Step 2.
Purge (wet)
•
Volatiles inmatrix diffuse into carrier
gas as gas is bubbled (purged) through
thematrix.Volatiles are transferred
to the trap.
•
Typical flow:
30–50mL/min.
for 10–15min.
Step 4.
DesorbPreheat
•
Trap is heatedwithout flow,
tominimize analyte desorb
time from packingmaterial.
•
Typical temp.:
5°C below desorb temp.
Sequences and FlowPaths of the Purge and Trap Unit
Purge and trap units are designed to have separate flow rates for the purge gas and the des-
orb (carrier) gas. The recommended gas for both purging and desorption is helium. The
purge gas flow typically is set at 30-50mL/min. The desorb gas flow ranges from 10-
80mL/min., depending on the column type andGC equipment used (see theApplications
section of this guide for example chromatograms). The desorb gas should be controlled
using a flow controller. The flow controller from the injection port of theGC commonly is
used, but a separate flow controller can be connected to the desorb gas bulkhead fitting on
the back of the purge and trap system. Hydrocarbon traps should be installed on the carrier
gas line prior to the purge and trap system. Thiswill prevent trace hydrocarbon or solvent
“ghost peak” contamination from interferingwith the analyses.
Purge and trap techniques involve the following series of steps that must be followed to
ensure accurate and reproducible results:
Step 1. Standby
During the standbymode, the purge gas flow is stopped, the trap is cooled, and the system is
readied for the start of an analysis. The desorb gas bypasses the trap and is directed onto the
column as the carrier gas flow. The gas flow rate through the column can bemeasured.
Step 2. Purge (wet)
During thewet purge, the purge gas flow passes through the purge vessel, removes volatile
analytes from the sample, and sweeps the analytes through the heated valve onto the adsor-
bent trap. The analytes are collected on the trap and the purge gas exits through the purge
vent. The purge gas flow typically is set at 30-50mL/min. and can bemeasured at the purge
vent. Samples usually are purged for 10-15minutes. During the purgemode, the desorb
(carrier) gas is directed onto the column.
Step 3. Purge (dry)
During thewet purge, a large amount of water is removed from the sample and collects on
the trap. The dry purge removes the excesswater that accumulated. During the dry purge,
the purge gas bypasses the purge vessel and is directed to the trap. The dry purge gas
removeswater and carries it out the exit vent. The desorb (carrier) gas is directed onto the
column. Only traps that incorporate hydrophobic adsorbents can be dry purged.
Step 4. DesorbPreheat
Once the analytes have been trapped and excesswater removed, the purge gas flow is
stopped. During this static period, the trap is rapidly heated to ~5°C below the desorb tem-
perature of the adsorbent materials used. The desorb preheat step uniformly volatilizes the
sample to create a narrow sample band and amore efficient sample transfer onto theGC col-
umn.Without a desorb preheat step the peakswould tail, resulting in poor chromatography.
During the desorb preheat step the desorb (carrier) gas is directed onto the column.
x
x
Step 3.
Purge (dry)
•
Trap is dried by purgingwith
gas only.
•
Typical time: 1-4min.
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