10
Figure 6.
The 6-port automated switching valve (V) controls purge and desorb flow rates.
Valves
The purge and desorb flows are controlled by an automated switching valve (Figure 6). The
valve is contained in a heated compartment to prevent sample condensation inside. By rotat-
ing the valve, the purge and desorb flow paths can be changed during the purge and trap
sequence (Figure 7).
purge
gas in
1
2
4
5 6
3
1
2
4
5 6
3
Tekmar 3100 in
DesorbMode
heated transfer line toGC
(4) 0.32mm ID fused
silica transfer line
(protectedwithin
0.53mm ID
Silcosteel
®
line)
(3) desorb/carrier gas
(2) from outlet end of trap
(1) vent
(5) from
inlet end
of trap
x
(6) purge
GC column
Valve positioned to purge sample onto trap.
trap
vent
Valve position changed to
desorb sample from trap
and into capillary column.
purge gas in
gas in
GC column
trap
backflush
AdsorbentMaterials and Traps
Adsorbent materials are used to trap theVOCs that have been purged from the sample. The
adsorbent must be able to retain compounds during the entire purging sequence and then
rapidly release them during the desorption step. Each adsorbent has a unique trapping capa-
bility for a specific class or classes of compounds. Therefore, a trapmay have several differ-
ent beds of adsorbents. Theweakest adsorbent material is placed at the inlet end of the trap,
then the next strongest adsorbent, and so on. Themore volatile compounds pass through the
weaker adsorbents and are retained by the stronger adsorbents, while the less volatile com-
pounds are retained on theweaker adsorbents and never reach the stronger adsorbents (from
which theywould be difficult to desorb). Once the compounds are collected, the trap is rap-
idly heated and backflushedwith carrier gas to drive the sample components into theGC
system. Ideally, the adsorbents in the trap retain polar and non-polar analyteswithout retain-
ingwater ormethanol, efficiently release the trapped compounds onto the analytical column,
andwithstand the temperatures required to desorb (i.e., “bake off”) the highermolecular
weight contaminants.A list of common traps used in purge and trap concentration can help
you choose the best one for your application (Table II, page 13).
gas in
V
vent
