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BY LIA UNRAU
Rice News Staff
March 26, 1998
Some call it the sniffer. Better than a bloodhound, this sensor can detect barely a trace of a particular gas, and it can tell exactly what it is.
Rice researchers have built a sensor that can be used to monitor the slightest changes in air quality in real-time. That means no bothering with samples, laboratory analysis and costly time delays.
Nobel prize winner and Wiess Professor of Natural Sciences Robert Curl and Frank Tittel, the Abercrombie Professor of Electrical and Computer Engineering, have collaborated for years on lasers for spectroscopy. Then one day several years ago, Tittel had a realization: the lasers could be adapted to monitor air quality. With the help of David Lancaster, a research fellow, and Dirk Richter, a graduate student in Tittel's lab, the researchers have begun field-testing their device and improving its design.
There are many existing technologies, but others can't discriminate between different types of gases with high precision and without sample collecting and analyzing, which can take up to two days depending on the gas and the application.
In addition to the key features of being highly selective and working in real-time, the sensor boasts a list of attributes: the sensor is rugged, reliable, power efficient, portable, lightweight and works at room temperature. The sensor currently measures 2 feet long, 1 foot wide and 1 foot high.
The sensor uses diode lasers, the kind used in compact disc players and classroom laser pointers, and a technique called difference-frequency generation. Two interacting laser beams are mixed in an appropriate crystal to create an infrared beam. Concentration of a gas can be measured directly by the optical absorption in the infrared beam.
Not only would this type of sensor have the potential to increase the safety and efficiency of various manufacturing and production processes, but it has particular appeal for monitoring environmental air quality.
It might find uses in monitoring "sick" buildings, or airplane cabins or cargo holds.
Its creators are especially proud of its ability to detect formaldehyde, a common gas emitted from plastics, and a substance that gives new cars their distinctive smell. Widely used in building materials like foams, paints, particle board, carpet and office equipment, formaldehyde can cause sore eyes, nose and throat, as well as nausea, dizziness and headaches. But formaldehyde is extremely difficult to measure, because it is hard to distinguish from similar gases at low concentrations, and it sticks to sampling surfaces.
Formaldehyde and other gases become dangerous quickly in confined spaces, such as on the space shuttle, or on the space station Mir, where the quality of re-circulated air must be kept high. In fact, last year NASA's Johnson Space Center field tested Rice's sensor, which was used to identify sources of formaldehyde from the plastics used in the construction of spacecraft cabins. At their test facility, JSC created a sealed practice chamber, 20 feet in diameter, three stories high, and filled with life support equipment. In one test, four people lived locked in the chamber for 90 days.
NASA considers a safe concentration of formaldehyde in space to be 40 parts per billion (ppb) over 7-90 days. Rice's sensor detected levels at 120-160 ppb prior to the human test. While concentrations up to 300 ppb may be safe on the ground, in space, formaldehyde becomes dangerous even at low concentrations. After an exhaustive search in which items were systematically removed, from printed posters to T-shirts, the culprit of the formaldehyde buildup was found--acoustic tiles used to muffle sound in the steel chamber.
John Graf, research engineer in the life support systems branch at JSC, sees the current sensor as a precursor to a usable model.
"If this instrument gets small enough and uses very little power, then we would very much like to use it in real-time in-flight assessment," Graf said. He cited the sensor's specificity, or the ability to distinguish between different gases, its ability to detect tiny concentrations, the fast response time and small sample required as key features that appeal to NASA projects. Currently, samples are taken and brought back to Earth to be analyzed weeks, sometimes months, after the fact.
In addition to formaldehyde, the sensor currently can be set to detect a number of other gases, including carbon monoxide, carbon dioxide, methane, sulfur dioxide and nitric oxide. The researchers plan to expand that repertoire by teaching the instrument different algorithms, as each gas has a "fingerprint." Also, they are looking at how to automatically sense a number of gases simultaneously.
"We're beginning to get a very clear vision of the end result," Curl said.
The sensors could also be used to monitor landfill emissions and the fence line around petrochemical plants to provide an early warning system of any leakage. The team is working to develop a way to compensate for changes in wind.
Another use is auto emissions control on highways. Located in the lab in the Space Sciences Building, the sensor detected rises in carbon monoxide on Rice Boulevard during morning rush hour and then much larger concentrations after 5 p.m.
Another use that reflects the researchers' interest in environmental protection is monitoring methane, a greenhouse gas. A relatively large but controllable source of methane is rice farming.
To measure methane now, another Rice professor, Ron Sass, a biogeochemist, goes out to the field, takes samples in plastic bags, then returns to the lab for analysis. With a real-time, remote-controlled sensor, he could observe methane gas emissions and fluctuation on a constant basis. But first the sensor must withstand the elements in a rice paddy--rain, wind, heat, standing water, and unattended operation, features the team is developing.
Studying the chemical composition of gases emitted from vents on the side of volcanoes may help scientists detect changes and perhaps help in predicting eruptions.
The research team is working to improve the sensor within a year by using fiber optics nearly throughout the system, which will reduce it to the size of a laptop computer and use less electrical power.
They also plan to create a version using optical waveguide technology, which will provide more infrared power for less electrical power. The technology is not quite here yet, but one of Tittel's former students, Konstantin Petrov, one of the original creators of the sensor, is working on this problem.
The researchers reported their findings last fall in journals such as Applied Optics and Applied Physics B.
The project was funded by Texas Advanced Technology Program awards and the Johnson Space Center, with contributions from the Welch Foundation.
For related information visit the following Web sites:
Rice Electrical and Computer Engineering Department: www-ece.rice.edu
Laser Science Group: www.ruf.rice.edu/~lasersci/
