Mechanistic Investigation of Charge Separation in Electrospray Ionization using Microparticles to Record Droplet Charge State

The mechanism of charge separation and droplet breakup during electrospray has been investigated using both biological and nonbiological microparticles as probes to preserve and record the charge state of droplets in the micron size range. Charged droplets and/or microparticles were observed using two-stage image charge detection after entering a differentially pumped, atmospheric pressure inlet. Microparticle probes allowed observation of not only droplets that did not fully evaporate but also product droplets small enough to fully evaporate, as well as the ability to distinguish between these two types of droplets. In both positive- and negative-mode electrospray ionization of aqueous suspensions, we observed roughly 20% of particles carrying charges opposite to the biased voltage on the capillary. Both positive and negative modes yielded distributions of charge states with maxima in the expected polarity but with tails extending into the opposite polarity of charge. Both polarities produced similar fractions of particles with opposite charge, with no preference for either polarity. This result is consistent with electric-field-induced charge separation and droplet breakup within the high-field region between the capillary and the counter electrode. These results suggest that electric-field-induced charge separation may be the dominant mechanism, at least in the micron size range and under the present experimental conditions, in which primary charged droplets of either polarity split into smaller progeny with a range of charges spanning both polarities. Implications for electrospray ionization mass spectrometry are discussed.