Seem to recall that the Birmingham 6 were initially convicted on the grounds of tests which were supposed to be for Nitro-Glycerine.
These were later discredited on the grounds that lots of innocent substances contain nitro-cellulose and could have given a false positive. Examples include lacquers, paints and varnishes commonly used in bars and restaurants and from memory (can't seem to verify it) even the coating on playing cards.
So if you want to fly don't go to a bar or restaurant for several days beforehand ?