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So there's a (usually spot check) inspection point at airports sometimes, and they do a check for traces of explosives/gunpowder residue.

I've never had any problems, but a couple of friends have had it go off innocently - where they've actually detected something, and searched their bags.

Once this was warranted - turned out his father had cleaned his shotgun over his bag, but the other has no idea how there could possible be anything.

How does this happen? Is it faulty equipment, or what?

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First of all, "explosive detectors", be it ion-mobility spectrometer or specially trained dog, do not typically detect explosives, they detect chemical markers (aka taggants), which have to be mix into all explosives produced legally since 1991.

The idea of these detectors is that they are used for screening, which means you want to have low rate of false negatives (ie. people who actually have explosives but came through undetected) sacrificing that for relatively high rate of false positives (ie. people who don't have explosives, but preliminary scan marked them for further investigation). Hence ion-mobility spectrometer also react to anything that pops up as "marker like", which can be cosmetics, some kinds of paint, fertilizers etc.

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There is also chemical reaction swabbing: cnn.com/2010/TRAVEL/02/17/tsa.hands.swabbing/index.html –  Karlson Apr 26 '13 at 20:39
    
@Karlson: as far as I know, it's not chemical reaction, they just place swabs in the ion-mobility spectrometer. –  vartec Apr 26 '13 at 20:46
    
@vartec and how would those detect homebrewn or foreign explosives lacking such traces? Things like that shoe with a Semtex sole? The detectors might detect those markers, but also trigger on things common chemicals found in explosives, which is why they tend to trigger on photographic film (celluloid and silver halides). –  jwenting Apr 29 '13 at 8:44
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@jwenting: Semtex is tagged with Ethylene glycol dinitrate. BTW. "foreign" as in extra-terrestrial? :-P The law obliging manufacturers to add taggants is a UN convention. As for homemade explosives, IIRC that is the problem, hence for example the ban on liquids, as homemade liquid explosives are not easily detectable. Anyway, airport "security" isn't really about security, it's about "security theater" –  vartec Apr 29 '13 at 8:55
    
@vartec I know it's about giving people a false sense of security and deterring the amateurs :) The professionals aren't going to try sneaking a bomb through security, they just get one of their own hired on the security team and drive it right through the main gate and onto the tarmac to be loaded in the cargo bay. –  jwenting Apr 29 '13 at 8:59
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The usual problem is a reaction on a generic "explosive"--which falsely reacts to glycerin. You'll find glycerin in a lot of skin products.

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Could you elaborate? Is it the glycerin they're detecting, or the 'explosive'? –  Mark Mayo Apr 26 '13 at 19:14
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@MarkMayo: It's a false positive. Glycerin looks a lot like an explosive. (After all, consider nitroglycerin.) –  Loren Pechtel Apr 26 '13 at 19:18
    
Ah right, I get it now. I misread the original sentence :) –  Mark Mayo Apr 26 '13 at 19:22
    
and celluloid, the base for photographic film, also triggers them. Even after moving to digital cameras I was flagged more often than not because there were still traces of celluloid in my camera bags from before the switch. –  jwenting Apr 29 '13 at 8:46
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It might be what you have described but it also might be a simple matter of where the bag had been but basically a false positive.

For example if you visit agricultural areas you might come in contact with Ammonium Nitrate (the stuff that blew up in West, Texas), which is a common fertilizer but also can be used to make gunpowder or other type of explosive materials. So that might be what is being chemically detected by the "sniffers" or using swabs. Additionally ammonium nitrate is derived from common ammonia (found in a lot of household cleaners), which if present in significant quantity could be construed as precursor for explosives.

There is also a great presentation from University of Wisconsin on what could be detected by the chemical methods, which basically expands the list of all matters of Nitrogen based compounds.

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If I had to guess, the equipment for detecting the chemicals has to be very sensitive so that it detects the faintest traces of it (to err on the side of caution). False positives are a hassle, but a false negative could have disastrous consequences.

Very sensitive equipment has to have a ton of gain to detect small signal levels. With a ton of gain comes a lot of noise. Separating signal from noise is a delicate business.

Then there is the problem that chemical compounds occur in vast combinations. The equipment has to identify some known explosives, from among billions of harmless molecules. It has to do this without actually reconstructing the molecular shape, which would require something like X-ray crystallography. It has to also work without having a sizeable sample of the substance which would be needed for Infra-Red or mass spectroscopy; just a whiff from a suitcase.

It's pretty amazing that this stuff works at all, really. It's amazing what a human, or for that matter, canine nose is capable of, too.

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