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Closed-loop water recycle systems for future manned space mission will required sensitive analytical methods to determine the quality of the water and insure the health and safety of the crew. As part of a NASA-funded study, we have been investigating techniques for the measurement of Total Organic Carbon (TOC) in water for use on the Space Station. The prototype system developed employs a membrane-based conductometric CO2 sensor which provides a gravity-independent means for measuring the amount of CO2 produced from the oxidation of organic compounds. Recently, we have been developing a system for the oxidation of organic compounds that does not require the use of chemical oxidizing agents. The reagentless oxidation reactor uses a combination of electrolytic and photolytic oxidation to convert organic compounds to CO2.

Development and testing of a high sensitivity monitor for the measurement of Total Organic Carbon (TOC) in water without gravity-dependent components and using minimal chemical reagents has been performed. A breadboard instrument was constructed and tested for linearity, selectivity, and day-to-day reproducibility. The instrument has a linear response for a wide range of organic compounds over a range from 20 ppb C to 50 ppm C, with excellent day-to-day reproducibility. The upper level can be extended to ∼100 ppm C by changing the operating conditions of the analyzer. The selectivity of the instrument has been determined and no interference is observed except for high concentrations (> 10 ppm) of iodine, hypochlorous acid, sodium nitrite and sodium sulfide.

The development of a high sensitivity, compact monitor for the measurement of total organic carbon (TOC) in water with no gravity-dependent components is discussed. The system is based on a combination of photo-catalzyed oxidation of organic compounds to form carbon dioxide, which is selectively measured using a gas permeable membrane and conductometric detection. This unique combination permits the development of a TOC analyzer with significant advantages over existing methods for TOC analysis including high sensitivity (i.e., detection limits at low parts per billion TOC concentrations), a linear response over a wide range of TOC concentrations (at least four orders of magnitude), long-term stable calibration, compact design, and performance with minimal maintenance for semi-continuous and continuous monitoring capabilities. The results from our preliminary investigations on the development of the TOC monitor are presented.