Improving the foundation for particulate matter risk assessment by individual nanoparticle statistics from electron microscopy analysis

Abstract

Air pollution is one of the major contributors to the global burden of disease, with particulate matter (PM) as one of its central concerns. Thus, there is a great need for exposure and risk assessments associated with PM pollution. However, current standard measurement techniques bring no knowledge of particle composition or shape, which have been identified among the crucial parameters for toxicology of inhaled particles. We present a method for collecting aerosols via impaction directly onto Transmission Electron Microscopy (TEM) grids, and based on the measured impactor collection efficiency and observed impact patterns we establish a reproducible imaging routine for automated Scanning Electron Microscopy (SEM) analysis. The method is validated by comparison to scanning mobility particle sizer (SMPS) measurements, where a good agreement is found between the particle size distributions (PSD), ensuring a representative description of the sampled aerosol. We furthermore determine sampling conditions for achieving optimal particle coverage on the TEM grids, allowing for a statistical analysis. In summary, the presented method can provide not only a representative PSD, but also detailed statistics on individual particle geometries. If coupled with Energy-dispersive X-ray spectroscopy (EDS) analysis elemental compositions can be assessed as well. This makes it possible to categorize particles both according to size and shape e.g. round and fibres, or agglomerates, as well as classify them based on their elemental composition e.g. salt, soot, or metals. Combined this method brings crucial knowledge for improving the foundation for PM risk assessments on workplaces and in ambient conditions with complex aerosol pollution.

Analysis of Electron Transparent Beam-Sensitive Samples Using Scanning Electron Microscopy Coupled With Energy-Dispersive X-ray Spectroscopy

“…The appropriate particle coverage was achieved by sampling for 5 s at a total number concentration of approximately 3.5 × 10 5 cm −3 , as measured by the SMPS. This fits well with the sampling recommendation of Brostrøm et al (2019). Particles were only observed on the lowest impactor stage, consistent with the expected 100–200 nm sizes, as the lowest stage collects particles with diameters from 73 to 590 nm (Brostrøm et al, 2019, 2020).…”

Complex Aerosol Characterization by Scanning Electron Microscopy Coupled with Energy Dispersive X-ray Spectroscopy

“…The EDS analysis was only performed on the complex impactor samples, while SE imaging was used for analysis of all samples containing a single primary www.nature.com/scientificreports www.nature.com/scientificreports/ particle type. To give a representative sample description, all impactor samples were analyzed by acquiring a series of images or maps in a straight line going through the center of impaction, according to the method described by Brostrøm et al 36 . Previously the imaging routine was only verified for the 3 rd impactor stage, but as we observed similar deposition patterns on the 2 nd stage in this work, we chose to apply the method here as well.…”

“…The final outlet was used to collect aerosol particles for SEM/EDS analysis, using a three stage cascade impactor (MINI). The three stages have cut-off diameters (D 50 ) of 1.36, 0.59, and 0.073 µm, respectively 36 . The first impactor stage was smeared with impactor grease (Dekati Ltd, Finland) to remove large particles and ensure minimal bounce to lower stages.…”

“…The first impactor stage was smeared with impactor grease (Dekati Ltd, Finland) to remove large particles and ensure minimal bounce to lower stages. The second stage was equipped with a Nickel disc, as the substrate of the TEM grids were found to break upon impaction of the larger particles 36 . The final MINI stage was installed with commercially available 400 mesh nickel TEM grids coated with a 25-50/1 nm Formvar/Carbon film (Electron Microscopy Sciences (EMS), USA).…”

Insights into coarse particle optics based on field evidence of particle morphology, chemical composition and internal structure

“…Conventional techniques for single particle analysis like; Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and Transmission Electron Microscopy (TEM) give surfacial characteristics, and sometimes skewed elemental distributions due to x-ray selfabortion or matrix effects (Wentzel et al, 2003;Arndt et al, 2016;Patterson et al, 2016;Fraund et al, 2017;Bahadar Zeb et al, 2018;Genga et al, 2018;Brostrøm et al, 2019;Ching et al, 2019). They are not capable of revealing the actual three-dimensional (3D) structure of the aerosol particle.…”