Quantitation of protactinium, 231Pa in abyssal carbonate

Abstract

Quantitation of protactinium, ²³¹Pa in abyssal carbonates at ∼10⁻¹² g g⁻¹ levels, presents certain challenges because it is the only isotope of this element that is present in the natural environment and its half-life of 32 800 y is too long for useful measurement using alpha-particle or gamma-ray spectroscopy. However, because it is an isotope in the ²³⁵U decay chain, closely linked to the ²³⁸U decay chain, quantitation can be achieved relative to a carbonate standard in secular radioactive equilibrium. The ²³⁸U, ²³⁵U, ²³⁴U, ²³⁰Th and ²³²Th isotopes in the carbonate standard are determined with a mixed ²³⁶U–²²⁹Th spike using isotope dilution protocols and a Multi-Collector Inductively Coupled Plasma Mass Spectrometer.