Mass and size determination of nanometer particles by means of mobility analysis and focused impaction

Particles in the size range of a few nanometers are characterized by means of a differential mobility analyzer (DMA) of the Eichler type in tandem with a focusing impactor with electrostatic blowing. One application of the DMA-impactor combination is determination of particle mass mp without knowledge of the relation Z(dp) between particle mobility Z and diameter dp. With Z(dp) known, also the size and density ρp of spherical particles can be determined. A mobility versus diameter relation Z(dp) for the range of a few nanometers is derived from the literature. It considers the effect of the finite diameter d of gas molecules, neglected in conventional studies. According to this relation, plots of Z−1/2 versus mp1/3 yield straight lines, and the intersection with the Z−1/2 axis is linearly related to the effective diameter d of the carrier gas molecules. Experimental data recorded with the DMA-impactor combination for silver particles are consistent with this relation. The same applies for values from literature for fullerenes and proteins. For air, d is approximately 0.53 nm, not far from the 0.6 nm estimated by Tammet (1995). The particle density ρp is derived from mp and the diameter inferred from Z(dp). Measurement of ρp on silver particles showed that charging devices may introduce contamination leading to formation of an adsorbed layer on the particles, reducing their measured density. The DMA-Impactor combination applied has sufficient resolving power to observe occurrence of different shapes of particles prepared by vapor condensation.