Synthesis of an ultrafine iron and soot aerosol for the evaluation of particle toxicity

Abstract

A diffusion flame system was used to generate an aerosol of soot and iron oxide. The primary fuel was ethylene. Iron was introduced by passing ethylene over liquid iron pentacarbonyl. The aerosol emission from the flame was diluted by secondary air to a level that could be used in animal exposure studies. The system was designed to operate at a constant soot production rate while the iron loading was varied from 0 to 50 μg m-3 in the diluted postflame gases. The impact of the iron on soot production was counteracted by the addition of acetylene to the fuel. Particles were collected on carbon grids and were examined via transmission electron microscopy. Electron energy loss spectroscopy was employed to characterize the aerosol. A differential mobility analyzer was used to measure the size distribution of the aerosol. The iron particles were typically 40 nm in diameter and often appeared in isolation from the soot aerosol, suggesting that either they were not formed concurrently with the soot or they remained after oxidation of the surrounding soot. Samples collected from within the flame, and downstream of the flame, indicated that the iron may have been present as very small particles comingled with the soot. The iron particles apparently melted and coalesced as they passed through the high temperature flame tip. Crystallization of the iron proceeded as the postflame gases cooled by mixing with external air. The flame system was shown to be capable of consistently producing steady concentrations of soot and iron for delivery to animals, without the confounding presence of toxic gaseous compounds.