In core/satellite noble metal particles consisting of small satellites and a large core, strong plasmonic coupling between the two particle species leads to hot spots in the gaps between the two building blocks. Hot spots are tiny regions, where the electric field is enhanced by orders of magnitude. Enhancement factors that are sufficiently high for single-particle detection are generated in such hot spots. Combined with Raman-active molecules, these highly plasmonically active particles can be used as SERS nanotags with single-particle SERS brightness. SERS nanotags coupled to antibodies are suitable labels for SERS-based lateral flow assays and immunoSERS microscopy. Before particles can be used as SERS labels, the sample must have high uniformity and purity. In the case of C/S particles, difficulties arise from an excess of unbound monomeric satellite particles added during synthesis. These small satellite particles are not sufficiently SERS-active and endanger the quality of LFAs and immunolabelings due to false-negative results. The most common approach to separate gold nanoparticles based on their size or shape is centrifugation. This work presents asymmetric flow field flow fractionation (AFFF) as a more sophisticated separation method for complex gold nanoparticles. AFFF is representative of field flow fractionation techniques, introduced by Giddings in 1965. As in chromatography, a sample is loaded at one end of a channel and transported to the other end with a carrierflow. A crossflow perpendicular to the carrierflow is applied, causing the sample to accumulate along the accumulation wall, which consists of a semipermeable membrane. The sample is subject to a transverse drift speed towards the accumulation wall, balanced by molecular diffusion at a certain point. This steady-state profile is given by