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-23 4 in seawater is discussed. Samples were collected either from a shore-base d 600 m water source, or from standard bottle casts in deep waters off Hawa ii and the Southern Ocean. We compared large-volume techniques, which rely upon Mn cartridges for the collection of dissolved Th-234 and its detection via gamma counting (> 200-1 samples), with small volume methods that emplo yed either direct beta counting, or beta counting after radiochemical purif ication (2-20-1 samples). Unique to this study is the presentation of small volume (2 and 5 l) Th-234 methods. This new technique is an adaptation of 20-1 methods that are based on the coprecipitation of thorium with Mn oxide s 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 uncertaint ies, (C) 2001 Elsevier Science B.V. All rights reserved.