Droplet evaporation and discharge dynamics in electrospray ionization

We present measurements of the distributions of droplet size and charge along with, for selected droplets, the variation of droplet size and charge with time for electrosprays of methanol, acetonitrile, and water, as well as for methanol at different polarities and electrolyte concentrations. These measurements are performed using a new technique for measuring droplet size and charge that uses phase Doppler interferometry for obtaining droplet size and inferring droplet charge from comparison of measured and calculated droplet mobility in a constant electric field. For selected droplets, multiple measurements of the size and charge are performed by repeated reversal of the drift field. This "ping-pong" experiment tracks droplet size and charge for loss of up to 99.9% of the initial droplet volume. We observe that droplet instability, referred to as a discharge event, mainly occurs near or above the Rayleigh limit of charge, resulting in a charge loss of 15-20% for methanol and acetonitrile and 20-40% in the case of water. Each discharge event is accompanied by a small mass loss, and droplet size evolution is dominated by evaporation. Discharge dynamics for negatively charged droplets are similar to those observed, for positively charged droplets. The addition of up to 10(-4) M of NaCl to the solution does not significantly alter discharge dynamics. Measured size-charge correlations for droplets from electrosprays of methanol at low electrolyte concentrations (< 10(-5) M), and to a lesser degree acetonitrile with similar electrolyte levels, fall into discrete groupings of size and charge that can be attributed to an initially monodisperse distribution of size and charge, followed by discharge events in which a nearly constant fractional charge loss occurs as a result of the Rayleigh instability.