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Deep-Ocean Mass Spectrometer

Fig. 1.  Schematic diagram of deep-ocean mass spectrometer (DOMS) layout.

The entire system fits within a 6.5 inch outside diameter pressure housing that is approximately five feet long.  It consists of a 1 to 200-amu range quadrupole mass spectrometer equipped with Faraday and electron multiplier detectors, compact turbo-molecular and backing diaphragm vacuum pumps, internal rechargeable batteries, and internal waste vacuum chamber.  Sample routing past the MIMS (Membrane Introduction Mass Spectrometry) is accomplished by computer-controlled solenoid valves.  We designed the pressure housings of both 6AL4V and type 2 titanium alloys that are rated to working depths of >4000 m and are essentially corrosion proof over long-term deployments.  We designed and integrated a fail-safe valving system for both rapid response to high-pressure MIMS failure and a pressure-switch circuit and high-pressure solenoid valve to detect and protect against slow leaks of the MIMS.  To route sample waters to the MIMS-based instrument, we also designed and built a rugged plastic plenum that couples to the face of the sampler head, the latter of which consists of the MIMS inlet and a full-ocean rated thermistor temperature probe, with operational range from -5 to 50 degrees C.

Fig. 2.  A typical 1-50 amu mass spectrum of the linear quadrupole mass spectrometer employed.  The spectrum was obtained in a closed system, and represents residual air gases plus those from vacuum wall outgassing.

We are currently bench testing the response of the system to various standard gas saturation partial pressures in water, including helium, nitrogen, argon, methane and propane.  For this work we are using a nearly-identical, bench-type quadrupole mass spectrometer and a custom-built MIMS for bench work.  The goals are to determine instrument sensitivity, background, clearing time or sample hysteresis, and the effect, if any, of mass/charge ratios upon these parameters.  Determination of varying temperature and pressure effects upon the MIMS and acquired spectra are also planned.

Fig. 3.  Photograph of instrument chassis, showing membrane introduction mass spectrometry (MIMS) sample inlet (near bottom) and full-ocean-rated thermistor temperature probe, in titanium front end cap.  Open hole is nitrogen purge pathway (unplugged).  Sample plenum (black) is resting on bench to left.


Fig. 4.  Photograph of DOMS instrument chassis with most system components installed, October, 2004.  When completed, the all-titanium system will be deployable to 4000 m water depth.


We have also designed and constructed a shallow-water or corrosive gas-resistant terrestrial housing for the mass spectrometer system, using similar designed but thinner-walled titanium alloy (grade 2). We plan to deploy the instrument in sulfatara fields to test its capability of long-term monitoring of volcanic gases (chiefly water vapor, carbon dioxide, sulfur dioxide, hydrogen sulfide, hydrogen and helium, with or without air gases).  We will initially deploy the instrument on Kilauea in early 2005 (in collaboration with USGS-HVO staff), and on active land volcanoes in Costa Rica (with project collaborators David Hilton, Kevin Brown and Geoff Wheat), in conjunction with our scheduled ALVIN dive program to long-term monitor cold seeps off the Costa Rica active margin beginning in June, 2005.  We presented our initial results in a paper contributed to the ORION special session at the fall meeting of the American Geophysical Union in December (Bossuyt & McMurtry, 2004).


Fig. 5.  Schematic diagram of planned deployment on cold seeps at 2000 m water depth off Costa Rica in June, 2005.



Prof. Gary M. McMurtry
Department of Oceanography
School of Ocean and Earth Science and Technology
University of Hawaii, Manoa


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