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

Abstract

Gas phase reactions between hydrated protons H+(H2O) 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+(H2O) n ], rate constants k 1 to form protonated monomer (MH+(H2O) x ) and k 2 to form proton bound dimer (M2H+(H2O) z ), and diffusion constants. At 1 × 1010 cm–3 (0.4 ppb) for [H+(H2O) 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+(H2O) x and 7.8 μs to 46 ms for M2H+(H2O) z . Maximum yield for protonated monomer for a reaction time of 1 ms was ∼40% for k 1 from 10–11 to 10–8 cm3·s–1, for k 2/k 1 = 0.8, and specific values of [M]. This model demonstrates that ion distributions could be shifted from [M2H+(H2O) z ] to [MH+(H2O) x ] using excessive levels of [H+(H2O) 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.

Quantitative Distributions of Product Ions and Reaction Times with a Binary Mixture of VOCs in Ambient Pressure Chemical Ionization

“…Since the limiting reagent is hydrated protons, maximum signals for compounds occurs before vapor reactants are totally consumed. The patterns in Figure can be understood through eqs and where product ion formation follows k M ·[M] or k N ·[N] in the absence of cross reactions . At equal concentrations (=1 ppb), rate coefficients or reactivity of the substance govern the product ion abundances.…”

“…While vapor concentration was shown to control the formation of higher clusters such as proton bound dimers, 6 this second pathway (cross reaction) also occurs proportionally. Such cross reactions will certainly be present with substances that do form higher clusters, and we can anticipate heterogeneous proton bound dimers (i.e., MNH + ) along with M 2 H + and N 2 H + .…”

“…in the absence of cross reactions. 6 At equal concentrations (=1 ppb), rate coefficients or reactivity of the substance govern the product ion abundances. Rate coefficients of M and N affect the time in which reactant ions are depleted and the relative abundances of formed product ions.…”

“…3 Reactions are relatively fast with collision frequencies of roughly 7 × 10 9 s −1 for hydrated protons and neutrals following second order kinetics as described recently with a general model for ion abundances, reaction times, rate coefficients, and vapor concentrations. 6 Findings from this computational model demonstrated that reactions reach completion in tens to hundreds of milliseconds at parts per billion (ppb) vapor concentrations, and these are significantly longer than sample residence times in commercial ion sources for MS and ion mobility spectrometry (IMS). Ionization of sample neutrals M and N to product ions follow second order rate equations (eqs 3 and 4):…”

Time-Resolved Ion Mobility Spectrometry with a Stop Flow Confined Volume Reaction Region

“…6-Methyl-5-hepten-2-one was identified to contain octane-2,7dione (PA 892 kJ mol −142 ) with 2% by mass. 37 Because of this, the sulcatone -2-butanone mixture may be referred to as a ternary mixture.…”