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Intricacies of comparative testing of explosives detectors at the ultra-trace level

This paper shall demonstrate the reduced lifetime of ultra-trace explosive residues when subjected to standard laboratory conditions, citing examples of flawed experimental design. The traditional view of "trace" level residue may lie within the detection limit capabilities of bench-top instrumentation. Gas chromatography / mass spectrometry, often the main stay of many trace evidence analysis laboratories can readily deliver nanogram and now potentially upper picogram detection limits. Today, emerging technologies continue to push the limits of detection, and sub-nanogram restrictions give way to picogram and femtogram opportunities. As instrument technologies become more sensitive, the need to work at continually lower detection levels is expressed. Generation of reliable, reproducible ultra-trace samples for the testing, analysis and evaluation of those technologies is challenged by the chemical properties of the very samples under investigation. Unlike testing against bulk quantities of explosives, at the picogram level unforeseen sublimation and sorption phenomena may potentially disrupt an otherwise well-planned test. While it may be valid to assume that the properties of bulk samples of most explosives are relatively constant with respect to time, it may not be safe to assume the same is true of ultra-trace level deposits of explosive residue. The vapor pressures of many common military explosives are low, but they are not zero. This fact cannot be ignored when working with trace levels of explosive residue. Failure of an inexperienced technician to consider these factors when conducting an evaluation may unnecessarily introduce bias into the data, and may result in the misrepresentation of a sensor's capabilities. The analyst is now faced with the complication of working with amounts of explosive so potentially low, that loss of a few picograms of material due to evaporation, air currents, poor laboratory technique or some other diluting factor represents a significant percentage of the total sample mass. Added to the complication are sample and substrate matrix, carry-over, and potential cross contamination effects that may now pose a significant effect rather than a slight background nuisance.