10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Methods to Minimize Diffusion Losses for sub-3nm SMPS Measurements
JACOB SCHECKMAN, Modi Chen, Hee-Siew Han, Juergen Spielvogel, TSI Incorporated
Abstract Number: 1562 Working Group: Instrumentation
Abstract Scanning Mobility Particle Sizer (SMPS) systems employing a Differential Mobility Analyzer (DMA) for sizing and a Condensation Particle Counter (CPC) for counting are widely used for measurement of aerosol particle size distributions. Such measurements of particles as small as 1nm in diameter are needed in academia and industry for applications including nucleation and growth, engineered nanoparticle synthesis, reaction kinetics and combustion research. Diethylene glycol (DEG) has been used as a working fluid to grow sub-2.5nm particles to a size detectable with a butanol-based CPC, thus enabling SMPS size distributions in this size range. A 1nm SMPS system employing a DEG-based Nano Enhancer along with a butanol-based CPC has been commercially available from TSI since early 2016.
As particle size decreases, diffusion loss rates increase dramatically. Less than 10% of 20nm particles are lost in a 1 L/min laminar flow through a straight 3m tube. For the same conditions, the loss rate of 2nm particles is nearly 95%, and greater than 99% at 1.5nm. Further, because of the size-dependent nature of diffusion losses, the mode of a monodisperse (σg=1.15) size distribution shifts from 1.5nm entering the 3m tube to 1.8nm on exit. Although diffusion losses can be corrected for, below 2nm the required correction factor can be several hundred or higher for a typical SMPS system. To minimize uncertainty due to counting statistics, care must be taken (minimize unnecessary tubing lengths, use transport flows, etc.) to configure the SMPS system to maximize particle penetration.
Here we present a compact SMPS setup for the TSI 1nm SMPS system. The components are assembled with the neutralizer coupled directly to the DMA, which is installed directly onto the Nano Enhancer inlet. This configuration eliminates Electrostatic Classifier internal plumbing, and neutralizer-to-DMA and DMA-to-CPC transport tubing. This configuration maximizes particle penetration through the system and minimizes uncertainties in the final corrected concentrations.
In addition, we derive a semi-empirical diffusion correction for the optimized, compact 1nm SMPS system configuration. For each system component, particle losses were measured experimentally under limited flow rate/particle size conditions. The straight-tube laminar flow loss equations were used to fit an “effective length” that is then used to calculate losses for other flow rates and particle sizes. We also present experimental validation of the correction at sizes <2nm, and evaluate its impact on real-world size distribution data generated by such a system.