Development and Validation of a Field-Deployable Optical Spectrometer for Real-Time, Natural Abundance Measurements of O and H Isotopes of Water

Water isotopes provide critical knowledge about the history and age of water supplies, degree of water mixing, location of water recharge, and velocity of ground water flow. Current technology involves collecting individual water samples, transporting them to a laboratory, and quantifying the isotope ratios via an Isotope Ratio Mass Spectrometer (IRMS). For water vapor samples, the issue is further complicated by the need to first freeze and preconcentrate the sample prior to analysis. In practice, this procedure has made long-term continuous monitoring of water supplies prohibitively time-consuming. We will report on the development and independent validation of a novel water isotope ratiometer based on cavity-enhanced laser absorption spectroscopy techniques (Off-Axis Integrated Cavity Output Spectroscopy) to quantify 18O/16O, 17O/16O, and D/H to better than 0.3 per mil, 0.5 per mil, and 2 per mil respectively. The analyzer provides continuous monitoring of either ambient water vapor (without preconcentration) or repeatedly injected liquid samples. Independent validation of the instrument's capabilities will be presented and results from recent field tests will be discussed.