The deep-ocean laser Raman
in-situ spectrometer (DORISS)
In 2003, after several years of hard work, MBARI engineers and scientists completed the first successful field experiments using a device that seems like something out of a science fiction novel. At the heart of this device is a green laser. Simply by pointing the laser at an object, scientists can find out what that object is made of. This amazing device is called a laser Raman spectrometer.
By bouncing a specially tuned laser beam off of almost any object or substance—solid, liquid, or gas—a laser Raman spectrometer can provide information about that object's chemical composition and molecular structure. For example, by moving the spectrometer's laser beam across a block of granite, scientists can identify the minerals in the rock, as well as the crystalline structure of those minerals. Similarly, by shining the laser into a beaker of seawater, scientists can tell how much carbon dioxide and other gases are dissolved in the water.Since the late 1980s, laser Raman spectrometers have been used on land for tasks such as medical diagnoses, environmental monitoring, and criminal investigation. In 2000, MBARI chemist Peter Brewer and engineer George Malby took on the daunting challenge of adapting one of these delicate instruments to the crushing pressure and near-freezing temperatures of the deep sea. Many researchers in the field said that it could not be done, and suggested that, for a variety of technical reasons, this instrument would not be practical in the deep sea.
Working with geochemist Jill Pasteris and her team at Washington University in Saint Louis, the MBARI researchers purchased a commercial bench-top spectrometer from Kaiser Optical Systems, Inc. Electrical engineer George Malby worked with mechanical engineer Mark Brown to package this instrument so that it could be carried by MBARI’s remotely operated vehicles (ROVs) 4,000 meters (13,000 feet) below the ocean surface.
In addition to developing a housing to resist the immense pressure at this depth, the engineers and scientists had to minimize the system’s weight and power consumption while cushioning the spectrometer’s delicate optics from the vibrations of the ROV. The engineering team also had to design a software and communications system that would allow scientists to control the spectrometer through two and a half miles or fiber-optic cable.To address these challenges, the team split the spectrometer into three sections, each with its own pressure housing. With its thick pressure cases, the initial prototype weighed about 180 kg (400 pounds) in air. It took many months and many test dives to successfully adapt the original lab-based instrument into a working oceanographic sensor, which they affectionately named DORISS, for "deep-ocean Raman in-situ spectrometer."
In 2002, the designers finally got the system into the water, testing it off Monterey Bay at a depth of 3,500 meters. The instrument not only survived the engineering test dives, but performed so well that scientists on board were eager to try it out. They had hoped to use the spectrometer the next day to examine a pool of liquid carbon dioxide that they had placed on the seafloor.
Unfortunately, because of high winds and rough seas, the researchers could not launch the ROV to perform their follow-up experiment at 3,500 meters. After waiting 36 hours for the weather to clear, the team decided to move inshore to run a modified version of their experiment within the relatively sheltered waters of Monterey Bay. For this modified experiment, they had to find a glass vessel to hold a small volume of liquid CO2 in front of the laser. Eventually they "liberated" a not-quite-empty jelly jar from the ship’s galley. With this improvised equipment, the team obtained the first in-situ laser Raman spectra of liquid CO2 and CO2 hydrates at the high pressures and near freezing temperatures of the deep sea.
In 2003, the research team obtained laser Raman spectra from gases emerging from deep sea vents in the Gulf of California. Although the gas measurements were quite successful, the scientists were frustrated that they could not use the spectrometer to study solid objects, such as the small pieces of methane hydrate they found dispersed within the seafloor sediment near the vents.
The laser Raman spectrometer could only make measurements on a tiny area (much smaller than the head of a pin) where the laser beam was precisely in focus. This was not a problem for liquids and gases that could be held in large glass containers, where any focus point within the container was acceptable. However, focusing the beam on a specific spot on the surface of a solid object proved much more difficult, because the laser had to be positioned to within one tenth of a millimeter and held absolutely still for several minutes. It soon became obvious that in order to study solid objects, the researchers needed a more precise positioning system than the manipulator arm on the ROV.
To solve this problem, MBARI postdoctoral fellow Sheri White spent most of 2003 working with a team of engineers and scientists to develop the "precision underwater positioner" (PUP). The PUP is a small, sturdy, three-legged platform that supports the laser probe-head. Set directly on the ocean bottom but connected by a fiber-optic cable to the ROV-mounted spectrometer, this device allowed scientists to remotely control the position of the laser beam to within a tenth of a millimeter.
By August of 2004, the team completed the first complete field test of DORISS and PUP, successfully analyzing a wide variety of solid objects, including rock samples, shells, and even bacterial mats. They also studied a semi-solid, ice-like substance called carbon dioxide clathrate hydrate, which forms when carbon dioxide is released at certain depths in the deep ocean. In both cases, the PUP performed flawlessly, a testament to the team's perseverance and dedication to this challenging project.
In 2005, having proven the instrument’s potential usefulness, the team sought to increase its sensitivity, reduce its size and weight, and eliminate its continual need for maintenance and calibration. In order to satisfying this last requirement they had to completely redesign the optical bench of the spectrometer. Fortunately, the engineers at Kaiser Optical were willing to work with MBARI to create a completely new spectrometer with a new optical bench design. In a strong testament to the Kaiser engineers, who addressed all of MBARI's needs for the new spectrometer, this new design proved to be virtually maintenance free.
Mechanical engineer Farley Shane was assigned the task of designing a pressure case to contain the new DORISS2 system. After exhaustive testing of DORISS2, it was discovered that the pressure of the deep sea was compressing the fiber-optic cables that carried light to the spectrometer, causing them to lose much of the Raman signal. By repackaging the optical fibers into an oil-filled pressure compensated housing, MBARI engineers solved this problem, making the instrument ten times more sensitive when operated under pressure. This allowed scientists to detect new chemicals and measure much smaller concentrations of these chemicals than was previously possible.
In 2006 the team took DORISS2 and PUP to an undersea area off Vancouver Island where methane hydrates are exposed on the seafloor. They used the spectrometer to study the composition of these hydrates and to study how the chemical structure of the hydrates changed when they were heated or combined with other compounds such as carbon dioxide.
In coming years, oceanographers will no doubt find many more uses for the laser Raman spectrometer as MBARI researchers further improve and refine their amazing new oceanographic tool.
MBARI contributors to Laser Raman project: Peter Brewer, Clark Brecht, Mark Brown, Danelle Cline, Rachel Dunk, Cheri Everlove, John Ferreira, Rich Henthorn, Scott Jensen, Bill Kirkwood, George Malby, Ed Peltzer, Karen Salamy, Jim Scholfield, Farley Shane, Alana Sherman, Duane Thompson, Peter Walz and Sheri White.
- MBARI laser raman spectrometer web page
- MBARI feature story: The laser raman spectrometer— shedding new light on ocean geochemistry
- Close-up photo of an experiment using the laser raman spectrometer