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

摘要

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 laminar-izers 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 R1=4 mm and R2=6 or 7 mm (gap=R2-R1=Δ=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/Δ =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.