Application of Half Mini DMA for sub 2 nm particle size distribution measurement in an electrospray and a flame aerosol reactor

Abstract

Conventional Differential Mobility Analyzers (DMA) have had limited success in classifying sub 2nm particles with high resolution, primarily due to diffusion broadening. High flow DMAs have been able to overcome this limitation and achieve high-resolution classification of sub 2nm particles, by maintaining laminar flow at high Reynolds numbers. A Half Mini DMA was compared with a Nano DMA (TSI model 3085) for sub 2nm size distribution measurements of organic ions generated by electrospray of tetra-heptyl ammonium bromide (THAB) solution and aerosols generated by a premixed flat flame reactor. Obscurities in ion peak measurements with the Nano DMA indicated a higher diffusion effect. Calculations of the diffusing transfer functions indicated that the Half Mini DMA deviated significantly from ideal conditions, possibly due to the higher demands for a more precise electrode placement and a smaller surface roughness, since it is operated at a high sheath flow rate.The Half Mini DMA was then applied to study the formation of flame-generated aerosols in the sub 2nm range. This is the first reported measurement of these clusters in an aerosol reactor. The effects of the flame synthesis precursor (titanium (IV) isopropoxide), the Kr-85 radioactive neutralizer, and the tubing materials on the flame aerosol size distribution measurements were investigated. After the introduction of titanium (IV) isopropoxide, several discrete peaks were detected, suggesting discrete cluster formation during the initial stages of TiO2 particle synthesis. The addition of a radioactive neutralizer balanced the size distribution of flame aerosols by changing the charging mechanisms. Compared with copper tubing, silicone conductive tubing generated extremely high positive ion contamination after neutralization.