김민수 (POSTECH)

김민수 (POSTECH)

황민욱 (POSTECH)

남경민 (POSTECH)

Electrospray ionization (ESI) transforms solution-phase samples into gaseous ions via Coulombic explosion under a strong electric field. As droplets shrink during this process, they exhibit molecular behaviors distinct from those in bulk solution—such as the formation of unusual molecular clusters and the spontaneous reduction of analytes. However, due to the ultrafast and nanoscale nature of these droplets, the underlying mechanisms remain largely unexplored. To probe this “no-man’s-land” of ion formation, we developed a custom vacuum ESI/X-ray scattering setup and employed ultra-short, super-bright X-ray pulses from PAL-XFEL. This system enables simultaneous wide-angle (WAXS) and small-angle (SAXS) X-ray scattering, allowing real-time, multi-scale observation of electrosprayed nanodroplets under vacuum.

Using this approach, we aimed to determine nanodroplet size distributions and investigate size-dependent molecular behaviors. Water and methanol were used as solvents, with an IR laser introduced to prevent water from freezing under vacuum conditions. Phosphotungstic acid hydrate (PTA), chloroauric acid (HAuCl₄), and lithium chloride (LiCl) were chosen as analytes. LiCl served as a control due to its low atomic number, while HAuCl₄ was selected for its known tendency to spontaneously form nanoparticles during spray. SAXS measurements provided electron density contrast for droplet size estimation, while WAXS offered complementary structural and compositional insights.

Distinct scattering signatures were observed for PTA and, HAuCl4. These differences suggest that molecular-level behaviors can be distinguished based on analyte identity. Although full interpretation of the WAXS patterns is ongoing, preliminary results reveal analyte-specific structural features. Future work will examine the effects of voltage polarity, solution flow rate, and emitter geometry on droplet evolution and ionization dynamics.