Preparation and size determination of monodisperse silica microspheres for particle size certified reference materials

Five samples of monodisperse SiO2 microspheres used for certified reference materials for particle size were synthesized by the seed-growth method, that is, through hydrolysis and condensation of tetroethylorthosilicate in a mixture of ethanol and water, using amoniua as catalyst. Their nominal diameters were 0.15 μm, 0.3 μm, 0.5 μm, 1 μm, and 2 μm. The average diameters of these microspheres were accurately determinated by SEM array-sizing method, and the microspheres with 1 μm and 2 μm nominal diameters were determined by both SEM array-sizing method and optical array-sizing method, while the particle size distribution of these spheres was determined by SEM. The measurement errors for each sample were quantitatively assessed. The SEM magnification was calibrated with a SEM calibration standard traceable to CNIM (China National Institute of metrology), while the optical microscopy magnification was calibrated with a NIST stage micrometer (SRM 2800). The average diameters determined by SEM and optical microscopy were identical to each other. For comparison, the average diameter of these microspheres was also measured by other independent techniques -light scattering and photo-correlation methods, and the obtained data agreed well with that obtained by the array sizing method.