A new instrument having a cross-flow configuration for aerosol and sheath flows is introduced for sizing of nanoparticles and referred to as nanoparticle cross-flow differential mobility analyzer (NCDMA). The flow and electrical fields in the NCDMA are analyzed theoretically, and its sizing characteristics are obtained in relation to the instrument geometry and operating conditions. The theoretical transfer functions compare well with numerical predictions for non-diffusing particles. For smaller particles, the focusing provided by the combination of the cross-flow configuration and electric field results in reduced Brownian diffusion broadening of particle position distribution in the classification region. From the analysis of instrument resolution based on the migration Peclet number, the NCDMA design is seen to provide a higher resolution than that of the TSI short column DMA (nano-DMA) for the same applied voltage. Monte Carlo simulation results of the Langevin equation confirm the theoretical prediction of improved resolution of NCDMA over conventional designs in the Brownian diffusion regime. (C) 2007 Elsevier Ltd. All rights reserved.