Separation and measurement of Pa, Th and U isotopes in marine sediments by microwave-assisted digestion and MC ICP-MS

Abstract

This manuscript describes a new protocol for determination of Pa/Th/U in marine sediments. Based on microwave-assisted digestion, it represents an important reduction of working time over conventional hot-plate digestion methods, and the use of HClO4 is avoided. While Th and U are completely dissolved with a first microwave step, around 40% of 231 22 Pa remains un-dissolved and a short hot-plate step with reverse aqua regia is required to achieve total digestion and spike equilibration. Next, the method involves a separation of these elements and a further purification of the Pa fraction using Dowex AG1- X8 resin. Separation with Bio-Rad and Sigma-Aldrich resins was compared; while both perform similarly for Th and U, Pa yields are higher with Bio-Rad. Finally, samples are measured using a Nu instruments Multiple Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS). Overall chemical yields range around 50% for Pa, 60% for Th and 70% for U.