Evaporation and Discharge Dynamics of Highly Charged Multicomponent Droplets Generated by Electrospray Ionization
We investigate the Rayleigh discharge and evaporation dynamics of highly charged two-component droplets consisting principally of methanol with 2-methoxyethanol, tert-butanol, or m-nitrobenzyl alcohol. A phase Doppler anemometer (PDA) characterizes droplets generated by electrospray ionization (ESI) according to size, velocity, and charge as they move through a uniform electric field within an ion mobility spectrometer (IMS). Repeated field reversals result in droplet "ping-pong" through the PDA. This generates individual droplet histories of solvent evaporation behavior and the dynamics of charge loss to progeny droplets during Rayleigh discharge events. On average, methanol droplets discharge at 127% their Rayleigh limit of charge, q_R, and release 25% of the net charge. Charge loss from methanol/2-methoxyethanol droplets behaves similarly to pure 2-methoxyethanol droplets which release ~28% of their net charge. Binary methanol droplets containing up to 50% tert-butanol discharge at a lower percent q_R than pure methanol and release a greater fraction of their net charge. Mixed 99% methanol/1% m-nitrobenzyl alcohol droplets possess discharge characteristics similar to those of methanol. However, droplets of methanol containing 2% m-nitrobenzyl evaporate down to a fixed size and charge that remains constant with no observable discharges. Quasi-steady-state evaporation models accurately describe observed evaporation phenomena in which methanol/tert-butanol droplets evaporate at a rate similar to that of pure methanol and methanol/2-methoxyethanol droplets evaporate at a rate similar to that of 2-methoxyethanol. We compare these results to previous Rayleigh discharge experiments and discuss the implications for binary solvents in electrospray mass spectrometry (ESI-MS) and field-induced droplet ionization mass spectrometry (FIDI-MS).
© 2009 American Chemical Society. Received: July 27, 2009; Revised Manuscript Received: October 1, 2009. Publication Date (Web): October 22, 2009. This article is part of the A: Carl Lineberger Festschrift special issue. We thank Dr. James N. Smith for his ping-pong experimental design, construction, and refinement as well as Professor Richard C. Flagan for his useful discussions. Yannika Marrs in Professor John Hemminger's laboratory at UC Irvine provided assistance in the surface tension measurements. This material is based on work supported by the National Science Foundation under Grant No. CHE-0416381 and Grant No. CHE-9727566.