Hyperaccumulating plants typically accumulate particular trace metals in their biomass without showing symptoms of toxicity. Metal concentrations in the aboveground biomass of hyperaccumulators often exceed background levels of regular plants 100-1000 times and may reach >10 g kg-1. Metal mobilisation mechanisms in the rhizosphere by hyperaccumulating plants are still mostly unexplored. Hyperaccumulation of Nickel was observed to lead to isotope fractionation (1). Consequently, studying the difference (Δ60Ni/58Ni) of Ni isotope compositions between bedrock, soil, plant material and plant exudates may be a powerful tool to study and further understand metal mobilisation processes by Ni-hyperaccumulating plants.
Special attention needs to be paid to the critical steps of the analytical protocol and understanding the fundamentals and tuneable instrument parameters of multi collector inductively coupled plasma mass spectrometry (MC-ICP-MS)to assure accurate Ni isotope ratio measurements with low bias from the true value and low uncertainty. In this study, a microwave-assisted digestion protocols using tetrafluoroboric acid (HBF4) was investigated, as well as a two-step isolation protocol separating iron (Fe) and Ni using Bio-Rad AG MP-1 and AG 1-X8 anion exchange resins respectively (2). The MC-ICP-MS used (Nu Sapphire, Nu instruments, Wrexham, UK) offers the application of a hexapole collision/reaction cell (CRC) to remove spectral interferences.
The MC-ICP-MS was further optimized for simultaneous measurement of 58Ni – 64Ni target isotopes, 57Fe for interference correction on 58Ni and 65Cu/63Cu as a ratio for instrumental isotopic fractionation correction based on the regression model.
The IAG MUH-1 certified reference material was used to validate the digestion and isolation protocols as it closely resembled the soil and rock samples. The HBF4 digestion demonstrated ~100 % Fe and Ni recovery, and the isolation protocol has shown ~95 % Fe and Ni recoveries even in presence of high boron contents as a consequence of the digestion. The CRC allowed to remove the 40Ar18O+ interference on 58Fe/58Ni using H2 and He gases but further investigation is needed to fully remove the 40Ar16O1H+ interference on 57Fe (3).

(1) Zelano et al., Plant Soil, 2020, 454, p. 225–243
(2) Beunon et al., J Anal At Spectrom, 2020, 35, 2213-2223
(3) Arnold et al., Spectrochim Acta Part B At Spectrosc, 2008, 63, 666-672