Mini-mass spec

Analyzing chemicals underwater with the mass spectrometer. The sampling port is at the end of the robotic arm.

Courtesy of Scott Wankel

There’s a lot going on 2500 meters below sea level. It’s dark, temperatures can climb to 300 degrees Celsius near thermal vents, and the pressure is about 250 atmospheres. If humans could swim at that depth, the pressure exerted on the body would be equivalent to the weight of about 30 Boeing 747 jumbo jets, says Peter Girguis at Harvard University. And yet the water at this depth is teeming with life, with more biomass in 1 cubic meter than in a cubic meter of the richest rain forest (Mar Ecol Progr, 148:135-43).

Scientists know relatively little about what that biomass is, however. “There’s so much we don’t know because we can’t culture these [deep sea] microbes,” says Girguis. “We know they’re down there making RNA, but we don’t really know what they’re doing.” The animals that live down there are fascinating, he says: Most lack mouths or anuses, and feed by taking in seawater rich with microbes through their hypertrophied gills. These microbes do “something” using chemical energy (not sunlight, of course) that sustains life and fuels the animals’ growth, he says, and that something is what Girguis wants to figure out.

Working at such depths is not easy, but Girguis and his colleagues—Scott Wankel, also at Harvard, and Jon Erickson of the Monterey Bay Aquarium Research Institute in California—have come up with a miniaturized mass spectrometer to do just that. Miniaturization is “an essential [next] step for these exotic environment applications,” says mass spec expert R. Graham Cooks, a professor of analytical chemistry at Purdue, in an email. The big advance to this one, he says, is “its ability to function in high-pressure environments.”

The trick, says Girguis, was to find a way to transfer the enormous pressure exerted by thousands of meters of water away from the device itself and instead to a container housing it. The solution: A custom-made titanium tube almost 1 meter long and 20 centimeters in diameter, which holds the device. A Teflon™ membrane just 10 microns thick—permeable to gas but not water—covers one end of the tube, and inside are two fist-sized pumps that create the high-vacuum conditions. A supply wand on the outside brings samples of water from different areas around the vent holes so “we can get different samples without moving the spectrometer,” Wankel explains.

Once the gas molecules pass through the membrane into the vacuum created by the pumps, they are ionized by a filament. The ions are then channeled into the spectrometer, where identification occurs. The whole thing runs off power supplied by a nearby deep-sea submersible such as the Alvin, operated by the Woods Hole Oceanographic Institution. “We wanted to create a tool that could be constructed inexpensively with most components off the shelf so that others could make and use the instruments as well,” said Girguis. The total cost, including the titanium housing (the custom-made, expensive part) is $40,000. Girguis and Wankel hope to publish a how-to paper on the topic in the near future.

There’s just one other miniature mass spec device out there, developed by Tim Short of SRI International, a research institute in Menlo Park, Calif. “We’ve been doing similar work in the Gulf of Mexico, although only at depths of 1000 meters, so this is a nice step forward,” Short says. In Girguis’s tool, “the design is improved so it can go deeper. The application sounds very interesting.”

Prior to deploying their little treasure, the researchers tested it in a high-pressure facility in Girguis’s lab. The real test, though, will come next year, in the North Pond area of the Sargasso Sea in the North Atlantic Ocean, where the sediment is several hundred meters thick. Girguis and his colleagues plan to install a newer, battery-powered version of their miniaturized mass spectrometer into a drill hole there, leaving it to collect data for as long as 6 months or a year before the batteries conk out. “I am often asked why we go to all the trouble (and incur all the expense) of sending instruments to the bottom of the ocean,” says Girguis. Here, he wants to show “how processes in the deep sea shape our world, and what role it plays in keeping our planet running.”