posted on 2021-07-15, 13:36authored byElie Lattouf, Osmo Anttalainen, Tapio Kotiaho, Hanna Hakulinen, Paula Vanninen, Gary Eiceman
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.