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Aerosol Spray to Identify Bombing Suspects
RedHanded writes "Forensic chemists at the Hebrew University of Jerusalem have developed a color-changing spray that can identify people suspected of making or planting bombs. The chemical turns from yellow to bright red when it comes into contact with urea nitrate, an explosive residue that may be left behind on the hands of someone who has handled an improvised device."
In the UK, the Birmingham Six were falsely imprisoned for 16 years (one chap died in prison) largely because of the Griess test. The trouble is, anything nitrated will give a positive. The playing cards the men had been using on the train when they were arrested were probably what set it off. Ping pong balls certainly would. Imagine Forrest Gump in the Twenty First Century, "And then I met the President again, then they tasered me, then I went to prison for life." The Griess test is now completely discredited. Its re-introduction would be on a par with admitting polygraphs, or examining chickens' giblets as evidence, whether it's packaged as an aerosol or anything else.
I believe it is used to detect tripwires. Silly string sprayed ahead of you will drape over tripwires without being heavy enough to trip them. makes sense to me anyway.
man, I feel like mold.
Unless you make it yourself, I think it is unlikely you have urea nitrate in your fertilizer. You probably have urea + potassium nitrate (or ammonium nitrate). To form urea nitrate, you need a strongly acidic conditions.
From the article, the "amazing" new molecule is just commercially available p-dimethylaminocinnamaldehyde. The chemistry involved is already well-known. It is used for, among other things, indirectly detecting biotin (by way of the urea in the molecule). Basically you mix your urea-containing compound with a strong acid (sulfuric acid works), which promotes enol tautomerization and makes the normally unreactive nitrogens of the urea reactive toward electrophiles. One of the nitrogens will react with the aldehyde to form an imine, and due to the availability of a quinoid resonance contributor, turn color (red in the case of dimethylaminocinnamaldehyde and yellow in the case of dimethylaminobenzaldehyde).
What's special here, and why this won't result in a thousand false positives from detection of any urea-containing compound, is that urea nitrate is a stable salt and acidic enough on its own to react with dimethylaminocinnamaldehyde without the addition of acid. So a wipe test, drop it in isopropanol, add some of the aldehyde and see if it changes color. It's a fairly elegant application of old chemistry to forensic analysis.