The Role of Aerosol Liquid Water in Droplet-Assisted Ionization Mass Spectrometry

Chemical analysis of aerosols by mass spectrometry is challenging because aerosols contain little mass and have complex compositions. Consequently, relatively few approaches allow online molecular (i.e., minimal fragmentation) analysis on aerosols, particularly for ultrafine (<100 nm) particles. Droplet-assisted ionization (DAI) mass spectrometry is a promising and straightforward approach for aerosol molecular analysis. In DAI, liquid aerosol droplets are delivered directly to the mass spectrometer inlet, where the droplets break up and form molecular ions similar to those observed in electrospray ionization. This study explores how aerosol liquid water, serving as a matrix, controls ion generation in DAI for systems spanning simple binary and more complex ternary ones. The results demonstrate that ion yields are tightly coupled to the aerosol's hygroscopic response, leading to humidity- and phase-dependent ionization efficiencies. When the water to analyte molar ratio is less than ∼2, ion formation is relatively insensitive to water content, but once the matrix to analyte molar ratio exceeds ∼2, ion yield is highly sensitive to aerosol liquid water. These dependencies can be easily minimized by normalizing the environmental relative humidity experienced by the aerosol immediately before delivery to the MS inlet. This study enables a critical evaluation of the factors controlling ionization by this inlet-based approach and identifies experimental designs that will facilitate a more widespread implementation of DAI for aerosol chemical analysis in different application domains. Lastly, this work is an interesting example of how knowledge of aerosol physicochemical properties can answer questions about ionization mechanisms in mass spectrometry.