Direct Collection of Aerosols by Electrostatic Classification for Size-Resolved Chemical Analysis

The performance of an inlet for the size-resolved collection of aerosols onto a heating filament for subsequent thermal desorption is presented. The device resembles a cylindrical Differential Mobility Analyzer (DMA) in that a sample flow is introduced around the periphery of the annulus between two concentric cylinders, and charged particles migrate inward towards the inner cylinder in the presence of a radial electric field. Instead of being transmitted to an outlet flow, the monodisperse sample is collected on a nichrome filament that is flush with the inner cylinder. The primary benefit of this mode of sampling, as opposed to sampling into a vacuum using inertial separation, is that chemical ionization of the vapor molecules is feasible. In this study, we present a model of the device that is similar to that used to characterize the DMA. A prototype was constructed and tested at atmospheric pressure and at 18 Torr. The collection efficiency was determined indirectly by counting particles not collected by the device; and also by vaporization of the particles from the filament, chemical ionization of the vapor, and low-pressure ion mobility spectrometry of the ionized sample. The data demonstrate that the device is indeed size selective, but the collection efficiency curves are broader than predicted by the model.