Dynamics of field-induced droplet ionization: time-resolved studies of distortion, jetting, and progeny formation from charged and neutral methanol droplets exposed to strong electric fields.

We recently reported that strong electric fields may be employed to directly extract positive and negative ions for mass analysis, including intact proteins, from neutral droplets. The present study investigates the dynamics of this process using switched high electric fields to enable time-resolved studies of droplet distortion, Taylor cone formation, and charged progeny droplet extraction from neutral and charged 225 microm methanol droplets. After a specific time in the field, a flashlamp is triggered to record droplet distortions using shadow photography. At a critical field strength E(c)0 corresponding to the Taylor limit, neutral droplets exhibit a prolate elongation along the field axis forming symmetric cone-jets of positive and negatively charged progeny droplets, approximately 10 microm in diameter. This process is termed field-induced droplet ionization (FIDI). Because the time scale of FIDI is related to the frequency of shape oscillations that occur below the Taylor limit, models of field-dependent oscillation become an important predictor of the time scale for progeny jet formation. Droplets with a net charge q distort into asymmetric tear shapes and emit a single charged jet of progeny at a critical field E(c)(q) that is less than E(c)0. The measured decrease in droplet stream charge indicates that total charge loss can be greater than the original charge on the droplet, resulting in oppositely charged droplets. Interestingly, above E(c)0, charged droplets sequentially emit a jet of the same polarity as the net charge followed by a jet of reverse polarity emitted in the opposite direction. For both neutral and charged droplets, increasing the electric field decreases the time to form jets and the combination of net charge and higher-than-critical fields has a compound effect in accelerating progeny formation. The implications of our results for using switched fields in FIDI-mass spectrometry for on-demand ion sampling from neutral and charged droplets are discussed.