Optimizing the Analysis of Volatile Organic Compounds

Purge and Trap Theory
Concentration of Volatile Organics
Volatile organic compounds can be concentrated by either static headspace or dynamic
headspace (i.e., purge and trap) sampling. In static headspace concentration, a sample is
placed in a closed sample chamber. Molecules of the volatile compounds in the sample
migrate to the headspace above the sample and equilibrium is established between the concentration
of the compounds in the vapor phase and in the liquid phase (Figure 3). Once
equilibrium is reached, an aliquot of the headspace above the sample is injected onto the GC
column. A major problem with static headspace techniques is that the sample matrix significantly
affects equilibrium. Analyses for compounds that show high solubility in the sample
matrix often yield low sensitivity as a result of matrix effects. Further, static headspace
analysis only samples an aliquot of the volatiles (i.e., 1mL, 2mL, or whatever the size of the
sample loop), which also affects sensitivity.
volatile
analyte
Figure 3.
Volatile analyte in equilibrium between the gas and sample phases.
G=gas phase (headspace)
The gas phase, commonly referred to as the headspace,
is above the sample phase.
} G S=sample phase
The sample phase contains the compound(s) of interest,
usually in the form of a liquid or solid in combination
}
with a dilution solvent or a matrix modifier.
S
Once the sample phase is introduced into the vial and
the vial is sealed, molecules of the volatile component(s)
diffuse into the gas phase until the headspace reaches a
state of equilibrium, as depicted by the arrows. An
aliquot is then taken from the headspace.
Purge and trap concentration is a dynamic headspace technique that reduces matrix effects
and increases sensitivity, relative to static headspace techniques. Samples containing VOCs
are introduced into a purge vessel and a flow of inert gas is passed through the sample at a
constant flow rate for a fixed time. Volatile compounds are purged from the sample into the
headspace above the sample and are transferred to and concentrated on an adsorbent trap
(Figure 4). After the purging process is complete, the trap is rapidly heated and backflushed
with carrier gas to desorb and transfer the analytes to the GC column.
Figure 4.
The purging process transfers the VOCs from the sample to the GC column.
GC
4
3
5
2
gas
source
6
1
trap
out to vent
The purge and trap concentrator in “purge” mode.
The 6-port valve allows carrier gas to bubble
through the aqueous sample, transferring volatiles
to the adsorbent material.
4
3
The purge and trap concentrator in
“desorb” mode. VOCs concentrated on the trap
are desorbed to the chromatograph for separation,
identification and quantification.
5
2
6
1
Sample purging in progress in a
Tekmar 3100 concentrator.
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