Case 5: Ultrafine Particles in Air Pollution
There is increasing interest in the measurement and health effects of Particulate Matter (PM), and especially Ultrafine PM (UFPM). UFPM have particle diameters <100nm. These are at the lower limits of practical resolution for most SEM systems. Ultrastructural examination of Ultrafine PM requires higher resolution than most standard SEM provides (see SEM basics). In order to examine UFPM at higher resolution, it is necessary to use Transmission EM (TEM) or very high resolution SEM, such as Field Emission SEM (FE-SEM). The morphology of such particles and their aggregates may provide important clues as to their generation and behavior in ambient and laboratory environments. This information may supplement traditional information collected by particle counting and sizing equipment, and by chemical analysis of bulk samples or even individual particle analysis by SEM/EDS or Time-of-flight-mass-spectrometry (TOFMS).
Another problem is the collection of UFPM on suitable substrates for TEM or SEM. Standard filters will allow many UFPM to penetrate through the pores of the filters, and although HEPA type filters will trap UFPM, their fibrous and uneven topography and composition make them a very poor substrate for SEM and inapplicable for TEM.
The images shown (top two images) of UFPM easily detectable in ambient aerosol from Syracuse, NY [Abraham, ME (1999)] demonstrate two possible approaches to high resolution examination of UFPM. The standard Nuclepore® type filter traps some of the UFPM by diffusion on its surface and especially around and inside the pores. The holey grid has uneven but very tiny openings in the carbonaceous formvar film on a standard TEM metal grid. Mounting such a grid in a filter system seems to be able to collect UFPM in a manner in which they can be then directly examined in a TEM*. Note that both images show chain-like aggregates of UFPM typical of emissions from diesel engines. The image at lower right shows the similar UFPM identified in the lungs of victims of the 1952 London Smog.
Much more work is needed to develop and test the optimal method for collection of UFPM for ultrastructural examination and analysis.
*special thanks to Maureen Barcza for painstaking TEM preparation and imaging.
