A condensation nucleus counter (CNC) sensitive to singly charged sub-nanometer particles

The mixing-type particle size magnifier (PSM) of Okuyama el al. (1984, Aero sol. Sci. Technol. 3, 353-366) has recently shown an ability to detect posi tively charged particles with mobility diameters as small as 1.5 nm (Seto e t al., 1997, J. Chem. Phys. 107, 1576-1585), well below the sensitivity lim it of 2.3 nm available in commercial CNCs. Gamero-Castano (1999, Ph.D. thes is, Yale University) has shown that the activation of even smaller particle s is limited in this PSM by homogeneous nucleation due to cooling of the sa turated dibutyl phthalate vapor stream before it mixes with the aerosol. Th is problem is overcome here by means of a reheater, whereby ions with mobil ities (in air) higher than 2 cm(2) V-1 s(-1) can be activated at supersatur ations well below the threshold for homogeneous nucleation. The critical su persaturation required to activate ions whose mobility exceeds 0.78 cm(2) V -1 s(-1) (mobility diameter smaller than 1.6 nm) is seen to be independent of size, indicating that this instrument can detect arbitrarily small (char ged) particles. The reheater makes it possible also to run the PSM at fairl y small vapor flow rates with essentially no vapor loss. Varying the vapor flow rate hence provides a rapid means to scan over the supersaturation in the mixing region, and hence infer the particle diameter (or its charge) wi th the PSM alone. Coupled to an optical detector this PSM constitutes a con densation nucleus counter (CNC) suitable for the analysis of charged sub-na nometer particles, ions, as well as charged biomolecules and their fragment s, with effectively no lower size limit. (C) 2000 Elsevier Science Ltd. All rights reserved.