The control suspension-prepared without sodium silicate or hexametaphosphate-did not contain iron colloids that were detectable by FFF-ICP-MS (Figure ). This aligns with research showing that, in the absence of a stabilizing agent-a sequestrant, for instance-iron oxide nanoparticles tend to form aggregates with diameters greater than 450 nm, the upper size limit considered here. The estimated radii of gyration characterizing colloids in the primary and secondary peaks ranged from 22-26 and 70-91 nm in sodium silicate suspensions, and 21-23 and 72-76 nm in hexametaphosphate suspensions (Table ). While the maximum scattering intensities were similar between the two peaks, the second one accounted for an estimated 5%-or less-of the total iron concentration. This is explained by the much larger particle sizes represented in the secondary peak, and the dependence of scattering intensity on both particle concentration and size.