Membrane Inlet Mass Spectrometer for the Quasi-Continuous On-Site Analysis of Dissolved Gases in Groundwater

We developed a stand-alone system based on a membrane inlet mass spectrometer (MIMS) for measuring dissolved gas concentrations in groundwater under field conditions. The system permits the concentrations of dissolved gases (He, Ar, Kr, N₂, and O₂) in groundwater to be determined quasi-continuously (every 12 min) with a precision of better than 4% for He and Kr, and with a precision of 1% for Ar, N₂, and O₂ in air-saturated water. The detection limits are below 3 × 10^–9 cm_STP³/g for the noble gases and below 400 × 10^–9 cm_STP³/g for N₂ and O₂. The results of a first deployment of the system in the field indicate that changes in the concentration of Ar that result from diel fluctuations of 3 °C in the river water temperature were still able to be resolved in groundwater, although the corresponding temperature signal almost vanished