Analysis of osmium in seawater presents complex challenges, linked to its very low (femtomolar) concentrations and multiplicity of possible oxidation states. Early insights were provided by Karl Turekian's group at Yale where it was realized that osmium tends to concentrate both in oxidizing Fe-Mn nodules and in reducing organic-rich marine sediments. Efforts to directly measure the seawater osmium isotope composition and concentration began in earnest following the developments in early 1990s of highly sensitive N-TIMS and ICP-MS. Initial techniques that attempted to pre-concentrate osmium using column chromatography (Minoru Koide and collaborators at Scripps Institution of Oceanography) and co-precipitation (Mukul Sharma and collaborators at Caltech) were only partially successful due a lack of equilibrium between seawater and tracer osmium. A breakthrough came in 1998, when Sylvain Levasseur in Claude Allegre's group in Paris simultaneously oxidized and pre-concentrated osmium in liquid bromine at 90°C. Oliver Woodhouse and coworkers at the Woods Hole Oceanographic Institution developed another procedure of directly distilling osmium from seawater and sparging it into an ICP-MS. These procedures appeared robust but yielded conflicting results. Subsequent work at Dartmouth (Sharma and collaborators) and Nancy (Maxence Paul and collaborators) has demonstrated that much higher temperatures and longer durations are required to fully equilibrate sample and tracer osmium. The complexities involved in storage of seawater osmium have also become apparent (see link to Eos report below). These findings resulted from U.S. National Science Foundation funded GEOTRACES intercalibration efforts in the Pacific and Atlantic oceans. The new insights call into question much of the earlier data on the marine distribution of this important biogeochemical tracer and raise new issues: How actively is osmium cycled in the water column? What is the relative importance of the various sources? How important are anthropogenic inputs? The workshop on "Dissolved Osmium Isotope Analysis" held at the Palais de Congrès de Montreal on 24 June 2012 before the annual Goldschmidt Conference summarized the latest recommendations for successful seawater osmium analyses.
Peucker-Ehrenbrink, B., M. Sharma, and L. Reisberg (2013), Recommendations for Analysis of Dissolved Osmium in Seawater, Eos Trans. AGU, 94(7), 73. Click here to access the Eos Workshop report.