Experimental evidence for space-charge effects between ions of the same mass-to-charge in Fourier-transform ion cyclotron resonance mass spectrometry

Abstract

It is often stated that ions of the same mass-to-charge do not induce space-charge frequency shifts among themselves in an ion cyclotron resonance mass spectrometry measurement. Here, we demonstrate space-charge induced frequency shifts for ions of a single mass-to-charge. The monoisotopic atomic ion, Cs + , was used for this study. The measured frequency is observed to decrease linearly with an increase in the number of ions, as has been reported previously for spacecharge effects between ions of different mass-to-charge. The frequency shift between ions of the same m/z value are compared to that induced between ions of different m/z value, and is found to be 7.5 times smaller. Control experiments were performed to ensure that the observed space-charge effects are not artifacts of the measurement or of experimental design. The results can be rationalized by recognizing that the electric forces between ions in a magnetic field conform to the weak form of the Newton's third law, where the action and reaction forces do not cancel exactly.

Comparison of particle-in-cell simulations with experimentally observed frequency shifts between ions of the same mass-to-charge in fourier transform ion cyclotron resonance mass spectrometry

“…Contrary to prior published proposals regarding space-charge interaction between ions of the same mass-to-charge [2, 12], experimental results in our lab have demonstrated frequency shifts among monoisotopic populations of Cs + ions [13]. A linear decrease in frequency with increasing ion abundance was observed over most of the range of ion abundance, and attributed to space charge induced frequency shifts between Cs + ions.…”

“…Over the range of ion numbers, 500 to 250,000, the frequency for Cs + present by itself decreases by only 3 Hz whereas the frequency for Cs + when ions of a second m/z value are present (same total number of ions) decreases by 21 Hz, for an orbital radius of 35%. The resulting ratio of the slopes is 1:7, in excellent agreement to the experimental value of 1:7.5 [13]. The much steeper slope of the simulated frequencies for two different mass-to-charge values corresponds to the addition of ion-ion space charge effects to the ion-image charge interaction.…”

“…Recently, our laboratory has made an experimental investigation of frequency shifts amongst monoisotopic populations of Cs + ions [13] and attributed the source of these shifts to be due to space charge interaction between like ions. These findings contradict previous theoretical studies of space charge interactions of ions contained within a Penning trap where like ions have been proposed to have no coulombic effect on cyclotron frequency [2, 12].…”

Space Charge Frequency Shifts of the Cyclotron Modes in Multi-Species Ion Plasmas

“…Plotted in Figure 6 are m =0, 1, and 2 cyclotron frequencies versus f E for 24 Mg + , 25 Mg + , and 26 Mg + . The shift of the cyclotron frequency increases linearly as space charge is increased, as seen in previous experiments [18][19][20][21]. However, this increase with f E is also proportional to the relative charge concentration δ s of the species.…”

Fast image–charge calculations for multi-particle simulations in FT-ICR analyzer cells of arbitrary geometry

“…Even though we cannot accurately determine the number of ions in the analyzer cell during an experiment, we can check that the observed frequency shift corresponds to a reasonable number of ions. Wong and Amster have measured a cyclotron frequency shift as a function of Cs + ion intensity, with a 50 fold difference in ion intensity between the highest and lowest ion numbers used in this experiment [28]. The frequency shift between the largest and smallest population of ions is 0.7 Hz, roughly corresponding to 350,000 ions, using the PIC derived value for frequency shift per ion of 2:0 Â 10 À6 Hz.…”