Atom Probe Tomography Method for Determination of Iron Isotope Compositions at Nanoscale

Accurate isotope compositions of nanoparticles in planetary materials provide critical constraints on the evolution of the early solar system. Atom probe tomography (APT) can analyze isotopes in situ with the highest spatial resolution, making it ideal for nanoscale planetary materials. However, significant deviations between isotope ratios determined using APT and a traditional mass spectrometer have hindered the wide application of APT in isotope analysis. Here, we propose that these isotope ratio discrepancies arise from different uncounted rates of the isotopes in APT analysis. Theoretical assessment indicates that such discrepancies can be corrected using a suite of reference materials. Iron isotope analyses of Fe-Mn-Ni steels and meteoritic irons using APT and a multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) confirm a strong linear correlation between the two data sets, supporting the establishment of ⁵⁶Fe/⁵⁴Fe and ⁵⁷Fe/⁵⁴Fe calibration curves for APT data correction. The calibration curves were subsequently validated using four meteoritic irons with known Fe isotope ratios. Combining all of the errors, our corrected APT results achieve a 2σ uncertainty of <0.15‰ for δ⁵⁶Fe. This precision demonstrates that our calibrated APT protocol enables accurate isotope determination, unlocking the potential of APT for precise isotope analysis of nanoscale planetary materials.