Eclipse, a versatile system for the separation and sizing of macromolecules and sub-micrometer particles in aqueous solutions, uses the asymmetric flow Field Flow Fractionation (AFFF) technique. The system can be used for very large macromolecules, and gentle separation assures that shear forces are eliminated or reduced to a minimum. It interfaces seamlessly with proprietary DAWN multi-angle light scattering instruments and software to determine absolute particle size distributions and molecular weights without reference to any standards.