Measurement of Condensed-Phase Reaction Kinetics in the Aerosol Phase Using Single Particle Mass Spectrometry

We applied a recently developed single particle mass spectrometer to analyze the elemental composition of individual aerosol particles and applied the technique to study the kinetics of thermal decomposition of metal nitrate aerosols (aluminum, calcium, silver, and strontium). Such decomposition processes on the industrial scale is known as spray pyrolysis and is a common method for making metal and metal oxide particles. Metal nitrate aerosols were passed through a tube furnace to induce thermal decomposition and transformation into oxides, and were delivered with high efficiency into the vacuum system of the single particle mass spectrometer using an aerodynamics lens arrangement. The particles were ablated and torn down to atomic ions with a tightly focused, high-power pulsed laser in the extraction field of the time-of-flight mass spectrometer. The mass spectra thus obtained are shown to carry a quantitative signature of the elemental composition of individual particles, and allow for on-line measurement of the stoichiometry transition from nitrate to oxide as the particles passed through a tube furnace. The results were used to calculate the reaction rates and activation energies for the decomposition reaction. In parallel, reaction rates were obtained by conventional thermogravimetric analysis, and a comparison revealed significant differences in measured reaction rates between the aerosol and conventional methods. In particular, it was found that the reaction rates determined in the aerosol phase were significantly higher than those obtained by traditional thermal methods, which we believe is associated with heat and mass transfer limitations associated with bulk methods. This new aerosol-based technique may offer an alternative approach to study condensed-phase reactions with minimal influence of artifacts caused by heat and mass transfer effects known to plague conventional thermal analyses.