CHROMIUM AND NI ISOTOPE ABUNDANCE RATIO MEASUREMENTS IN LOW RESOLUTION USING THE SAPPHIRE DUAL PATH MC-ICP-MS WITH COLLISION/REACTION CELL

The isotope abundance ratios of the transition metals Cr and Ni provide insights into a wide range of terrestrial processes, and enable environmental and biogeochemical tracing. Moreover, they hold geochronological importance thanks to the presence of short-lived chronometers ⁵³Mn-⁵³Cr and ⁶⁰Fe-⁶⁰Ni, which allow accurate dating of early solar system processes. These isotopic systems pose unique challenges due to their small radiogenic variations and the scarcity of precious samples, necessitating high-precision isotope abundance ratio measurements of small sample aliquots.

Plasma-based Cr and Ni isotope measurements suffer from several isobaric interferences such as ⁴⁰Ar¹²C⁺ on ⁵²Cr⁺, ⁴⁰Ar¹⁴N⁺ on ⁵⁴Cr⁺, or ⁴⁰Ar¹⁸O⁺ on ⁵⁸Ni⁺. To resolve these interferences, both isotope systems are usually measured in medium resolution mode (M/ΔM > 5-95% peak height > ~ 5000), which comes at the cost of reduced sensitivity. In this study, we will present isotope abundance ratio data for Cr and Ni measured in low resolution on a Nu Sapphire Dual Path MC-ICP-MS with Collision/Reaction Cell to remove isobaric interferences.

First results using different gas mixtures (He-H­₂, He-O­₂, and He-N­₂) within the Collision/Reaction Cell reveal residual plasma-based interferences when H₂ and O₂ are used as reaction gases and potential hydride formation for H₂. In contrast, the use of N₂ allows for interferences-free measurements of all Cr and Ni isotopes, as evidenced by mass-dependent isotopic variations for both systems. The achieved sensitivity is > 600 V/ppm for either isotope system under dry plasma conditions.

Preliminary consecutive measurements indicate that the 2σ SD short-term repeatability (N = 60) is < 30 ppm for ⁵³Cr/⁵²Cr_50/52 and < 70 ppm for ⁵⁴Cr/⁵²Cr_50/52 for repeated 200 s measurements of a 50 ng/ml Cr standard solution. Further data will be presented on the short-term repeatability of Ni isotope abundance ratio measurements, the robustness of the method for either isotope system against concentration mismatching, and the measurement trueness for the isotope standard reference materials NIST SRM 979 (Cr) and NIST SRM 986 (Ni). Results for selected meteoritic samples of known isotopic compositions will be presented as well.