Raman Spectrometer Probes the Deep

Daniel S. Burgess

As NASA rovers equipped with high-tech instruments again investigate the surface of Mars, it can be difficult to appreciate that places much closer to home have resisted such in situ analyses of rocks and minerals. But the deep-sea floor remains as inhospitable a place for investigators as the "final frontier," and ocean scientists interested in performing geochemical studies have depended on techniques such as coring and dredging that can damage the recovered samples.

More than two miles below the surface, a curious Pacific grenadier watches the Deep-Ocean Raman In Situ Spectrometer interrogate a pool of liquid CO₂ on the sea floor. Courtesy of Peter G. Brewer.

Researchers from Monterey Bay Aquarium Research Institute in Moss Landing, Calif., and from Washington University in St. Louis hope their Deep-Ocean Raman In Situ Spectrometer will change this. The commercially available Raman spectrometer modified for use on a remotely operated vehicle can collect spectra from gas-, liquid- and solid-phase specimens 3600 meters below the surface.

In the proof-of-principle work, the team employed a HoloSpec f/1.8i holographic imaging spectrograph from Kaiser Optical Systems Inc. of Ann Arbor, Mich., equipped with the company's Mk II holographic filtered probe head with two sampling optics. A 532-nm frequency-doubled Nd:YAG laser from Coherent Inc. of Santa Clara, Calif., provided excitation, and a 2048 × 512-pixel cooled CCD camera from Andor Technology of Belfast, Northern Ireland, collected the spectra.

To ready the spectrometer for deployment, the researchers replaced two stock instrument alignment mechanisms and broke down the device into three sections: the power inverter, single-board computer and laser; the optical bench and camera; and the probe head. Each section was enclosed in a pressure vessel rated to withstand pressures of 41 MPa and connected to the others by cabling.

In a series of deep-water tests, the 211-kg spectrometer assembly was mounted on either of two remotely operated vehicles used by the institute. The results indicated that the system could detect the Raman signals of seawater, real-time changes in the composition of a CO₂/N₂ gas bubble, liquid CO₂ on the ocean floor and semitransparent calcite crystals on the ocean floor.

Peter G. Brewer, a senior scientist at the institute, said that the quality of the collected spectra was better than the researchers had hoped but that preparing a commercial spectrometer for this application was more difficult than they had expected. Specifically, the CCD camera was oriented at a right angle to the optical components of the spectrograph, complicating the packaging of the optical bench. Kaiser has since introduced a linear version of the instrument, he said, and Jobin Yvon has a compact design employing a grism that interests the team.

Brewer said that the group plans to talk with vendors about the design of the second-generation instrument to produce a system that is lighter, easier to assemble and more robust. "We just cut our teeth with the first tests," he said. "Now we're looking to be more adventurous."