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

Gas phase reactions between hydrated protons H + (H 2 O) n 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 + (H 2 O) n ], rate constants k 1 o form protonated monomer (MH + (H 2 O) x ) and k 2 o form proton bound dimer (M 2 H + (H 2 O) z ), and diffusion constants. At 1 × 10 10 cm -3 (0.4 ppb) for [H + (H 2 O) n ] 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 + (H 2 O) x and 7.8 μs to 46 ms for M 2 H + (H 2 O) z . Maximum yield for protonated monomer for a reaction time of 1 ms was ∼40% for k 1 from 10 -11 o 10 -8 cm 3 ·s -1 , for k 2 / k 1 = 0.8, and specific values of [M]. This model demonstrates that ion distributions could be shifted from [M 2 H + (H 2 O) z ] to [MH + (H 2 O) x ] using excessive levels of [H + (H 2 O) n ], 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.