Technology News - March 11, 2004

Monitoring water bodies with mass spectrometry

A portable mass spectrometer capable of performing underwater analyses could greatly enhance the way in which pollution is monitored and unlock some of the secrets of aquatic biogeochemical processes, according to researchers at the University of South Florida (USF) and the Massachusetts Institute of Technology (MIT) who are developing similar devices.

Deployed on a remotely guided surface or underwater vehicle or hung from a buoy, the instrument can simultaneously monitor in situ for a broad suite of dissolved gases and volatile organic compounds (VOCs), says R. Timothy Short, a sensor development engineer at USF. This capability is its primary advantage over other types of environmental monitoring systems that typically involve collecting samples for analysis back in a laboratory.

“You get real-time, in situ information” that can be merged with the vehicle’s Global Positioning System observations to create high-resolution chemical distribution maps, Short explains. And such data are difficult to obtain with traditional grab-sampling methods or other in situ instruments that target only one or very few compounds. Plus, “you get the information almost immediately [through a wireless Ethernet connection], so you can make intelligent choices about where to sample next,” he adds.

The technology may prove useful for addressing a wide range of problems, from identifying point sources of anthropogenic chemicals to spill cleanups, industrial outfall management, and water intake protection to underwater oil and gas exploration to basic marine and freshwater science.

“We’re at the point where [it's difficult to] advance our understanding or improve water resource and coastal zone management without the ability to make more comprehensive measurements,” says Harold Hemond, a biogeochemist at MIT. “We need to be able to observe chemical conditions as they vary over the day, over the seasons, or in response to perturbations such as rainstorms, water withdrawals, or chemical inputs.”

Another potential application would be to map metabolic gases in a water column to help determine the state of eutrophication. To do this, a mobile mass spectrometer could be integrated into a networking system of sensors hung from buoys or mounted on lake or ocean bottoms to collect detailed data.

Although portable mass spectrometers have been around for more than a decade, getting them to function underwater has been a major challenge because of the need to perform the mass spectrometry in a vacuum, according to Short. Depth has also posed a problem. “For every 10 meters you go down, the water pressure increases by an atmosphere,” requiring an especially rugged system to withstand such a harsh environment, he notes. And the instrumentation itself, especially the vacuum pumps, is extremely power hungry, so getting the mass spectrometer to run on batteries at very low power has been another barrier, Hemond adds.

To date, Short and his colleagues have constructed and deployed systems based on both linear quadrupole and ion trap mass analyzers—both well-known commercial designs. A 275-micrometer-thick polydimethylsiloxane membrane inlet separates the water environment from the high vacuum inside the instrument. Volatile gases diffuse across the membrane, providing parts-per-billion detection limits for a number of VOCs, including toluene, benzene, and chloroform, and sensitive detection of dissolved gases, such as methane, oxygen, and carbon dioxide up to 200 atomic mass units.

The instrument’s standard configuration is 45 inches (in.) in length, 7.5 in. in diameter, and 73 pounds (lb)—not including the lead–acid batteries used to power it. Short and his colleagues have evaluated its performance in a variety of field deployments, including in situ monitoring of municipal sewage effluents, motorboat exhaust in a marina, and hydrothermal vent waters in the Gulf of Mexico and Yellowstone Lake in Wyoming at depths of up to 80 meters (m). Recent refinements of the membrane interface design and sampling pump have extended the system’s analysis capability down to more than 200 m, however. The USF technology has been licensed to Applied Microsystems Ltd. and is commercially available.

Hemond and his graduate student Richard Camilli have deployed their system, which is based on a cycloidal analyzer using a polymer membrane inlet, in coastal marine systems and stratified lake ecosystems. The analyzer is a mass filter that uses both a magnetic and an electric field to separate ions. Encased in a glass pressure sphere, the MIT system measures 17 in. in diameter and weighs 45 lb with batteries.

The MIT researchers are still field-testing the instrument, which is capable of measuring in the sub-parts-per-million level at depths of up to 75 feet. They have focused primarily on metabolic gases in an effort to better understand geochemical processes and how best to manage them, but they're also interested in practical management questions, according to Hemond.

Both groups note that they are still documenting the capabilities of the instrumentation, its endurance, and the limits of its depth deployments.

The USF researchers in particular are working on increasing the number of chemicals the unit can detect. In addition to VOCs and dissolved gases, they’d like to detect more polar compounds, such as pesticides, PCBs, PAHs, and fuels. An alternative membrane interface, namely an automated solid-phase extraction interface, is expected to help with this. Likewise, the group would like to expand the system’s analysis capability down to much greater depths (on the order of 1500–2000 m) to look for ocean bottom sources of methane locked up in gas hydrates.

Other challenges that both groups face include reducing the instrumentation’s size and increasing its endurance through miniaturization. Currently, the USF system “consumes 100 watts, so if you’re running it on batteries, you need a lot of them for it to last more than a few hours,” Short says.

The vacuum pumps draw the most power, and to reduce the consumption, Short and his colleagues are replacing the roughing pumps with a newer, lower-power design. He admits, however, that since most of the system’s components are off-the-shelf, “There’s only so much we can do because most of them weren’t designed to save power.” As a result, they’re building a system from scratch—which is still in the laboratory stage—that should give them more flexibility on this end.

The MIT system, which was mostly custom built in the lab, consumes 20 watts and can be put into sleep modes at intervals to conserve power on longer deployments, according to Hemond.

Bio-fouling, too, poses a problem for long-duration deployments but has not yet been an issue for either group. The longest deployment to date for the USF and MIT instruments has been three days and five hours, respectively.

Ultimately, both groups envision outfitting multiple unmanned, underwater vehicles with these portable mass spectrometers to serve as a network that is constantly monitoring chemicals of interest, sniffing them out, and even tracking them in the event of something like an underwater pipeline leak. —KRIS CHRISTEN