Nitrate clusters with H20 and/or HN03 such as N0^(HN03)„ are common in the atmosphere (Perkins and Eisele, 1984). Proton transfer to such clusters can occur, but clearly, the trace gas must be more acidic than HN03. This limits the number of trace gases that can be ionized through this mechanism but includes the important atmospheric species H2S04 and methanesulfonic acid, CH3S03H (Tanner and Eisele, 1991; Viggiano, 1993).
In short, positive and negative ions in air containing the trace gases of interest can be formed through discharge techniques and ions of the trace gases of interest generated via ion-molecule reactions. As discussed in more detail later, this approach has been used quite successfully to measure a number of species in air, including formic acid, acetic acid, dimethyl sulfide, and CI2 (Spicer et al., 1994a, 1998). An alternate method is to add another compound to the mass spectrometer inlet, ionize this added species, and use its ion-molecule reactions to form ions and/or ion adducts of the species of interest. This has been used to measure HONO, for example, in air where a chloride ion adduct of HONO is formed when CHC13 is added in the corona discharge region (Spicer et al., 1993a). Other examples include the measurement of HN03. For example, as described in Section A.4a(5), radioactive ionization of added SFft generates daughter ions that react with SiF4 to give SiF5~. The SiF5~ forms an adduct with HN03 and this adduct can be used to measure HN03 in air (Huey et al., 1998).
(2) Laser photoionization Another ionization method with great potential for ambient air applications is
laser photoionization (see Letokhov (1987) and Pfab (1995) for reviews). Trace gases can be ionized if sufficient energy in the form of light is pumped in; for example, polycyclic aromatic hydrocarbons (PAH; see Chapter 10) in combustion mixtures have been measured by two-photon ionization at 248 nm (e.g., Castaldi and Senkan, 1998).
In practice, for application to ambient air, efficient photoionization requires the use of pulsed lasers and multiphoton absorption methods. The terms "multiphoton ionization," or MPI, and "resonance-enhanced multiphoton ionization," or REMPI, are used to describe these processes.
Figure 11.18 illustrates the principles of application of REMPI to NO (discussed in more detail later). The electronically excited states of NO are shown in Fig. 11.18a and some potential ionization schemes in Fig. 11.18b (Pfab, 1995). Pulsed tunable lasers with wavelengths from ~190 to fOOO nm and spectral resolutions of 0.1 cm~' are readily available. To ionize NO, the absorption of two, three, or four photons is needed. The first photon excites the NO into an intermediate state from which it is ionized using a second or, in some cases, two more photons. The transitions are described as an (n + m) transition, where n is the number of photons that need to be absorbed simultaneously to reach the intermediate state and m is the number of photons to ionize the molecule from that state. The wavelength/energies of the photons involved in the various steps may be the same, which is referred to as a "one-color" process, or different, a "two-color" process. In the two-color case, the second photon is primed to indicate it is a different wavelength than the first photon. For example, in Fig. 11.18b, ionization via the A state can occur either by a (1 + 1) process using 226 nm or by a (1 + 1') process, where the A state is reached using 226 nm and ionization from this state occurs using 308 nm (Pfab, 1995; Hip-pier and Pfab, 1995). (The dashed arrows show transitions used for detecting NO by laser-induced fluorescence; see Section A.4a(l).)
The high spectral resolution of laser radiation provides selectivity. For example, Figure 11.19 shows the REMPI spectrum of the NO X (0,0) -> A band using a (f + f) process with 226- and 308-nm light to pho-toionize NO (e.g., see Pfab, f995; and Lee et ai, 1997). As the laser is tuned into resonance with a particular rotational transition in this band, ions are generated and detected using a conventional electron multiplier. Clearly, high selectivity is possible by tuning on and off
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