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

journal contribution

posted on 2021-07-15, 13:36 authored by Elie Lattouf, Osmo Anttalainen, Tapio Kotiaho, Hanna Hakulinen, Paula Vanninen, Gary EicemanGas 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}to form protonated monomer (MH^{+}(H_{2}O)_{x}) and*k*_{2}to 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}to 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.