news
Suppose you wanted to design a device that could detect potential terrorist attacks in a public place. Ideally, the system should detect several classes of potential threat agents—radioactive, chemical, explosive, and biological— at low concentrations. To be fast and practical, it would need to be autonomous and reagentless. At the same time, it must minimize false positives, lest the system become like a high-tech car alarm: always beeping but never heeded. In a new paper published in AC (2008, 80, 4583–4589), Matthias Frank and colleagues at the Lawrence Livermore National Laboratory show that their single-particle aerosol mass spectrometry (SPAMS) system is in many respects close to meeting the qualifications of that idealized detector. Unlike systems that collect aerosol particles for later analysis in a lab, SPAMS operates entirely onsite. Particles ranging from 0.7–10 μm are concentrated and focused in a high-flow aerodynamic lens. They then pass a battery of six laser beams, which size and track the particles. A second series of lasers quantifies the particles’ laserinduced fluorescence (LIF). Finally, if sizing and LIF data suggest more thorough analysis is needed, the particles enter a bipolar TOFMS, in which positive and negative ions are analyzed simultaneously. The system then identifies the particles and logs the results. The system is completely autonomous and can track and analyze as many as 10,000 particles/s by size and LIF. Lasers fire based on onboard tracking; the decision to analyze a given particle by MS, which can process 50 particles/s, can be made on the fly. Alarms are triggered only if the number of particles of a given type exceeds carefully set thresholds. “Detecting things is not hard,” observes Frank. “The really hard part is to detect nothing when no threat agents are present, and that’s where most other technologies struggle today.” Fluores4784
A n a ly t i c a l C h e m i s t r y / J u ly 1 , 2 0 0 8
JACQUELINE MCBRIDE
SPAMS: fast, autonomous, and accurate multiagent threat detection
Audrey Martin and George Farquar, two members of the SPAMS development team, with an early version of the device.
cence-based analyzers, for instance, often have a high false-positive rate. Technologies such as PCR are more specific but take longer and require sample processing, he adds. Lead author Paul Steele cites three key parameters for detectors such as SPAMS: sensitivity, response time, and false-alarm rate. “What we have accomplished is to make an instrument that is very sensitive, with a very low falsealarm rate, but is very fast,” he says. “That’s unique.” Key to that speed are the system’s underlying electronics and algorithms, explains Steele. For instance, a typical single-particle mass spectrum contains about 60,000 data points. “That’s a lot of data to handle in real time,” he says. The team’s solution: reduce the data to a more manageable “vector” comprising 350 or so elements, which can be checked against a database. “We compare its vector to the vector library, and in real time, we can tell you if the particle is a spore, or explosive, or
background,” Steele says. Frank and his team first tested SPAMS in the lab, challenging the instrument with spores from a nonpathogenic strain of Bacillus anthracis (which causes anthrax), cobalt powder (a surrogate for 60Co, which has potential applications in dirty bombs), diethyl phthalate (a nerve agent surrogate), pseudoephedrine (used to synthesize methamphetamine), and trinitro-1,3,5triazinane (RDX, a highly explosive agent). In both single- and multipleagent scenarios, SPAMS correctly identified each compound and triggered the correct alarms, on average 34 seconds after release in the lab. The researchers also tested SPAMS at San Francisco International Airport, analyzing the air and logging data, but with the alarms offline, over about 7 weeks. Later, the system logs were studied to determine if any alarms—false or otherwise—would have been triggered. None would have, the authors found, despite scrutiny of nearly 1 million particles. SPAMS has not yet been deployed to protect public facilities, Frank notes; that would require further testing and approval by the U.S. Department of Homeland Security. But implementing SPAMS at a mall or airport would not be difficult, he says. “The system is very practical. . . . It doesn’t take much training to use it.” Yet SPAMS is expensive, Frank concedes, “because it is one-of-a-kind, and it was really pushed to a high-end, military-type application. For civilian facility protection, certain things could be scaled down.” In a forthcoming AC paper (DOI 10.1021/ac8002825), Frank’s team shows that the system can detect tuberculosis in surrogates of human effluent. Other potential applications include detecting other diseases, scanning baggage at airports, or even monitoring to ensure compliance with nuclear nonproliferation agreements. a —Jeffrey M. Perkel
