After trapping, FRF was adjusted to ensure that the NP was stably trapped (η < 0.3 ), resulting in harmonic motion of the NP in both the axial and radial directions. Measurement of the axial motional frequency (Fz,) was done using a resonance excitation method where a weak AC "drive" potential was applied to a lens just outside the trap, sweeping Fdrive through the expected frequency range for Fz. When Fdrive = Fz, the amplitude of the NP motion increases, resulting in a dip in the APD signal, which is fit to extract the value of Fz. As noted by Eqn. 1, Fz is proportional to the NP's charge-to-mass ratio (Q/M).