Validation of Field-Dependent Ion–Solvent Cluster Modeling via Direct Measurement of Cluster Size Distributions

Abstract

Ion mobility spectrometry is widely used in analytical chemistry, either as a stand-alone technique or coupled to mass spectrometry. Ions in the gas phase tend to form loosely bound clusters with surrounding solvent vapors, artificially increasing the collisional cross section and the mass of the ion. This, in turn, affects ion mobility and influences separation. Further, ion–solvent clusters play an important role in most ionization mechanisms occurring in the gas phase. Consequently, a deeper understanding of ion–solvent cluster association and dissociation processes is desirable to guide experimental design and interpretation. A few computational models exist, which aim to describe the amount of clustering as a function of the reduced electric field strength, bath gas pressure and temperature, and the chemical species probed. It is especially challenging to model ion mobility under high reduced electrical field strengths due to the nonthermal conditions created by the field. In this work, we aim to validate a recently proposed first-principles model by comparing its predictions with direct measurements of cluster size distributions over a range of 20–120 Td as observed using a High Kinetic Energy Ion Mobility Spectrometer coupled to a mass spectrometer (HiKE-IMS-MS). By studying H+(H2O) n , [MeOH + H + n(H2O)]+, [ACE + H + n(H2O)]+, and [PhNH2 + H + n(H2O)]+ as test systems, we find very good agreement between model and experiment, supporting the validity of the computational workflow. Further, the detailed information gained from the modeling yields important insights into the cluster dynamics within the HiKE-IMS, allowing for better interpretation of the measured ion mobility spectra.

On the Arrival Time Distribution of Reacting Systems in Ion Mobility Spectrometry

“…To gain an overview of the fragmentation behavior of protonated EtOAc in HiKE-IMS (as opposed to PTR-MS), we first recorded the abundance of each species involved as a function of reduced field strength. This also allows us to validate whether the MC modeling predicts the correct ion chemistry as a function of reduced field strength . As can be seen in Figure , the fragments known from PTR-MS appear above ca.…”

“…Previously, a first-principles model was presented, which is able to simulate the ion mobility and ion chemistry of a reacting system during the transit through an IMS device. , For the ion chemistry, the relative populations of each species at a certain time, P ( t ), are propagated in time via a state-transition matrix, i.e., through a Markov-chain process: bold-italicP ( t + Δ t ) = bold-italicϕ ( Δ t ) · bold-italicP ( t ) Here, entry ϕ ij describes the reaction probability from species j to species i in time interval Δ t , based on the respective rate constant (see below). For the ion motion, eq is used, i.e., the ensemble moves a step of d incr = false⟨ v D false⟩ ens normalΔ t = false⟨ K false⟩ ens E normalΔ t false⟨ K false⟩ ens = ∑ i K i P i per Δ t through the device.…”

“…Previously, a first-principles model was presented, which is able to simulate the ion mobility and ion chemistry of a reacting system during the transit through an IMS device. 50,51 For the ion chemistry, the relative populations of each species at a certain time, P(t), are propagated in time via a state-transition matrix, i.e., through a Markov-chain process:…”

“…As stated above, we estimate the background water concentration in the drift region to be around 70 ppm V , giving H + (MeOH) the opportunity to form water clusters. In fact, we studied this system previously 51 and Figure 2 shows the 2D-IMS-MS data for this system at a reduced field strength of 70 Td as obtained from the experiment and MC data. Note that the experimental data (Figure 2A) also shows the reactant ion peak (RIP), here consisting of the H + (H 2 O) 2 species (m/z 37), which is not considered in the calculations.…”

“…100 Td in successive order, as expected from the reaction mechanism (Scheme 1). The high reduced field 51 and can be explained by a ligand-switching mechanism between the monohydrate and proton-bound dimer and their different stabilities (see Figure S12). After some fine-tuning of the fragmentation reaction rates (see Section S4.3 of the SI for details), the modeled relative populations closely match the observations over the whole range of reduced field strengths very well.…”