Ion Formation from Rapidly Heated Aqueous Droplets by Droplet-Assisted Ionization

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Chemical Physics

Background:

• Droplet-assisted ionization is a novel method for online aerosol characterization using mass spectrometry.
• The technique relies on droplet breakup to generate charged progeny droplets, leading to gas-phase molecular ions.

Purpose of the Study:

• To investigate the conditions governing droplet breakup in a temperature-controlled capillary.
• To elucidate the relationship between droplet breakup, ion formation, and capillary wall temperature.
• To determine the activation energies for ion formation based on solute type.

Main Methods:

• Computational fluid dynamics (CFD) simulations with a droplet evaporation model were employed to study droplet breakup.
• Experimental measurements of ion current as a function of capillary wall temperature were conducted.
• CFD results were correlated with experimental data to interpret ion formation processes.

Main Results:

• Droplet breakup modes (aerodynamic/thermal) are temperature-dependent below ~150 °C and become temperature-independent above this threshold.
• Ion signal intensity above 150 °C is linked to ion formation from charged progeny droplets.
• Two main activation energies for ion formation were identified: ~41 kJ mol⁻¹ for nonionic solutes (suggesting charge residue process) and ~24 kJ mol⁻¹ for salts (suggesting ion evaporation process).

Conclusions:

• The study clarifies the physical processes governing droplet breakup and subsequent ion formation in droplet-assisted ionization.
• Understanding these mechanisms allows for optimization of aerosol mass spectrometry analysis.
• Distinct ion formation pathways are confirmed for different solute types, providing insights into molecular ion generation.