A new particle mass classifier, the Couette centrifugal particle mass analyzer (CPMA), is described and compared to the existing aerosol particle mass (APM) analyzer. The Couette CPMA and APM use centrifugal and electrostatic forces to classify particles according to their mass-to-charge ratio. The APM consists of two rotating coaxial electrodes, rotating at the same angular velocity, between which charged particles pass. Depending on the voltage and rotational speeds of the electrodes, particles of different mass-to-charge ratios will be classified. The Couette CPMA uses a similar operating procedure, but the inner electrode rotates slightly faster than the outer. This produces a stable system of forces that dramatically improves the transfer function of the classifier. Two theoretical models of the classifiers are developed. One model neglects the effects of diffusion and the other model does not. The models show that the shape of the transfer function for both devices is determined by two non-dimensional constants. The models also show that the Couette CPMA has better performance characteristics than the APM when transfer functions are compared. (C) 2005 Elsevier Ltd. All rights reserved.