Negative Gaseous Ions in Positive-Voltage Electrospray Ionization Mass Spectrometry

Area of Science:

• Analytical Chemistry
• Mass Spectrometry

Background:

• Electrospray ionization (ESI) is a key technique in mass spectrometry.
• Previous research indicated positive voltage ESI can produce negatively charged droplets.
• The generation of negative ions from positive voltage ESI was not directly confirmed.

Purpose of the Study:

• To report the direct observation of negative ions generated during positive voltage ESI.
• To investigate the influence of analyte structure on negative ion formation.
• To explore potential mechanisms for this phenomenon.

Main Methods:

• Utilized positive voltage electrospray ionization (ESI) with mass spectrometry (MS) detection in negative-ion mode.
• Employed spray current measurements using a concentric ring electrode.
• Compared MS intensities of positive and negative ions under varying voltage conditions.

Main Results:

• Direct MS detection of negative ions generated from positive voltage ESI was achieved.
• Negative ion formation was observed both with and without nebulizing gas.
• Analyte structure was found to affect the yield of negative ions.

Conclusions:

• Positive voltage ESI can indeed produce observable negative ions.
• These findings offer novel mechanistic insights into the ESI process.
• The results may guide future developments in ESI techniques for mass spectrometry.