Precise Re isotope ratio measurements by negative thermal ionization mass spectrometry (NTI-MS) using total evaporation technique

High precision rhenium isotope ratios, Re-187/Re-185, have been determined by negative thermal ionization mass spectrometry (NTI-MS) using a total evaporation technique. The salient features of this method are evaporation of the entire sample and simultaneous integration of the signal from each isotope, which effectively eliminates isotope fractionation effects during the evaporation process. The Re-187/Re-185 ratio is obtained with a high reproducibility (1.6755 +/- 0.0014 (2sigma), R.S.D. = 0.083%, n = 28) for 50 pg-1 ng of a Re natural standard using the total evaporation with NTI-MS. This value is within analytical uncertainty of the previously reported accurate Re-187/Re-185 ratio (1.6740 +/- 0.0011) adopted by IUPAC as the Re isotopic composition, and is significantly more precise than the ratio obtained from conventional NTI-MS isotopic measurements in our laboratory (1.6772 +/- 0.0037 (2sigma), R.S.D. = 0.22%, n = 34). Based on these results, the total evaporation technique allows us to precisely determine Re isotope ratios, even for small sample amounts. In addition, this method is effective for highly precise Re abundance determinations using isotope dilution. (C) 2004 Elsevier B.V. All rights reserved.