Both atmospheric and laboratory-generated particles can be semisolid in phase under many conditions.  The slow molecular diffusion that occurs within these highly viscous particles may lead to heterogeneous composition in which the particle core is different from its surface.  We demonstrate that Easy Ambient Sonic-Spray Ionization - Mass Spectrometry (EASI-MS) can provide molecular level information on both the surface and the bulk of organic particles, depending on the experimental configuration.  EASI-MS is a solvent-based spray ionization method that generates ions from the friction of a sheath gas against the solvent spray without the need for heat or voltage.  Bulk composition of particles can be obtained by mixing the analyte particles with the µm-sized charged solvent droplets close to the nebulizer (the "droplet mode").  Alternatively, the surface of the particles is preferentially probed by allowing the solvent droplets to evaporate significantly before interacting with the particles (the "orthogonal mode").  We present the application of EASI-MS to detecting the bulk and surface composition of model core-shell organic particles using the droplet and orthogonal modes, respectively.  The probe depth of EASI-MS in orthogonal mode is quantified by controlling the thickness of the particle coating.  Ongoing studies include application of EASI-MS to understanding the bulk and surface composition of solid and liquid particles undergoing heterogeneous chemical reactions with gaseous oxidants.  These measurements will provide insight into products under different conditions of viscosity and mixing, and how these might change under atmospheric conditions.

Using easy ambient sonic-spray ionization mass spectrometry (EASI-MS) for depth profiling organic particles

2019 Fall National Meeting

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