Potassium and rubidium isotopic analysis using Neoma MC-ICPMS with the collision/reaction cell

Abstract

Potassium and rubidium isotopes are critical for understanding various geological processes that shape our planet. However, their analysis often encounters significant challenges. This study introduces methods for the simultaneous purification of K and Rb using three-step ion exchange column chromatography, coupled with high-precision isotopic analyses using Thermo Fisher Scientific Neoma MC-ICPMS/MS. This instrument features a double Wien filter and a collision/reaction cell. Using an Apex Omega desolvating nebulizer and low-resolution mode, we achieved sensitivities of 345 V per ppm for ³⁹K and 800 V per ppm for ⁸⁵Rb and obtained high precision (≤0.05‰; 95% confidence interval) for low-concentration solutions of K (≥20 ng g⁻¹) and Rb (≥5 ng g⁻¹). Our tests indicate that K isotope measurements are largely insensitive to sample/standard concentration mismatches within ±15% but are significantly affected by acid molarity and matrix effects. In particular, elevated concentrations of Mg and Ca introduce a positive δ⁴¹K bias, emphasizing the need to maintain total matrix concentrations below 2% for accurate results. Rb isotope measurements, on the other hand, show minimal sensitivity to concentration mismatch within ±20%, and various acid strength from 0.15 to 0.75 mol L⁻¹ HNO₃, but are influenced by matrix effects, especially K/Rb ratios. For reliable Rb isotopic data, K/Rb ratios should be kept below 1 with 20% magnetic field of the MS/MS module (B-field). To validate our method, we processed geostandards through column purification, yielding isotopic compositions consistent with those from other measurement techniques. Overall, our findings demonstrate that simultaneous K and Rb isotopic analyses for low concentration samples are feasible using Neoma MC-ICPMS/MS.