Precise measurement of 41 K/ 39 K ratios by high‐resolution multicollector inductively coupled plasma mass spectrometry under a dry and hot plasma setting

Rationale Stable K isotope geochemistry is becoming an important tool for various applications. Developments in analytical methods for K isotopes based on multicollector inductively coupled plasma mass spectrometry (MC‐ICP‐MS) without collision cell will bring research capability of K isotopes to many existing MC‐ICP‐MS labs. Methods Stable K isotopes were analyzed without applications of “cold plasma” and collision cell on a Nu 1700 Sapphire high‐resolution multicollector inductively coupled plasma mass spectrometer. A conventional dry and hot plasma setting is used for analysis to maintain high K sensitivity and signal stability, and high mass resolution was applied to provide interference‐free shoulders of 39 K + for isotopic measurement of 41 K/ 39 K ratios. 40 Ar + ion beam generated in ICP was neutralized in the ion guide rail for the Daly detector. Results Under such operating conditions, an external reproducibility of <±0.1‰ (2 standard deviation) for 41 K/ 39 K is achieved for K solutions of 1 ppm or above. Tests were carried out to evaluate the influence of total K loading, K concentration and acid molarity mismatch, matrix effects, and 40 Ar + and 40 Ar 1 H + tailing on K isotope analysis. We found that the accuracy of K isotope analysis can be compromised by concentration mismatch of sample and standard K, by 0.007‰ in δ 41 K per 1% mismatch of K content. By contrast, mismatch of HNO 3 molarity or existence of HCl in HNO 3 exerts negligible influences on the analytical precision and accuracy of K isotope analysis. Furthermore, K isotope analytical results remain accurate when Na/K, Mg/K, Ca/K, Rb/K, V/K, and Cr/K ratios are below 3%. Conclusions The high‐precision K isotope analytical method reported here is robust for studies on K isotopic variations in geological, cosmochemical, and biological samples. The f 41 K values of six international geostandards measured using our method are consistent with data measured using different analytical methods from other laboratories.