Controlling the charge of single nanoparticles in an ion trap

Control over the charge of a nanoparticle (NP) in a radiofrequency ion trap is crucial for mass spectrometric and charge dependent investigations of single NPs. We show how this is achieved for positively charged silica NPs (nominal diameter 100 nm, 350-1400 e) with a simple experimental realization using a standard cold cathode gauge. The change of the NP charge is the result of processes, where electrons and cations interact with the trapped NP. We investigated how NP charging depends on pressure and gas type as well as on the ion trap amplitude and waveform, which can be used for charging and discharging in a wide NP charge range. The measurement of average charging rates as a function of the NPs' charge for widely varied experimental parameters allows us to capture essential relationships between gas pressure, NP charge, trap potential and net charging rates. The acceleration of gas cations by the trap potential is shown to be the driver of electron abstraction from the NP by gas cations and thereby makes high NP charges accessible.