Elucidation of formation mechanism responsible for charge-transfer reagent ions in microwave induced plasma...

Charge transfer reagent ions NO⁺ and O₂⁺ were major observed in microwave-induced plasma desorption ionization (MIPDI) source. Ions NO⁺ was prosperous in the discharge region while ions O₂⁺ generated in the afterglow region. Fundamental aspects of conditions that controlled MIPDI were addressed here to help understand the specific role of atmospheric components for charge transfer reagent ions formation. In the presence of trace nitrogen in Ar-MIPDI plasma, the production of NO⁺ ions increased significantly, while ions N₂⁺ were not observed. The controlled afterglow environment obtained by connecting the plasma source to the mass spectrometer using a quartz reaction chamber found that ions O₂⁺ produced from the interaction between plasma-generated energy particles and oxygen molecules. With the aid of mass spectra and optical emissions spectra, the ionization capacity of Ar-MIPDI plasma was revealed. Both the discharge region and afterglow region are not energetic enough to ionize N₂ molecules. Charge transfer reagent ions in the MIPDI source have vital ionization ability for aromatic compounds. A deeper understanding of fundamental aspects, especially of condition-controlled ionization in a MIPDI source, will be vital for future practical applications, particularly with regard to difficult to ionize (e.g., nonpolar) species.