Differential Electrical Mobility Analysis (DMA)
is using an electric field for the classification and analysis of charged aerosol particles ranging from about 1nm to about 1µm in a gas phase. It mainly consists of four parts: (1) a pre-separator, which limits the upper size to a known cut-off size; (2), a particle charge conditioner, which charges the aerosol particles to a known electrical charge (a function of particle size). A bi-polar diffusion particle charger is commonly used. The gas is ionized either by radiation from a radioactive source (e.g. 85Kr), or by ions emitted from a corona electrode. Gas ions of either polarity diffuse to the aerosol particles until charge equilibrium is reached. (3) a differential electrical mobility spectrometer (DEMS), which discriminates particles with different electrical mobility by particle migration perpendicular to a laminar sheath flow. The voltage between the inner cylinder and the outer cylinder (GND) is varied to adjust the discrimination level. (4) an aerosol particle detector, using e.g. a continuous flow condensation particle counter (CPC) or an aerosol electrometer (AE).
A typical set-up of the DEMS is shown in FIG. 1. It shows the flow rates of the sheath flow F1, the polydisperse aerosol sample F2, the monodisperse (classified) aerosol exiting the DEMS F3 and the excess air F4.
The electrical mobility Z depends on the particle size x and the number of elementary charges e:
 |
(1) |
With the number of elementary charges p, Kn the Knudsen number 2l/x, l is the mean path of gas molecule, h is the dynamic fluid viscosity and A, B, C, are numeric constants determined empirically.
FIG. 1 Equipment used in the pipet method of size analysis
Commercial instruments are available for a variety of applications in aerosol instrumentation, production of materials from aerosols, contamination control, etc. [ISO/CD 15900 2006: Determination of particles Size Distribution – Differential Electrical Mobility Analysis for Aerosol Particles].
