Dynamic Energy Measurements in Charge Detection Mass Spectrometry Eliminate Adverse Effects of Ion–Ion Interactions

Abstract

Ion−ion interactions in charge detection mass spectrometers that use electrostatic traps to measure masses of individual ions have not been reported previously, although ion trajectory simulations have shown that these types of interactions affect ion energies and thereby degrade measurement performance.Here, examples of interactions between simultaneously trapped ions that have masses ranging from ca. 2 to 350 MDa and ca. 100 to 1000 charges are studied in detail using a dynamic measurement method that makes it possible to track the evolution of the mass, charge, and energy of individual ions over their trapping lifetimes. Signals from ions that have similar oscillation frequencies can have overlapping spectral leakage artifacts that result in slightly increased uncertainties in the mass determination, but these effects can be mitigated by the careful choice of parameters used in the short-time Fourier transform analysis. Energy transfers between physically interacting ions are also observed and quantified with individual ion energy measurement resolution as high as ∼950. The mass and charge of interacting ions do not change, and their corresponding measurement uncertainties are equivalent to ions that do not undergo physical interactions. Simultaneous trapping of multiple ions in CDMS can greatly decrease the acquisition time necessary to accumulate a statistically meaningful number of individual ion measurements. These results demonstrate that while ion−ion interactions can occur when multiple ions are trapped, they have negligible effects on mass accuracy when using the dynamic measurement method.

Ion emission from 1–10 MDa salt clusters: individual charge state resolution with charge detection mass spectrometry

“…Ion frequencies that increase above this +10 Hz threshold can occur due to ion–ion interactions. 48 Traces that cannot be associated with an initially trapped ion are discarded.…”

“…There are also small variations in frequency due to ion–ion interactions. 48 In order to distinguish frequency changes due to charge loss from those caused by weak ion–ion interactions, the first derivative with respect to time of each trace of the fundamental frequency of each ion is computed. A 25 ms wide moving average of the first derivative signal stepped forward in 5 ms intervals is then used to smooth the trace.…”

“…34 CDMS has the advantage that the m / z , charge, and mass of ions are measured on an individual basis, thereby circumventing issues associated with the spectral congestion of heterogeneous samples analyzed using conventional mass spectrometers. 18,34–48 This capability has extended the size range of ionic clusters that can be investigated by more than 100-fold. The extent of charging on aqueous nanodrops relative to the Rayleigh limit charge for an aqueous nanodrop depends on the identity of ions contained in the droplet.…”

“…Here, the dissociation behavior of salt clusters formed from aqueous solutions containing KCl, CaCl 2 , and LaCl 3 is investigated using a method that makes it possible to obtain individual charge state resolution for the discrete emission of +1 and +2 charged species from 1–10 MDa ions. Cluster ion energy, m / z , charge, and mass are measured throughout the entire time that ions are trapped, which can range from 100 ms up to 5+ s. 41,43–46,48 These capabilities have previously been used to monitor pure aqueous nanodrops up to 600+ MDa (50–120 nm in diameter) and measure relatively small ion emission events that preceded or followed large fission events. 18 These results demonstrate that CDMS is well-suited for investigating the dynamics of large clusters or nanodrops and can enable further insight into the late stages of gaseous cluster ion formation from charged droplets.…”

Combined Multiharmonic Frequency Analysis for Improved Dynamic Energy Measurements and Accuracy in Charge Detection Mass Spectrometry

“…This enables dynamic measurements of ion energies and makes it possible to track changes in the ion energy during the trapping period. Phenomena such as energy losses due to collisions with background gas, ,, mass losses due to solvent evaporation, ,, ion fragmentation, spontaneous charge emissions, , and energy transfers due to ion–ion interactions have all been analyzed using CDMS combined with the dynamic energy measurement method. An additional advantage of this method is that unlike filter-based ion energy determination where the energy resolution is solely determined by the passband of the filter, the precision of the dynamic method depends on the signal-to-noise ratio (S/N) of the harmonic amplitudes.…”