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