A CONDENSATION NUCLEUS COUNTER (CNC) SENSITIVE TO SINGLY CHARGED SUB-NANOMETER PARTICLES

The mixing-type particle size magnifier (PSM) of Okuyama et al. (1984, Aerosol. Sci. Technol. 3, 353-366) has recently shown an ability to detect positively charged particles with mobility diameters as small as 1.5 nm (Seto et al., 1997, J. Chem. Phys. 107, 1576-1585), well below the sensitivity limit of 2.3nm available in commercial CNCs. Gamero-Casta?o (1999, Ph.D. thesis, Yale University) has shown that the activation of even smaller particles is limited in this PSM by homogeneous nucleation due to cooling of the saturated dibutyl phthalate vapor stream before it mixes with the aerosol. This problem is overcome here by means of a reheater, whereby ions with mobilities (in air) higher than 2cm2V^-1s^-1 can be activated at supersaturations well below the threshold for homogeneous nucleation. The critical supersaturation required to activate ions whose mobility exceeds 0.78cm2V^-1s^-1 (mobility diameter smaller than 1.6nm) is seen to be independent of size, indicating that this instrument can detect arbitrarily small (charged) particles. The reheater makes it possible also to run the PSM at fairly 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) with the PSM alone. Coupled to an optical detector this PSM constitutes a condensation nucleus counter (CNC) suitable for the analysis of charged sub-nanometer particles, ions, as well as charged biomolecules and their fragments, with effectively no lower size limit.