SPACE-CHARGE EFFECTS IN THE DIFFERENTIAL MOBILITY ANALYZER

This paper presents a theoretical analysis of the effect of space-charge on the performance of differential mobility analyzers. The mathematical treatment, which is carried out for non-diffusing monodisperse aerosol particles, is based on the calculation of the electric field (applied plus space-charge) along particle trajectories, and leads to a simple expression for the classification voltage as a function of two dimensionless parameters. The effect of aerosol properties (mobility, concentration), DMA geometry and operating conditions on the classification performance is discussed. The model predicts that space-charge affects the DMA performance for values of n0Z(tau) larger than about 22,000 V-1cm-1 (n0 is the particle number concentration at DMA inlet; Z the electric mobility; (tau)the mean aerosol residence time).