A method for 231Pa analysis by thermal ionization mass spectrometry in silicate rocks

We describe a new technique for analyzing 231Pa in silicate rocks by isotope dilution mass spectrometry. This technique is different from the technique developed by previous studies (Pickett et al., 1994) and also permits the determination of down to a 100 fg of 231Pa with a 1–2% uncertainty at the 2σ level. While its performance is comparable to the LANL technique (Pickett et al., 1994), a number of modifications may help extend the application of 231Pa–235U to a wider number of geochemistry labs: (1) this technique does not require 237Np, a hazardous α-emitter for 233Pa spike preparation, (2) Pa is run as a double-oxide on a tungsten filament with a sensitivity comparable to the metal method.