The Study Of Ion/ion Interactions Within Ion Traps

Quadrupole ion traps have been widely used in mass spectrometry(MS) systems as mass analyzers and/or ion trapping, reaction devices. With a large number of ions trapped in a quadrupole ion trap, the effects of ion/ion interactions can not be ignored. On one hand, Coulomb force between ions will modify ion trajectory, and result in chemical mass shift, resolution degradation, and etc, with increasing number of ions trapped inside a quadrupole ion trap. As a mass analyzer, the performance of a quadrupole ion trap is greatly influenced by space charge effects. On the other hand, ions could react to dissociate into fragement ions. In ETD experiments, quadrupole ion traps are usually utilized as the reaction devices.For the space charge effects, though there are lots of experiments have observed and studied the effects, it is still lacking effective theoretical methods to study space charge effects on ion motion in quadrupole ion traps. In this work, a theoretical method was proposed to study space charge effects in quadrupole ion traps, including ion trapping, ion motion frequency shift and nonlinear effects on ion trajectories. To this end, the spatial distributions of ion clouds within quadrupole ion traps were first modeled for both 3D and linear ion traps. It is found that the electric field generated by space charge can be expressed as a summation of even-order fields, such as quadrupole field, octopole field, etc. Ion trajectories were then solved using the harmonic balance method. Similar to high-order field effects, space charge will result in an “ocean wave” shape nonlinear resonance curve for an ion under a dipolar excitation. However, the nonlinear resonance curve will be totally shifted to lower frequencies and bend towards ion secular frequency as ion motion amplitude increases, which is just the opposite effect of any even-order field. Based on theoretical derivations, methods to reduce space charge effects were proposed.For the ion/ion reactions, many experiments were performed to study the reaction types and reaction rates. However, there is still lack of theoretical model and simulation tools to provide us a deeper understanding of the reaction process. In this study, a gas-phase ion-ion reaction model was developed, and it was integrated into an ion trajectory simulation program. GPU parallel computation techniques were also applied to accelerate the simulation process. With this simulation tool, the dependence of ion-ion reaction rate within 3D quadrupole ion traps on both ion trap operation parameters and the characteristics of reaction pair were investigated. It was found that the m/z values and charge states of ions have significant influences on the reaction rate. Moreover, higher ion-ion reaction rate was achieved under higher trapping voltages and higher buffer gas pressures. Furthermore, secondary reaction and/or neutralization of ETD fragment ions were observed from simulation. The reaction and/or neutralization rate depends on the charge state and m/z ratio of each fragment ion, and more than 50% product ions could be lost due to this secondary reaction.