A system and method for measuring particle mass distributions as a function of particle diameter. The system includes a nebulizer, a particle size classifier and a particle detector. The system may include a dilution module. It may also include a particle mass classifier. The system and method are useful for distinguishing particle populations of similar size and different densities as well as determining the interaction between materials (binding) in mixed colloid systems with varying material ratios. The system and method are also useful for determining particle density in both homogeneous and heterogeneous colloids.