Nano particles have been sized based on their electrical mobility. This is not efficient because most of the nano particles are electrically neutral. Measuring the particle size distribution typically consist of detecting size selected particles with an aerosol particle detector such as Condensation Particle Counter (CPC). This is also the working principle of Scanning Mobility Particle Sizer. The particles are first neutralized to a known charge distribution, then they are selected according to their electrical mobility after which they are counted with a CPC. From the electrical mobility one can calculate so called mobility diameter of the particle.
Measuring the sub 5 nm particle size distribution is really challenging. In the resent years the CPC technology has been developed really intensively and thus the detection of particles as small as 1 nm in diameter is now possible (Iida et al. 2009, Vanhanen et al. 2011, Jiang et al. 2011). The most challenging part of the size distributions measurement below 5 nm is the size selection of the particles. Table 1 presents the charging efficiencies, activation efficiencies, and the penetration efficiencies of the size classifier (DMA) and the detectors for particle sizes from 1.1 to 5 nm based on Jiang et al. 2011. It can be seen that the charging efficiency is clearly the bottle neck for the SMPS system (from only 0.65% to 2%). Also the DMA penetration can be quite low especially for sub 2 nm particles and clusters.
To improve detection of the smallest nano particles, a different approach has been used for the Airmodus A11 nCNC. Table 1 and figure 1 also show the total detection efficiencies for both SPMS and A11. From these detection efficiencies the data correction factors have been calculated. It can be seen that for 1.47 nm particles the SMPS needs a correction factor of 6838 while the A11 it is only 1.82. This means that there needs to be almost 7000 #/cc only in the size range of 1.47 nm in order for the CPC of the SMPS system to give a reading of 1 #/cc. For CPCs the false count rate of 0.01 #/cc is typically acceptable. This would already result concentration of 70 #/cc for the SMPS system.
The nCNC is a combination of Particle Size Magnifier (PSM) and a CPC. The PSM is used to grow particles that can’t be detected with the CPC to sizes that are in the size range of CPCs. The main benefit of the PSM is that it uses mixing type principle to grow the particles. The mixing ratio can be changed rapidly which results a change to the smallest size that can be grown by the PSM. Scanning the mixing ratio continuously allows the user to measure size distribution of particles in the size range of 1 to 4 nm. This approach minimizes the losses for the smallest particles because no size selection before the PSM is needed. If we take the same example that we used for the SMPS; with A11 the 7000 #/cc of 1.47 nm particles would result a reading of about 3800 #/cc making the instrument three orders of magnitude more sensitive in the sub 2 nm size range.
References
Iida, K., Stoltzenburg, M. R., and McMurry, P. H. (2009). Effect of Working Fluid on Sub-2nm Particle Detection with a Laminar Flow Ultrafine Con- densation Particle Counter. Aerosol Sci. Technol. 43:81–90.
Jiang, J., Chen, M., Kuang, C., Attoui, M., and McMurry, P. H. (2011). Electrical Mobility Spectrometer Using a Diethylene Glycol Condensation Particle Counter for Measurement of Aerosol Size Distributions Down to 1 nm. Aerosol Sci. Tech., 45:510–521.
Vanhanen, J., Mikkila, J., Lehtipalo, K., Sipila, M., Manninen, H. E., Siivola, E., et al. (2011). Particle Size Magnifier for Nano-CN Detection. Aerosol Sci. Tech., 45:533–542.
