kit can be important: a broad range is generally
EnSys RISc, ENVIROL, USACE, Idetek Quantix,
more desirable. The importance of the range de-
Ohmicron RaPID Assay, and EnviroGard (Plate)
pends on the range of concentrations expected in
kits are quantitative methods, providing a numeri-
samples, the ability to estimate the approximate
cal value. The CRREL 2,4-DNT method is consid-
concentration from the sample extract, the amount
ered semiquantitative and provides a somewhat
of effort required to dilute and rerun a sample, and
less accurate numerical value. The DTECH and
the sampling and analytical objective. Some test
EnviroGard (Tube) test kits are semi-
kits have a range factor (upper limit of range/
quantitative (concentration range), and indicate
lower limit) of just one order of magnitude (10),
that the level of an analyte is within one of several
while other methods span two or more orders of
ranges. For example, the DTECH TNT soil kit,
magnitude (100 to 400). Because explosives con-
without dilution, indicates a concentration within
centrations in soil may range five orders of mag-
one of the following ranges: less than 0.5, 0.5 to
nitude (100,000), reanalyzing many out-of-range
1.5, 1.5 to 2.5, 2.5 to 4.5, 4.5 to 6.0, and greater than
samples may be necessary. The DTECH immu-
6.0 ppm.
noassay methods require an additional test kit to
run each sample dilution.
Samples per batch
Other immunoassay methods can run dilutions
Several of the available test kits are designed to
in the same analytical run, but one must prepare
run batches of samples or single samples or both.
the dilutions without knowing whether they are
Using a test kit designed for analyzing a large
needed. The CRREL, USACE, and EnSys RISc colo-
batch to analyze one or two samples may not be
rimetric procedures for TNT provide sufficient
very cost-effective or efficient. In most cases,
reagent to run several dilutions at no additional
samples may easily be batched for extraction and
cost. For the EnSys RISc TNT kit, the color devel-
processed simultaneously.
oped can simply be diluted and reread in the spec-
trophotometer. The procedures that the test meth-
Sample size
ods use for samples requiring dilution should be
The size of the soil sample extracted contrib-
evaluated as part of the site-specific data quality
utes to the representativeness of a sample. Explo-
objectives.
sives residues in soil are quite heterogeneously
The detection range of a kit becomes much less
distributed (Jenkins et al. 1996a, b; 1997), and as
relevant when the objective is to determine
the subsample size actually extracted decreases,
whether a soil is above or below a single action
heterogeneity increases. Although sample prepa-
limit; the same dilution can be used for all samples.
ration procedures such as drying, mixing, sieving,
In some cases, changing the range of a kit may be
and splitting can reduce within-sample heteroge-
desirable to facilitate decision-making. If a method
neity, such procedures can be time-consuming.
has a range of 1 to 10 ppm and the contamination
Based on work by Jenkins et al. (1996b, 1997), field
level of concern is 30 ppm, diluting all samples
compositing and homogenization greatly improve
(using acetone or methanol or as directed by the
sample representativeness. The commercial test
instructions) by a factor of five would change the
kits use 2 to 10 g of soil, while the CRREL meth-
test kit range to 5 to 50 ppm and permit decisions
ods extract 20 g of soil to improve the representa-
to be made without additional dilutions.
tiveness of the results. For some test kits, it is pos-
Cleanup levels for explosives in soil vary con-
sible to extract a larger sample using solvent and
siderably depending upon the site conditions,
glassware not provided in the kit, and then using
compounds present and their relative concentra-
the required volume of extract for the analytical
tion, threats to groundwater, results of risk assess-
steps. The smaller the sample size, the more im-
ments, remedial technology, etc. (EPA 1993). Based
portant is the homogenization of the sample be-
on a review of data from many sites, Craig et al.
fore subsampling, although this procedure alone
(1995) suggested preliminary remediation goals of
cannot completely compensate for loss of repre-
30 ppm for TNT, 50 ppm for RDX, and 5 ppm for
sentativeness due to smaller samples.
2,4-DNT and 2,6-DNT.
Sample preparation and extraction
Soil extractions procedures for most of the
Type of results
The type of results provided by the various
screening methods are similar, shaking 2 to 20 g
screening methods are quantitative or semi-quan-
of soil in 6.5 to 100 mL of solvent (acetone or metha-
titative. The CRREL (TNT, RDX, and AP/PA),
nol) for 1 to 3 minutes. This may be followed by
16
Previous Page
