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

• Published in: Rapid communications in mass spectrometry Vol. 36; no. 11
• Main Authors: An, Shichao, Luo, Xianglong, Li, Weiqiang
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 15.06.2022
• Subjects: Biological properties; Cold plasmas; Emission spectroscopy; Geochemistry; Guide rails; Inductively coupled plasma mass spectrometry; Ion beams; Isotopes; Mass spectrometry; Mathematical analysis; Plasma; Sapphire; Scientific imaging; Stability analysis
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.9289

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 39K+ for isotopic measurement of 41K/39K ratios. 40Ar+ 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 41K/39K 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 40Ar+ and 40Ar1H+ 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 δ41K 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 f41K values of six international geostandards measured using our method are consistent with data measured using different analytical methods from other laboratories.