The absolute uncertainty in M values extracted from equation 1 depends on the accuracy with which the parameters are known. F RF is set by a digital waveform generator with negligible uncertainty. The value of z 0 (2.96 mm) has an estimated uncertainty of ±0.04 mm due to the accuracy of trap machining and assembly, contributing ~1.3 % to the absolute uncertainty. V 0 is monitored continuously during experiments by a pair of 5½ digit multimeters with stated V AC accuracy better than 0.3 % in the frequency range of interest. For Q initial < ~70 e, the F z resolution is usually sufficient to allow exact Q determination from fitting a series of Q steps, but if Q initial is larger, Q is typically uncertain by ~1.5%. Thus, the absolute uncertainty in M is typically ~1.5%, increasing to ~3% for highly charged NPs. In principle, the absolute mass scale could be calibrated, however, it is not obvious what calibration standard to use for the M/Q >10 5 Da/e range of interest.