To obtain the NP mass from the measured Q/M values, we need to know Q, and because Q can change during kinetics experiments, it is necessary to follow changes in Q under reaction conditions. Q is determined by measuring the discrete steps in F z (ΔF z ) that result from single electron changes in Q (i.e., ΔQ = ± e). Such charge steps can be driven using a VUV lamp to irradiate the trap center with 11.8