Optimizing the Analysis of Volatile Organic Compounds

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Step 2.
Purge (wet)
• Volatiles in matrix diffuse into carrier
gas as gas is bubbled (purged) through
the matrix. Volatiles are transferred
to the trap.
Typical flow:
30–50mL/min.
for 10–15 min.
Step 3.
Purge (dry)
Trap is dried by purging with
gas only.
Typical time: 1-4min.
Step 4.
Desorb Preheat
Trap is heated without flow,
to minimize analyte desorb
time from packing material.
Typical temp.:
5°C below desorb temp.
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Sequences and Flow Paths of the Purge and Trap Unit
Purge and trap units are designed to have separate flow rates for the purge gas and the desorb
(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 and GC equipment used (see the Applications
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 the GC 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. This will prevent trace hydrocarbon or solvent
“ghost peak” contamination from interfering with 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 standby mode, 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 be measured.
Step 2. Purge (wet)
During the wet 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 adsorbent
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 be measured at the purge
vent. Samples usually are purged for 10-15 minutes. During the purge mode, the desorb
(carrier) gas is directed onto the column.
Step 3. Purge (dry)
During the wet purge, a large amount of water is removed from the sample and collects on
the trap. The dry purge removes the excess water that accumulated. During the dry purge,
the purge gas bypasses the purge vessel and is directed to the trap. The dry purge gas
removes water 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. Desorb Preheat
Once the analytes have been trapped and excess water removed, the purge gas flow is
stopped. During this static period, the trap is rapidly heated to ~5°C below the desorb temperature
of the adsorbent materials used. The desorb preheat step uniformly volatilizes the
sample to create a narrow sample band and a more efficient sample transfer onto the GC column.
Without a desorb preheat step the peaks would tail, resulting in poor chromatography.
During the desorb preheat step the desorb (carrier) gas is directed onto the column.