New analytical techniques and instrumentation | Zheng Isotope Geochemistry Laboratory

High precision analysis of stable potassium (K) isotopes by the collision cell MC-ICP-MS “Sapphire” and a correction method for concentration mismatch

Stable potassium (K) isotopes (⁴¹K/³⁹K) have shown great promise as novel chemical tracers for a wide range of bio-, geo-, and cosmo-chemical processes, but high precision stable K isotope analysis remains a challenge for plasma source mass spectrometry due to intense argon-related interferences produced directly from argon plasma. Here we provide an assessment on the analytical figures of merit of a new generation collision-cell equipped multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Sapphire from Nu Instruments, for K isotope analysis based on our extensive tests over a duration of ∼8 months.

Because use of helium and hydrogen as collision/reaction gases can reduce argon-related interferences to negligible levels at optimal flow rates, the collision-cell mode can operate at low mass resolution during K isotope analysis, providing >2 orders of magnitude higher K sensitivity (>1000 V per μg mL⁻¹ K), as compared to the widely used “cold plasma” method, and the capability for direct ⁴⁰K measurement.

One challenge of the collision/reaction cell analysis on Sapphire is its higher susceptibility to matrix effects, requiring effective sample purification prior to analysis. Also, the collision-cell mode on Sapphire shows a pronounced effect associated with concentration (or ion intensity) mismatch between the sample and the bracketing standard during analysis, and this effect may not be fully eliminated through conventional concentration matching practice. Instead, we developed a correction method for this concentration/ion intensity mismatch effect.

Our method reduces the burden to the operator and increases sample throughput. This method allows for accurate K isotope analysis with an intermediate precision of ≤0.05‰ (2SD) to be routinely achieved using the collision cell on Sapphire, representing a major advance to stable K isotope analysis.

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

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 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.