Determination of the size distribution of polydisperse nanoparticles with single-particle mass spectrometry: the role of ion kinetic energy

We develop a method to determine size and size distribution (30-150 nm) of polydisperse nanoparticles using a laser ablation/ionization time-of-flight single-particle mass spectrometer that extends the work first described by Reents and Ge. We found a composition independent "power law" dependence between the total peak area and original particle volume that enables one to determine particle volume directly from a particles mass spectrum. This power-law relationship suggests that some ions ablated and ionized from a particle are selectively lost during transport from the laser ablation/ionization region to the detector. A numerical calculation of ion trajectories shows that ion loss is highly dependent on the initial kinetic energy of ions. We show that the size-dependent energetic ions formed by the laser-particle interaction lead to power-law relationship between the cube root of peak area and particle diameter. The results demonstrate that particle size distributions measured with the mass spectrometer are in good agreement with those measured with a scanning mobility particle sizer.