Regulated in situ generation of molecular ions or protonated molecules under atmospheric-pressure helium-plasma-ionization mass spectrometric conditions

In an enclosed atmospheric-pressure helium-plasma ionization (HePI) source engulfed with dehumidified ambient gases, molecular cations are generated from compounds such as toluene, bromobenzene, and iodobenzene. Evidently, the ionization is effected by a direct Penning mechanism attributable to interactions of the gas-phase analyte with metastable helium atoms. It is widely known that secondary ions generated from ambient gases also play an important role in the overall ionization process. For example, when the ambient gases bear even traces of moisture, the analytes are ionized by proton transfer reactions with gaseous H₃O⁺. In this study, we demonstrate how a controlled variation of experimental conditions can manipulate the abundance of molecular ions and protonated molecules in a HePI source. [Figure not available: see fulltext.]