ADELPHI, Md. (Sept. 5, 2012) -- Scientists at the U.S. Army Research Laboratory are exploring ways to quickly alert Soldiers to deadly gaseous nerve agents in the air using a new approach termed multi-wavelength photoacoustics.
Kristan Gurton, an experimental physicist in the Battlefield Environment Division, or BED, Computational and Information Sciences Directorate at U.S. Army Research Laboratory, or ARL, recognized early in his career that the photoacoustic technique was an extremely sensitive spectroscopic method, able to detect chemical compounds at parts-per-million or parts-per-billion concentrations.
But he also realized that traditional laser photoacoustic spectroscopy would not be appropriate as a viable approach for solving a real-time chemical/biological detection problem.
Photoacoustic spectroscopy, or the photoacoustic effect, is the measurement of light's absorption by matter, its subsequent heating and conversion to sound. It is traced to Alexander Graham Bell, who among other things, proved that light could have an effect on matter.
"What we needed was a low-cost, easy-to-use method that would quickly and accurately detect one of about 12 common, deadly gasses," Gurton said. "It had to be fast and it had to be extremely sensitive -- to detect hazardous gases at trace levels."
Because traditional laser photoacoustic spectroscopy is designed to measure a single absorption parameter at a time, it was not suited to produce the type of detailed information needed to detect and identify complex gaseous molecular compounds, Gurton said.
The solution was a non-traditional approach to conventional laser photoacoustic spectroscopy that is pretty straight-forward.
"We used multiple laser sources in a single, untuned, flow-through hollow cylindrical cell, like a straw, filled with the gas and equipped with a small microphone in the center of the cell. Each laser modulated at a different frequency, which caused the gas to heat."
"The effect was a faint acoustic wave that could detect gaseous nerve agent stimulants at trace levels in real-time," he said.
The Optical Society of America's journal, Optic Letters accepted Gurton's research paper last month, "Selective real-time detection of gaseous nerve agent stimulants using multi-wavelength photoacoustics," was written with co-authors Melvin Felton and Dr. Richard Tober, both from ARL.
The novelty of the method is the propagation of multiple laser beams through a single, untuned, flow-through photoacoustic cell, Gurton said.
A goal of the basic research is to find ways to reinforce Soldiers' ability to detect minute levels of harmful gases on the battlefield, said physicist, Dr. Yongle Pan, the authors' BED colleague. "The sensor brings us closer to protecting the Soldier from potentially lethal hazards."
Pan has five patents related to the technologies for the detection and characterization of hazardous aerosols. He said, "The technology takes time to develop."
In 1997, the Defense Against Weapons of Mass Destruction Act directed the Department of Defense to establish a domestic preparedness program to improve the ability of local, state and federal agencies to respond to biological-related incidents.
"As we study, we get closer to better solutions," Pan said. "Basic research methods being developed at ARL are designed to compliment each other, since we recognize that no single method is capable of producing a complete solution for protecting the Soldier."
The biological threat to military forces and civilian populations is now more likely than at any point in history, according to Medscape Reference, a medical authority for health care professionals.
The photoacoustics method could be useful in detecting chemical agents on the battlefield -- or in cases such as the Sarin attack on the Tokyo subway in 1995 -- on the homefront, Pan said.
"The next step in commercializing a photoacoustics-based detection product involves further prototype testing and evaluation," Gurton said. "Such a device would most likely utilize a quantum cascade laser array with at least six mid-infrared laser wavelengths. The cell itself is surprisingly inexpensive to produce. The cost and size would be driven by the packaging of the quantum cascade laser array.
"I envision a ruggedized device about the size of a small milk carton one day."
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