Storage and Preservation of Soil Samples
for Volatile Organic Compound Analysis
ALAN D. HEWITT
mechanism to continue (Hewitt 1998a). We illus-
INTRODUCTION
trate in Figure 1 how quickly VOCs are lost from
Most samples collected to identify and quantify
the center of silty-sand soil held at ambient tem-
peratures (18 2C) in an uncovered 3.6-cm-i.d.
analytes in hazardous waste require some form of
preparation (e.g., extraction, subsampling, etc.)
5.1-cm-long metal core barrel liner stored in a plas-
prior to instrumental analysis. This part of the to-
tic bag (Hewitt and Lukash 1996). The initial rapid
tal measurement process has traditionally taken
loss of trichloroethylene (TCE) may represent TCE
place in an off-site laboratory. Therefore, samples
that was in a gaseous state at the time of sample
obtained during the characterization stages of a
collection. The change to a slower loss rate may
site investigation or when monitoring the progress
represent this analyte when it must first go
of a remediation activity often experience trans-
through a phase change, e.g., be desorbed or vola-
portation and storage, in addition to collection,
tilized, prior to escaping.
preparation, and analysis. During the last decade
Another mechanism that can influence VOC
there has been a growing awareness of the many
concentrations in samples that are transported and
stored at 4 2C is biological degradation (Brad-
problems that can be encountered when attempt-
ing to maintain representative concentrations of
ley and Chapella 1995, Hewitt 1997a). In general,
hazardous waste constituents throughout the to-
this loss mechanism is not expected to be as large
tal measurement process. Volatile organic com-
a source of determinate error as volatilization. This
pounds (VOCs) have been especially suspect with
premise is based on the observation that losses of
regard to their identification and quantification in
an order of magnitude can occur on a time scale
samples removed from the vadose zone (Hewitt
of minutes to hours (see Fig. 1), due solely to dif-
et al. 1995).
fusion and advection. In contrast, losses of a simi-
In most contaminated soils and other solid waste
lar magnitude due to biological processes usually
materials, VOCs coexist in gaseous, liquid, and
require days to weeks (Hewitt 1995a). Figure 2 is
solid (sorbed) phases (Conant et al. 1996). Of par-
an example of the changes in concentration ob-
ticular concern to the collection, handling, and
served for several analytes in samples held in
storage of samples for VOC characterization is the
sealed glass ampoules and either stored at room
retention of the gaseous component. This phase
temperature or in a refrigerator. This experiment
exhibits molecular diffusion coefficients that allow
was run under aerobic conditions, which is typi-
for their immediate loss from a freshly exposed
cal of most samples that are transported and
surface, and continued losses from within the body
stored. Under these conditions biological mecha-
of the porous matrix (Siegrist and Jenssen 1990).
nisms favor the degradation of aromatic hydro-
Furthermore, once the gaseous phase becomes
carbons over halogenated compounds. Therefore,
depleted, nearly instantaneous volatilization from
besides giving a slower rate of analyte loss, bio-
the liquid and sorbed phases occurs in an attempt
degradation is compound selective.
to restore the temporal equilibrium that often ex-
To limit the influence of volatilization and bio-
ists, thereby allowing the impact of this loss
degradation losses, the U.S. Environmental Pro-
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