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

Abstract

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. 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. Under such operating conditions, an external reproducibility of <±0.1‰ (2 standard deviation) for 41 K/39 K is achieved for K solutions of 1ppm 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 Ar1 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 δ41 K 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%. The high-precision K isotope analytical method reported here is robust for studies on K isotopic variations in geological, cosmochemical, and biological samples. The f41 K values of six international geostandards measured using our method are consistent with data measured using different analytical methods from other laboratories.