Characterization of a Nitrogen-Based Dielectric Barrier Discharge Ionization Source for Mass Spectrometry Reveals Factors Important for Soft Ionization

Atmospheric pressure plasma-based ionization coupled to mass spectrometry is a powerful analytical technique. However, the characteristics of existing ionization sources, especially regarding the reactive species and the effect of the discharge type on the soft ionization, are often not well described. In this work, the active capillary plasma ionization source, which is based on a dielectric barrier discharge, was characterized by optical emission spectroscopy and mass spectrometry. To obtain a better understanding of the requirements for a soft ionization, several reactive species and the energy of the ionization reaction were identified. Charged reactive species such as H3O+, N2 +•, N2H+, NO+, N3 +, and N4 +• as well as uncharged species (most probably the excited neutral nitrogen) were all found to contribute to the soft ionization process in dielectric barrier discharge. The energy in the plasma was determined to be in the range from 8 to 16 eV, based on the ionization energy of nitrogen and the measurements of tungsten hexacarbonyl. Furthermore, not only was the type of reactive species relevant for achieving a soft ionization, it was also crucial that the sample was injected through the inner electrode, which leads to nearly no direct contact with the discharge filaments.