Another approach is to detect trapped NPs optically, with mass determination based on the NP motion in a trap. For example, Gerlich and co-workers developed three dimensional quadrupole traps with good optical access to the trap center, and then used light scattering to detect and carry out mass measurements on individual trapped NPs. Q for the highly charged NPs was determined by observing changes in the motional frequency when the trapped NP charge changed due to collisions with background electrons or cations. This approach typically has been done under conditions with modest mass resolution (M/ΔM ≤ 10 4 ), but resolution can exceed 10 6 , albeit at the cost of slow measurement times. On the other hand, individual NPs can be studied for long time periods as they are heated, cooled, or exposed to radiation, allowing studies of dynamical processes such as gas adsorption/desorption or electron emission. H.-C. Chang and coworkers used a different approach to optical detection to perform high precision mass measurements of single microbes, among other particles . The experiments described below also use optical detection of both the NP mass and temperature over time, to measure quantitative kinetics for surface reactions of NPs at high and ultra-high temperatures. The Viggiano group has recently completed construction of an instrument of this type, which they also plan to use for surface chemistry studies, related in this case to combustion processes of solid propellants. The optically-detected trapped NP approach has also been used by several groups to carry out spectroscopic measurements on individual trapped NPs, with simultaneous mass determination. For example, Bieske and co-workers used an ion trap with particle detection by laser-induced fluorescence to detect, mass analyze, and measure fluorescence spectra for individual trapped NPs such as dye-doped polystyrene nanospheres or dye-functionalized alumina or titania NPs. The Asmis group has developed a cryogenic nanoparticle trap experiment, allowing the study of single NP absorption spectra via action spectroscopy, in which optical absorption is detected by the change in NP mass as weakly adsorbed "tag" species (e.g. rare gas atoms) desorb due to heating by photon absorption. They have reported use of this approach to study adsorption and desorption processes and to measure plasmonic spectra of trapped gold NPs, for example.