Determination of Iridium in Natural Waters by Clean Chemical Extraction and Negative Thermal Ionization Mass Spectrometry

Abstract

Methods for the precise, routine measurement of Ir in seawater, riverwater, and estuarine water using isotope dilution negative thermal ionization mass spectrometry (ID-NTIMS) have been developed. After equilibration with a ^(191)Ir-enriched spike, Ir is separated from solution by coprecipitation with ferric hydroxide, followed by anion exchange chromatography using a reductive elution technique. UV irradiation is employed for the decomposition of trace organics, which interfere with negative ion production. IrO_2- 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 these procedures to determine the concentrations of Ir in 4 kg samples from the Pacific Ocean, the Atlantic Ocean, the Baltic Sea, and the rivers supplying the Baltic. Our chemical procedures introduce a total blank of ~2 × 10^8 atoms per sample. The distribution of Ir in the oceans is fairly uniform, averaging ~4 × 10^8 atoms kg^(-1). The concentrations in the rivers supplying the Baltic Sea range from (17.4 ± 0.9) × 10^8 for a pristine river to (92.9 ± 2.2) × 10^8 atoms kg^(-1) for a polluted river. The distribution, speciation, and transport of Ir in natural waters can now be subjected to intensive study.