Are existing analytical methods for the characterization of nanoplastic (NP) and microplastic (MP) suitable for meeting the requirements coming from impeding legislation? In particular, the implementation of the proposal from the European Chemical Agency (ECHA) to restrict the NP and MP use in consumer products will require reliable methods to perform size and mass-based concentration measurements. Analytical challenges arise: (i) with highly polydisperse samples in the nanometric range, and also (ii) at the sub-micron interface, e.g. between 800 nm and 5 µm, where techniques applicable at the nanometre scale reach their upper limit of applicability and analytical approaches applicable at the micrometre scale have to be pushed to their lower limits of detection. In this work we compared the performances of nine analytical techniques by measuring the particle size distribution and mass-based concentration of quadrimodal polystyrene nanoparticle mixtures of 100-250 nm and of bimodal mixtures containing both nano and microparticles. We tested which analytical approaches can accurately and reliably measure highly polydisperse samples and bridge the gap at the sub-micrometre interface, with the educational aim to underline advantages, disadvantages and limitations of each technique. The results show that widely used light scattering based measurements, such as dynamic light scattering (DLS), do not have the resolution power to distinguish multiple populations in very complex samples. Nanoparticle tracking analysis (NTA), nano flow cytometry (nFCM) and asymmetric flow field flow fractionation coupled to static light scattering (AF4-MALS) measure the sample accurately in the nano-range (