With appropriate values of V 0 and F RF , it is possible to trap charged species ranging from molecular ions to micron-size particles. Indeed, in our initial experiments, we started by trapping Rhodamine 6G cations, detecting them by fluorescence, and also used fluorescence to trap semiconductor quantum dots with sizes as small as ~5 nm. For the purposes of measuring NP surface reaction kinetics, however, we tend to work with NPs containing between a few hundred thousand and a few million atoms, with M/Q in the 10 5 to 10 6 Da/e range. These are large enough to have bulk-like densities and electronic properties, but as shown below, they still have interesting surface chemistry that can differ strongly from bulk surfaces. Much larger NPs could be studied, however, if M is increased without also increasing Q, F z drops, and this slows the measurement times, while if Q is also increased substantially, the effects of ±1 e charge steps become increasingly difficult to distinguish from the effects of changing mass.