Passage FourDespite their names, satin and
问题：
Passage Four
Despite their names, satin and soman are exceptionally ugly sisters. They are organophosphorous nerve gases. They are cheap and simple to manufacture. And mere milligrams—just a drop—of either is enough to kill an adult in a couple of minutes. They therefore make particularly fine weapons of mass destruction, equally popular with rogue dictators who have not been able to build nuclear bombs and with weird cults such as Aum Shinrikyo, which gassed the Tokyo subway in 1995.
Detecting them soon enough to prevent their effects can be difficult. Even detecting them after the event—if you are, for example, a United Nations weapon inspector—can be haphazard. But help may be at hand. A paper in this month’s edition of Analytical Chemistry, by George Murray and his team at John Hopkins University’s Applied Physics Laboratory in Laurel, Maryland, reports a new way of picking up minute qualities of the two gases without the risk of false alarms from legitimate organophosphorous compounds such as insecticides.
Dr Murray’s detector consists of a fiber-optic cable that has one end plugged into a laser and the other coated with a metal called europium. The laser generates blue light, and europium has the property of shining red when exposed to blue light of the correct wavelength—an effect that is exploited in many optical devices. But the metal possesses a second property that makes it uniquely suitable for Dr. Murray’s purpose: it reacts ply with organophosphates, and when it does so, the wavelength of the light that is emitting changes perceptibly.
To stop his detector going off in response to the wrong signals such as insecticide on the flea collar of a dog, Dr Murray has resorted to a second trick. The europium is embedded in a plastic film that binds specifically to sarin or soman (they are very similar molecules), using special pockets called molecular imprints that have been chemically etched into it. The organophosphates commonly used as pesticides do not fit into these pockets; and so fail to react with the metal. The result is a detector that is both sensitive ( it can pick up concentrations of as seven nerve-gas molecules in a trillion) and reasonable fast (it is able to sound the alarm within 30 seconds).
So far, Dr Murray has tested his device only on soman dissolved in water. This is mainly a safety measure, because water-borne nerve agents are easier to handle than those in gaseous form. But soman or sarin-contaminated water supplies are a real hazard in themselves—and not just in far-flung war zones. In America, for example, there are occasional leaks from military bases. The most recent was from Tooele Chemical Disposal Facility in Utah, one of the sites where the country’s chemical weapons stockpile is being destroyed. At the moment it would be hard to work out if any material from such a spill had found its way into the water supply until people started to become ill.
Dr Murray seems confident, however, that his technology will work just as well on sarin and saman gas, and has prepared "smart" cards coated with the mixture of europium and plastic to detect airborne nerve gases. Unfortunately, he does not, at the moment, have access to any place where the safety regulations will permit him to try them out. But if UN inspectors were ever allowed back into Iraq, he might have a chance.
Which part of the detector is the decisive factor that enables it avoid to false alarm ？（）