Mechanistic Understanding Leads to Increased Ionization Efficiency and Selectivity in Dielectric Barrier Discharge Ionization Mass Spectrometry: A Case Study with Perfluorinated Compounds

Perfluorinated compounds have unique properties and many practical applications, but are difficult to ionize efficiently with soft ionization methods. An active capillary plasma ionization source based on dielectric barrier discharge ionization (DBDI) coupled with mass spectrometry was used to study the ionization pathway of perfluorinated compounds (PFCs), with the aim of both increasing the ionization efficiency and influencing the selectivity for generating product ions in negative ion mode. Cyclic and linear perfluorinated alkanes were found to mainly form [M – F]⁻ and [M – F + O]⁻ ions, respectively; the [M]^−• ion was only obtained at low discharge voltage. Additionally, fluorine attachment [M + F]⁻ was observed mostly for perfluorinated alkenes. An isotope labeling experiment with ¹⁸O₂ showed that the primary source of oxygen in the substitution reaction is molecular oxygen, reacting with the analyte in the form of O^–• ions. The abundance of [M – F + O]⁻ ions can thus be enhanced by increasing the plasma voltage to produce a higher O^–• ion density. The loss of the fluorine (without substitution by oxygen) was mainly observed at high frequency, a fact which can be exploited for tuning the ionization toward specific product ions. Overall, the mechanistic understanding of the ionization of PFCs allowed to increase the selectivity of the product ions, resulting in increased ionization efficiency.