The study of amorphous phase-separated Dielectric Nano-Particles (DNPs) smaller than 10 nm is a great challenge for the materials community. In conjunction with Transmission Electron Microscopy (TEM) and Electron-Probe Micro-Analysis (EPMA), we took advantage of a recent technology, Tri-Dimensional (3D) Atom Probe Tomography (APT) to investigate the variations of the chemical composition in sub-20-nm oxide nanoparticles, grown in silicate glass through heat treatments, at their early stages of nucleation. More precisely, we are investigating the core of an optical fiber drawn from a preform prepared according to the Modified Chemical Vapor Deposition (MCVD) process. We provide here a comprehensive set of experimental data obtained from direct measurements of the concentration for P, Mg, Ge and Er within amorphous dielectric nanoparticles (DNP) of radii ranging from 1 nm to 10 nm. We report on an increase of the concentration of Mg and P with the size of the DNPs. Most importantly, we also demonstrate that erbium ions are partitioned in these small DNPs and their environment changes with the size of the nanoparticles. Molecular dynamics simulations were also implemented to discuss the structural modifications of the Er environment. This presentation highlights the trade off on the size of the DNPs: smaller to reduce light scattering vs bigger to modify luminescence properties.