Ion collision crosssection measurements in quadrupole ion traps using a time-frequency analysis method.

In this study, a method for measuring ion collision crosssections (CCSs) was proposed through time-frequency analysis of ion trajectories in quadrupole ion traps. A linear ion trap with added high-order electric fields was designed and simulated. With the presence of high-order electric fields and ion-neutral collisions, ion secular motion frequency within the quadrupole ion trap will be a function of ion motion amplitude, thus a function of time and ion CCS. A direct relationship was then established between ion CCS and ion motion frequency with respect to time, which could be obtained through time-frequency analysis of ion trajectories (or ion motion induced image currents). To confirm the proposed theory, realistic ion trajectory simulations were performed, where the CCSs of bradykinin, angiotensin I and II, and ubiquitin ions were calculated from simulated ion trajectories. As an example, differentiation of isomeric ubiquitin ions was also demonstrated in the simulations.