Description and theoretical analysis of a differential mobility spectrometer

Electrical mobility analysis is the most efficient technique for measuring aerosol particle size distributions in the submicron size range. Recent advances in aerosol science underline the need of fast measurements of particle size spectra in this range, and therefore a great amount of effort has been focused towards this direction. This paper provides the description and a theoretical framework for the analysis of a fast-response differential mobility spectrometer (DMS). In common with other instruments of its category, it consists of a particle charger, a classification column, and a series of detectors. Passing the sample flow first through a corona-wire diffusion charger that sets a charge on the particles, the aerosol is introduced around the central rod of an inside-out cylindrical classifier equipped with a series of isolated electrode rings connected to sensitive electrometers. Current readings produced by deposition of the charged particles on the electrometer rings are then translated to particle number concentrations corresponding to the electrical mobility range collected on every channel. Combining Fuchs' limiting-sphere theory to predict the number of charges on the particles downstream of the charger with a nondiffusing transfer function of the classifier, we present calculations of the kernel of the DMS, and show how changing the operating conditions affects the overall performance of the instrument.