Equations 1 and 2 explain the size-dependency of the retention time. Specifically, particles with smaller hydrodynamic radius (r H ) and larger diffusion coefficients elute faster. A more detailed theoretical overview and a schematic figure explaining the separation principle of AFFF can be found in the literature. In most cases, an AFFF system in combination with a multi-detector setup is used to characterize particles online in their native state. While UV/Vis extinction spectroscopy and dynamic light scattering (DLS) are used in almost every setup, additional methods, such as inductively coupled plasma mass spectroscopy (IC-MS), are cutting-edge technologies. These methods allow us to comprehensively characterize nanoscale objects, particularly metal-based nanoparticles.