Electrosprayed material passes through a pair of skimmer cones, with differential pumping between them, and then passes into a larger differential pumping chamber equipped with a radio-frequency (RF) hexapole ion guide operating at 106 kHz and 750 V amplitude, with the hexapole chamber pressure set to ~1.6 mTorr (of N 2 ) to damp motion of the NPs. At the end of the hexapole guide, an einzel lens focuses NPs leaving the hexapole through a differential pumping aperture into the main trap chamber. The aperture is built into the final electrode of the einzel lens, and to allow the hexapole and trap chambers to be isolated, the lens also incorporates a ~1.5 mm thick slide valve, similar to one we developed for a windowless vacuum-ultraviolet lamp. The isolation valve is opened only for NP injection so that the trap vacuum has minimal contamination by species from the ESI source.