Effective Removal of Ar-Based Spectral Interferences Using Sulfur Hexafluoride in ICP-MS/MS: Applications to In Situ K–Ca Geochronology

Inductively coupled plasma-mass spectrometry (ICP-MS) using Ar as the plasma gas has been used successfully in many diverse fields; however, Ar-based spectral interferences (e.g., ³⁸Ar¹H⁺, ⁴⁰Ar⁺, ⁴⁰Ar¹⁶O⁺, and ⁴⁰Ar⁴⁰Ar⁺) limit the precise and accurate measurement of the isotopes with which they interfere (e.g., ³⁹K, ⁴⁰K, ⁴⁰Ca, ⁵⁶Fe, and ⁸⁰Se). We demonstrate that using sulfur hexafluoride (SF₆) as a reaction gas in ICP-MS/MS can effectively remove Ar-based spectral interferences (up to 9 orders of magnitude for ⁴⁰Ar⁺) through charge exchange between SF₆ and ⁴⁰Ar⁺, ⁴⁰Ar¹H⁺, ⁴⁰Ar¹⁶O⁺, and ⁴⁰Ar⁴⁰Ar⁺, with the generation of a range of SF ions (e.g., ³²S¹⁹F₃⁺, ³²S¹⁹F₅⁺). The SF₆ gas is inert, safe, and easier to handle than the commonly used H₂, and it does not produce any potential ArH⁺ and CaH⁺ spectral interferences in the reaction cell. Moreover, SF₆ can react with specific elements (e.g., Ca) to form higher mass fluorides (e.g., CaF), making it more powerful in ICP-MS/MS. We used SF₆ as a reaction gas for the in situ K-Ca dating of K-rich minerals (K-feldspar and muscovite) and obtained precise ages (1-10%). Three K-feldspar and three muscovite samples were analyzed, and the results are consistent with the recommended ages. SF₆ has great potential in the precise analysis of isotopes (e.g., ³⁹K, ⁴⁰Ca, ⁵⁶Fe, ⁸⁰Se) that Ar interferes with using MC-ICP-MS/MS.