Optimization of Electrospray-Scanning Mobility Particle Sizer (ES-SMPS) Residual Control and Comparative Evaluation of Low-Concentration Silica Nanoparticle Characterization

Reliable characterization of low-concentration colloidal nanoparticles remains challenging due to aerosolization-induced residuals. In this study, an electrospray (model: ES 3480) coupled with SMPS (ES-SMPS: electrospray-scanning mobility particle sizer) has been optimized for residual free particle characterization. We first conducted residual-control study using 20 nm Au nanoparticles because they showed a stable particle peak and then subsequent validation was done using 20 nm SiO2. The effects of sample electrical conductivity, sample flow rate, capillary inner diameter (ID), and sheath flow rate were investigated. The optimized conditions were found to be 480 μS/cm conductivity, 0.35 μL/min sample flow rate, 40 μm ID, and 20 L/min sheath flow rate. The optimized electrospray (model: ES 3480) conditions were then used to characterize SiO2 of different size (20–200 nm) and the characterization results were compared with those obtained using another electrospray (model: ES 3482), an atomizer (KanoA, model: Kanomax 3250), and nanoparticle tracking analyzer (NTA). Increasing the ES 3480 capillary ID from 40 to 75 μm, increased the measured airborne particle concentration by only 1.55-fold, compared with a theoretical 12-fold increase, indicating substantial particle losses during aerosol transport. ES 3482 generated 10-fold lower airborne particle concentrations than ES 3480. The dynamic linear ranges were found to be 7.9 × 107–2 × 1012 #/mL for ES 3480, 6.7 × 108–2 × 1012 #/mL for ES 3482, 9.9 × 107–2 × 1012 #/mL for KanoA, and 8.2 × 106–1.9 × 109 #/mL for NTA with the lower boundary depending on silica particle size. Light scattering technique NTA showed greater concentration and particle-size-dependent variability and 1.73-fold overestimation of particle concentration with 50 nm SiO2 nanoparticles, whereas the aerosol-based systems showed better agreement with prepared concentrations. Overall, this study provides useful guidance for better control of residues in electrospray systems and helps users select appropriate instruments for colloidal nanoparticle characterization.