Hearing chemical compounds in real time

Hearing chemical compounds in real time. The ability to detect and identify gaseous compounds quickly and accurately has many applications, whether for real-time pollution monitoring or for sensing chemical weapons on the battlefield. A common, sensitive method for measuring a trace gas is laser photoacoustic spectroscopy. In LPAS, the absorption of laser light by a sample generates local heating, which in turn generates acoustic waves; those waves can be detected by a sensitive microphone and analyzed (see Physics Today, May 2009, page 34 ). Now Kristan Gurton and colleagues at the Army Research Laboratory have demonstrated a way to expand LPAS to multiple absorption signals, which allows the presence of a particular gas species to be detected in real time. The team’s approach is facilitated by the increased availability of lasers—particularly quantum cascade lasers—in the spectrally rich mid-IR. The researchers filled a photoacoustic cell with the gas to be analyzed and then illuminated it with three lasers of different wavelengths simultaneously. By modulating each laser at a different frequency, Gurton and company could separate out from the microphone signal the absorption at each laser wavelength; the ratios of the absorption signals yielded concentration-independent metrics. Tests on varying concentrations of different compounds—acetone, isopropyl alcohol, and five chemical nerve agents—exhibited clear absorption-ratio distinctions for all but two of the species, as seen here, with parts-per-million sensitivity. Adding additional lasers at different wavelengths should allow greater discrimination. The researchers envision that a sufficiently rugged device for field use would be about the size of a milk carton. (K. P. Gurton, M. Felton, R. Tober, Opt. Lett. 37, 3474, 2012.)