settling or filtration or both. One test kit
TNT; however, oven drying of less-contaminated
(EnviroGard) specifies air drying; for the EnSys
samples for only 1 hour resulted in little loss of
RISc colorimetric test kits, drying to less than 10%
TNT, and 30 minutes of drying was estimated to
moisture is optional. For the CRREL methods,
be sufficient for analytical purposes. Microwave
samples must contain 2 to 3% water by weight;
drying was not recommended because of spotty
therefore, water must be added to the extract for
very dry soils or incomplete color development
ing should not be used because it may present a
will occur, resulting in a false negative.
safety hazard and such drying degrades thermally
The solvent extraction times of 1 to 3 minutes
unstable explosives in the soil. The effective pro-
used in on-site methods may result in incomplete
duction rate depends on the number of reruns re-
extraction of explosives compared with the 18-
quired because a sample is out of the detection
hour ultrasonic bath extraction step used in EPA
range.
Method 8330. The percent of explosives extracted
is sample-specific but is generally higher for high
Interferences and cross-reactivity
concentration samples, higher for sandy soils,
One of the major differences among the field
lower for clayey soils, and lower if 1-minute ex-
methods is interference for colorimetric methods
tractions are used relative to 3-minute extractions.
and cross-reactivity for immunoassay methods.
For many soils, a 3-minute extraction time is ad-
The colorimetric methods for TNT and RDX are
equate; ratios of 3-minute versus 18-hour extrac-
broadly class sensitive; that is, they are able to de-
tions of TNT and RDX using acetone or methanol
tect the presence of the target analyte but also re-
range from 66 to 109% as reported by Jenkins et
spond to many other similar compounds
al. (1996c). Jenkins recommends at least a 3-minute
(nitroaromatics and nitramines/nitrate esters, re-
solvent extraction procedure for explosives. When
spectively). For colorimetric methods, interference
pinpointing concentrations, a short kinetic study
is defined as the positive response of the method
should be conducted of the specific soils encoun-
to secondary target analytes or co-contaminants
tered at a site (Jenkins et al. 1996c). The kinetic
similar to the primary target analyte. The
study would involve analyzing an aliquot of ex-
ENVIROL colorimetric TNT method utilizes solid
tract after 3 minutes of shaking, and again after
phase extraction and liquidliquid transfer to re-
10, 30, and 60 minutes of standing followed by an-
duce interferences from other nitroaromatics. Im-
other 3 minutes of shaking. If the concentration of
munoassay methods are relatively specific for the
explosives increased significantly with the longer
primary target analytes that they are designed to
extraction time, a longer extraction period is
detect. For immunoassay methods, cross-reactiv-
needed. Jenkins et al. (1996a) found that 30-minute
ity is defined as the positive response of the
extraction times worked well for clay soils at the
method to secondary target analytes or co-con-
Volunteer Army Ammunition Plant, Chattanooga,
taminants similar to the primary target analyte.
Tennessee. Where multiple analytes are of inter-
The cross-reactive secondary target analytes for
est in each sample, a common extract may be used
TNT are mainly other nitroaromatics. The cross-
for both the colorimetric and immunoassay test
reactivity to these compounds varies considerably
methods.
among the four TNT immunoassay test kits. The
immunoassay test kit for RDX is quite specific,
with only 3% cross-reactivity for HMX.
Analysis time
The analysis time or throughput for the colori-
Depending upon the sampling objectives, broad
metric and immunoassay procedures ranges from
sensitivity or specificity can be an advantage or
3 to 11 minutes per sample for batch runs. The
disadvantage. If the objective is to determine
EnviroGard kits specify air drying of samples
whether any explosives residues are present in soil,
(which would add considerable time), and dry-
broad sensitivity is an advantage. For the CRREL
ing is optional with the EnSys RISc colorimetric
and the EnSys RISc colorimetric methods for TNT,
kits. Cragin et al. (1985) investigated various pro-
the color development of the extracts can give the
cedures for drying explosives-contaminated soils,
operator an indication of what types of com-
including air, oven, desiccator, and microwave
pounds are present in soil; for example, TNT and
drying. Air and desiccator drying appear to result
TNB turn red, DNB turns purple, 2,4-DNT turns
in only minor losses of explosives. Oven drying
blue, 2,6-DNT turns pink, and tetryl turns orange.
of highly contaminated soil (15% TNT) at 105C
For the CRREL method and the EnSys RISc RDX
for an unspecified period resulted in a 25% loss of
kit, RDX as well as HMX, nitroglycerine, PETN,
17
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