Determination of lead isotope ratios in Mn–Fe-rich nodules by laser ablation multi-collector inductively coupled plasma mass spectrometry

Abstract

In this study, Pb isotope ratios in ferromanganese nodules were determined with good repeatability and minimum laboratory bias using laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). One certified ferromanganese nodule reference material, NOD-A-1, in the form of pressed powder pellets (PPPs) produced upon ultrafine processing, was homogeneous enough to be used as an in-house matrix-matched standard or quality control sample for Pb isotopic microanalysis. The Tl normalization exponential law correction method with C_{Tl-NIST SRM 997}/C_Tl-sample ≥ 15 (method 1) and the standard-sample-bracketing (SSB) method (method 2) were utilized to determine Pb isotope ratios with good repeatability and laboratory bias. In situ measurements of the Pb isotopes yielded a good relative standard deviation (2RSD) of <0.036% for ^207,208Pb/²⁰⁶Pb and <0.066% for ^206,207,208Pb/²⁰⁴Pb. In addition, the matrix effect was insignificant when the silicate glass reference material was employed as the external standard in the ferromanganese nodules using these two methods, resulting in relative errors (RE) of <0.064% for ^206,207,208Pb/²⁰⁴Pb, and <0.046% for ^207,208Pb/²⁰⁶Pb. The in situ Pb isotopic compositions of three Chinese ferromanganese nodules, GBW07249, GBW07295, and GBW07296, were in good agreement with those obtained by solution nebulization MC-ICP-MS (SN-MC-ICP-MS) within 2s (standard deviation) measurement uncertainties, demonstrating the reliability and robustness of our proposed analytical protocol.