DEVELOPMENT OF A MASS SPECTROMETER FOR MEASUREMENTS OF DISSOLVED NOBLE GASES IN GROUNDWATER

A mass spectrometric system was developed to measure quantities of dissolved fixed and noble gases (N_2, O₂, He, Ne, Ar, Kr, and Xe) in groundwater samples. Measurements of dissolved gases in groundwater are used to estimate quantities of excess air trapped during recharge, recharge temperature, amounts of degassing, and many other applications in the hydrologic sciences. These values are particularly critical for determining apparent groundwater age based on measured amounts of dissolved gas tracers such as CFCs and SF₆. While it is possible to obtain reliable results for excess air and recharge temperature using gas chromatography based N₂-Ar measurements, the determinations can be easily confounded by processes that enhance dissolved N₂ or Ar after recharge (for instance, microbial denitrification, artificial addition of gases from drilling, or injection of specific gases as hydrologic tracers). Measurements of fixed and noble gases can overcome these difficulties by providing alternative determinations, and additionally giving insight into He accumulation, gas entrapment during recharge, and quantification of degassing processes. Measuring this array of compounds in a sample presents several challenges, as their relative abundances range over seven orders of magnitude and their limited reactivity restricts the avenues available to quantitative isolation and measurement. Several mass spectrometry-based analytical techniques have been applied by different research groups to measure dissolved fixed and noble gases. These techniques typically involve different combinations of reactive getters and cryogenic traps to separate subsets of gases for analysis. These different approaches carry different benefits and tradeoffs with regard to precision, speed, cost, and effort of analysis. Our instrument is designed to provide fast and low-cost analysis of dissolved noble gases. We employ a sequence of getters and liquid nitrogen traps to separate the different gases, and leverage automation to increase precision, and employ measurements from manometers and a quadrupole mass spectrometer as detectors. Critical design considerations including the automation scheme, sample handling, sensitivity, and precision will be presented.