Chemical Analysis of Individual Alkali-Containing Aerosol Particles: Design and Performance of a Surface Ionization Particle Beam Mass Spectrometer

A mobile particle beam mass spectrometer has been developed to measure the alkali metal content in individual submicron aerosol particles. The instrument employs an aerodynamic inlet system for efficient sampling of particles into vacuum, and the detection of individual particles is based on decomposition and surface ionization on a hot platinum surface. A boxlike design of the hot ionizing surface is shown to limit problems associated with particle bounce effects and incomplete ionization, and the decomposition/ionization process is not sensitive to detailed particle properties. High transmission efficiencies and quantitative determination of the alkali metal content in individual particles with diameters down to 14 nm are demonstrated. Experiments with particles doped with alkali salt show that the size range may be extended down to a few nanometers after further improvements of the inlet system. High size resolution can be achieved with the instrument for particle sizes down to tens of nanometers, as illustrated by the detection of multiply charged particles passing through a DMA. The robustness of the instrument makes it suitable for field measurement applications, and the technique is demonstrated at a 12 MW biomass combustion facility. The performance of the instrument and further refinements of the technique are discussed, and potential applications in field and laboratory studies are outlined.