Precise Gas Sensor Could Monitor Pollution and Detect Disease

A portable nitric oxide (NO) gas sensor, suitable for large-scale deployment, could be of great value to atmospheric science, pollution control, biology, and medicine.

Researchers at Princeton University (NJ, USA) and Rice University (Houston, TX, USA) developed an ultrasensitive NO detector that uses lasers and sensors that are inexpensive, compact, and highly sensitive. The device, a transportable Faraday rotation spectroscopic system, is based on a tunable external cavity quantum cascade laser. A broadly tunable laser source allows targeting the optimum molecular transition of the NO fundamental band. For an active optical path of 44 cm and a one second lock-in time, constant minimum NO detection limits of 4.3 parts per billion by volume (ppbv) and 0.38 ppbv are obtained by using a thermoelectrically cooled mercury–cadmium–telluride photodetector and liquid nitrogen-cooled indium–antimonide photodetector, respectively. The laser light passes through polarizing filters that block all light unless NO is present.

Preliminary tests of the device were conducted during the 2008 Olympic games held in Beijing (China). The researchers believe their device could find uses ranging from the study and control of car and truck emissions to monitoring human exposure to pollutants in urban and industrial environments. For medical uses, the device is particularly attractive because the results are not corrupted by water vapor, which is present in breath samples. Testing for nitric oxide in a patient's breath, for example, could reveal chronic obstructive pulmonary disease and inflammation. The study was published in the August 4, 2009, issue of the Proceedings of the National Academy of Sciences (PNAS).

"The sensor we have developed is much more accurate and sensitive than existing systems, yet is far more compact and portable,” said coauthor Gerard Wysocki, Ph.D., an assistant professor of electrical engineering at Princeton. "The more nitric oxide, the more light makes it through the filters. There's no background signal to worry about.”

Existing systems to detect NO and other trace gases have a variety of drawbacks. Some, such as carbon monoxide (CO) sensors for homes, are compact and inexpensive, but not very sensitive. High-end systems, such as mass spectrometers and gas chromatographs, are much more sensitive, but are slow, bulky, complicated and expensive, making them impractical for use outside of a laboratory. Intermediate sensitivity optical systems pass a laser beam through a gas sample and detect whether some of the laser light is absorbed by the gas sample; however, the amount of absorption is very small compared to the overall amount of laser light, so the signal is hard to detect. Further, conventional optical sensors tend to be bulky, use large amounts of the sample, and require frequent operator intervention.

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