Methods for onboard monitoring of silver biocide during future human space exploration missions

Silver ions (Ag+) have been proposed as a biocide to treat the water in NASA's next generation of human space exploration vehicles/habitats. One advantage of Ag+ is that it is effective as a biocide in a range (200 to 500 ppb) safe for human consumption. So, monitoring Ag+ is essential to ensure the safety and health of the crew. Here we present two analytical methods based on capillary electrophoresis and capacitively coupled contactless conductivity detection (CE-C4D) to address the need to monitor Ag+ levels in water. Using 5 M acetic acid as a background electrolyte (BGE), 100 ppb of Ag+ could be detected in a simulant of the International Space Station (ISS) water. In addition to Ag+, other inorganic cations (K+, Ca2+, Na+, Mg2+, Ni2+, and Zn2+) frequently found in the ISS potable water can be detected simultaneously using this BGE in less than 4.5 min. By using a BGE composed of 0.5 M acetic acid, levels of Ag+ as low as 25 ppb could be detected in the ISS water simulant in less than 2.5 min. Although in this condition none of the other cations interfered with the detection of Ag+, some of them co-migrated, which could prevent obtaining additional information about the sample composition.