Adsorbers for in-situ collection and at-sea gamma analysis of dissolved Thorium-234 in seawater

Abstract

Two polypropylene cartridge types (Beta Pure and Hytrex II) were tested in the laboratory as adsorbers for in-situ collection of dissolved Thorium-234 (234TH) in seawater. Using a uranyl nitrate tracer, we determined that a Mn02 impregnated 3.25-inch Hytrex II cartridge with a flow rate of 8 liters/minute would collect 234TH with a greater than 60% effciency. The smaller size and composition of the 3.25 inch Hytrex II cartridge enabled it to be pressed into a permanent 1-inch "puck" for direct gamma counting. This protocol significantly reduced the handling between collection and counting. When field tested in the Gulf of Maine as a large volume (>500 liters) collector, the new adsorbers produced a greater than 80% collection effciency and a dissolved 234TH concentration which was consistent with independent samples collected at the same station and depth. These adsorbers were used successfully for the in-situ collection of 234TH in large volumes of seawater during the 1992 NOAA and NSF sponsored JGOFS EqPac program, with a mean collection efficiency of 0.79 +/- 9% (n=104 cartridge pairs).