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. 2019 Dec;30(12):2741-2749.
doi: 10.1007/s13361-019-02343-y. Epub 2019 Nov 1.

Ion-Ion Interactions in Charge Detection Mass Spectrometry

Daniel Y Botamanenko  1 , Martin F Jarrold  2
Affiliations

Ion-Ion Interactions in Charge Detection Mass Spectrometry

Daniel Y Botamanenko et al. J Am Soc Mass Spectrom. 2019 Dec.

Abstract

Charge detection mass spectrometry (CDMS) is a single-particle technique where the masses of individual ions are determined by simultaneously measuring their mass-to-charge ratio (m/z) and charge. Ions are usually trapped inside an electrostatic linear ion trap (ELIT) where they oscillate back and forth through a detection cylinder, generating a periodic signal that is analyzed by fast Fourier transforms. The oscillation frequency is related to the ion's m/z, and the magnitude is related to the ion's charge. In early work, multiple ion trapping events were discarded because there was a question about whether ion-ion interactions affected the results. Here, we report trajectory calculations performed to assess the influence of ion-ion interactions when multiple highly charged ions are simultaneously trapped in an ELIT. Ion-ion interactions cause trajectory and energy fluctuations that lead to variations in the oscillation frequencies that in turn degrade the precision and accuracy of the m/z measurements. The peak shapes acquire substantial high and low m/z tails, and the average m/z shifts to a higher value as the number of trapped ions increases. The effects of the ion-ion interactions are proportional to the product of the charges and the square root of the number of trapped ions and depend on the ions' m/z distribution. For the ELIT design examined here, ion-ion interactions limit the m/z resolving power to several hundred for a typical homogeneous ion population.

Keywords: CDMS; Charge detection mass spectrometry; ELIT; Electrostatic linear ion trap; Ion-ion interactions.

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Figures

Figure 1.
Figure 1.
SIMION representation of the electrostatic linear ion trap used in the simulations with the x-, y-, and z-axes superimposed
Figure 2.
Figure 2.
Examples of the two limiting forms of trajectories found for single trapped ions. (a) A planar trajectory that resulted for an ion that started with a radial offset of 0.75 mm in the y-direction and had an entrance angle of 0°. (b) A cylindrical trajectory that resulted for an ion that started with a radial offset of 0.75 mm in the y-direction and had an entrance angle of 1.2° in the x-direction. In both cases, the ions had a mass of 5 MDa, a charge of 200 e, and an initial kinetic energy of 130 eV/z
Figure 3.
Figure 3.
Plot of the ion energy against time for two ions simultaneously trapped for 100 ms. The second ion trajectory was initialized 6 μs after the first. In that time, the first ion had moved 4 mm from the center of the detection cylinder where the trajectories were initiated
Figure 4.
Figure 4.
More detailed information about the ion-ion interaction that causes the energy jump in Figure 3 at around 93 ms. (a) Views of the trajectories in the xy-plane (i.e., looking down the axis of the trap) before (black lines) and after (red lines) the encounter that causes the energy shifts. (b) A plot of the distance between the two ions as a function of time. (c) The change in the oscillation frequency as a function of time. (d) The change in the ion energy
Figure 5.
Figure 5.
Average frequency deviations for two ion trapping events. (a) The average frequency deviation as a function of the m/z difference for two ion trapping events. The upper scale is the average initial frequency differences that correspond to the m/z differences given in the lower scale. The points are the average from 2500 simulated trapping events. (b) The average frequency deviations and the average energy deviations are linearly related
Figure 6.
Figure 6.
Results for trapping of multiple ions with the same m/z (25,000 Da) and charge (200 e). (a) Plots of the m/z distributions obtained for trapping of one to eight ions. (b) A plot of the m/z peak width versus the number of trapped ions. The red points are determined from the FWHM of the m/z distributions in (a). The black points are determined from 2.355 times the RMSD of the distribution. Both methods yield the same value for a Gaussian distribution. (c) A plot of the average frequency deviation from ion-ion interactions against the number of trapped ions
Figure 7.
Figure 7.
Results for trapping of multiple ions with the same mass (5.00 MDa) and a Gaussian charge distribution centered on 200 e. (a) Plots of the mass distributions obtained for trapping of one to eight ions. (b) A plot of the mass peak width versus the number of trapped ions. The red points are determined from the FWHM of the mass distributions in (a). The black points are determined from 2.355 times the RMSD of the distribution. Both methods yield the same value for a Gaussian distribution
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