Reactive Ions in Aqueous Electrospray Microdroplet Experiments Originate from Electrical Discharge.

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Mass Spectrometry

Background:

• Electrospray ionization (ESI) experiments are often interpreted through microdroplet-specific redox chemistry, evidenced by detected radical ions.
• Previous studies proposed that unusual chemical reactivity in ESI is linked to reactions occurring within microdroplets.

Purpose of the Study:

• To investigate the origin of low-m/z radical ions observed in electrospray ionization (ESI).
• To determine if microdroplets are necessary for the formation of these reactive ions.
• To clarify the role of electrical discharge versus droplet-specific chemistry in ESI.

Main Methods:

• Accurate-mass measurements using internally referenced standards to determine elemental compositions of low-m/z ions.
• Comparison of ion formation under conditions with and without solvent infusion and electrospray-generated droplets.
• Analysis of ion distributions under varying background gas compositions and applied voltages across different ion source configurations.

Main Results:

• Radical cations (e.g., H₂O^+•, N₂^+•, O₂^+•) form at electrospray onset independent of microdroplet presence.
• The commonly reported m/z 36 ion is identified as NH₄⁺(H₂O), not (H₂O)₂^+•.
• Low-m/z ion distributions correlate with gas-phase discharge processes, not droplet-phase reactions, even during protein ion generation.

Conclusions:

• Electrical discharge in the gas phase is the primary source of small radical ions in ESI experiments, not microdroplets.
• Observed ion distributions can be explained by discharge-driven chemistry, negating the need for droplet-specific mechanisms.
• Experimental designs must differentiate between discharge and droplet-phase processes for accurate mechanistic assignments in ESI.