High-speed imaging-based evidence for scarce microdroplet interactions in dual-spray ionization systems

While electrospray ionization is a prominent ionization technique in mass spectrometry, a number of other atmospheric pressure ionization techniques have also been developed. Some of them use two sprayers. For example, in extractive electrospray ionization, one sprayer is used to produce charged droplets, while the second sprayer is used to deliver sample droplets. Some insights on the microdroplet interactions were previously obtained by phase Doppler anemometry. Here, we used a high-end high-speed camera to directly visualize the interactions between microdroplets in dual-spray systems. Various (electro)spray arrangements were tested, including: positive-neutral, positive-positive, positive-negative, and neutral-neutral. Pressurized auxiliary gas was used for nebulization. Surprisingly, the observations of direct physical interactions between microdroplets were scarce. The observed interactions included coalescence, disruption, and fragmentation. Liquid bags were sporadically observed. Furthermore, the microdroplets in the gas-assisted sprays often had irregular shape, which complicated numerical analysis. The scarcity of physical contacts between microdroplets calls for reconsideration of the dual-spray ionization mechanism, which may also involve interactions between nanodroplets and gas-phase species as well as interactions with surfaces.