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...