Overview of methods for analysing single ultrafine particles

Increasing awareness that structures and attributes on a nanometre scale within aerosol particles may play a significant role in determining their behaviour has highlighted the need for suitable single ultrafine particle analysis methods. By adopting technologies developed within complementary disciplines, together with the development of aerosol-specific methods, a basis for characterizing single sub-100 nm (ultrafine) particles and features in terms of size, morphology, topology, composition, structure and physicochemical properties is established. Size, morphology and surface properties are readily characterized in the scanning transmission electron microscope (STEM), while high-resolution transmission electron microscopy (HRTEM) allows structural information on particles and atomic clusters to sub-0.2 nm resolution. Electron energy loss spectroscopy (EELS) and X-ray emission in the STEM allow the chemical analysis of particles and particle regions down to nanometre diameters. Scanning probe microscopy offers the possibility of analysing nanometre-diameter particles under ambient conditions, thus getting away from some of the constraints imposed by electron microscopy. Imaging methods such as atomic force microscopy and near-field scanning optical microscopy (NSOM) offer novel and exciting possibilities for the characterization of specific aerosols. Developments in aerosol mass spectrometry are providing the means for chemically characterizing size-segregated ultrafine particles down to 10 nm in diameter on-line. By taking a multi-disciplinary approach, the compilation and development of complementary tools allowing both routine and in-depth analysis of individual ultrafine particles is possible.