Laser desorption ionization of size resolved liquid microdroplets

Mass spectra of single micrometer-size glycerol droplets containing organic and inorganic analytes were obtained by on-line laser desorption ionization. Aerosol droplets entered the mass spectrometer through an inlet where they were detected by light scattering of a continuous laser beam and then ablated with a pulsed excimer laser operating at 248 nm. Inorganic analytes consisted of iron (III), sodium, and chloride ions at concentrations ranging from 10−3 to 10−2 M . When a high laser fluence was used for laser desorption ionization, the analyte signal intensities increased linearly with analyte concentration and droplet volume. These dependences suggest a volume desorption mechanism. Organic analytes consisted of heptanoic acid and benzoic acid in basic solutions. A lower laser fluence was needed to prevent extensive fragmentation of the molecular ions. For benzoic acid, the analyte signal intensity increased linearly with concentration, but increased only slightly with increasing droplet size. The concentration dependence can be rationalized on the basis of a simple surface adsorption model assuming that most of the analyte signal intensity arises from species at or near the droplet surface. The droplet size dependence is not consistent with a surface desorption mechanism and is probably the result of complex processes during laser desorption ionization that affect ion formation and/or ion collection efficiencies as a function of droplet size.