ACCURATE METHODOLOGY FOR IRON ISOTOPES ANALYSIS IN GEOLOGICAL AND ENVIRONMENTAL SAMPLES.

Iron isotopes have been extensively utilized in various applications within the fields of geology and environmental sciences (Conway et al., 2019). Nonetheless, isotopic analysis poses significant challenges concerning sample purification and spectrometer measurements, primarily due to isobaric interferences and the need to correct instrumental fractionation (Douphas et al., 2009). Consequently, it is imperative for a robust methodology to demonstrate precise and accurate measurements across diverse sample matrices.

In this study, we validate a methodology for iron isotopes analysis based on a purification procedure utilizing AG-MP1 resin, as established in a previous study (Souto-Oliveira et al., 2019). Additionally, the isotopic composition measurements using MC-ICP-MS Neptune Plus (Thermo Scientific) were developed as part of this research. All chemical procedures were conducted in a clean room laboratory (ISO 7) under a laminar flow hood (ISO 5), utilizing 18.2 MΩ H2O (Milli-Q System®) and ultra-pure acids obtained from the Acid Purification System (Savillex®, DST-100) at the Research Center of Isotope Geochemistry and Geochronology (CPGeo), University of São Paulo (USP).

Samples were introduced dissolved in a 0.3 M HNO₃ medium using a stable introduction system (SIS: double Scott/cyclonic spray chamber) coupled with a low-flow PFA nebulizer (50 μL min⁻¹). The entire methodology, encompassing both purification and measurements, was validated using geological reference materials (BCR-2, BHVO-2, AGV-2). Iron data collection involved 25 cycles with 8.4 seconds integration each. Samples were measured bracketed by standards. Instrumental baseline and on-peak blank correction were conducted using 0.3 mol HNO₃. External normalization for Fe analysis was achieved by doping samples and bracketing with Ni_USP solution. To correct for instrumental fractionation in Fe, Direct Sample Standard Bracketing was employed.

Analyses of BCR-2 at CPGeo yielded d⁵⁶Fe_IRMM = 0.07 ± 0.03‰ (n=23). Douphas et al. (2009) reported the following values for this reference material: d⁵⁶Fe_IRMM = 0.07 ± 0.023‰. Analyses of AGV-2 at CPGeo yielded d⁵⁶Fe_IRMM = 0.10 ± 0.11‰ (n=4). Douphas et al. (2009) reported the following values for this reference material: d⁵⁶Fe_IRMM = 0.105± 0.017‰. Analyses of BHVO-2 at CPGeo yielded d⁵⁶Fe_IRMM = 0.09 ± 0.07‰ (n=4). Sossi et al. (2017) reported the following values for this reference material: d⁵⁶Fe_IRMM = 0.13± 0.02‰.

Accuracy of the methodology concerning isobaric interferences was confirmed by a three Isotope Plot (δ⁵⁶Fe_USP vs. δ⁵⁷Fe_USP) of environmental and geological reference material samples, confirming mass-dependent fractionation trend. The robustness and quality assurance were demonstrated by intercomparison of δ⁵⁶Fe_IRMM values of geological reference materials (BRC-2, BHVO-2, and AGV-2) between CPGeo results and those reported in the literature. Hence, we can confidently assert the accuracy and robustness of the validated methodology for analyzing iron isotopes in both environmental and geological samples.

Conway, T. M., Hamilton, D. S., Shelley, R. U., Aguilar-Islas, A. M., Landing, W. M., Mahowald, N. M., & John, S. G. (2019). Nature Communications, 10(1), 1–10. https://doi.org/10.1038/s41467-019-10457-w

Dauphas, N., Pourmand, A., & Teng, F. (2009). Chemical Geology, 267(3–4), 175–184. https://doi.org/10.1016/j.chemgeo.2008.12.011

Souto-Oliveira, C. E., Babinski, M., Araújo, D. F., Weiss, D. J., & Ruiz, I. R. (2019).Atmospheric Environment. 198, 427–437.

Sossi, P. A., Moynier, F. (2017). Earth and Planetary Science Letters, 471 (125-135).