An intercomparison of small- and large-volume techniques for thorium-234 in seawater

In this paper, an intercomparison of methods for the determination of Th in seawater is discussed. Samples were collected either from a shore-based 600 m water source, or from standard bottle casts in deep waters off Hawaii and the Southern Ocean. We compared large-volume techniques, which rely upon Mn cartridges for the collection of dissolved Th Ž . and its detection via gamma counting )200-l samples , with small volume methods that employed either direct beta Ž . counting, or beta counting after radiochemical purification 2–20-l samples . Unique to this study is the presentation of small Ž . 234 volume 2 and 5 l Th methods. This new technique is an adaptation of 20-l methods that are based on the coprecipitation of thorium with Mn oxides followed by direct beta counting of the precipitate. The small volume Mn coprecipitation methods were found to be superior to other methods due to ease of sample collection, processing and low overall analytical uncertainties. q 2001 Elsevier Science B.V. All rights reserved.