Evaluation of Commercial Enzyme Immunoassays for the
Field Screening of TNT and RDX in Water
PHILIP G. THORNE AND KAREN F. MYERS
INTRODUCTION
of the degree of contamination, where present,
can assist site managers in prioritizing their sam-
Contaminated groundwater on and around
pling efforts.
U.S. military installations is a serious problem.
Field methods for the detection and quantifi-
Besides the usual toxic and hazardous materials
cation of TNT and RDX in soil and water have
common to many large industries, the military is
been developed that rely on the colorimetric anal-
a unique source of nitroaromatic and nitramine
ysis of highly specific reaction products (Jenkins
and Walsh 1992, Jenkins et al. 1994). The detection
able intake limits for these classes of compounds
limits for these tests are 1 g/g TNT and RDX in
in drinking water (EPA 1988a,b,c, 1989, 1992). To
soil and 0.9 g/L TNT and 3.8 g/L RDX in water.
comply with EPA guidelines and to protect the
Several samples an hour can be processed using
public safety, the U.S. military must identify and
these methods.
remediate sources of contamination.
Enzyme immunoassay methods for
Conventional laboratory analyses
TNT and RDX
The traditional approach used to delineate the
Enzyme immunoassays are analytical meth-
extent and degree of explosives contamination
ods based on highly selective binding reactions of
has been to analyze monitoring well samples at
antibodies with specific target analytes. Antibod-
off-site laboratories using SW846 Method 8330
ies are proteins produced in response to foreign
(EPA 1994). Off-site analysis requires several
substances as part of the vertebrate immune re-
days' turnaround and delays on-site decision
sponse system. Methods developed for small
making. Site managers cannot optimize sampling
molecules are usually formatted as competitive
strategies without data on initial samples. In
enzyme-linked immunosorbent assays (ELISAs).
addition, laboratory analyses generally cost over
In one common form of ELISA (Fig. 1), the target
0 per sample. This cost limits the number of
analyte is bound to an enzyme through a spacer
samples that can be analyzed, decreasing the spa-
molecule to form an enzymeanalyte conjugate.
tial resolution of the investigation. A survey of
Antibodies are bound onto the surface of a solid,
results from 46 military installations (Walsh et al.
such as the walls of a microtitre well or test tube,
1993) showed that approximately two-thirds of
or onto small spheres. When a known amount of
the soil samples and three-quarters of the water
enzymeanalyte conjugate and sample contain-
samples analyzed were not contaminated with
ing the free target analyte are mixed with the anti-
secondary explosives or transformation prod-
bodies, they compete with each other for binding
ucts. Of the samples that were contaminated, 95%
sites on the antibodies. Upon the addition of the
contained TNT (2,4,6-trinitrotoluene) and/or
appropriate substrate, the enzyme catalyzes a
RDX (1,3,5-hexahydro-1,3,5-trinitrotriazine). Sig-
reaction that changes a chromogen from colorless
nificant cost reductions can be achieved if nega-
to colored. Quantitation is accomplished by com-
tive samples could be identified without off-site
paring color intensity to a standard curve. The
analysis. Furthermore, rapid field determination
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