7
gas
source
5
4
3
2
1
6
Figure 3.
Volatile analyte in equilibrium between the gas and sample phases.
Sample purging in progress in a
Tekmar 3100 concentrator.
G=gas phase (headspace)
The gas phase, commonly referred to as the headspace,
is above the sample phase.
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 amatrixmodifier.
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.
volatile
analyte
}
}
G
S
6
1
2
3
4
5
Purge and Trap Theory
Concentration ofVolatileOrganics
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 con-
centration 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 theGC
column.Amajor problemwith static headspace techniques is that the samplematrix signifi-
cantly affects equilibrium.Analyses for compounds that show high solubility in the sample
matrix often yield low sensitivity as a result ofmatrix 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.
Purge and trap concentration is a dynamic headspace technique that reducesmatrix effects
and increases sensitivity, relative to static headspace techniques. Samples containingVOCs
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 theGC column.
out to vent
trap
GC
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.
The purge and trap concentrator in
“desorb”mode.VOCs concentrated on the trap
are desorbed to the chromatograph for separation,
identification and quantification.
Figure 4.
The purging process transfers theVOCs from the sample to theGC column.
