Negative Gaseous Ions in Positive-Voltage Electrospray Ionization Mass Spectrometry

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.