In 1993, scientists at Britain's Chemical and Biological Defence Establishment (CBDE), at Porton Down, proved the Iraqis had used sarin against the Kurds. They analysed material collected from Kurdish villages by James Briscoe, working for the US group Physicians for Human Rights. Although the samples were gathered four years after the attack, the CBDE chemists found traces of chemicals derived from sarin in the soil of bomb craters.
Dr Graham Pearson, director-general of the CBDE, said their findings were conclusive: "We even detected sarin itself on a bomb fragment where it had been protected by being absorbed into the paint. Our tests are sensitive enough to detect less than a billionth of a gram of nerve gas."
Sarin has been described as the poor man's atomic bomb because of the numbers that can be killed by a relatively small amount. It is not a gas but a colourless liquid which boils at 147C, yet it is volatile enough to contaminate the air to lethal levels. It disrupts the central nervous system of those who breathe it and, like the commuters of Tokyo, they would not realise the danger because sarin does not smell.
In theory, any competent chemistry graduate can make sarin from commercially available chemicals, but the danger comes in handling the finished product because any direct contact with sarin, even a drop of liquid on the skin, can be lethal. As little as 1mg is enough to kill a human and 1oz would be enough to wipe out a town of 30,000 inhabitants.
The German chemist Gerhard Schrader discovered the nerve gases tabun and sarin in 1937 when he was working for the chemical combine IG Farben, testing phosphorus compounds for insecticidal properties. Schrader was not the first to make these molecules - they had been reported in technical journals in 1902 - but he was the first to realise their lethal nature and found a better way of making them, which was patented by IG Farben in 1938.
When the Nazis realised how toxic the molecules were, they became a closely guarded secret, code-named N-Stoff. Testing moved on from guinea pigs to apes, and finally to concentration camp inmates. Fifty years ago, in the closing stages of the Second World War, the staff at IG Farben's headquarters in Frankfurt destroyed all their records, but details of the tests leaked out during the Nuremberg trials. In any case, the Allies found the massive stockpiles of nerve gases at chemical plants, which had been making hundreds of tons of them per month.
In his memoirs, Inside the Third Reich, Albert Speer, minister for armaments and war production, says he considered killing Hitler early in 1945 by releasing nerve gas into the ventilation shaft of his underground bunker in Berlin. His plan was thwarted when the inlet shafts were suddenly redesigned to guard against possible gas attacks.
British chemists in the war worked on similar phosphorus molecules but failed to find either tabun or sarin. Although they made similar molecules, these turned out to be no more toxic than the phosgene and mustard gas used in the First World War.
Sarin is an example of an organophosphorus compound. The term is often confused with organophosphate, the chemicals commonly used as pesticides, such as in sheep dips. Both types of compound are insecticides, and sarin was once tested against the plant-louse phylloxera, which attacks grapevines. A 0.1 per cent solution spread on the ground near the vine root completely eliminated the infestation. Effective as it is, sarin is too dangerous to use as an insecticide.
The sarin molecule consists of a phosphorus atom joined to an oxygen, a fluorine, a propoxy group and a methyl group (CH3). This last component confirms the molecule as an organophosphorus compound because it has a direct carbon-to-phosphorus bond. Organophosphate insecticides are similar, but lack such a bond, which renders them less toxic to mammals but still highly toxic to insects.
Sarin acts in the body by paralysing the enzyme cholinesterase. This is needed to remove the chemical messenger acetylcholine after it has transmitted a signal from a nerve-ending to, say, a muscle. If acetylcholine is not removed it continues to stimulate the muscle unchecked, resulting in twitching, convulsions and possibly death if it is the heart or lung muscle. One of the first symptoms of sarin poisoning is semi-blindness caused by its effect on eye muscles. The antidote for nerve gas is atropine, which counteracts the uncontrolled messenger molecules, together with oxime, which re-releases the sarin-blocked enzyme so it can function again.
Antidotes are likely to be needed because sarin will continue to appeal as a weapon of mass terror, but it can be neutralised at relatively little cost, either in antidote treatment or in clean-up operations. The molecule is destroyed by alkaline solutions, and a mixture of washing soda and household bleach will erase it by knocking off its fluorine atom, rendering it non-volatile and non-toxic.
Dr John Emsley is science writer in residence, Imperial College, London.