In this work we focus on the characterization of micro- and nano-powders typically adopted for chemical mechanical polishing, extensively used whenever the global and local planarization of surfaces is required as in nanoelectronic fabs. We present an innovative method for the accurate characterization of water suspensions of nanoparticles. It relies upon the combination of a new approach to extract light-scattering information from single particles and the recently developed diagnostic tool named Single Particle Extinction and Scattering. It can be used in line. Data interpretation becomes independent of any a-priori assumptions about the samples. The results of accurate measurements performed on ceria as well as aluminium oxide slurries are reported. We show the strong advantages of this method compared with traditional ones by explicitly reporting experimental results on calibrated spheres made of different materials. We discuss possible applications for in-line characterization of ultrapure water, chemicals, slurries for abrasive processes, for example, as well as the detection of any undesired particles – which could be the key for future improvements to advanced process control systems.