Membrance interface evaluations for underwater mass spectrometers.

A component that has enabled the development of underwater mass spectrometry is a mechanically supported membrane interface probe. Our two research groups have used metallic porous frits that support polydimethyl siloxane (PDMS) membranes embedded in a heated membrane probe assembly, allowing the deployment of the underwater membrane introduction mass spectrometer (MIMS) instruments to ocean depths of 2000 meters. The fabrication of such frits has consisted of shaping larger Hastalloy C porous frits to the size required to support a PDMS capillary of 0.64 mm ID and 1.19 mm OD using a diamond‐coated wheel and Dremel tool. This procedure is time‐consuming and cumbersome, and the porosity of the final frits is likely not reproducible. To facilitate the fabrication of the membrane assembly, we report on the use of new porous metallic structures. Frits with diameters of approximately 3.0 mm (1/8”) and known porosities (48.3 % and 32.5%) were produced by the Fraunhofer Institute in Dresden, Germany, using powder metallurgical processes. We used these frits to fabricate new membrane interface assemblies. Using a new custom‐heated membrane probe with the new porous frits, we performed calibrations relating dissolved methane concentrations to mass spectrometer response (m/z 15) using linear least‐squares fitting procedures. Both the limit of detection (methane concentration in the tens of nanomolars) and the sensitivity (on the order of 10‐1 pico‐amps/nanomole of methane) were found to be comparable with those obtained with the previously fabricated Hastelloy C frits. The calibration parameters for the new assembly were also found to be a function of the flow rate, temperature, and sample hydrostatic pressure

Shell Malformation in Stressed \u3cem\u3eAmphistegina\u3c/em\u3e Populations: Relation to Biomineralization and Paleoenvironmental Potential

Beginning in summer 1991 and continuing through 1996, Amphistegina populations in the Florida Keys and elsewhere have exhibited symptoms of stress. Previous studies have reported progressive loss of symbiont color in most adult individuals during spring and summer months. In addition, commonly 15-35% of living individuals in afflicted populations have malformed tests that are chipped, broken, or broken and repaired, in contrast to approximately 5% of the living individuals in populations sampled during the 1970\u27s and 1980\u27s. This study illustrates observed malformations including test breakage, uneven external surfaces, abnormal shapes, bioerosion, and loss of outer chambers. Internal anomalies include poorly defined pore cups, excessive calcification, and minimal organic matrix. Previous cytological studies of these stressed Amphistegina revealed loss of Golgi apparatus and endoplasmic reticulum. Because these organelles are sites of glycoprotein and glycosaminoglycan synthesis, we postulate that reduced synthesis of these major organic matrix components of the foraminiferal shell may account for the observed biomineralization anomalies. Documentation of these anomalies can provide the basis for determining if similar stress events occurred in the fossil record

Mg/Ca Ratios in Stressed Foraminifera, \u3cem\u3eAmphistegina gibbosa\u3c/em\u3e, from the Florida Keys

Since 1991, significant proportions of Amphistegina populations in the Florida Keys and elsewhere have exhibited stress symptoms that include loss of symbiont color (‘bleaching’), anomalous shell breakage and reproductive damage. Previous studies of other taxa have reported elevated Mg/Ca ratios in tests from pollution-stressed foraminiferal populations. The purpose of this study was to test the hypothesis that anomalous shell breakage in stressed Amphistegina gibbosa is the result of loss of control of calcification, resulting in elevated concentrations of Mg that weaken the crystal structure of the test. Analysis of Mg and Ca concentrations in A. gibbosa tests, using an Inductively-Coupled Plasma Mass Spectrometer, revealed normal Mg/Ca (2–5 mol%) in all specimens analyzed, including normal specimens collected in 1982 (prior to the onset of the stress event), and both normal and broken specimens collected quarterly from afflicted populations in 1996. Analysis of specimens from the high Mg calcite taxon, Archaias angulatus, revealed Mg/Ca of 10–14 mol%. This study, which presents an ICP-MS procedure that can be used to assess Mg/Ca in individual foraminifera, does not support the hypothesis that shell breakage in stressed Amphistegina results from disruption of calcification at the ionic level

In Situ Determination of Porewater Gases by Underwater Flow-through Membrane Inlet Mass Spectrometry

An underwater membrane introduction mass spectrometer was deployed in permeable sandy sediment on the Georgia continental shelf (depth = 27 m) to measure in situ dissolved gas concentrations in sediment porewaters. Over a 54-h period, 30 profiles (up to 18 cm deep) were sampled using an automated sediment probe coupled with an underwater positive displacement syringe pump. Porewater was analyzed with a flow-through membrane assembly at constant sample flow rate (0.35 mL/min) and membrane temperature (45°C). Calibration was performed on-site using ambient seawater equilibrated with gas standards. Measurements of methane, nitrogen, argon, oxygen, and carbon dioxide concentrations were used to produce depth-time contours and demonstrate the dynamics of dissolved gases in the porewater. Profiles indicated a well-oxygenated surface layer (1 to 2 cm depth) and anoxia below −3 to −5 cm. Elevated concentrations of methane below the oxycline reveal active methanogenesis in shelf sands despite low (0.05%) organic carbon content. Chemocline depth and sediment ripple height were correlated, suggesting that the porewater environment is controlled by advection-driven interactions between boundary-layer flow and bottom topography. By coupling in situ concentration profiles to independent estimates of sediment-water exchange, it was estimated that maximal oxygen consumption at this site occurs \u3e 2 cm below the interface. Oxygen consumption at this site is estimated as 2.3 mmol m−2 d−1 based on combined dissolved profiles and advection estimates. Raw data and data analysis scripts (Matlab) are available electronically in a Web Appendix

Field-deployed Underwater Mass Spectrometers for Investigations of Transient Chemical Systems

The mass spectrometer developments and underwater deployments described in this work are directed toward observations of important reactive and influential inorganic and organic chemicals. Mass spectrometer systems for measurement of dissolved gases and volatile hydrocarbons were created by coupling a membrane analyte-introduction system with linear quadrupole and ion trap mass analyzers. For molecular masses up to 100 amu, the in situ quadrupole system has detection limits on the order of 1–5 ppb. For masses up to approximately 300 amu, the underwater ion trap system detects many volatile hydrocarbons at concentrations below 1 ppb. Both instruments can function autonomously or via interactive communications from a remote control site. Continuous operations can be sustained for up to approximately 12 days. Deployments have initially involved shallow water proof-of-concept operations at depths less than 30 m. Future modifications are planned that will allow operational depths to 200 m