The Evolution of Electrospray Generated Droplets is Not Affected by Ionization Mode

Area of Science:

• Analytical Chemistry
• Physical Chemistry

Background:

• Electrospray ionization (ESI) efficiency depends heavily on mobile phase properties.
• Droplet composition in the ESI plume changes due to electrochemical reactions, drying, and fission.
• Limited research exists on how ESI mode polarity affects mobile phase composition within droplets.

Purpose of the Study:

• Investigate changes in organic solvent content, pH, and droplet size within the ESI plume.
• Evaluate the impact of ESI mode polarity (ESI+ vs. ESI-) on these properties.
• Introduce a new metric, absolute pH (pHabsH2O), for comparing droplet acidity across different mobile phase compositions.

Main Methods:

• Studied droplet properties (organic solvent content, pH, size) in the ESI plume.
• Employed both positive (ESI+) and negative (ESI-) ionization modes.
• Introduced and utilized the absolute pH (pHabsH2O) metric to account for organic solvent content.

Main Results:

• Observed surprisingly similar changes in mobile phase properties for both ESI+ and ESI- modes.
• Demonstrated that droplet evolution is largely independent of ESI mode polarity.
• Evaporation of organic modifiers was identified as the primary driver of droplet evolution, followed by fission.

Conclusions:

• The dynamics of mobile phase property changes in ESI droplets are independent of the ionization mode.
• Findings from ESI+ mode can be reliably extrapolated to ESI- mode for understanding ESI processes.
• Droplet evolution is primarily governed by solvent evaporation and secondarily by droplet fission.