Ion-Ion Interactions in Charge Detection Mass Spectrometry

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.

Dramatic Improvement in Sensitivity with Pulsed Mode Charge Detection Mass Spectrometry

“…In our early work using CDMS, we restricted the measurements to single trapped ions and trapping events where more than one ion was trapped were discarded. It is feasible to analyze the multiple ion-trapping events and determine m / z values and charges for a few simultaneously trapped ions. , However, when two or more highly charged ions are simultaneously trapped in an ELIT, ion–ion interactions cause trajectory and energy fluctuations which degrade the m / z resolving power . Because high m / z resolving power is not required for these studies, the multiple ion-trapping events were analyzed.…”

“…31,59 However, when two or more highly charged ions are simultaneously trapped in an ELIT, ion−ion interactions cause trajectory and energy fluctuations which degrade the m/z resolving power. 59 Because high m/z resolving power is not required for these studies, the multiple ion-trapping events were analyzed. However, the trapping of multiple ions with similar m/z values can lead to errors in the data analysis and so the measurements reported here were restricted to samples where on average one ion is trapped per trapping event.…”

Applications of Charge Detection Mass Spectrometry in Molecular Biology and Biotechnology

“…With these traps, it is feasible to trap up to 10 ions at a time without causing a significant degradation in performance. In the future, traps could be designed to optimize performance for multiple ions …”

Higher Resolution Charge Detection Mass Spectrometry

“…55,63,64 Self-bunching cannot occur in CDMS; the ion density is too low, and ion−ion interactions are thought to be detrimental. 65 Efforts to improve the m/z resolving power of CDMS ELITs have focused on reducing the dependence of the ion's oscillation frequency on the ion energy and the ion's radial offset and angular divergence. 66 In this manuscript we describe studies where recent improvements in the accuracy of the charge measurements in CDMS 53 are combined with higher resolution m/z measurements 66 to obtain a mass resolving power almost an order of magnitude better than that achieved in previous CDMS measurements.…”

“…In the ELITs designed for ion bunches, efficient trapping is less important, and ions with trajectories that move slightly off-axis can be lost and hence not degrade the resolving power. In addition, self-bunching (through ion–ion interactions) can reduce the effects of the ions’ energy distribution. ,, Self-bunching cannot occur in CDMS; the ion density is too low, and ion–ion interactions are thought to be detrimental . Efforts to improve the m / z resolving power of CDMS ELITs have focused on reducing the dependence of the ion’s oscillation frequency on the ion energy and the ion’s radial offset and angular divergence …”