they are present in the sample/methanol slurry.
the analyte concentration needs to be expressed
The correction factor is the total liquid volume of
on a dry weight basis. This sample should be col-
the sample (methanol plus water) divided by the
lected within a couple of centimeters and from the
original volume of the methanol added to the
same stratum as the subsample taken for VOC
sample vial. Therefore the correction factor is
analysis. Likewise, the location adjacent to where
greater than or equal to 1.
the subsample for VOC analysis was removed
When a vapor partitioning (i.e., purge-and-trap
should be inspected visually and its characteris-
or headspace) method is the analytical method of
tics logged. In addition, the adjacent material can
choice, the sample is placed into a tared VOA vial
be retained for determining other relevant prop-
from which the vapor is removed for analysis
erties, such as the presence of oils, other visible
without the container being opened. Water that
signs of contamination, grain-size distribution,
contains no detectable levels of VOCs can either
organic carbon content, etc. Collection of these
be present in the VOA vial prior to introducing
ancillary samples should be performed after
the sample, or it can be added via a needle as de-
subsamples for VOC analysis have been collected.
scribed above for methanol, after a sample has
When a subsample is prepared by methanol
been obtained. However, in the case of purge-and-
extraction, an aliquot of the extract is transferred
trap analysis, a Teflon-coated stir bar should be
to a VOA vial containing water for either purge-
present prior to the sample introduction step. Fur-
and-trap or headspace analysis. Before an aliquot
thermore, since elevated temperatures and some
of the extract is transferred, the sample should be
form of mechanical mixing are recommended dur-
completely dispersed in methanol, and then al-
ing or before the removal of vapors, special auto-
lowed to settle so that an aliquot of clear superna-
mated equipment is often necessary. The current
tant can be removed for analysis. However, since
automated equipment for purge-and-trap and
studies have shown that extraction kinetics can be
headspace systems use 44- (or 40-) and 22-mL VOA
slow, it may be advisable to assist extraction with
heat (40C) and vibrational energy (sonication for
vials, respectively (U.S. EPA 1996a and 1996c). The
volume of water used for these two different sys-
30 minutes) when a total recovery is necessary
tems is typically 10 mL or less. The difference in
(Askari et al. 1996, Ball et al. 1997, Hewitt 1998).
weight of the container, measured before and af-
Ensuring the completeness of analyte extraction
ter the sample is added, is used to determine the
from a given matrix requires an analysis of addi-
sample's wet weight.
tional aliquots after further treatment. For purge-
If aromatic compounds are of concern and the
and-trap analysis, methanol aliquot volumes of 0.1
sample has been taken from an area receiving treat-
mL or less are typically transferred, while for
ment to increase its biological activity, then some
headspace analysis, methanol aliquot volumes as
form of preservation besides refrigerated (4C)
large as 1.0 mL can be transferred, depending on
storage should be implemented soon (within a
the detector and analytes of concern.
couple of hours) after collection. If a sample is
Vapor partitioning involves the direct analysis
placed into an empty VOA vial or one that con-
of a sample by either a purge-and-trap or a
tains only a Teflon-coated stirring bar and a limited
headspace method. In both cases, the sample is
amount of water (vessel less than 1/3 filled) the
placed into a tared VOA vial from which the va-
sample can be preserved by placing in a freezer
por is removed for analysis without the container
(12 3C) (Hewitt 1999). When methanol is
being opened. Water is usually used to assist with
present in the VOA vial, no additional preservation
the partitioning of the VOCs from the sample.
measures are necessary other than 4 2C storage.
Before being placed on the autosampler carousel,
For vapor partitioning methods of analysis, preser-
the sampled materials should be completely dis-
vation can also be achieved by making this solution
persed in water, if possible (vortex mixing or soni-
acidic (e.g., a pH of 2 or less with either sodium bisul-
cation can be used). This mixing of the solid ma-
fate or hydrochloric acid), when carbonates are not
terial with the aqueous solution not only helps
present. Frozen storage should be used when car-
prevent the plugging of the sparging needle used
bonates are present and low level concentrations of
by purge-and-trap systems, but the mixing also
aromatic compounds are of interest.
assists in attaining an equilibrium state by com-
Because the subsample is placed directly into a
pletely exposing the sample to the partitioning
tared container for both of these procedures
solution.
(methanol extraction and vapor partitioning), a
Vapor partitioning methods of sample prepa-
separate collocated sample should be collected if
ration for analysis are much more likely to be af-
6
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