November 25, 2004

The ENose Knows

ONBOARD THE SPACE STATION, ammonia is a good thing. It flows through pipes, carrying heat generated inside the station outside to space. Ammonia helps keep the station habitable.

It is also poisonous. If it leaks, astronauts need to know quickly. The problem is ammonia becomes dangerous at a concentration of a few parts per million (ppm). Humans, however, can't sense it until it reaches about 50 ppm.

On the shuttle and space station ammonia is just one of about 40 or 50 necessary compounds, which cannot accumulate in a closed environment.

What if an electrical fire breaks out? The increasing heat releases a variety of signature molecules. Humans can't sense them either until concentrations become high.

Because of all those reasons, NASA decided to develop an electronic nose, or ENose. It's a device that can learn to recognize almost any compound or combination of compounds. Like a human nose, the ENose is versatile, yet it's much more sensitive.

"ENose can detect an electronic change of 1 part per million," said Dr. Amy Ryan, who heads the project at NASA's jet propulsion laboratory. She and her colleagues are teaching the ENose to recognize those compounds-like ammonia-that cannot accumulate in a space habitat.

The ENose uses a collection of 16 different polymer films. Researchers designed these films to conduct electricity. When a substance-such as the stray molecules from a glass of soda-is absorbed into these films, the films expand slightly, and that changes how much electricity they conduct.

Because each film is made of a different polymer, each one reacts to each substance, or analyte, in a slightly different way. And, while the changes in conductivity in a single polymer film wouldn't be enough to identify an analyte, the varied changes in 16 films produce a distinctive, identifiable pattern.

Electronic noses already see use here on Earth. In the food industry, for example, they can detect spoilage. There's even an electronic tongue, which identifies compounds in liquids.

Copyright Instrument Society of America Nov 2004