Fig. plots F z , Q, and M for a hafnium NP trapped with 2 mTorr of UHP Ar buffer gas, then held at 1500 K for an hour. Note that during this period F z increased in obvious steps, attributed to thermionic emission, which is facile for Hf NPs due to hafnium's low work function (~4 eV, varying with surface conditions and nanostructure ). By fitting a series of such steps, taking into account the slopes of F z vs. time before and after each step, the corresponding Q values can be determined, and knowing Q allows M to be calculated from F z . In this simple example, the initial mass corresponds to ~1.55 x 10 6 Hf atoms and decreased at an average rate corresponding to loss of ~0.53 Hf atoms/second, attributed mostly to etching by reaction with background O 2 present in the trap atmosphere in the 10 -8 Torr range (see below).