PARTICLE-BEAM MASS-SPECTROMETRY OF SUBMICRON PARTICLES CHARGED TO SATURATION IN AN ELECTRON-BEAM

A particle beam mass spectrometer has been developed for measuring the size distribution of submicron (in this study similar to 0.02-0.3 mu m) particles in low-pressure environments. Particles are sampled into a differentially pumped system and focused into a narrow, low-divergen ce beam using aerodynamic lenses, charged to saturation in an electron beam, and then deflected electrostatically into a Faraday cup for mea surement of the particle current. Measurements of particle current as a function of deflection voltage are combined with measured relationsh ips between particle velocity and diameter, charge and diameter, and m ass and diameter, to determine the particle size distribution. Results of measurements on KCl particles are in good agreement with size dist ributions obtained by differential mobility analysis. The maximum diam eter resolution (D-p/Delta D-p) is about 10, a complete size distribut ion can be measured in about 2-3 min, and the detection limit is about 10 particles cm(-3) when sampling from a pressure of 270 Pa.