Assessment of the Contribution of Electron Microscopy to Nanoparticle Characterization Sampled with Two Cascade Impactors

Abstract

This study assessed the contribution of electron microscopy to the characterization of nanoparticles and compared the degree of variability in sizes observed within each stage when sampled by two cascade impactors: an Electrical Low Pressure Impactor (ELPI) and a Micro-Orifice Uniform Deposit Impactor (MOUDI). A TiO₂ nanoparticle (5 nm) suspension was aerosolized in an inhalation chamber. Nanoparticles sampled by the impactors were collected on aluminum substrates or TEM carbon-coated copper grids using templates, specifically designed in our laboratories, for scanning and transmission electron microscopy (SEM, TEM) analysis, respectively. Nanoparticles were characterized using both SEM and TEM. Three different types of diameters (inner, outer, and circular) were measured by image analysis based on count and volume, for each impactor stage. Electron microscopy, especially TEM, is well suited for the characterization of nanoparticles. The MOUDI, probably because of the rotation of its collection stages, which can minimize the resuspension of particles, gave more stable results and smaller geometric standard deviations per stage. Our data suggest that the best approach to estimate particle size by electron microscopy would rely on geometric means of measured circular diameters. Overall, the most reliable data were provided by the MOUDI and the TEM sampling technique on carbon-coated copper grids for this specific experiment. This study indicates interesting findings related to the assessment of impactors combined with electron microscopy for nanoparticle characterization. For future research, since cascade impactors are extensively used to characterize nano-aerosol exposure scenarios, high-performance field emission scanning electron microscopy (FESEM) should also be considered.

Keywords:

• electron microscopy
• ELPI
• MOUDI
• nanoparticle characterization
• TiO₂ nanoparticles

ACKNOWLEDGMENTS

This work was supported by a grant from the Agence Française de sécurité sanitaire de l’environnement et du travail (AFSSET). Alexandra Noël is a recipient of PhD scholarships from the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) and from the Fonds de la recherche en santé du Québec (FRSQ). Special thanks to Annie Ouellet of the IRSST for technical support.