Optimizing the Analysis of Volatile Organic Compounds

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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 rotating
the valve, the purge and desorb flow paths can be changed during the purge and trap
sequence (Figure 7).
Figure 6.
The 6-port automated switching valve (V) controls purge and desorb flow rates.
heated transfer line to GC
(4) 0.32mm ID fused
silica transfer line
(protected within
0.53mm ID
Silcosteel ® line)
(3) desorb/carrier gas
(2) from outlet end of trap
Tekmar 3100 in
Desorb Mode
Figure 7.
The flow paths for purging and desorbing are determined by the valve position.
purge
purge gas in
gas in
GC column
gas in
5 6
4 1
3 2
vent
Valve positioned to purge sample onto trap.
trap
Adsorbent Materials and Traps
x
(1) vent
GC column
gas in
5 6
4 1
3 2
(5) from
inlet end
of trap
(6) purge
trap
backflush
Valve position changed to
desorb sample from trap
and into capillary column.
Adsorbent materials are used to trap the VOCs 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 capability
for a specific class or classes of compounds. Therefore, a trap may have several different
beds of adsorbents. The weakest adsorbent material is placed at the inlet end of the trap,
then the next strongest adsorbent, and so on. The more volatile compounds pass through the
weaker adsorbents and are retained by the stronger adsorbents, while the less volatile compounds
are retained on the weaker adsorbents and never reach the stronger adsorbents (from
which they would be difficult to desorb). Once the compounds are collected, the trap is rapidly
heated and backflushed with carrier gas to drive the sample components into the GC
system. Ideally, the adsorbents in the trap retain polar and non-polar analytes without retaining
water or methanol, efficiently release the trapped compounds onto the analytical column,
and withstand the temperatures required to desorb (i.e., “bake off”) the higher molecular
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).
V
vent