The results of this work provide a clearer picture of the relationship between particle composition and laser desorption ionization mass spectra. For micron-size particles, the surface layer of the particle is preferentially analyzed. The consequence is that relative signal intensities of marker ions in the mass spectra may not indicate the total composition of the particle if a concentration gradient exists between the surface layer and interior of the particle. For dry particles that have previously undergone one or more deliquescence-efflorescence cycles, the surface layer composition will reflect the eutonic composition that is given by a phase diagram of water activity vs. composition. This process tends to enrich the surface layer with minor components and enhance the signal intensities of marker ions for these species in the mass spectra.
Aqueous particles give very different spectra from their dry particle counterparts.
In addition, quenching of methanesulfonate and sulfate marker ions in the mass spectra of aqueous multicomponent particles is observed. Thus, it is recommended that ambient measurements in humid environments be performed in a manner such that water is removed from the particles prior to analysis by laser desorption ionization. Simple focusing mechanisms. Annual Meeting, Providence, RI, 10197.