Elemental and morphological diversity of individual magnetic particles from urban surfaces: implications for adverse health outcomes

Urban magnetic dust particles (MDPs) are heterogeneous materials containing elemental iron (Fe⁰), magnetite (Fe₃O₄) and trace elements, which potentially pose health risks upon inhalation. The composition of nanoscale MDPs, which have risks of passing the blood brain barrier, has only recently been the subject of quantitative characterization at single-particle level. This study investigates the heterogeneity of hundreds to thousands of MDPs collected from urban parking garages at the individual particle level using both single-particle inductively coupled plasma time-of-flight mass spectrometry (spICP-TOFMS) and scanning transmission electron microscopy coupled with energy-dispersive X-ray microanalysis (STEM-EDX). spICP-TOFMS analysis reveals that only ∼8.6% of MDPs contain Fe, while STEM-EDX shows Fe in over 99% of particles. The discrepancy is attributed to “Fe-missing particles” enriched with other elements, which, because of their small diameter and low MS response, fall below the spICP-TOFMS detection limit. In contrast, EDX identifies fewer trace-level metals, due to its higher detection limit for metals (0.1%) than TOFMS. Operationally, spICP-TOFMS exhibits higher throughput of particles, while STEM-EDX requires more labor and time-intensive procedures. Three key differences between these methods significantly influence the identification and significance of heterogeneous MDPs: (1) oxygen impacts iron oxidation state interpretation and is detected by X-ray diffraction (XRD) analysis of bulk MDPs or single particle using EDX but not by spICP-TOFMS; (2) spICP-TOFMS exhibits varying detection limits for iron versus potentially catalytic elements (e.g., Cu, Pt, etc.); and (3) spICP-TOFMS has higher particle surveying efficiency compared to EDX. Using both methods reveals complementary insights into the size, shape, composition and potential redox state of MDPs that impact pollution, and potentially respirable particles that lead to adverse human health impacts.