Particle diameter (|im)
FIGURE 11.62 Experimental calibration curves for a commercial single-particle counter and two types of calibration aerosols: dioctyl phthalate (DOP) and coal dust (adapted from Whitby and Willeke, 1979).
and the latter for particles from ~ 0.001 /¿m and larger. The particles must be on the surface of the substrate and form less than a monolayer to minimize overlap of the particles. For electron microscopy, the sample and substrate must also be able to be subjected to high vacuum, heat, and electron bombardment without degradation over a period of time sufficient to make the measurement.
Because of the instrumental requirements, these are usually not routine monitoring techniques. However, unlike other methods, they give detailed information on particle shapes. In addition, chemical composition information can be obtained using transmission electron microscopy (TEM) or scanning electron microscopy (SEM) combined with energy-dispersive spectrometry (EDS). The electron beam causes the sample to emit fluorescent X-rays that have energies characteristic of the elements in the sample. Thus a map showing the distribution of elements in the sample can be produced as the electron beam scans the sample.
For example, Fig. If .63 shows the TEM image of an NaCl crystal (Fig. 11.63A) and of the same crystal after exposure to gaseous HN03 (Fig. 11.63B) and then small amounts of water vapor (Fig. 11.63C) (Allen et al., 1996). After the crystal is dried, the formation of new microcrystallites attached to the NaCl is observed. These can be shown using EDS to be crystals of NaN03. Thus Fig. ff.64b shows the EDS spectra obtained from the larger, original NaCl crystal (but after exposure to 11 NO , and water vapor) and of small microcrystallites attached to it (Fig. 11.64a). Only Na and CI are seen in the first case, but Na, N, and O in the second, and in the correct ratio for NaN03. The HN03 has reacted with the NaCl surface to generate metastable surface nitrate ions. Exposure to water vapor generates a mobile quasi-liquid layer on the surface that contains Na+, Cl~, and NO^ ions. On drying, the ions segregate to form separate microcrystallites of NaN03 and a fresh NaCl crystal (Allen et al., 1996).
The combination of SEM with EDS has also been applied to atmospheric particles (e.g., Posfai et al., 1995; Anderson et al., 1996; McMurry et al., 1996; Ganor et al., 1998). For example, individual sea salt particles were analyzed using TEM combined with EDS as well as selected-area electron diffraction (SAED) by Posfai et al. (1995) and Anderson et al. (1996). The crystal shapes correlated well with the chemical composition determined using EDS and SAED. For example, cubic crystals of NaCl were observed. Sulfate occurred in either rod-shaped crystals, which had significant concentrations of (Mg + K + Ca) compared to Na, or tubular crystals, with much smaller concentrations of these three metals. In the latter case, the EDS showed
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