to achieve this goal: (i) time-of-flight mass spectrometry Aerocolloidal particles have been trapped from an uncharged (TOF-MS), and (ii) vibrational spectroscopies (Raman or source aerosol using an electrodynamic balance. Graphite and soot fluorescence). In TOF-MS, particles are differentially particles were charged photoelectrically using a Xe 2 (172 nm) excipumped into a vacuum chamber where single particles elasmer lamp, while particles of titanium dioxide, sodium nitrate, tically scatter laser light onto a photosenstitive detector. and diethylhexyl sebacate (DEHS) were charged using a unipolar When the particle is sensed by the detector, an ultraviolet corona charger prior to injection into the chamber. It was found (UV) laser is triggered to ablate the particle in flight. During that the Stokesian drag force produced by convection in the balablation, ions are generated which travel through a drift tube ance chamber can destabilize the levitated microparticle when it
to an ion multiplier. The time it takes the ions to travel to exceeds the electrostatic force required to center the particle. Although the electrostatic restoring force can be increased by increas-the multiplier is related to the ion mass, so the concentrations ing either the particle charge or the ac field strength, charging of of species which comprised the particle can be inferred from the particles is more difficult as the particle diameter is decreased, the intensities associated with the ionic fragments. A number which gives rise to a trapping limit. Monodisperse DEHS particles of studies have been performed using TOF-MS including were used to determine the experimental trapping limit for unipochemical characterization of model aerosol particles conlar charging. For the experimental apparatus used in this study, sisting of inorganic and biological compounds and chemical a diameter of about 1 mm was found to be the trapping limit for analysis of atmospheric aerosol particles (1-13).
DEHS. Results are compared to the theoretical trapping limit
Time-of-flight measurements have yielded the composicalculated by a force balance on a particle exposed to motion of tion of particles having diameters down to 100 nm. However, the surrounding gas. แญง 1997 Academic Press for atmospheric particles, a diameter of 100-200 nm is con-