Application of stable isotopes and AF4/ICP-SFMS for simultaneous tracing and quantification of iron oxide nanoparticles in a sediment–slurry matrix

Abstract

One major challenge in nanomaterial analysis, especially in complex environmental matrices, is the unambiguous differentiation between natural and engineered nanomaterials (ENMs). Particularly with regard to the investigation of ENM's/engineered nanoparticle's (ENPs) fate, analytical methods are needed allowing for tracing and sensitive quantification. Several ENPs are metal-based and contain elements being omnipresent in environmental matrices (e.g., Al, Ti, Zn, Fe and non-metal Si) – hence, high background levels of these elements are expected, compromising sensitive detection. In this work we developed successfully a combined approach of stable isotope labeling (tracing) and reverse post-channel species-unspecific on-line isotope dilution (quantification) in combination with AF4/sector-field ICP-MS (AF4/ICP-SFMS). The approach was successfully applied to iron oxide nanoparticles isotopically enriched in ⁵⁷Fe coated with a SiO₂ shell (⁵⁷Fe@SiO₂ ENPs). Upon method development and validation the ⁵⁷Fe@SiO₂ ENPs were spiked into a filtered re-suspended river sediment–slurry matrix for proof-of-concept. Our approach allowed for unambiguous tracing of the ⁵⁷Fe@SiO₂ ENPs among natural iron colloid fractions as well as simultaneous sensitive quantification via reverse on-line ID despite high “natural” iron background. A limit of detection of 2.4 μg Fe L⁻¹ and a limit of quantification of 7.8 μg Fe L⁻¹ in real matrix was achieved. Furthermore, a good reproducibility in terms of peak areas was obtained (0.4–3.0%); the RSDs for elution times vary between 0.1 and 0.7%. To the best of the authors' knowledge, for the first time a reverse post-channel on-line isotope dilution AF4/ICP-SFMS approach in combination with isotopically labeled ⁵⁷Fe@SiO₂ ENPs was developed, allowing for (i) unambiguous tracing, and simultaneous (ii) sensitive quantification of the ⁵⁷Fe@SiO₂ ENPs within a sediment slurry matrix in the presence of natural iron colloid fractions. Based on the experience gained in this work, we feel certain that the approach is deployable to further stable isotope labeled metal- and nonmetal-based ENMs and shows great potential in ENMs studies.