Precise measurement of 41K/39K 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 K-39(+) for isotopic measurement of K-41/K-39 ratios. Ar-40(+) 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 parts per thousand (2 standard deviation) for K-41/K-39 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 Ar-40(+) and (ArH+)-Ar-40-H-1 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 parts per thousand in delta K-41 per 1% mismatch of K content. By contrast, mismatch of HNO3 molarity or existence of HCl in HNO3 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.

Automated summary

Stable K isotope geochemistry is becoming an important tool for various applications. The development of analytical methods based on multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) without collision cell allows for the analysis of K isotopes in many existing labs. A robust high-precision K isotope analytical method has been developed, which can be used for studying K isotopic variations in geological, cosmochemical, and biological samples.