The use of Pb isotope ratios determined by magnetic sectorICP-MS for tracing Pb pollution in marine sediments near Casey Station, East Antarctica

Abstract

Magnetic sector inductively coupled plasma mass spectrometry (ICP-SMS) was used to measure lead concentrations and isotope ratios in marine sediments and other samples collected from near the Australian Antarctic Station Casey. Precisions obtained from the repetitive analysis of a standard Broken Hill Pb sample at a concentration of ∼40 ng g⁻¹ in solution were <±0.2% for ratios involving ²⁰⁴Pb, and <±0.1% for those referenced to ²⁰⁶Pb or ²⁰⁷Pb (n = 12 replicates over 2 days, values as 1s). Ratios were accurate to within ∼±0.1% for the analysis of this standard sample. Comparative measurements between ICP-SMS and TIMS had typical differences in values of <0.4% for contaminated samples, irrespective of ratio. For marine sediment samples with Pb concentrations in the sample digest of >10 ng g⁻¹, instrumental capability was characterised by isotopic precisions ranging from 0.1-0.5% (1s) for ratios involving ²⁰⁴Pb, and <0.25% (1s) for ratios with ²⁰⁶Pb or ²⁰⁷Pb as the basis (typically found from triplicate analyses). For sediments of low Pb concentration (<10 ng g⁻¹ in the sample digest), isotope ratios to ²⁰⁴Pb were found to be limited by instrument counting statistics when using standard ICP-SMS. To help overcome this problem, Pb isotope ratios for these samples were measured with a capacitive decoupling Pt guard electrode employed, offering considerable signal enhancement (5–10×). These natural background sediments were found to display typical Pb isotope ratios of 40.5, 15.5, 18.6 and 1.19 for ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁷Pb. For comparison, the most contaminated samples had Pb isotope ratios of approximately 36.2, 15.4, 16.4 and 1.06 for ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁷Pb, respectively. Evidence of simple two component mixing between anthropogenic and natural geogenic Pb was found near Casey Station. Runoff from the Thala Valley tip site, adjacent to the bay, was identified as a clear source of Pb pollution, with impacted sediments displaying an isotopic signature approaching that of abandoned lead batteries collected from the tip. These batteries possessed Pb isotope ratios identical to Australian Broken Hill lead. In this study, the use of Pb isotope data has proved to be a sensitive method of assessing contamination levels in the Antarctic marine environment adjacent to a waste disposal site. Lead isotope ratios have proved superior to simple elemental concentration determinations when distinguishing between impacted and non-impacted samples. ICP-SMS has been shown to offer relatively fast, accurate and cost effective Pb isotope ratios, with precisions suitable for many environmental applications.