Existing aerosol particle mass classifiers (PMCs) can classify particles having masses down to ca. half an attogram (i.e., 10^-18 g), which corresponds to a diameter of ca. 10 nm for spherical particles with standard density (1 g/cm³). Here, we describe an improved design of such a classifier, namely, the nano-PMC, which can classify particles with masses down to 20 zeptograms (10^-21 g). The response of the classifier was characterized with spherical polystyrene-latex and ammonium sulfate particles, produced by atomization and mobility classification. Measured responses were compared with predictions by a numerical trajectory-based model that considers particle diffusivity. Measurements and predictions of the mean mass of the particles penetrating the classifier agreed within experimental uncertainty (<6%). Differences in the spectrum width could be attributed to recirculation flows occurring in the classification channel.

To demonstrate the capabilities of a nano-PMC, we used it in a tandem configuration with a differential mobility analyzer to determine (1) the size-dependent shape factor of cubic sodium chloride particles having diameters from 15 to 120 nm, and (2) the apparent density and mass–mobility coefficient of coalesced and aggregated silver particles generated by spark ablation. Measurements of the shape factor of the cubic sodium chloride particles show good agreement with previous observations. Coalesced silver particles exhibited an apparent density that was lower compared with that of bulk silver, suggesting a slightly non-spherical particle shape. The mass-mobility scaling exponent of aggregated silver particles determined by the measurements was 2.3 ± 0.1.

The complete article "The Nano-Particle Mass Classifier (Nano-PMC): Development, Characterization, and Application for Determining the Mass, Apparent Density, and Shape of Particles with Masses Down to the Zeptogram Range" can be downloaded at the website of the Journal "Aerosol Science and Technology", Volume 49, Issue 7, pp. 495-507 (charges may apply).