Membrane Inlet Mass Spectrometry, vertical O2/Ar profiles

Methods

Membrane Inlet Mass Spectrometry

Sample seawater was pumped aboard ship via tubing with one end fixed to a cable being lowered to progressively deeper depths. A custom-made MIMS cuvette consisted of a stainless-steel baseplate in which a frit inlay (gas permeable 50 µm PTFE), overlaid with a 10 µm PTFE membrane (RTC Reichelt Chemietechnik GmbH, Germany) and a flow through cuvette which transported water along the membrane. To avoid large temperature changes, a heating coil was wrapped around the MIMS cuvette which kept the stainless-steel base plate at 45 °C. For calibration purposes, a 4 L polycarbonate bottle of filtered seawater was equilibrated continuously using an aquarium air pump. The calibration bottle was placed in the flow-through water intake reservoir to keep water temperatures close to the environmental temperatures. Every 4 hours, automated solenoid valves switched the water intake from the flow through to the air equilibrated water which was subsequently measured by the MIMS. A calibration step lasted 25 minutes after which the system changed back to the seawater intake. These automatic calibrations were used for the atmospheric O2/Ar ratio baseline correction. Subsequent to each calibration, the calibration water-bottle was automatically topped off with filtered water from the seawater intake using a peristaltic pump. Equilibration with room air for >3 hours and constant monitoring of the O2 concentration using O2 optodes (Firesting, P2 optodes) ensured that the water was equilibrated. For further details, see Tortell, 2005 and Kaiser et al., 2005.