Rubidium Isotope Measurements of Low‐Rb Geological Materials by <scp>MC</scp>‐<scp>ICP</scp>‐<scp>MS</scp>

The rubidium (Rb) isotope system has the potential to trace planetary evolution, magmatic‐fluid interaction and chemical weathering. These applications are based on Rb isotope measurement results with precisions fit for purpose, but measurements of low‐Rb geological materials are challenging due to large sample consumption and overload of ion‐exchange resin. Here we developed a measurement procedure for Rb isotope data (δ 87 Rb SRM984 ) of low‐Rb geological materials using MC‐ICP‐MS. Using an Aridus III desolvator and Ni standard sampler + Ni X skimmer cone combination, the Rb loading amount was reduced significantly to 20 ng. A comparison between two methods for instrumental mass‐bias correction, the sample‐standard bracketing and combined sample‐standard bracketing and internal (Zr) normalisation (C‐SSBIN), shows that C‐SSBIN could produce Rb isotope data with better intermediate measurement precisions but strictly restricted to optimal Zr/Rb ratio. The robustness of this method was demonstrated by monitoring δ 87 Rb SRM984 data of two in‐house Rb isotope standards, replicates, some reference materials with δ 87 Rb SRM984 values previously reported, and element‐doped and matrix‐spiked synthetic solutions. Based on repeated measurements of Rb isotope standards and reference materials, the long‐term (over one year) intermediate precision was better than 0.05‰ (2 s , standard deviations). We additionally recommend thirteen reliable reference materials for future Rb isotope ratio measurements.