Iron is the most abundant transition metal in particulate matter. The bulk element compositions of iron and other transition metals are traditionally examined to articulate their role in particulate toxicity. However, particles collected from ambient air available in microgram quantities are not sufficient for some bulk spectroscopy techniques. Here, we used spatially resolved scanning transmission electron microscopy (STEM) equipped with energy electron loss spectroscopy (EELS) and electron dispersive x-ray spectroscopy (EDS) to examine the chemical speciation of iron. Particles (PM 2.5) collected from highly polluted Central Valley locations in California (Fresno and Visalia) were examined in this study. Submicron to nanoscale particles were identified from electron micrographs, and elemental composition of these particles were investigated from STEM-EDS microanalysis. The oxidation state of iron in the particles was assessed by Fe-L core edge spectroscopy. The collected Fe white line spectra were compared to standard compounds with known iron oxidation state. Quantitative analysis for oxidation state of iron was realized by comparing the Fe-L3/L2 intensity ratio, by following the double atan EELS background subtraction approach. The Fe-L3/L2 ratios of the reference compounds were validated by synchrotron soft x-ray spectroscopy, operated under ambient conditions. We found four distinct types of iron speciation in the particles: oxide/oxyhydroxide, metallic, substituted in phyllosilicate minerals, and coordinated/deposited on carbon or silica surfaces. The chemical nature of iron in atmospheric particles are subject to location, weather, and local anthropogenic and geogenic sources. These iron phases have significantly different biochemical profiles upon inhalation and this is an essential factor to consider in toxicology models. The study methods discussed here are applicable to a wide variety of engineered and natural nanophases.

Fe-L edge spectroscopy reveals the chemistry of iron in nanoscale particulate matter

255th National Meeting (2018)

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