# Negative Gaseous Ions in Positive-Voltage Electrospray Ionization Mass Spectrometry

## TL;DR

This paper reports the unexpected detection of negative ions in positive-voltage electrospray ionization mass spectrometry, offering new insights into the ionization process.

## Contribution

The first direct mass spectrometry measurement of negative gaseous ions from standard positive electrosprays is presented.

## Key findings

- Negative ions were detected in positive-voltage electrospray ionization mass spectrometry.
- Analyte structure affects the ability of positive electrospray to generate negative ions.
- Negative ions were observed both with and without nebulizing gas.

## Abstract

Here, we report a serendipitous observation of negative
ions in
positive-voltage electrospray ionization (ESI) mass spectrometry (MS).
Several previous studies have demonstrated that electrospray operated
with positive voltage can generate droplets carrying a negative net
charge. Our spray current measurements on a counter electrode with
concentric conductive rings indicate the presence of negative species
within the charged aerosol produced by positive voltage electrospray.
MS measurements performed in negative-ion mode captured negative ions
generated by positive electrospray, both in the presence and absence
of nebulizing gas. Analyte structure influences the ability of positive
electrospray to generate the corresponding negative ions. The MS intensities
produced by positive and negative ions were compared when positive
and no voltage was applied to the electrospray. Possible mechanisms
underlying these observations are discussed in the context of recent
literature. These findings represent the first direct MS measurement
of negative desolvated gaseous ions from standard positive electrosprays
and provide new mechanistic insights that may inform future ESI development.