FINDING a needle in a haystack is hard, but spotting a molecule on a battlefield is even harder. When it comes to detecting chemical-weapon attacks, the most common approach in use today is known as ion-mobility spectrometry. This involves zapping the particles floating in a sample of air with electrons from a radioactive source, so that they become electrically charged. The charged particles are then timed as they are drawn through a filter. The heaviest (and potentially most dangerous) molecules move the slowest. When such molecules are spotted, the alarm is sounded.
The problem with ion-mobility spectrometry is that it is not very good at distinguishing between different types of large molecule. Diesel fuel, cigarette smoke, detergents and pesticides all contain heavy molecules in the same range. That means that current chemical-weapons detectors can sound both false positives, which are expensive and frustrating, and false negatives, which are potentially fatal.
Robert Levis, a chemist at the Centre for Advanced Photonics Research at Temple University in Philadelphia, believes that he and his colleague Herschel Rabitz, of Princeton University, have come up with a better solution. Building on the separate research efforts of Kent Wilson, another chemist at the University of California in San Diego, and Gustav Gerber, a physicist at the University of Würzburg in Germany, Dr Levis and Dr Rabitz are using lasers in a novel fashion. Their new detector, which is now being turned into a commercial product, uses extremely intense laser pulses to break up incoming molecules into smaller pieces.
Dr Levis found that specific types of pulses tend to break up specific molecules in a specific way. The system figures out which chemicals are present by running through a number of pulses of different types that are known to break up chemical agents in predictable ways, and then identifying the resulting fragments.
A good analogy for this process, says Dr Levis, is to compare the laser pulses to music. Each type of molecule responds to a certain type of pulse, but not to others, just as people respond to their favourite type of music, but not to musical genres they dislike. “You can find a combination of frequencies of laser light that turns on a given molecule and makes it dance in a very specific way,” says Dr Levis. Given a specific chemical target, a process of trial and error can identify the right type of pulse to make even the deadliest chemicals jump.
The musical analogy is apt, since each laser pulse, like a snippet of music, is made up of many different frequencies. Certain components of these “shaped” pulses act rather like enzymes in a chemical reaction: they create an environment that allows other, simultaneous reactions to proceed more easily than they otherwise would. The result, Dr Levis says, is a process that should allow chemical identification within five minutes, with far more reliability than existing systems.
The new laser-based detector has attracted interest from America's army and navy, and from the National Science Foundation. A unit the size of a large suitcase could be ready for use as soon as early 2005, says Dr Levis.