High Precision and High Sensitivity Measurements of Osmium in Seawater

We present improved chemical separation and mass spectrometric procedures that permit precise and accurate determination of Os concentration and isotopic composition in seawater samples. Water samples (50 mL) with added Os-190 tracer are heated to 300 degrees C at 128 bar in quartz carius tubes with (CrO3)-O-VI. This allows tracer-sample equilibration via complete oxidation of all Os species to OsO4, which is then separated and purified using distillation. Samples are measured using negative thermal ionization mass spectrometry. A new loading technique using face-to-face double-filament geometry is developed that reduces organic interference and improves effective ionization efficiency, which is about 5% for 200 fg of Os (1 fg of Os = 3 x 10(6) atoms). The total yield of the chemical procedure is similar to 90%, and the total procedural blank is 3.6 fg. We applied the new procedure on seawater depth profiles from the Atlantic, Pacific, and Southern Oceans and found that (1) the improved procedure results in an increase in Os concentration of 25-35% over previous methods, (2) Os in seawater does not appear to behave conservatively, and (3) surface seawater has lower Os-187/Os-188 values than deep waters.