extraction materials with a 70-year-old, qualita-
Table 3. Cross-reactivities of
tive colorimetric assay. The chemical confirmation
commercial TNT immunoassays.
reagents used were also derived from work done
Millipore
D-TECH
by chemists around the turn of the century (re-
viewed in Jenkins 1990). The resulting methods
Tetryl
+
are both sensitive and relatively free from inter-
2Am-DNT
+
ferences produced by humic substances or other
4Am-DNT
+
nitroaromatics that are likely to occur at military
TNB
+
+
2,6-DNT
+
sites. A single extract can be used to screen for
2,4-DNT
+
picric acid, ammonium picrate, TNT, 2,4DNT and
RDX in soils. The estimated cost of a few dollars
per sample is very low. Single assays can be run in
about 20 minutes. Multiple samples can be pro-
IMMUNOASSAYS FOR
cessed in less time using cartridge or membrane
PICRATE IN SOIL AND WATER
Immunoassays are gaining increased accep-
Immunoassays are available commercially for
tance in screening applications, although the costs
detecting TNT in soil extracts and water. A disad-
and time requirements can be significantly higher
vantage of all immunoassays is that they also de-
than spectrophotometric methods. The positive
tect compounds that are structurally related to the
response to picric acid of two TNT immunoassays
target analyte. This phenomenon is called cross-
presents two interesting possibilities. When a field
reactivity. Antibodies are produced in response to
site is known to contain no nitroaromatics other
small molecules (molecular weight less than 200)
than picric acid or ammonium picrate, immunoas-
only after they have been conjugated to a large
says could be used to screen for picrate. Although
carrier protein. Exactly how this conjugate is made
this may be unusual at military sites, it should be
determines the sensitivity of the assay and the de-
the case at industrial sites contaminated with
gree of cross-reactivity to various compounds
picric acid. Conversely, if a TNT immunoassay
(Harrison et al. 1991). In the case of TNT, conju-
gives a response that is greater than the sum of
gates could be made by coupling a protein to ei-
TNT and other known cross-reactive analytes as
ther a reactive moiety at the 1- position (e.g.
determined by Method 8330, unsuspected con-
trinitro-sulphonic acid) or at the 2- or 4- position
tamination by picric acid may be indicated.
(2- or 4-aminodinitrotoluene). The antibody would
then tend to recognize either a trinitro-aromatic or
a dinitro-toluene, respectively. Judging from the
LITERATURE CITED
cross-reactivities listed on two commercial TNT
kits (Table 3), it was assumed that they were pro-
Anderson, D., S.F. Tsang, T. Jackson and P.
duced using these different schemes. A test of their
Marsden (1993) Dinoseb analysis in the field and
cross-reactivies to picrate showed that the
the laboratory. In Proceedings of the 9th Annual Waste
EnviroGard TNT plate kit was mildly responsive,
Testing and Quality Assurance Symposium Proceed-
with a detection limit of about 2.5 g/g in soil and
ings, American Chemical Society, p. 347359.
5 g/L in water (the detection limit of TNT is 0.25
Bagnato, L. and G. Grasso (1986) Two-dimensional
g/g in soil and 0.5 g/L in water). The D-TECH
thin-layer chromatography for the separation and
kit was equally sensitive to TNT and picrate, with
identification of nitro derivatives in explosives.
detection limits of 0.2 g/g in soil and 5 g/L in
Journal of Chromatography, 357: 440444.
water.
Barnabas, J. (1954) Identification of phenols by
circular paper chromatography. Chemical Abstracts,
49: 10128.
CONCLUSIONS
Barral, E. (1915) Picric acid and malingering.
Chemical Abstracts, 10: 2100.
The proposed methods (App. A and B) for field
Colman, D.M. (1962) Paper chromatography of
screening for residues of picric acid or ammonium
nitro compounds. I. Substituted trinitrobenzenes.
picrate in soil and water resulted from the com-
Journal of Chromatography, 8: 399403.
bined application of contemporary solid-phase
Deniges, G. (1923) Nature and application of the
10
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