Hand-held differential mobility analyzers of high resolution for 1-30 nm particles: Design and fabrication considerations

Little has been published on the details of design and fabrication of differential mobility analyzers (DMAs) classifying 1 nm particles with resolving powers of 30 or more. These DMAs must operate at Reynolds numbers Re >> 2000 (supercritical), requiring laminarizers and diffusers that have tended to make them large and heavy. Here we discuss design and fabrication criteria as well as observed performance for several variants of a miniature supercritical DMA weighing 2.7 kg. In spite of the narrow working sections associated to small electrode radii of R-1=4 mm and R-2=6 or 7 mm (gap=R-2-R-1=Delta=2 or 3 mm), a large enough Re (hence resolution R down to 1 nm diameter particles) is achieved via a diffuser enabling up to transonic sheath gas speeds. Several axial distances L between the inlet and outlet aerosol slits have been tested to facilitate classification of particles with diameters above 20 nm. R is limited by concentricity errors, magnified by the small gaps used. Nonetheless, short models with L/Delta=2 achieve consistently R > 40. The long 2 and 3 nm gap model reaches typically R > 25-30, and exceptionally up to R=40. This performance is with 1 nm particles, and would probably be better with larger particles. While prior long supercritical DMAs have used slightly conical inner electrodes to accelerate and stabilize the flow, a cylindrical version of our long DMAs shows no signs of turbulent transition. (C) 2012 Elsevier Ltd. All rights reserved.