Continuous monitoring of dissolved gas concentrations in groundwater using a quadrupole mass spectrometer

An apparatus has been developed for fully automated, continuous measurement of the concentration of selected gases in groundwater using a quadrupole mass spectrometer. The apparatus has several advantages over conventional methods: (1) since a gas-permeable microporous film is attached to the head of a sampling tube, gases dissolved in groundwater are separated from water by putting the sampling tube into water; (2) by continuously introducing gases separated from water into an on line mass spectrometric system, we have successfully made isotopic measurements such as 40Ar/36Ar in groundwater on a continuous basis; and (3) very short intervals of measurement, less than several minutes, are attained by switching the electric voltage of the mass spectrometer discontinuously from peak to peak of arbitrarily chosen gas species. Some experiments are described to evaluate the basic performance of the apparatus such as the reproducibility in repeated measurements, dependence on the total amount and chemical composition of gases, and the response time to a sudden change of dissolved gas contents. An example is presented of observation of distilled water, which demonstrates that the new apparatus is sufficient for continuous monitoring of selected dissolved gases in water for various purposes.