CONTINUOUS MONITORING OF DISSOLVED-GAS CONCENTRATIONS IN GROUNDWATER USING A QUADRUPOLE MASS-SPECTROMETER

An apparatus has been developed for fully automated, continuous measur ement of the concentration of selected gases in groundwater using a qu adrupole mass spectrometer. The apparatus has several advantages over conventional methods: (1) since a gas-permeable microporous film is at tached 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 measure ments such as Ar-40/Ar-36 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 di scontinuously from peak to peak of arbitrarily chosen gas species. Som e experiments are described to evaluate the basic performance of the a pparatus such as the reproducibility in repeated measurements, depende nce on the total amount and chemical composition of gases, and the res ponse 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. (C) 1997 Elsevier Scien ce Ltd.