Evaluating use of laser ablation quadrupole inductively coupled plasma mass spectrometry for low concentration lead isotope measurements

Abstract

Lead (Pb) concentrations and isotopic compositions in environmental samples can provide important insights into Pb sources, pathways, and distributions. However, low Pb levels in many environmental samples present challenges for precise Pb isotopic measurements. This study evaluates the use of laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) combined with collisional focusing and an Aerosol Rapid Introduction System (ARIS) for measuring Pb isotopes in reference materials with variable Pb concentrations (0.3–11 μg g⁻¹) and environmental samples (i.e., Pacific oyster shells) with Pb concentrations below 1 μg g⁻¹. Accuracy and precision were assessed using three glass reference materials (BCR-2G, BHVO-2G, and GOR128-G). For ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb ratios, measurements were within 0.2 % and 2.8 % of reported values for all reference materials, with decreased accuracy associated with low count rates. The precision was 0.4%–1.8 % relative standard deviation (RSD) for certified reference materials. Analytical uncertainty was lower for reference materials (∼1.2 % RSD for ²⁰⁶Pb/²⁰⁷Pb and ∼1.3 % RSD for ²⁰⁸Pb/²⁰⁷Pb) compared to oyster shells (∼3.6 % RSD for ²⁰⁶Pb/²⁰⁷Pb and ∼3.8 % RSD for ²⁰⁸Pb/²⁰⁷Pb), likely due to low Pb concentrations and sample heterogeneity. While Pb isotopic measurements demonstrated good repeatability across parallel transects of the same oyster shell (r = 0.7), the higher analytical uncertainties (>0.5 % RSD) limit reliable source identification or apportionment in these natural samples. In approaching new studies with the methods presented here, researchers will need to determine if for a given application the uncertainties on measured sample Pb isotope ratios are sufficiently small.