Technology▼Solutions
A portable mass spectrometer capable trial outfalls; protecting water intakes; of performing underwater analyses supporting underwater oil and gas could greatly enhance the way in exploration; and conducting basic which pollution is monitored and unmarine and freshwater science. lock some of the secrets of aquatic “We’re at the point where [it’s diffibiogeochemical processes, according cult to] advance our understanding or to researchers at the University of improve water resource and coastal South Florida (USF) and the Massazone management without the ability chusetts Institute of Technology (MIT) to make more comprehensive meawho are developing similar devices. surements,” says Harold Hemond, a 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. The portable mass spectrometer mounted on the “You get real-time, in underside of this remotely guided boat can prositu information” that vide far more information about chemical plumes can be merged with the than traditional technologies. vehicle’s Global Positioning System observations to create biogeochemist at MIT. “We need to high-resolution chemical distribution be able to observe chemical condimaps, Short explains. And such data tions as they vary over the day, over are difficult to obtain with traditional the seasons, or in response to perturgrab-sampling methods or other in bations such as rainstorms, water situ instruments that target only one withdrawals, or chemical inputs.” or very few compounds. Plus, “you Another potential application get the information almost immediwould be to map metabolic gases in a ately [through a wireless Ethernet water column to help determine the connection], so you can make intellistate of eutrophication. To do this, a gent choices about where to sample mobile mass spectrometer could be next,” he adds. integrated into a networking system The technology may prove useful of sensors hung from buoys or for addressing a wide range of probmounted on lake or ocean bottoms to lems, including identifying point collect detailed data. sources of anthropogenic chemicals Although portable mass spectromeand spill cleanups; managing industers have been around for more than a © 2004 American Chemical Society
decade, getting them to function underwater has been a major challenge because of the need to perform the mass analysis 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 wellknown commercial designs. A 275-micrometerthick 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 dectection 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 (lbs)—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 hydroCOURTESY OF R. T. SHORT
Monitoring water bodies with mass spectrometry
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thermal 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 lbs (including batteries). The MIT researchers are still fieldtesting the instrument, which is capable of measuring in the sub-partsper-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, ac-
cording 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, 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 re-
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placing 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. The MIT system, which was mostly custom built in the lab, consumes 20 watts and can be put into sleep modes 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 deployments to date for the USF and MIT instruments have 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
