The large research effort in nanotechnology over recent years has brought to the forefront the lack of easy-to-use tools for characterizing nanoparticles. While there are several established techniques in the field of particle sizing that can be used to measure the particle size distributions of nano-dispersions, few of them can provide quantitative information in the range of 0.1 to 1 microns. As it turns out, there is a great need for this information. For example, this size range corresponds to where protein aggregates are expected to be, which is of interest to pharmaceutical companies that formulate protein-based drugs because of the safety and efficacy issues posed.Recently, SPOS technology has undergone a quantum leap in capability with respect to the lower size limit of single-particle detection and sizing. Based on a new and innovative approach that uses a focused laser beam to achieve a much smaller and more intensely-illuminated view volume, this new FX-Nano technology can count and size particles as small as 150 nanometers. Importantly, the FX-Nano sensor functions at much higher particle concentrations than those allowed by traditional SPOS sensors – above 1,000,000/mL. As a result, little or no dilution is required for most protein suspensions of interest. SPOS technology made it possible over the years to quantify oversized particles found in many types of dispersions, allowing manufacturers to improve products as diverse as IV emulsions and microprocessors. The new innovation presented by the FX-Nano means that tails of much smaller nanoparticles can be characterized which will lead to the introduction new and improved protein drugs and therapies. This is an ideal system to quantify the size and concentration of aggregated protein.