DETERMINATION OF IRIDIUM IN NATURAL-WATERS BY CLEAN CHEMICAL-EXTRACTION AND NEGATIVE THERMAL IONIZATION MASS-SPECTROMETRY

Methods for the precise, routine measurement of Ir in seawater, riverw ater, and estuarine water using isotope dilution negative thermal ioni zation mass spectrometry (ID-NTIMS) have been developed. After equilib ration with a Ir-191-enriched spike, Ir is separated from solution by coprecipitation with ferric hydroxide, followed by anion exchange chro matography using a reductive elution technique, UV irradiation is empl oyed for the decomposition of trace organics, which interfere with neg ative ion production, IrO2- ions are produced in the mass spectrometer by heating the sample on a Ni-wire filament in the presence of Ba(OH) (2). Detection efficiencies ranged from 0.1% to 0.3%. We have used the se procedures to determine the concentrations of Ir in 4 kg samples fr om the Pacific Ocean, the Atlantic Ocean, the Baltic Sea, and the rive rs supplying the Baltic, Our chemical procedures introduce a total bla nk of similar to 2 x 10(8) atoms per sample, The distribution of Ir in the oceans is fairly uniform, averaging similar to 4 x 10(8) atoms kg (-1). The concentrations in the rivers supplying the Baltic Sea range from (17.4 +/- 0.9) x 10(8) for a pristine river to (92.9 +/- 2.2) x 1 0(8) atoms kg(-1) for a polluted river, The distribution, speciation, and transport of Ir in natural waters can now be subjected to intensiv e study.