For most NPs in this study, Q was low enough (<70 e) to be determined exactly by fitting the series of Q steps. In that case, the largest source of uncertainty in Eqn. 1 is from the trap "size" parameter, z0, which is estimated to be ±0.04 mm, giving rise to uncertainty in M of ~±1.3%. For NPs with large Q, there is an additional ~1% uncertainty in M based on the uncertainty in Q. Note that both the z0 and Q uncertainties only affect the absolute value of M. Neither factor contributes to the relative uncertainty for comparing different mass measurements, which is estimated to be ~±0.2%, mostly due to noise and the resulting error in the fitting of the Fz resonances.