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Chemistry Principles of Detection Kits

Nitroaromatics such as TNT, TNB, picric acid and its derivatives form highly colored compounds (Meisenheimer Complexes) upon reaction with alkali. Common alkali formulations for explosive analysis contain 5 to 10 percent tetrabutylammonium hydroxide in ethanol or water.


Nitrate esters such as nitroglycerine (NG), EGNG, PETN, and nitrocellulose (NC) under similar conditions will undergo alkaline hydrolysis producing nitrate ions (NO2), which can be readily detected with the Griess reaction. This includes the action of nitrate ions on an aromatic amine, usually sulfanilic acid, in an acidic medium. The diasonium ion that is then formed is complexed with an aromatic nucleophile to produce a colored azo dye.


Nitramines such as RDX, HMX, and tetryl also undergo alkaline cleavage to form nitrate ions, which produce the same colored azo compound by the Griess reaction. Dimethyl sulfoxide (DMSO) is used as a solvent for the alkaline reagent. This solvent accelerates the color formation with plastic explosives. A combination of sulfanilamide and N-(l-naphthyl) ethylenediamine gives a fast and intensified color reaction.


As for the inorganic nitrates, a fine zinc dust is used to reduce them to nitrite ions that then react with the Griess reagent.


DropEx A is a highly acidic solution containing a color producing oxidable reagent. The acid releases chlorine- oxides from the chlorate ions and they oxidize the reagent to produce the color reaction.


DropEx B is a solution that decomposes TATP producing a chemical that reacts with the color reagent in DropEx A


There is no way of distinguishing between innocent fertilizers and nitrate-based explosives, so care should be taken when interpreting the results.


The order or reagent application must not be altered and all three should be used when testing, in order to perform a complete test. If Expray/DropEx-2 is sprayed after a positive result was obtained with Expray-1, a change to pink color is an indication of a double base or a triple base explosive (such as Composition B or triple base gunpowder). Even when Group B explosives only are tested, one must start with EXPRAY/DropEx-1 and only then apply with EXPRAY/DropEx -2. If nitrate based explosives are suspected, one should still start with EXPRAY/DropEx -1, then move to EXPRAY/DropEx -2 and only then apply EXPRAY/DropEx -3.

Stability and Storage

All three reagents have been tested after accelerated aging experiment (120ºF, 8 weeks). Like all aerosol cans, Expray cans may loose pressure if stored at elevated temperatures so, it is recommended that when not in use, that the cans be stored a cool, dry place. The specially designed T1 case protects a three aerosol can kit from temperature extremes. The cases withstood a maximum temperature of 167ºF degrees for up to one hour (testing simulated a sunrise to sunset cycle with a peak temperature for an hour) and should provide adequate protection of the kits under extreme conditions.


The cans should always be stored in an upright position, and the carrying case of the kit should be stored in standing position.


Both DropEx and Expray are guaranteed for 12 months from receipt. Expray packaging is such that it can be used all at once or throughout that year. Although the Expray cans are guaranteed for one year after purchase; the reagents should remain effective almost indefinitely. DropEx 1 must be used within 30 days of breaking to seal on the dropper bottle.

False Positives

Any fertilizer containing nitrates will react to EXPRAY-3 (after 'E' and 'X'). If nitrite compounds (such as sodium nitrite) are tested, a color reaction will be obtained even after applying EXPRAY-1 and EXPRAY-2. No other false positives are known, but one should note that only the colors listed should be observed.


If a different color appears in any stage, it should be disregarded. Furthermore, an unreacted test paper left in the open-air will gradually change in color to light pink. Note that there are some varnishes and lacquers made of nitrocellulose (which is a group B explosive), and if the kit is applied directly to a surface treated with such coatings, a positive (pink) reaction will appear. The varnishes do not disperse residues, so that touching a varnished surface will not cause positive reaction on the touching hand or surface.

Health and Safety

In general, all spray products should be treated carefully. Do not: puncture the can, throw into fire, or expose to extreme heat. Spray cans should be held in upright position, and exposure to direct sunlight (especially in a parked car in the summer) should be avoided. Usually it is recommended to use the sprays in a ventilated area and to avoid inhalation of the spray.


Avoid contact of the Expray/DropEx reagents with skin or eyes. In case of accidental contact, flush with plenty of water.


The aerosol propellants are flammable as is DropEx-3. Do not use near fire.

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