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Mobility analysis

The mobility analyzer of a parallel electrical aerosol instrument consists of a number of cylindrical electrodes. The central electrode is divided into several sections as well, each biased at a different fixed electric potential to extend the mobility range while limiting the size of the instrument.

attachment:mobility_analyzer.svg

The charge collecting outer electrodes are connected to electrometric amplifiers at near ground potential which in turn are connected to a data acquisition system.

The charged aerosol is sucked into an circular opening near the central electrode. The clean sheath air is introduced near the outer electrode. The particles travel along the analyzer and those with the same charge electrical polarity as the central electrode are pushed towards the outer electrodes by a radial electric field. The particles that have been deposited on a collecting section pass their charge on to the respective amplifier.

The electric mobility analysis follows the generic aspiration theory [Tammet1970]. According the theory, the movement of a charged particle in a cylindrically symmetrical analyzer is determined by the air flow rate (Φ) and the electric field (CU product, where C is the analyzer capacitance, U is the analyzer voltage) that is crossed by the particle while traveling inside the analyzer.

z = (Φ⋅ε0)/(CU)

The formula determines the relationship between three parameters: flow rate Φ, particle mobility z and analyzer capacitance C. Fixing one of them fixes the ratio of the other two. So an ion with the mobility z attaching to an outer electrode has passed the flow rate Φ and the electric field CU. This allows to calculate the attachment locations of ions with different mobilities entering the analyzer at different flow rates. The formula can be modified for the analyzer, consisting of multiple sections as follows:

zn = (a + Φs)⋅ε0)/(ni = 0CiUi)

where Φa is the sample aerosol flow rate and Φs is the sheath flow rate, zn is the lowest (limiting) mobility of particles reaching the the section n.

[Tammet1970]Tammet, H. (1970). The aspiration method for the Determination of Atmospheric-Ion Spectra. The Israel Program for Scientific Translations Jerusalem. Washington, D.C.: National Science Foundation.

Docs/MobilityAnalysis (last edited 2011-11-03 16:42:33 by Sander Mirme)