Evaluation of silica-gel activator for lead isotopic measurements in polar ice and snow at picogram per gram concentrations using TIMS

초록

The analysis of the isotopic compositions of lead (Pb) at the picogram per gram (1pg/g = 10-12 g/g) level in snow and ice from polar region remains as the technical challenges. Ultra-clean procedures have to be used to minimize contamination problems and to accurately determine Pb isotopic compositions at the very low Pb concentration level. In the case of the analysis of Pb isotopes using thermal ionization mass spectrometry (TIMS), ionization efficiency can significantly vary depending on sample loading on the filament with silica-gel activator. Using a simple and low contamination procedure, Gerstenberger and Haase (1997) improved the silica-gel emitter by preparing silica-gel with a colloidal silicic acid produced by Merck, However, a colloidal silicic acid from Merck is no longer available for purchase. Another ion emitter, which is produced by the hydrolysis of silicon tetrachloride (SiCl4), has been used for the analysis of Pb isotopes at Curtin university in Australia. However, the silicon tetrachloride has high reactivity that makes its handling very difficult. Miyazaki et al. (2005) made silica-gel emitter using PL-7 produced by Fuso chemical, but there is no data for blank of PL-7. In this study, we report the procedural blank when silica-gel emitter of PL-7 is used for the analysis of Pb isotopes. The amount of Pb contamination introduced during the whole sample treatment with reagents is 0.5±0.2 pg Pb. This value was determined by isotope dilution mass spectrometry (IDMS) using 205Pb spike. The bias introduced into the isotopic ratios by evaporation of the sample and other unknown factors was determined by repeated measurements of 50~100 pg of the NIST SRM 981. Based on the 207Pb/206Pb ratio, the correction is observed to be 0.22±0.24% per mass unit.