Understanding and Optimizing the Ionization of Polycyclic Aromatic Hydrocarbons in Dielectric Barrier Discharge...

Fundamental aspects of undoped and dopant-assisted dielectric barrier discharge ionization (DBDI) were here addressed to help overcome the suboptimal ionization efficiency of nonpolar species. In the absence of dopants, polycyclic aromatic hydrocarbons (PAHs) mostly form [M + H]⁺ rather than [M]^+• ions. Humidity was shown to further increase the relative amount of protonated PAHs, while the introduction of other dopants, such as fluorobenzene and chlorobenzene, shifted the ionization toward radical cations. The source of the protons was clarified through the use of deuterated species and confirmed both water as well as dopants as potential sources. The type of plasma gas also influenced the ionization, with the relative amount of radical cations increasing from CO₂, to N₂, to air. The addition of SF₆ gas and the relative amount of low-energy (0-11 eV) electrons in the source suggests that Penning ionization leads to the relatively high amount of radical cations in air plasma. Penning ionization of excess dopant to form radical dopant species, and subsequent charge exchange with the PAHs, is proposed as the principal ionization mechanism for the formation of radical cationic PAHs in dopant-assisted DBDI. Benzylamine "thermometer molecules" were used to evaluate the energy involved in the ionization process and showed that dopants can affect the internal energy (-39.4 kJ mol^-1 with humidity in a nitrogen plasma and +15.7 kJ mol^-1 with fluorobenzene in an air plasma). A deeper understanding of fundamental aspects, especially of dopant-assisted ionization in a dielectric barrier discharge source, will be vital for future method optimization, particularly with regards to difficult to ionize (e.g., nonpolar) species.