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 k1 to form protonated monomer (MH+(H2O)x) and k2 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 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)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.
Highlights
Analytical response in atmospheric pressure chemical ionization (APCI) mass spectrometry (MS)[1−4] and ion mobility spectrometry (IMS) at ambient pressure[5,6] is governed by gas phase reactions between analyte neutrals and reagent ions, often derived from proton clusters in positive polarity
Residence times and collision frequencies are often sufficient to result in larger cluster ions such as proton bound dimers shown in eq 2
Reactions with hydrated protons are considered in models developed here recent studies have demonstrated that precursor ions including O2+ can be utilized at high electric fields and reduced pressures.[31]
Summary
Analytical response in atmospheric pressure chemical ionization (APCI) mass spectrometry (MS)[1−4] and ion mobility spectrometry (IMS) at ambient pressure[5,6] is governed by gas phase reactions between analyte neutrals and reagent ions, often derived from proton clusters in positive polarity. The abundances of protonated monomer and proton bound dimer with a single neutral substance (M) at a vapor concentration of 2.46 × 1011 cm−3 (or 10 ppb) are shown in Figure 1 for reaction. A complete description for ion distributions, reaction times, and vapor concentrations is shown as a contour plot in Figure 3 and encompasses parameters of modern APCI MS and IMS. Under these conditions, diffusion losses for DMMP increase with decreasing volume (radii) and are below 1% for time intervals
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