Expanded flow rate range of high-resolution nanoDMAs via improved sample flow injection at the aerosol inlet slit

• Published in: Journal of aerosol science Vol. 113; pp. 265 - 275
• Main Author: Fernandez de la Mora, Juan
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.11.2017
• Subjects: Differential mobility analyzer; Flow instability; Inlet slit; Mixing layer; Nanoparticles; Upper size range; Nanoparticles; Upper size range; Differential mobility analyzer; Flow instability; Inlet slit; Mixing layer
• DOI: 10.1016/j.jaerosci.2017.07.020
• ISSN: 0021-8502

High-resolution DMAs requiring hundreds of l/min of sheath gas flow Q to classify 1nm particles have not been previously examined and optimized under the modest Q values (tens of l/min) needed to classify particles well above 10nm. Here we study the resolving power ℛ (based on the relative width of the transfer function) of the Halfmini DMA, at sample flow rates q >1 l/min. The charge-reduced electrosprayed ovalbumin protein used as test aerosol has an accurately determinable Gaussian mobility distribution, which limits the measurable ℛ to at most 25–30. Non-ideal DMA response can however be precisely probed by comparing measured peak widths with the convolution of the protein's mobility distribution with the triangular Knutson-Whitby distribution associated to the finite qi/Q ratio. For the unmodified Halfmini DMA, ℛ departs considerably from ideal at qi/Q values as modest as 2%, revealing a problem with the aerosol inlet flow. This imperfection is not removed by either increasing or decreasing the slit width, nor by reorienting axially the sample flow jet as it merges with the sheath gas. By introducing a ring with many perforations upstream of the inlet slit, which favors a more symmetric distribution of the sample flow over the slit perimeter, ℛ values close to ideal are approached with qi/Q ratios as large as 6–12%. This improvement enables (in principle) classification of particles larger than 30nm. •Existing high resolution nanoDMAs have limited upper size range due to mixing layer flow instabiliites.•The ideal range of operation may be extended by making the sample aerosol stream axisymmetric.•Inlet flow symmetry achieved by a perforated ring etends the size range of the halfmini DMA past 30nm.