ABSTRACT

We report the origin of significant fractionation of Pb isotope ratios in Ca-doped 0.08 mol L^–1 HNO₃ solutions. We applied Tl-spiked standard bracketing mass fractionation correction using solution-based multiple collector-inductively coupled plasma mass spectrometry. The addition of matrix elements into the NIST SRM 981 Pb standard solution resulted in strong isotope fractionation of up to –150 ppm per AMU (atomic mass unit), when Ca was doped at 200 times the molar concentration of Pb. In contrast, only +20–60 ppm per AMU fractionation occurred upon addition of the same amounts of Mg, Al, or Fe. Strong fractionation by Ca only occurred when doping into a test tube containing Pb. In contrast, insignificant fractionation (+20–60 ppm) occurred when Ca was mixed immediately before sample nebulization. The results suggest that strong Pb isotope fractionation was probably due to precipitation of a Pb-Ca complex in the test tube, which is insoluble in the sparse 0.08 mol L^–1 HNO₃. Mixing of Ca immediately prior to sample nebulization prevented precipitation and caused a small (+22 ppm) positive fractionation, similar to that seen for other major elements. A small amount of fractionation was common in major-element-doped samples. This suggested different degrees of isotopic fractionation of Tl and Pb, perhaps by the space charge effect. In order to test the effect of Ca-Pb precipitation in practical sample analysis, a conventional anion exchange chromatographic separation of Pb after HF-HNO₃ sample digestion was applied to 200–500 × Ca-doped SRM 981 and a large amount of Ca-doped basalt standard of JB-2 samples. No isotopic fractionation was observed, showing decomposition of the Ca-Pb precipitate by strong acid attack and during column chromatography. The results suggest the negligible effect of Pb isotope fractionation, even in Ca-rich samples after acid digestion.