Single-Particle Laser Ionization Techniques

Subsequently, Marijnissen et al. (1988) proposed a single-particle system in which the amount of scattered 

Subsequently, Carson et al. (1995) and Neubauer et al. (1996, 1997) reported the ability to provide specia-tion of some aerosol components such as ammonium sulfate, ammonium sulfite, and methanesulfonic acid through control of the ionizing laser pulse energy, and Reents et al. (1995) showed that parent peaks could be obtained even for components such as Si02 that are difficult to ionize. Hinz et al. (1996) reported the simultaneous detection of both positive and negative ions produced by laser ionization of a single particle using a dual TOF system. 

As illustrated below, the mass spectra of particles in ambient air can be (not surprisingly) quite complex. The use of tandem mass spectrometry would therefore be quite valuable, and indeed, such an instrument has 

FIGURE 11.69 Schematic diagram of single-particle laser ionization mass spectrometer (adapted from Gard et at., 1997). 

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In summary, the use of mass spectrometric methods, combined with various approaches to vaporizing and ionizing the particles, is gaining increasing popularity and interest for the analysis of continuous sources of particles or single particles. The problem of quantification of the components seen by single-particle laser ionization techniques remains to be solved. On the other hand, the vaporization approaches can provide quantitative data on some volatile and semivolatile components but cannot measure the nonvolatile species and, at present, do not provide a full mass spectrum for a single particle. 

Another very sensitive detection scheme is based on resonant two- or three-photon ionization of atoms and molecules in the gas phase (Sect. 6.3). With this technique even liquid or solid samples can be monitored if they can be vaporized in a furnace or on a hot wire. If, for instance, a heated wire or plate in a vacuum system is covered by the sample, the atoms or molecules are evaporated during the pulsed heating period and fly through the superimposed laser beams L1+L2 (+L3) in front of the heated surface (Fig. 15.2). The laser LI is tuned to the resonance transition /> - k) of the wanted atom or molecule while L2 further excites the transition k) f). Ions are formed if Ef is above the ionization potential IP. The ions are accelerated toward an ion multiplier. If L2 has sufficient intensity, all excited particles in the level / ) can be ionized and all atoms in the level [/ flying through the laser beam during the laser pulse can be detected single-atom detection) [15.10-15.12]. If... 

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