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

We describe a new technique for analyzing Pa-231 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 Pa-231 with a 1-2% uncertainty at the 2 sigma level. While its performance is comparable to the LANL technique (Pickett et al., 1994), a number of modifications may help extend the application of Pa-231-U-235 to a wider number of geochemistry labs: (1) this technique does not require Np-237, a hazardous alpha-emitter for Pa-233 spike preparation, (2) Pa is run as a double-oxide on a tungsten filament with a sensitivity comparable to the metal method. (C) 1999 Elsevier Science B.V. All rights reserved.