Deep sea detection

Scientists and engineers at the

Center for Ocean Technology (COT) are developing equipment to detect dissolved gasses and volatile organic compounds in the deep marine environment. Using specialised mass spectrometry equipment in a protective underwater housing, COT’s Tim Short and colleagues are able to measure compounds at concentrations less that one part per billion.

“Mass spectrometry (MS) is a powerful tool for detecting a wide variety of compounds in very small quantities and at extremely low concentrations,” said Short.

Underwater MS can monitor a number of scientifically important gases, such as methane, oxygen and carbon dioxide as well as a range of harmful compounds such as benzene, chloroform and toluene. In addition, analysis in the field has important advantages over sample collection and analysis in the lab.

“On-site analysis lowers the risk of sample loss, contamination and degradation,” Short explained. “The quicker we can analyse a sample the better. Being able to analyse continuously in real-time is optimal for hazardous situations. Furthermore, field analysis greatly increases measurement rates and reduces cost per sample."

According to Short, mass spectrometers contain a vacuum within which individual electrically charged molecules (ions) can be analysed. When sample molecules enter the vacuum chamber through a membrane filter they are electrically charged and then sorted by their mass-to-charge ratio. The analysis produces a graph showing a spectrum of ion intensities for each sample. From this information the molecular structures of compounds can be identified.

Originally focused on shallow water environments, Short and colleagues are currently developing systems that can go to depths greater than half a mile. Changes in the membrane interface design have increased the depths at which they deploy the mass spectrometer and recent tests have indicated that eventually the MS instrument will be able to reach full ocean depths.

“We will soon be able to go to depths of three miles,” he predicted. “In the course of development, we first had divers take the instruments to 60 feet off the coast of Fort Meyers to study the gas composition of some hydrothermal seeps."

Next, the research group collected MS depth profiles to 600 feet in an inlet in Canada and, more recently, to 1140 feet in the Gulf of Mexico.

“We also used a remotely operated vehicle to take the MS equipment 240 feet down in Lake Yellowstone,” explained Short.

With MS equipment mounted onto an autonomous underwater vehicle (AUV), Short says a number of practical goals come into sharper focus. The environmental benefits of knowing what chemicals are down there, their concentrations, and where they go, are invaluable.

“Taking MS analysis into greater depths with roving vehicles has some very practical scientific and environmental purposes,” said Short.

For example, deep-water hydrothermal vents can be studied using MS and marine deposits of methane gas can be detected. In some cases, underwater methane can be hazardous because it can destabilise the seafloor, but if scientists learn how to convert methane hydrates – ice-like deposits of methane gas formed in low temperatures and under great pressure – into usable fuel, seeking out methane deposits using underwater MS might be another “gold rush.”

“Applications for on-site MS at extreme depths are varied and have potentially immense scientific and economic benefits,” speculated Short.