Conquering the outdoors with on-site mass spectrometry

Abstract

In recent years, mass spectrometers with a membrane inlet separating gases from water for final nalysis have been used successfully for the on-site quantification of dissolved gases in surface waters. I classical' membrane inlet mass spectrometers (MIMS), the membrane directly separates the water from the high-vacuum environment of the mass spectrometer. The gas equilibrium MIMS (GE-MIMS) that is described in this review, however, makes use of an intermediate pressure reduction stage after the membrane inlet. Hence, the gas concentrations after the membrane are at steady state, near solubility equilibrium with the water tobeanalyzed. This setup has several advantages over classical MIMS, which enable autonomous and continuousin-field operation. The GE-MIMS can be used to acquire noble gas concentration time series (NGTS). Noblegases are useful tracers for physical gas exchange and transport in groundwater and other aqueous systems. Hence NGTS enable the temporal dynamics of physical gas exchange and transport in groundwater and other aqueous systems to be investigated. To determine the 2 turnover that has occurred in groundwater since recharge, both the 2 concentration in situ and the total input of 2 to the groundwater since recharge is needed. Determination of the latter is only possible if the relevant physical exchange and transport mechanisms can bequantified. In particular, gas exchange between soil air and groundwater often significantly affects groundwater 2 concentrations. Determination of 2 turnover in groundwater therefore requires a combined analysis of 2 and noble gas concentrations. ©Schweizerische Chemische Gesellschaft.