Ultra-sensitive determination of Os isotopes at the femto-gram per gram level in Greenland snow samples

Depth variations in the ¹⁸⁷Os/¹⁸⁸Os ratio in polar snow and ice can quantitatively elucidate changes in the accretion rate of extraterrestrial matter over the last hundreds of thousands of years or more. However, such investigations still remain an analytical challenge, because the extremely low concentrations of Os to be measured, down to the sub-femto gram per gram level, make precise and accurate measurements of ¹⁸⁷Os/¹⁸⁸Os ratios very difficult.

Here we present highly sensitive and ultra-clean analytical procedures to achieve reliable determination of ultra-low Os concentration ( 10^-15 g g^-1) and isotopic composition in polar snow and ice. Each aliquot ( 50 mL) with ¹⁹⁰Os tracer solution was heated to 300 ℃ at 100 bar in quartz-glass reaction vessel with a mixture of H₂O₂ and HNO₃. This allows all Os species to be completely oxidized to volatile OsO₄ via sample-tracer equilibration. The bulk of resulting OsO₄ in aqueous solution was then separated and purified using distillation. We performed detailed and reliable blank determinations for the successive steps of the analytical procedures, which is essential to obtain the reliable measurement of Os concentrations and isotope ratios at extremely low concentrations levels. All samples were measured using negative thermal ionization mass spectrometry (N-TIMS) (TRITON, Thermo Scientific) under class 10 clean conditions. We present here preliminary results of the measurement of Os concentrations and its isotopes in Greenland snow. Our ultra-clean and sensitive methods offer a promising way to simultaneously determine both Os concentration and isotopic composition in polar snow and ice.