At the Bottom of the Sea
As Samuel Taylor Coleridge’s Ancient Mariner noted, there is water everywhere in the ocean, but none is fit to drink. The simple answer that the water is too salty is not informative enough for oceanographers. They want to know exactly what is in the water. In the past, that would have meant collecting samples via ship or submersible, with analysis done later in a laboratory.
An instrument based on laser-induced breakdown spectroscopy could directly measure chemical elements as they emerge from the seafloor and spew from hydrothermal vents. In this artist’s conception, a sensor is mounted on one of the manipulator arms of a remotely operated vehicle. Researchers hope to observe the composition of vent fluids in their natural environment, even at ultrahigh pressure. Illustration by E. Paul Oberlander, Woods Hole Oceanographic Institution.
That situation is changing, with a shift toward observatories and long-term studies, noted Anna P.M. Michel, a graduate student in the MIT and Woods Hole Oceanographic Institution (WHOI) Joint Program. “As a result, a critical need for in situ chemical and biological sensors is evolving,” she said.
Michel was part of a research team from the Massachusetts-based WHOI and from the University of South Carolina in Columbia that recently evaluated laser-induced breakdown spectroscopy as a tool to analyze water at the bottom of the ocean. The results were published in the May 1 issue of Applied Optics.
In the deep-ocean environment, laser-induced breakdown spectroscopy has several advantages. In the technique, a short-pulse laser beam is fired at a sample, typically ablating nanograms or less of material while forming a plasma. The glowing plasma yields spectroscopic information and an elemental fingerprint. No sample preparation is involved, and the technique is usable with solids, liquids or gases, with sensitivities in the parts-per-million range or better. The only thing that must reach the target and be collected from it is light. Measurements can be taken every second or more often.
For these reasons, instruments for laser-induced breakdown spectroscopy would be useful within observation stations that might someday be perched on the seafloor near hydrothermal vents. Also known as black smokers, these are spots where geothermally heated water of several hundred degrees centigrade spews forth. Vents are some of the most biologically productive areas in the deep ocean, in part because of the chemicals dissolved in the water.
Although laser-induced breakdown spectroscopy had been employed on other planets and on submerged materials, no one knew whether it would work on ocean floor water. According to Michel, very little attention has been paid to bulk liquid analysis or to the effect of oceanic pressures on signals generated by laser-induced breakdown spectroscopy.
Any pressure-dependent effects would be in addition to those that arise from creating a plasma in water. In a liquid, plasma lifetime is reduced to typically less than a microsecond, versus the 5 to 20 μs for plasmas in air. Another problem is that water reduces the intensity of the generated light.
For their investigation of the suitability of the technique, the researchers simulated the ocean in a high-pressure cell equipped with optical ports, through which they injected pulses from Nd:YAG lasers operating at 1064 nm. The same ports allowed them to view the plasma. They varied the pressure in the cell up to several hundred atmospheres — pressures similar to those found near hydrothermal vents.
They used a laser made by Continuum of Santa Clara, Calif., to fire a 5-ns pulse for single-pulse experiments. When performing dual-pulse studies, they used a laser from Quantel SA of Les Ulis, France, to fire a 9-ns pulse, followed by a second pulse at a variable delay from the Continuum device. In capturing the plasma spectrum, they used a spectrograph from Chromex (now part of Bruker Optics) and an intensified CCD detector made by Princeton Instruments.
They dissolved sodium, manganese, calcium, potassium and lithium in the water, finding that increasing the pressure, adding salt or changing the temperature did not affect their ability to detect these analytes. Laser-induced breakdown spectroscopy seems to be a viable chemical sensing method, Michel noted. Before the technique is put into practice, however, more work must be done. For example, the researchers might wish to account for the effect of particulates in the water.
As for the equipment, Michel said that off-the-shelf lasers and spectrometers would have to be smaller and more robust to go into a deep-ocean observatory. “One of the greatest challenges is being able to collect the tiny amount of plasma light that is being produced. Doing that at the bottom of the ocean would require a great deal of engineering to build a fiber optic probe that could both deliver the laser light and collect the plasma light.”
- A gas made up of electrons and ions.
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