PNNL

Abstract

The interpretation and qualification of measurements of particle composition by laser ablation based single particle mass spectroscopy is complex. Among the most difficult system to quantify are internally mixed particles containing alkali metals. The Alkali atoms in such particles tend to suppress the formation of other ions sometimes to below detection limit. Here we present a study of the behavior of the single particle mass spectral peak intensities as a function of the amount of the sodium containing compounds deposited on the surface of 240 nm PSL spheres. We generate three morphologically distinct and well defined coating types: uniform layer, cubic nodules and rounded nodules, and measure the individual particle mass spectra as a function of the vacuum aerodynamic diameter with nanometer resolution. We find that the probability of detecting the PSL spheres depends on the amount of the alkali metal on the PSL sphere surface and the ablation laser power. We also find that the morphological distribution of the sodium containing coating plays a role in determining mass spectral intensities. The data suggest that PSL spheres with localized Na-containing nodules are easier to detect than those completely encapsulated. We show that 80% of PSL spheres with nodules, whose weight fraction is close to 50%, can be detected at high laser power compared with 60% detection of encapsulated PSL particles with the same amount of coating. At low laser powers these detection limits drop to 35% and ~0% respectively.