Evidence for electron-based ion generation in radio-frequency ionization

Area of Science:

• Analytical Chemistry
• Mass Spectrometry

Background:

• Radio-frequency ionization (RFI) is a novel ionization technique for mass spectrometry (MS).
• The precise mechanism of ion generation in RFI, particularly electron emission, remains incompletely understood.
• Understanding RFI's ion generation is crucial for expanding its applications in MS analysis of volatile organic compounds (VOCs).

Purpose of the Study:

• To investigate the role of electron emission in the RFI process.
• To characterize the behavior of RF-generated electrons within a Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer.
• To quantify the electron attachment rate constant for hexafluorobenzene (C6F6) using RFI-MS.

Main Methods:

• Direct measurement of charged particle current.
• Indirect detection of electrons via reaction with hexafluorobenzene (C6F6).
• Utilized a 9.4-T FT-ICR mass spectrometer under ultrahigh vacuum conditions.

Main Results:

• Demonstrated that radio-frequency generated electrons can be trapped in the ICR cell.
• Observed the formation of C6F6 radical anions (C6F6•−) from trapped electrons reacting with C6F6.
• The intensity of C6F6•− correlated with trapped electron numbers and decreased with quenching time.
• Measured electron attachment rate constant for C6F6 agreed with literature values.

Conclusions:

• Experimental evidence supports the involvement of RF-generated electrons in the RFI process under ultrahigh vacuum.
• This finding enhances the mechanistic understanding of RFI-MS.
• Confirms RFI's capability for negative-ion mode analysis through electron attachment.