New sensor for detecting trace gases
DOI: 10.1063/PT.3.1155
New sensor for detecting trace gases. Practitioners in many fields, including environmental science, homeland security, and health care, are keenly interested in identifying exquisitely tiny amounts of certain gases in, for example, the atmosphere or one’s breath. Laser-based techniques are useful because they can selectively probe spectral lines with great sensitivity. In the mid-IR, where many molecules have a multitude of lines, the quantum cascade laser is fast becoming the instrument of choice. (For more on QCLs, see the article in PHYSICS TODAY, May 2002, page 34.) Typically, an optical cavity is loaded with the gas at millibar pressure and the laser light is finely tuned to resonate with both a single cavity mode and a spectral line of interest. In a new twist, Gottipaty Rao and Andreas Karpf (Adelphi University, New York) use gases at atmospheric pressure, combining so-called off-axis cavity enhanced spectroscopy with multiple-line integrated absorption spectroscopy, which they developed. The off-axis alignment of the laser excites a near-continuum of cavity modes in addition to reflecting many times for a long path length through the gas. After tuning the QCL to a dense spectral region, the spectroscopists enhance the sensitivity by integrating over the resulting absorption spectrum, which includes more than 100 transitions. Their proof-of-principle detection of nitrogen dioxide has a sensitivity of 28 parts per trillion, which compares favorably with other laser-based systems. Unlike other systems, this one works well at high pressure, is less sensitive to instrumental vibrations, does not require fast electronics, and is effective for molecules with dense, poorly resolved spectra. (
