J Spielvogel, M Richter, L Keck & H Grimm

The Differential Mobility Analyser (DMA) is a common instrument to classify fine and ultrafine particles. The classification is accomplished in the annulus between inner and outer electrode of the DMA, where the charged particles in the sample air drift through the particle free sheath air due to the voltage that is applied to the electrodes. This voltage determines the size of the
particles that reach a narrow slit at the bottom of the inner electrode and thus a downstream detection system. The DMA can be used either with a controlled stepwise or continuous change
of the voltage to measure size distributions, or with a fixed voltage as particle generator for particles of well defined mobility diameter. Conventional DMAs, typically operated with sheath air flow rates of 3 – 20 lpm, feature however several shortcomings which limit the field of application: (1) The resolution of particle size, being controlled by the ratio of sheath air flow to sample air flow (at least when this ratio not too high), is limited and typically around 10. (2) The diffusion of particles causes high losses of small particles and thus a low sensitivity of the instrument. (3) The size range of the DMA, being controlled by the sheath air flow rate, features a lower limit of typically 5 nm.

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