Parametric Sensitivity in a Generalized Model for Atmospheric Pressure Chemical Ionization Reactions

Gas phase reactions between hydrated protons H⁺(H₂O) and a substance M, as seen in atmospheric pressure chemical ionization (APCI) with mass spectrometry (MS) and ion mobility spectrometry (IMS), were modeled computationally using initial amounts of [M] and [H⁺(H₂O)], rate constants k₁ to form protonated monomer (MH⁺(H₂O)) and k₂ to form proton bound dimer (M₂H⁺(H₂O)), and diffusion constants. At 1 × 10¹⁰ cm^-3 (0.4 ppb) for [H⁺(H₂O)] and vapor concentrations for M from 10 ppb to 10 ppm, a maximum signal was reached at 4.5 μs to 4.6 ms for MH⁺(H₂O) and 7.8 μs to 46 ms for M₂H⁺(H₂O). Maximum yield for protonated monomer for a reaction time of 1 ms was ∼40% for k₁ from 10^-11 to 10^-8 cm³·s^-1, for k₂/k₁ = 0.8, and specific values of [M]. This model demonstrates that ion distributions could be shifted from [M₂H⁺(H₂O)] to [MH⁺(H₂O)] using excessive levels of [H⁺(H₂O)], even for [M] > 10 ppb, as commonly found in APCI MS and IMS measurements. Ion losses by collisions on surfaces were insignificant with losses of <0.5% for protonated monomer and <0.1% for proton bound dimer of dimethyl methylphosphonate (DMMP) at 5 ms. In this model, ion production in an APCI environment is treated over ranges of parameters important in mass spectrometric measurements. The models establish a foundation for detailed computations on response with mixtures of neutral substances.