spiking solution was prepared by combining the
Miscellaneous variables
appropriate volumes of individual aqueous stock
For many soils, the use of acetone and vigor-
standards after verifying their concentrations by
ous agitation (hand shaking) results in near-quan-
RP-HPLC analysis (Grant et al. 1993). One-millili-
titative recovery of explosive compounds within
ter aliquots of this solution were then diluted up
3 minutes (Jenkins 1990, Walsh and Jenkins 1991).
to 1.00 L, to make seven 1-L water samples. The
Noted exceptions, however, are heavy clays or
resulting analyte concentrations in these 1-L
high organic soils, which have demonstrated slow
samples ranged from 2.08 to 2.71 g/L. These
extraction kinetics. For this reason, extraction time
samples were preconcentrated (200 fold) using
for a given soil must be verified at each location
membrane solid phase extraction (SPE, [Empore
(Jenkins and Walsh 1998). Initially, when devel-
3M, SDB-RPS, 47 mm]) and eluted with 5 mL of
oping field screening methodologies, a 20-g soil
acetone, following the procedure outlined by
sample (undried) was extracted with 100 mL of
Jenkins et al. (1994).
acetone at a soil to solvent ratio 1:5, so as to en-
For this study an apparatus was used that con-
sure that the water content of the final extract was
sisted of three filtering funnels attached to a
not too high for the colorimetric based methods
common manifold. The manifold allowed three
of determination. Subsequent studies with this
samples to be prepared simultaneously and
field screening method determined that the water
all the filtrate to be collected in a 2-L vacuum flask.
content in the acetone extract was not an issue.
Briefly, after rinsing the extraction funnels
Therefore, to maximize delectability in soil, a
(Kontes), collection vessels, and tongs with
sample weight (g) to acetone volume (mL) ratio
acetone, 47-mm membrane disks was placed on
of 1:1 could be used.
each support screen and wetted with acetone
Following the same logic, sample preparation
before centering each funnel and clamping in
for the on-site analyses of explosive residues by
place. Each membrane was precleaned with two
GC also used a 1:1 ratio. To assess if soil moisture
10-mL aliquots of acetone. Prior to the sec-
would affect instrumental responses, solutions
ond aliquot being pulled completely through
were prepared to simulate the extracts that would
the membrane, a 30-mL aliquot of organic-
be obtained when a 1:1 ratio of sample weight to
free water was added to a funnel. Near the
acetone volume was used with soils of 5, 10, 20,
completion of filtering this aliquot of water, a sec-
30, 40, 50% moisture by dry weight. Furthermore,
ond 30 mL aliquot was added. With a small vol-
to assess the range of application of the method
ume remaining from the second water rinse, a 1-L
and potential use of non-reagent-grade acetone,
fortified water sample was added to the funnel. A
the following experiments were performed to (1)
small vacuum was applied throughout these
determine the detectability and retention times of
cleaning and rinsing cycles, then adjusted during
other common explosives, (2) estimate the upper
a sample preconcentration step so that the solu-
limit of linearity for the target analytes, and (3)
tion passed through the membrane at approxi-
examine the feasibility of using hardware-store
mately 100 mL/min. After the sample had com-
grade acetone for sample extraction. The addi-
pletely passed through a membrane the vacuum
tional explosives determined were nitrobenzene;
remained on for an additional 10 minutes to help
ortho-, para-, and meta- nitrotoluene; 1,3-dini-
remove all of the water.
trobenzene; 2,6-dinitrotoluene; 1,3,5-trinitro-
Once dried, the entire funnel and membrane
benzene; and methyl-2,4,6-trinitrophenyl
support was removed from the manifold and wa-
nitramine (tetryl). Acetone-based standards of
ter drops were removed from the tip of the drain
these analytes were prepared from archived stock
tube with a clean acetone wetted towel. Before
standards.
returning a funnel and membrane support to the
manifold, a 25- 200-mm test tube was positioned
to collect any further solution passing through a
RESULTS AND DISCUSSION
membrane in the filter assembly. Then 5 mL of
acetone was poured over the interior surface of a
GC separations
funnel and allowed to cover the membrane for
The GC and its configuration was selected
three minutes before applying a small vacuum and
based on the works of Walsh and Ranney (1998).
slowly pulling through. The volume of acetone
Here, we strived to meet the following goals: easy
recovered during a membrane extraction step was
field implementation, minimal consumable sup-
4.2 0.2 mL.
port, and rapid analysis time (less than 15 min.).
5
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