Precise isotope ratio determination of common Pb using quadrupole LA-ICP-MS with optimized laser sampling conditions and a robust mixed-gas plasma

Abstract

This work presents a method for the precise (0.2% RSE) determination of common (i.e. non-radiogenic) Pb isotope ratios using quadrupole Nd:YAG (266 nm) LA-ICP-MS at low (∼2 ppm) Pb concentrations. Laser sampling conditions significantly influence the precision of Pb isotope ratio measurements. This paper presents a set of optimum sampling conditions for the described system. Setting the laser focus above the sample surface significantly improves the precision of ratio measurements due to increased count rates and a reduction in the heterogeneity of particulate matter produced by fracturing at the site of ablation. In addition, using a more robust, mixed-gas (Ar–N2) plasma significantly increases sensitivity and reduces mass bias. With a mixed Ar–N2 plasma and optimized laser sampling conditions, single collector quadrupole LA-ICP-MS can be superior, for some applications (e.g., where micrometre-scale spatial resolution is important), to TIMS, and at low Pb concentrations is a cost-effective alternative to LA-MC-ICP-MS. The application of single detector, quadrupole LA-ICP-MS to the precise determination of common Pb isotope ratios in minerals has not been previously documented. This method is a powerful tool for use in isotope tracer studies of ore deposits and has potential applications to a range of environmental problems.