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Fast, Accurate Detection of Explosives
It doesn't come easy writes "Fast, highly reliable detection of residues that could indicate the presence of explosives and other hazardous materials inside luggage is now possible with technology under development at Purdue University. Recent improvements to a previously developed prototype have proven successful at detecting at the picogram (trillionths of a gram) level in lab tests, about 1,000 times less material than previously required. From the article: 'In the amount of time it requires to take a breath, this technology can sniff the surface of a piece of luggage and determine whether a hazardous substance is likely to be inside, based on residual chemicals brushed from the hand of someone loading the suitcase.'"
Ok, here's something I've always wondered about. If you have these exquisitly sensitive machines that can detect even a few molecules of material, aren't they by the same token super-vulnerable to being attacked by "chaffing" or overloading?
Its worse than that. You have to look at the false positive and negative rates for detection. If you have a test that is 99.9% specific, it will still fail in practical use in an airport, as that means that 1/1000 people will come up positive. (I think I have the right statistical measure here, but apologies if not). If you have alot of people going through you will still have a big problem -London had 1 000 000 FLIGHTS last year, so the equivalent of 1000 plane loads of people will come up positive per year. This is the same issue as using automatic detection of terrorists - Its one thing to match/no match a known ID (eg biometric passport) to a person, its another to match every passer by to every known terrorist.
Coming back to chemical detection, this level of sensitivity will mean that every person who uses GTN for angina (commonly known as "Anginine" tablets or sprays) runs the risk of coming up positive. This amounts probably about a million people in US, and lots more elsewhere in the world. GTN (used in microgram doses in the treatment of poor blood supply to the heart; the precursor to a heart attack) is actually tri nitro glycerine, and is just a wee touch explosive in larger quantities.
Just my 2c worth.
Michael
There is no cryptographic solution to the problem where the intended receiver and the attacker are the same entity.
Regarding the article, nanogram sensitivity (a trillion molecules of TNT) is utterly unimpressive. The vapor pressure of most explosives is so low that you need femtogram sensitivity to directly sense vapor. For an explosive like RDX that has an absurdly low vapor pressure, you really want attogram sensitivity (about a million molecules). You can heat up dust and surfaces to vaporize more explosive, but with a mass spectrometer you then run into a problem with selectivity: many ordinary boring compounds will have the same molecular weight as the explosive--the signal will be swamped by the noise. (Hmmm ... the article says they're using clever ionization, and tandem spectrometry. That helps a lot, but they still have a hell of a problem to solve.)
The article says "'If you tried to detect a particular compound out of a mixture of thousands of different substances, you might begin to see the limitations of this method,' Talaty said. 'But real-world explosives are not that complex.'" What, people walk through airports with purified blocks of luggage? No! You get a suitcase drenched with sweat (which includes urea), solvents, ammonium nitrate from natural sources, perfumes, plasticizers, plastic monomers and short chain polymers, various mineral oils, a whole boat-load of volatiles from living things, and many more. The background signal is a freaking nightmare. I work in the explosive detection field, and I sure wish it was as easy as they say.
If you have these exquisitly sensitive machines that can detect even a few molecules of material, aren't they by the same token super-vulnerable to being attacked by "chaffing" or overloading?
I think a few molecules might be a bit of an over statement. Nitroglycerin has a weight of 227g/mole. A mole is 6.02*10^23. So one molecule of nitroglycerin weighs 3.77 * 10^-22 grams.
A picogram = 1*10^-12 grams.
1*10^-12/(3.77*10^-22)=2.65* 10^9, or 2.65 billion molecules. That's a ways from a few.
I think your point still is valid though. Could someone contaminate an area such that it couldn't be cleaned sufficiently? My guess is it probbably could be. You don't have to get rid of all the material, just enough so that you're below the level of detection.
AccountKiller