The experiment has been described previously and is shown schematically in Fig. . In essence, the instrument has an atmospheric pressure inlet equipped with an electrospray ionization (ESI) source to inject NPs into the vacuum system. ESI is done from dilute suspensions of NPs in methanol, with ~200 -800 μM ammonium acetate added as a charging reagent. Spraying is typically done with a capillary potential of 3kV in an N 2 -purged environment. Ammonium acetate is used because it decomposes to volatile products well below our T NP range of interest, and methanol should also desorb at relatively low T NP , leaving the NP surfaces uncontaminated by ESI species. The process of removing oxides or other contaminants from prior air exposure is discussed below.