Magazine article Oceanus

A Laser Light in the Ocean Depths

Magazine article Oceanus

A Laser Light in the Ocean Depths

Article excerpt

Researchers studying rocks on Mars or suspicious white powders on the battlefield have a high-tech way to determine a sample's chemical composition without bringing it back to the laboratory. It's known as laser-induced breakdown spectroscopy, or LIBS.

Anna Michel, a student in the MIT/WHOI Joint Program in Oceanography, would love to use LIBS on the seafloor. The problem is, LIBS doesn't work in water ... at least not yet.

Michel and other scientists in the Deep Submergence Laboratory at Woods Hole Oceanographic Institution (WHOI) are working to develop a LIBS instrument that can be used in the sea.

Invented in the 1960s, LIBS focuses high-intensity pulses of laser light on a solid, liquid, or gas until it creates plasma (an electrically charged gaseous state) and a short-lived flash of light. Those light emissions come in different wavelengths, or "colors," of the spectrum, depending on the elements present in the sample. A spectrometer detects the wavelengths to identify a sample's chemical composition.

On the surface of Mars or in the deserts of Iraq, the process is relatively straightforward because rocks and dust are dry and laser light travels through air with little impediment. But at the bottom of the ocean, the intense pressure and the liquid environment suppress the plasma, producing less detectable light. The laser light and the return signal are also scattered, distorted, and diffused by seawater.

"Very few people work with LIBS in liquids," said Michel, who has been collaborating with terrestrial LIBS expert Michael Angel of the University of South Carolina. "Even fewer people work with LIBS at high pressure, and no one has worked with it in seawater."

Alan Chave, Michel's advisor from the WHOI Department of Applied Ocean Physics and Engineering, got a serendipitous phone call from Andrzej Miziolek, a former colleague working at the U.S. Army Research Laboratory in Aberdeen, Md., who had read an article about ocean observing technology and suggested that LIBS might prove valuable.

"The call came right around the time that Anna was looking for a Ph.D. project," Chave said. "Since she had a chemistry and chemical engineering background, it seemed like the perfect fit." The idea was to develop an instrument that could directly measure elements just as they emerge from the seafloor and spew from volcanic hydrothermal vents. For decades, ocean scientists have collected fluid samples to study underwater venting and volcanism--which build the Earth's crust, seed seawater with minerals, and fuel some of the planet's most primeval life forms. But sampling devices on the Alvin submersible or remotely operated vehicles (ROVs) such as Jason inevitably capture excess seawater that dilutes the fluid sample's chemical composition. …

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