Accurate and precise determination of Pb/Ca and lead isotopes (20XPb/207Pb) from mass-limited coral samples by ICP-QQQ-MS

Abstract

The Pb/Ca ratio of coralline aragonite is a recognized archive of marine lead (Pb) pollution, while the isotopic composition (^20XPb/²⁰⁷Pb) enables source tracing. The critical analytical challenge of Pb analyses is the extremely low concentration of aragonite lattice bound Pb (10⁻⁹ mol mol⁻¹). The extremely low levels of Pb in natural corals render the determination of Pb/Ca and Pb isotope ratios susceptible to potential contamination from preexisting detrital matter like clay and organic substances or during laboratory processing. We present a novel method for the accurate and precise determination of Pb/Ca and Pb isotopes (^20XPb/²⁰⁷Pb) from <2 mg of coralline CaCO₃ by triple quadrupole inductive coupled plasma mass spectrometry (Agilent® ICP-QQQ-MS). The external precision of our Pb/Ca ratio determination is better than 2% (n = 56, 2σ); whereas the Pb isotope ratios, determined on ∼50 pg of column purified Pb, have precision better than ±0.0008 (1.23%, 2σ), ±0.0043 (0.38%, 2σ), and ±0.0042 (0.17%, 2σ) for ²⁰⁴Pb/²⁰⁷Pb, ²⁰⁶Pb/²⁰⁷Pb, and ²⁰⁸Pb/²⁰⁷Pb ratios respectively. The determination of metals/Ca ratios of multiple trace metals (viz. Li/Ca, B/Ca, Mg/Ca, Sr/Ca, U/Ca, etc.) is also possible during the same instrumental session. Our Pb/Ca determination method utilizes matrix matching coupled with external calibration, whereas the Pb isotope method is based on concentration matching and standard-sample bracketing. Overall, the total mass of chemically cleaned coralline CaCO₃ required for Pb/Ca and Pb isotope determination is ∼2–3 mg. Our novel method addresses the key analytical challenges associated with the determination of the Pb concentration and isotope ratio in the coralline archive and outlines a simple method with key advantages compared to the other traditional methods for extremely low Pb concentration and high Ca matrix samples.