djm@castle.ed.ac.uk (D Murphy) (08/21/90)
From: D Murphy <djm@castle.ed.ac.uk> A description of this appeared in the magazine `Chemistry in Britain' in 1987, so it isn't classified. You can probebly buy these things over the counter in some countries too. The chemical attack warning system used by UK forces was developed at CDE at Porton Down, and is called NAIAAD - Nerve Agent Immobilised Assay Alarm and Detector (where *do* they get these names). It works by taking in air samples carrying vapour (nerve agents are not gases, they're normally deployed as sprays or aerosols) and passing them over a filter to which has been chemically bound the acetyl cholinesterase enzyme which organophosphorus agents attack. There is also a supply of substrate - acetyl thiocholine is used rather than the natural acetyl choline for sensitivity and reversibility reasons. The filter is monitored electrically - when the enzyme is operational the detector measures a constant potential as the acetyl thiocholine is split into acetic acid and thiocholine. A linked system regenerates the thioester. Organophosphorus agents (sarin, tabun, soman, VX, and probably others too) bind very tightly to the cholinesterase enzyme active site, thus stopping any more reaction. When this happens, the potential across the filter changes and an alarm goes off. Hydrogen cyanide produces a similar effect. I don't know if there is a specific detection circuit for blister agents - though since any chemical attack is cause to don protective clothing I doubt it. Blister agents react with many organic molecules, alkylating them, so exposing the NAIAAD filter to mustards or nitrosoureas would also likely generate a change in potential and thus set off the alarm. The whole thing sits in a battery operated box, man portable, about 1'x1'x6" dimensions. Murff...