Characterization of Mobility-Dependent Ion Confinement in Rotating Electric Fields

Here, we describe ion confinement based on mobility in rotating electric fields under low E/N conditions. To do this, we constructed a device with a stack of eight segmented ring electrodes to which sinusoidal waveforms were applied with a 45° phase shift. Ion confinement was characterized by monitoring ion intensities measured using a quadrupole time-of-flight mass spectrometer. The All Pressure Ion Confinement (APIC) device was operated at a pressure range of 3.8-8.0 Torr. Two mixtures of phosphazene and tetraalkylammonium ions covering a broad mobility range were used to evaluate APIC transmission. The results showed that ion confinement depends on ion mobility in rotating electric fields. As pressure increases, the electric field strength required for maximum ion intensity also increases. Highly mobile ions need lower electric fields at a given pressure, while less mobile ions require stronger fields to reach maximum intensity. We also observed that ion confinement depends on the rotational speed of the electric field, highlighting the importance of balancing ion velocity and the rotating field speed. We define a dimensionless parameter α that scales with the ratio of ion velocity to the field's rotational speed. Varying electric field strength, ion mobility, and field rotation speed revealed a strong correlation between ion confinement and α, with optimal confinement observed when 0.1 < α < 1.5. These findings are useful for predicting mobility-dependent behaviors in low fields within rotating electric fields and can guide the design and operation of ion optics using such fields.