Mobility analysers select particles according to their electrical mobility. In order to take into account the effect of gravity in these analysers, we introduce the settling velocity in equations governing the behaviour of particles. We deduce a new transfer function taking into account the effect of gravity. Based on these theoretical results, we developed a new application. By simultaneously measuring the relaxation time and the electrical mobility of aerosols, we determine their diameter, their mass and therefore their density. In order to validate this theory, we measure the mass of a latex particle (diameter between 0.1 and 2 mu m) with a radial how differential mobility analyser known under the name SMEC. The agreement between the theoretical and the experimental values is good and the uncertainty less than 5% for latex particles with a diameter greater than 0.5 mu m.