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

We describe a new technique for analyzing Pa-231 in silicate rocks by isoto pe dilution mass spectrometry. This technique is different from the techniq ue developed by previous studies (Pickett et al., 1994) and also permits th e determination of down to a 100 fg of Pa-231 with a 1-2% uncertainty at th e 2 sigma level. While its performance is comparable to the LANL technique (Pickett et al., 1994), a number of modifications may help extend the appli cation of Pa-231-U-235 to a wider number of geochemistry labs: (1) this tec hnique 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.