Sequential separation of cerium (Ce) and neodymium (Nd) in geological samples for high-precision analysis of stable Ce isotopes, and stable and radiogenic Nd isotopes by MC-ICP-MS

Abstract

Stable isotopes of cerium (Ce) and neodymium (Nd), two rare earth elements (REEs), have emerged recently as useful tracers for a range of geological and environmental processes, such as redox changes in environments or continental weathering. However, the coupled use of stable isotopes of two (or more) rare earth elements is often hampered by difficulties in sequential separation of individual REEs and analysis of their stable isotope ratios at high precision. Here, we report a new three-stage chromatographic procedure that allows for simultaneous separation of Ce and Nd from geological samples for high-precision analysis of their stable isotope ratios on MC-ICP-MS. This approach also allows for analysis of radiogenic ¹⁴³Nd/¹⁴⁴Nd ratios at high precision. Sequential separation of Nd and Ce in our procedure was achieved and optimized using α-hydroxyisobutyric acid (α-HIBA) as an eluent. Possible Ce and Nd stable isotope fractionations induced by the α-HIBA column were quantified. Total procedural blanks of our three-stage separation are low for Ce (∼46 pg) and Nd (∼2 pg). Extensive tests were conducted on MC-ICP-MS under dry plasma conditions to evaluate matrix effects associated with common cationic impurities, acid concentration, analyte/dopant ratio, and concentration mismatch between the sample and the bracketing standards. The presence of α-HIBA, and, occasionally, aluminum (Al) can affect measurements of both stable Ce and Nd isotopes, and acid concentration mismatch may affect stable Nd isotope measurements. An iterative correction method was developed to considerably increase the tolerance of stable Ce isotope analysis to Nd isobaric interferences, thereby improving the robustness of our measurement. Based on the analysis of various reference materials, we demonstrate that our new method is suitable for routine high-precision stable Ce and Nd isotope measurements on geological samples with a wide range of matrix compositions, yielding a long-term precision of ≤0.04‰ (2SD) for ¹⁴²Ce/¹⁴⁰Ce and ≤0.03‰ (2SD) for ¹⁴⁶Nd/¹⁴⁴Nd. This method also permits accurate radiogenic ¹⁴³Nd/¹⁴⁴Nd measurement with an external precision of ≤15 ppm (2SD). Because the α-HIBA column is known for its capability of separating individual REEs, our approach can be adapted to separate other REEs for high-precision stable and radiogenic isotope analysis with minor modifications.