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New sensor detects gaseous chemical weapon surrogates in 45 seconds

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Using lasers and tuning forks, researchers at PNNL have developed a chemical weapon agent sensing technique that promises to meet or exceed current and emerging defense and homeland security chemical detection requirements. The Quartz Laser Photo-Acoustic Sensing technique now is ready for prototyping and field testing.

PNNL has demonstrated QPAS's ability to detect gaseous nerve agent surrogates as part of a lab bench sensor system consisting of a vapor dilution system, a small volume thermally desorbed preconcentrator, and the QPAS detector tuned for organophosphorus compounds. One test used diisopropyl methyl phosphonate, which is a chemical compound similar to sarin. QPAS detected DIMP at the sub-part-per-billion level in less than one minute. The miniscule level is similar to letting one drop of liquid DIMP evaporate into a volume of air that would fill more than two Olympic-size swimming pools.

"QPAS-based system is an extremely sensitive chemical detection technique that can be miniaturized and yet is still practical to operate in field environments," said Michael Wojcik, National Security Directorate. "we're eager to take it to the next level."

The QPAS technique is based on Laser Photo-Acoustic Sensing and infrared Quantum Cascade Lasers. LPAS is an exquisitely sensitive form of optical absorption spectroscopy, where a pulsed laser beam creates a brief absorption in a sample gas, which in turn creates a very small acoustic signal. A miniature quartz tuning fork acts as a "microphone" to record the resulting sound wave.

Multiple QCLs were paired with the tuning forks, producing a QPAS sensor array, allowing simultaneous examination of a single sample at many infrared wavelengths. Nearly every molecule has unique optical properties at infrared wavelengths between three and 12 micrometers, and QCLs provide access to any wavelength in this region.

"Because of this access and the fact that QPAS is almost immune to acoustic interference, the QPAS array has potential for excellent chemical sensitivity and selectivity," Michael said.

QPAS's small components represent a major advance over previous LPAS measurement methods. Historically, LPAS instruments were physically large, often measuring a yard or more in length. The entire arrangement was cumbersome, power-hungry and prone to interference from external sound and vibration.

In the QPAS technique, several QCLs can fit on a 3 x 3 millimeter chip. And the tuning forks are identical to the kind used in wristwatches. A conceptual design for a battery-operated, prototype QPAS array sensor, which includes 10 pairs of QCLs and tuning forks, would fit into a briefcase that is 12 inches long, 12 inches wide and 6 inches high-and the entire thing would weigh less than 15 pounds.

QPAS is currently at Technology Readiness Level "four," meaning that while the technical components exist and initial testing is complete, the system still must be converted to a prototype.

Part of the research was done in collaboration with Rice University, Houston, Texas, where a portion of the QPAS technology originated.

Development of the technology was funded by the Defense Advanced Research Projects Agency.

Source: Pacific Northwest National Laboratory

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