Abstract
Samples for lead isotope measurements are generally loaded with silica-gel as the activator, and phosphoric acid. Then, the silica-gel, sample mixture is fused into glass on the filament by passing a high current for a short instant before mass-spectrometer analysis. Here, we use a timer to control the electric power supply for the sample loading unit thus insuring a constant condition for loading. The ion beam thus obtained was found to be more stable than the conventional method. No significant change of isotopic composition was observed within a single run under this condition. The overall range of mass fractionation is insignificant for a sample size between 250 and 25 ng. Repeated analyses (n = 15) of NBS SRM-981 gave the following grand means and reproducibility: 206Pb/204Pb = 16.8908 ± 0.0026 (1σ SD); 207Pb/204Pb = 15.4285 ± 0.0025; 208Pb/204Pb = 36.4982 ± 0.0066.
