Many of the other carbon NPs also etched in multiple waves punctuated by brief periods with near-zero EEO2, however, the EEO2 values tended to drop over time, such that the peak EEO2 values in succeeding waves remained well below the initial EEO2 values. The total mass lost before becoming inert was also much smaller than for graphite, and little mass loss occurred except in the first wave. For example, the initial EEO2 values for both carbon black NPs were ~10 -3 Da/O2 collision, similar to values for the graphite NPs, but because EEO2 decreased during the initial 3 to 4 hour etching wave, only ~6% of Minitial was lost. Each carbon black NP also had one or two additional etching waves lasting up to several hours each, however, the rates remained low and there was little additional mass loss. Similarly, the graphene oxide NP also had initial EEO2 similar to those for the graphite NPs, and for the first ~2.5 hours it evolved quite similarly to one of the graphite NPs (Fig. , open black), including a small increase in EEO2 at short times, followed by slowly decreasing reactivity. At the ~3.5 hour point, however, the graphene oxide NP rapidly became unreactive, reaching our sensitivity limit at ~4 hours after just ~16% mass loss, and although there were two additional etching waves during the next 11 hours, the EEO2 values remained low so that little additional mass was lost.