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ERDC/CRREL TN-05-2
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METHOD AND MATERIALS
Equipment
The Expray kit is composed of three aerosol cans, each of which contains
reagents that will react with explosives residues and give an easily discernable
color change. The first can is designed for the colorimetric detection of polynitro-
aromatics (TNT, tetryl, trinitrobenzene). The second can is for nitramines and
nitrate esters (RDX, HMX, NG, and NC). The third can is for the detection of
nitrate-based explosives, ammonium-nitrate and fuel oil (ANFO), and black
powder. The complete kit comes with a special collector test paper that has an
impervious backing and raised coarse areas that aid in sample retention and
swiping. It also contains tests strips to determine whether the reagents are still
reactive, and a reference card for quick identification of possible indicators. The
manufacturer states that a single can will perform a maximum of 110 tests, using
the included test paper. Our tests have indicated that approximately 20 batches
of samples with up to 50 samples on an 8.5- 11.0-inch sheet of paper can be
screened from one can. The determination of the type of explosive and relative
concentration is dependant on the sequence of the application. Expray 1 must be
applied first, followed by Expray 2, followed by Expray 3.
Expray 1 contains an alkali that reacts with polynitroaromatics and forms
Meisenheimer complexes, which are highly colored compounds. The alkali is
tetrabutylammonium hydroxide, and the colors range from dark brown for TNT
to orange for tetryl. A blue-green color indicates the presence of DNT (Plexus
Scientific, Silver Spring, Maryland).
Expray 2 contains compounds that produce a Griess reaction with the nitrate
ions from the reaction with Expray 1, and this yields an azo dye, giving a color
change. For nitramines (RDX, HMX), sulfanilamide and N-ethylenediamine are
used to produce the dye. For nitrate esters (NG, NC, PETN), sulfanilic acid is
used to produce diazonium ions, which are complexed with an aromatic nucleo-
phile to produce the dye. Dimethyl sulfoxide is also added as a solvent to acceler-
ate the reaction for nitramines. The color produced is pink for any nitramine and
nitrate ester.
Expray 3 is used for the detection of inorganic nitrates (ANFO, gun powder,
black powder). Zinc dust is used to reduce the nitrates to nitrite ions, which then
react with the Griess reagent. This will also yield a pink color with the presence
of nitrates. There is no way to distinguish between innocent fertilizers and
nitrate-based explosives.
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