Water condensation-based nanoparticle charging system: Physical and chemical characterization

A water condensation-based ion charging system has been developed to enhance both the charging efficiency and the concentration of sub-20 nm particles. This NanoCharger consists of a bipolar ion source followed by a parallel plate water-based condensation system, an embedded ion scavenger, and an aerodynamic focusing stage. Sufficient numbers of ions are transported through the system to attach to the formed droplets. An ion scavenger removes the ions immediately after the droplet formation to minimize multiple charging. A subsequent cold-walled condensation stage removes most of the water vapor, lowering the dew point to below 16 °C, while a set of focusing nozzles concentrates the droplets into ∼10% of the flow. The flow is then slightly heated to evaporate the droplets. The physical enhancement of electrical charging was evaluated in the laboratory using mobility-selected particles, and found to provide ∼40-fold enhancement over bipolar charging for 6–15 nm particles. Chemical artifacts were evaluated through thermal desorption chemical ionization mass spectrometry. Data comparing ion spectra for flow that passed through the NanoCharger to that obtained without it showed nearly equivalent ion spectra, indicating that no significant artifacts were introduced from the condensation–evaporation process. Copyright © 2018 American Association for Aerosol Research