HR: 1340h
AN: OS13B-0532    [Abstracts]
TI: Measurement of Relative Dissolved Gas Concentrations Using Underwater Mass Spectrometry
AU: * Bell, R J
EM: rbell@marine.usf.edu
AF: College of Marine Science, 140 Seventh Avenue South, St Petersburg, FL 33701 United States
AU: * Bell, R J
EM: rbell@marine.usf.edu
AF: Center For Ocean Technology, 830 First Street South, St Petersburg, FL 33701 United States
AU: Toler, S
AF: Center For Ocean Technology, 830 First Street South, St Petersburg, FL 33701 United States
AU: van Amerom, F H
AF: Center For Ocean Technology, 830 First Street South, St Petersburg, FL 33701 United States
AU: Wenner, P
AF: College of Marine Science, 140 Seventh Avenue South, St Petersburg, FL 33701 United States
AU: Wenner, P
AF: Center For Ocean Technology, 830 First Street South, St Petersburg, FL 33701 United States
AU: Hall, M
AF: Center For Ocean Technology, 830 First Street South, St Petersburg, FL 33701 United States
AU: Edkins, J
AF: Center For Ocean Technology, 830 First Street South, St Petersburg, FL 33701 United States
AU: Gassig, S
AF: Center For Ocean Technology, 830 First Street South, St Petersburg, FL 33701 United States
AU: Short, R
AF: Center For Ocean Technology, 830 First Street South, St Petersburg, FL 33701 United States
AU: Byrne, R
AF: College of Marine Science, 140 Seventh Avenue South, St Petersburg, FL 33701 United States
AB: The deployment of underwater mass spectrometer (UMS) systems in marine and lacustrine environments has provided chemical data of exceptional temporal and spatial resolution. UMS instruments operate moored, tethered, remotely, or autonomously, allowing users to customize deployments to suit a wide variety of situations. The ability to collect and analyze real-time data enables prompt, intelligent sampling decisions based on observed analyte distributions. UMS systems can simultaneously detect a wide variety of analytes generated by biological, chemical, physical, geothermal and anthropogenic activities. A polydimethylsiloxane (PDMS) membrane separates the sample-stream from the spectrometer's vacuum chamber. This membrane is selective against water and charged species, yet highly permeable to volatile organic compounds (VOC) and simple gases. Current detection limits for dissolved gases and VOCs are on the order of ppm and ppb respectively. Semi-quantitative proof-of-concept applications have included horizontal mapping of gas gradients, characterization of geothermal vent water, and observation of dissolved gas profiles. Horizontal gradients in dissolved gas concentrations were determined in Lake Maggiore, St Petersburg, Florida. The UMS was positioned on a remotely-guided surface vehicle, and real-time gas concentration data were transmitted to shore via wireless ethernet. Real-time observations allowed intensive sampling of areas with strong gas gradients. Oxygen and CO2 exhibited patchy distributions and their concentrations varied inversely, presumably in response to biological activity. The UMS signal for methane depended on the instrument's proximity to organic rich sediments. Geothermal vent water was characterized while the UMS was deployed in Yellowstone Lake, Wyoming, on a tethered Eastern Oceanics remotely operated vehicle (ROV). Observations of dissolved vent-gas compositions were obtained to depths of 30m. Distinct differences in dissolved vent-gas compositions at different sites point to diverse geothermal conditions beneath the lake. Oxygen concentrations were low at most vents, while hydrogen sulfide, methane and carbon dioxide concentrations were highly variable. Dissolved gas depth profiles were obtained using the UMS system in Saanich Inlet, Canada. Due to degradation of organic material, the inlet's deep water is typically anoxic, and rich in methane, carbon dioxide, and reduced sulfur compounds. Relative gas concentrations were obtained between the surface and 200m. A thermocline was detected as the instrument entered anoxic bottom water at 100m. Below this depth oxygen signal intensity declined sharply to background levels. In contrast, carbon dioxide increased sharply below 100m until a reproducible maximum was observed at 120m. Methane and hydrogen sulfide increased steadily with depth below 100 m, and exhibited no local maxima. Fully quantitative UMS measurements require characterization of the influence of salinity, and especially temperature and pressure, on the performance of the internal PDMS membrane. Temperature exerts a strong influence on gas diffusion across the PDMS membrane and the behavior of residual gases in the vacuum chamber; therefore, precise thermostating methods must be adopted. Other technical issues being examined in the laboratory include variations in UMS response attributable to pressure-induced membrane compression, and variable hydrodynamic conditions at the sample/membrane boundary. Experiments are being developed to address the issue of calibrating the ion signal intensity for dissolved gas concentrations.
UR: http://cot.marine.usf.edu/hems/underwater/
DE: 4820 Gases
DE: 4894 Instruments and techniques
DE: 4251 Marine pollution
DE: 4271 Physical and chemical properties of seawater
DE: 4802 Anoxic environments
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting