Q stepping often occurs spontaneously during experiments, and it is essential that these Q changes be detected so that M can be correctly determined from the F z measurements. Keeping track of changes in Q over time is straightforward as long as the F z steps from ±e changes in Q are large enough to clearly distinguish from F z changes due to changing M, as in Fig. , for example. Under some conditions, however, typically involving reactions at very high T NP or high reactant pressures, F z can change so rapidly that it is difficult to detect Q steps unambiguously. An extreme case, illustrated below, is when an NP fragments during reaction, resulting in a large step change in F z due to loss of fragments carrying both mass and charge. When any of these situations occur, we first reduce the rate of F z change substantially, which can usually be accomplished simply by reducing T NP substantially, although reactant pressures can also be reduced if necessary. Once a stable rate of F z change is established, the new value of Q is determined by measuring and fitting a new series of Q steps, which can either be spontaneous (Q often steps down when T NP is dropped substantially) or driven by the VUV lamp. As a precautionary measure, we often do Q stepping both at the beginning and end of kinetics experiments to ensure that no spontaneous charge steps were missed during periods of rapid change.