A novel tandem ion-mobility method for probing reaction kinetics at nanoscale is developed. The experimental system consists of two different ion-mobility schemes in series. The first mobility characterization is to size select particles with a differential mobility analyzer (DMA). The second mobility characterization employs an aerosol particle mass analyzer (APM) and measures changes in mass resulting from a controlled reaction of nanoparticles. The DMA, used in this experiment for size selection, consists of an annular region between two concentric cylinders, with the center cylinder held at high voltage and the outer one at ground. Charged particles of the right polarity feel an attractive force toward the center electrode and move radically inward at an electrophoretic velocity determined by the particle charge, and drag force which is a function of particle size. When charged particles flow between the cylinders the electric force on the particle is balanced by the drag force, and at a fixed voltage all particle exiting the instrument have (to the resolution of the instrument) equivalent mobility sizes. The APM can determine the particle mass distribution based on particle mass to charge ratio, and is used in our experiment to monitor changes to the particle mass resulting from evaporation. The APM consists of two concentric cylindrical electrodes that rotate together at a controlled speed. An electrical field is created by applying high voltage on the inner electrode while the outer one is held at ground. Charged particles flowing within the concentric cylinders experience opposing centrifugal and electrostatic forces and as a result particles exiting the instrument at fixed voltage and rotation speed all have the same nominal mass. By scanning either the voltage or the rotation speed, the particle mass distribution (independent of particle shape) can be determined. Based on operating conditions for the DMA and APM and the root-sum-square (RSS) method, we estimate the uncertainty of density calculation ~5%.

We demonstrate the use of the DMA-APM technique to measure the inherent density of nanoparticles, measuring the surface energy of unsupported nanocrystals, and the kinetics of aluminum and nickel nanoparticle oxidation.