Multiphysical description of atmospheric pressure interface chemical ionization in MION2 & Eisele type inlets

The inlets of chemical ionisation mass spectrometers are fundamental instrument components in chemical ionisation mass spectrometry (CIMS). However, the sample gas and reagent ion trajectories are often understood only in a general and qualitative manner. Here we evaluate two common atmospheric pressure chemical ionisation inlets (MION2 and Eisele type inlet) with 3D physico-chemical models regarding the reagent ion and sample gas trajectories and evaluate their efficiencies of reagent ion production, reagent ion delivery from the ion source volume into the ion-molecule mixing region, and the interaction between reagent ions and target molecules. The models are validated by laboratory measurements and quantitatively reproduce observed sensitivities to tuning parameters, including ion currents and changes in mass spectra. The study elucidates how the different transport and chemical processes proceed within the studied inlets, how space charge is already relevant at concentrations of as low as 10⁷ cm^-3, and compares the two investigated models. The models provide insights into how to operate the inlets and will help in the development of future inlets that further enhance the capability of CIMS.