DETERMINATION OF FEMTOGRAM QUANTITIES OF PROTOACTINIUM IN GEOLOGIC SAMPLES BY THERMAL IONIZATION MASS-SPECTROMETRY

We describe a procedure for measurement of Pa-231 in geologic samples by isotope dilution thermal ionization mass spectrometry, using Pa-233 as a spike isotope, which provides marked improvements in precision and sample size relative to established decay counting techniques. This method allows determination of as little as a few tens of femtograms of Pa-231 (approximately 10(8) atoms) with a conservative estimated uncertainty of +/-1% (95% confidence level). Applications of Pa-231-U-235 systematics to uranium-series geochemistry and geochronology should be greatly enhanced by this approach.