FIGURE 11.71 Typical laser ionization positive ion mass spectra of single particles in rural Colorado (adapted from Murphy and Thomson, 1997a,b).

Clearly, laser ionization-TOF mass spectrometry is a promising tool for real-time single-particle analysis. However, there are some important aspects of particle characterization on which data are not provided by these techniques at the present time. For example, while the identification and grouping of elements in single particles in ambient air using principal-component analysis provide insight into their sources (e.g., see Noble and Prather, 1996; Liu et al., 1997; Murphy and Thomson, 1997a, 1997b; Middlebrook et al., 1997; and Wood and Prather, 1998), independent quantification has not yet been achieved. Indeed, it may prove to be elusive due to the sensitivity of ion formation to the particular conditions, including the wavelength of the ionizing laser, the laser fluence, the chemical composition of the particle, etc. (e.g., see Thomson et al., 1997; and Ge et al., 1998b).

For example, Neubauer et al. (1998) have shown that the spectra can be very sensitive to the amount of water present and whether the particle is aqueous, i.e., above the deliquescence point, or solid (but holding adsorbed water on the surface). Figure 11.72 shows the

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