Optimization of mixed Pb–Tl solutions for high precision isotopic analyses by MC-ICP-MS

Abstract

Measurements of SRM 981 Pb isotope standard mixed with Tl for mass bias corrections were conducted with a MC-ICP-MS (Nu Plasma) using a Nu Instruments desolvating nebulizer. During the initial experiments, Pb and Tl from single-element, concentrated stock solutions were mixed in 2% HNO₃ prior to isotope ratio measurements. The results revealed relatively poor precision and accuracy of the Pb isotope measurements, large variations in ε²⁰⁵Tl in the same standard (ranging from −3.9 to +30.1), and large variations in the observed Pb/Tl intensity ratios. When analyses were restricted to freshly mixed (<1 hour) Pb–Tl solutions, however, highly precise isotopic ratios were obtained for lead (²⁰⁶Pb/²⁰⁴Pb = 16.9373 (±0.0011, 2σ), ²⁰⁷Pb/²⁰⁴Pb = 15.4907 (±0.0012, 2σ), and ²⁰⁸Pb/²⁰⁴Pb = 36.6935 (±0.0039, 2σ)) and for thallium (ε²⁰⁵Tl = 1.5 (±0.8, 2σ)). In addition, Pb/Tl intensity ratios were constant and corresponded to the mixing ratios of the prepared solutions. A series of experiments revealed that the poor precision and accuracy observed for the initial set of isotope ratio measurements resulted from variable photoxidation of Tl⁺ to Tl³⁺, which occurs in the presence of Pb and solar UV radiation. This reversible reaction generates Tl³⁺, which behaves distinctly from Tl⁺ during desolvation and leads to consistently higher measured Pb/Tl and ²⁰⁵Tl/²⁰³Tl ratios. The extent of the interaction between Pb and Tl and the subsequent effect on isotope ratio measurements is sensitive to a combination of factors, including differences in the acid matrix and molarity, desolvation conditions, and UV light exposure. It appears that the observed ε²⁰⁵Tl variations in the Pb–Tl³⁺-bearing solutions dominantly result from mass-dependant differential diffusion of Tl during desolvation. These experiments suggest that great care must be exercised during isotopic analyses of systems utilizing one element for mass-bias correction on another, such as Pb–Tl, Cu–Zn and Mo–Zr, which readily undergo redox reactions under laboratory conditions.