Application of membrane inlet mass spectrometry for online and in situ analysis of methane in aquatic environments

Abstract

A method is presented for the online measurement of methane in aquatic environments by application of membrane inlet mass spectrometry (MIMS). For this purpose, the underwater mass spectrometer Inspectr200-200 was applied. A simple and reliable volumetric calibration technique, based on the mixing of two end member concentrations, was used for the analysis of CH 4 by MIMS. To minimize interferences caused by the high water vapor content, permeating through the membrane inlet system into the vacuum section of the mass spectrometer, a cool-trap was designed. With the application of the cool-trap, the detection limit was lowered from 100 to 16 nmol/L CH 4 . This allows for measurements of methane concentrations in surface and bottom waters of coastal areas and lakes. Furthermore, in case of membrane rupture, the cool-trap acts as a security system, avoiding total damage of the mass spectrometer by flushing it with water. The Inspectr200-200 was applied for studies of methane and carbon dioxide concentrations in coastal areas of the Baltic Sea and Lake Constance. The low detection limit and fast response time of the MIMS allowed a detailed investigation of methane concentrations in the vicinity of gas seepages. aAm Soc Mass T he analysis of methane as well as other trace gases like carbon dioxide, nitrous oxide, or dimethyl sulphide in aquatic environments is a major objective of basic and applied research. This includes investigations of the air-sea exchange of these greenhouse gases as well as of the release of methane from gassy sediments, hydrocarbon reservoirs, and pipelines, or through dissociation of gas hydrates. Although the spatial extent of such discharge sites is often rather small, ranging from a few square meters to several square kilometers, they are considered major drivers for the marine methane cycle in the present and past [1][2][3][4].The localization of such discharge sites as well as the detection and quantification of trace gases in lakes or the ocean relies essentially on water sampling (e.g., by Rosette Water Samplers, Hydro-Bios, Kiel, Germany) and subsequent chemical analysis by gas chromatography (GC) or infrared-spectrometry (IR) onboard the research vessel. Application of GC as well as IR requires the phase transfer of gases from the dissolved to the gaseous phase. For this purpose, head-space techniques, vacuum-degassing, or spray chambers are applied [5].Head-space techniques and vacuum-degassing are very suitable for analysis of discrete water samples but or N 2 0 in surface waters sampled along transects by research vessels. Nevertheless, the time required for equilibration and subsequent GC or IR analysis is about several tens of minutes to hours [7]. Hence, highresolution investigations, in time and space, of trace gases at natural gas seepages like pockmarks (morphological depressions at the seafloor) or hydrocarbon leakages during surveys by research vessels are rather difficult and time consuming.Compared with such rather long measuring times, application of membrane inlet mass...

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