A newly developed device can detect ultra-low concentrations of gases such as nitrogen dioxide accurately and almost instantaneously—even when situated in a field environment where it is shaken by nearby machinery, passing vehicles, thermal changes or air currents.

“Our sensor is much faster and has the potential for much higher sensitivity—if employing better matched optical mirrors—than the previously reported results," says Gottipaty Rao, a physicist at Adelphi University. "It opens the door to interesting, real-time investigation of trace gas concentrations.”

The device measures ultra-low concentrations of gases such as nitrogen dioxide, a pollutant released by cars and power plants. Image credit: Pixabay. The detector uses a measurement technique called cavity ring-down spectroscopy (CRDS), in which a laser shoots a pulse of light into a precisely aligned cavity formed by mirrors. When the pulse ends, the light bounces around in the cavity and slowly leaks out.

The time it takes for the light to escape is called the ring-down time. If the cavity contains a small amount of gas that absorbs the wavelength of the laser, the ring-down time will decrease, since some light is lost to the absorption. Measuring the change in ring-down time indicates the concentration of the trace gas.

In order for the sensors to work, the laser must be resonant with the cavity, meaning that the wavelength of the light “matches” the cavity length in such a way that the light bounces around for a long time. Standard CRDS sensors are susceptible to vibration-induced errors, since small shifts in the length of the cavity can dramatically reduce the sensitivity. As a result, special vibration-isolation equipment must be employed to use CRDS in the field.

To solve the vibration sensitivity issue, Rao and colleagues used a high-powered broadband laser containing a wider range of wavelengths than typical for CRDS lasers. Any shift of the cavity length due to vibration simply shifts the cavity resonances to other wavelengths that the laser is already emitting.

The researchers tested the device by measuring trace concentrations of nitrogen dioxide. But if the wavelength of the laser were to be changed, the technique could also be applied to monitor other gases, such as methane, ammonia and sulfur dioxide, Rao says.

Currently, monitoring of nitrogen dioxide in the atmosphere is done using chemiluminenscence, a chemical reaction that generates light, Rao notes. It is not capable of real-time measurements and requires an elaborate calibration procedure to measure the absolute concentration of the gas. CRDS has advantages over chemiluminenscence, and Rao believes the new detector will make it a more practical tool for the field.

“More importantly, our approach may also prove useful in developing a highly sensitive explosive detector—especially applicable to security and air travel—that targets nitro group-based explosives, such as TNT, GN, RDX, HMX, PETN and TATB,” Rao says.

The team says the device’s sensitivity and response time could be even further improved by using higher-reflectivity mirrors and optimizing the design of the cavity. “This would open up new possibilities in atmospheric monitoring, chemical reaction studies and explosive detection,” Rao says.