A novel purification method for high precision measurement of Ni isotopes by double spike MC-ICP-MS

Abstract

Nickel (Ni) isotopes have been developed as a potential biosignature and proxy for tracing the occurrence of the Great Oxidation Event. However, the precise measurement of Ni isotopes is still hindered by the costly operation, complicated procedures and/or high procedural blanks of the chemical purification schemes. Here, a novel purification scheme, utilizing only dimethylglyoxime (DMG) and acetone as organic reagents, is proposed to separate Ni from samples through a four-step procedure with five chromatographic columns. Steps I, II and III are designed to effectively eliminate major matrix elements, including 99% Fe, Ca, Ti, and Al, 85% Mn, 55% K, even for samples with [Ca]/[Ni] ≈ 4600 and [Mg]/[Ni] ≈ 1900. Step IV utilizes the formation of Ni(DMG)₂ complexes in an aqueous medium containing 0.5 M HCl, 95% acetone and 0.1 M DMG, which can remove residual elements such as 45% K; this step avoids the use of Ni-specific resin, which requires careful pH tuning. Steps III and I can be repeated to obtain high-purity Ni. The Ni yield through these four steps is greater than 92%, with a maximum loss of ∼8% occurring in Step II for low-Ni samples. The total procedural blank is 0.4 to 1.2 ng. When double spiking (⁶¹Ni–⁶⁰Ni) is used to correct for instrument mass bias and purification-related isotope fractionation, small sample sizes (600–800 ng) are sufficient for achieving high-precision Ni isotope determination. Nickel isotopes for standards such as SRM 986 and geological reference materials (GRMs) such as BHVO-2, NOD-P-1, and NOD-A were determined on a Nu Plasma III MC-ICP-MS. The long-term reproducibility is 0.05‰ (2SD, n = 134) for pure solution and 0.06‰ (2SD, n = 18) for GRMs, and the average δ⁶⁰Ni values are in excellent agreement with previous studies. These results show that our new purification method can be applied to various samples. Using this new method, we measured a set of granite, sediment, diamictite and loess samples and preliminarily constrained the δ⁶⁰Ni of the upper continental crust to be 0.11 ± 0.14‰ (2SD, n = 17).