Let's stick to the facts please. IMS is widely believed to be
an excellent method for explosives detection but it will not give the right answer all the time. Automatically dismissing its results as "astrology" or automatically accepting its results without further analysis are both the wrong answers.
A recent review, "A critical review of ion mobility spectrometry for the detection of explosives and explosive related compounds" (Ewing and co-workers, 2001) makes the following statements:
Asselin reported the detection of ammonium nitrate via the nitrate ion, presumably a result of the slow decomposition to nitric acid and ammonia. Ammonia should provide superb response in the positive polarity and the nitrate ion was observed in negative polarity. Vapor detection from a solution of nitric acid also produced the nitrate ion. Although detection is possible, resolution and specificity problems exist in detection solely by means of the nitrate ion, as other explosives (and presumably other organo-nitrate compounds) provide a nitrate product ion at high temperatures. Also, NOx is a common airborne pollutant with a multitude of sources, including ubiquitous automobile exhaust.
And:
One special difficulty in the detection of explosives by IMS is the formation of fragments ions which constitute a reduction in detection limits. Fragmentation may also compromise the specificity of the method through the production of the common nitrate ion. This problem may be addressed through modifications of gas phase ionization reactions or possibly through new ionization schemes such as electrospray or laser ablation. Radically new ionization source designs might allow ion formation to be isolated from ion characterization.
It is very, very difficult to come up with an analytical or statistical method that will never have false negatives or false positives, and trying to eliminate one makes the other more likely. In this particular application, I would design a device that
reduces false negatives as much as possible so there is no chance that anything remotely like a bomb will get through the machine without detection. I imagine that's how the manufacturer designed it as well. This means false positives are left to sensible humans to have other evidence looked at and decisions made. I'm not sure that the right decision was made here, but as TSAMGR would point out I don't really have all the facts and would only be
...uming, so I can't be sure either way.
Unfortunately, neither I nor the passenger in question are likely to ever have all the facts as they will be suppressed under color of "sensitive security information" so essentially the TSA has no accountability to me directly.
Bibliography:
Ewing RG, Atkinson DA, Eiceman GA, Ewing GJ.
Talanta 2001 May 10;54(3):515-29. "A critical review of ion mobility spectrometry for the detection of explosives and explosive related compounds."