The Scanning Mobility Particle Sizer is based on the principal of the mobility of a charged particle in an electric field. Particles entering the system are neutralized (using a radioactive source) such that they have a Fuchs equilibrium charge distribution. The source is Kr-85, a beta emitter. Two sources are available, one with an activity of 74MBeq and the other 370MBeq. The University of Manchester has a mobile license and can operate it around the country. However, all users who require use of the SMPS must have their own site licence to house this source and have spare capacity on the licence. If this source is to be used away from the host institute, then users must have a mobile licence, with capacity for the source. Before any loan of the AMF SMPS is agreed, the users must supply proof of the licence and the spare capacity to the IS and a copy of the local rules for handling and storage of Radioactive substances for approval by University of Manchester local Radiological Protection Officer. The particles then enter a Differential Mobility Analyser (DMA) where the aerosol is classified according to electrical mobility, with only particles of a narrow range of mobility exiting through the output slit. This monodisperse distribution then goes to a Condensation Particle Counter which determines the particle concentration at that size.

The 3936 SMPS is widely used as the standard for measuring airborne particle size distributions. This system is also routinely used to make accurate nanoparticle size measurements of particles suspended in liquids. The National Institute of Standards and Ttechnology (NIST) uses a TSI DMA to size 60nm and 100 nm standard size reference materials. SMPS spectrometer sizing is a discreet technique in which number concentrations are measured directly without assuming the shape of the particle size distribution. The method is independent of the refractive index of the particle or fluid, and has a high degree of absolute sizing accuracy and measurement repeatability.