Electrical Mobility Classification

The stable polydisperse output of an atomizer can be used with an electrical mobility classifier to generate monodisperse streams of submicrometer aerosols with diameters ranging from 0.001 to 1 mm [25,26]. A schematic of a commercially available electrical mobility classifier is shown in Fig. 4. It consists of a controller platform for charge neutralization and flow control of the aerosol source to a differential mobility analyzer (DMA) unit. Three DMA units are available that provide classification over a range of two orders of magnitude in particle size for each unit. The polydispersed aerosol is passed through a bipolar charger to bring the charge state of the aerosol to a Boltzmann equilibrium. The neutralized aerosol is made to flow on the outer annular region between two coaxial cylinders. The outer cylinder is grounded, and the inner cylinder is charged to a selected potential. Clean-sheath airflows surround the inner electrode. Stable flowrates are maintained by mass flow controllers. The electrical field set up in the coaxial region cause charged aerosol particles to migrate toward the inner electrode. Only charged particles within a narrow mobility range determined by the potential on the inner electrode and the flow

Figure 4 Schematic of an electrostatic classifier (TSI Model 3080).

rates can exit the slit near the end of the inner electrode. Hence, a particular size of particle corresponding to an electrical mobility value can be selected by the appropriate choice of an inner electrode voltage. The monodisperse particle concentration output for a particular size from a constant-output atomizer-mobility classifier system can be optimized by the selection of an appropriate solution concentration or operating conditions to match the modal size of the atomizer output to the selected monodisperse size. Monodisperse particle output up to 106 cm"3 can be obtained in the size range from 0.05 to 0.08 mm. Diffusion losses for smaller particle sizes restrict the concentrations to less than 104cm"3 for particle sizes of about 0.01 mm. The output for particle sizes larger than 0.1 mm is limited by the output of the atomizer. Frequently, an atomizer evaporation-condensation technique is used to generate a polydisperse aerosol with particle sizes larger than 0.1 mm. Extension of particle size to 1 nm can be achieved using a DMA modified for ultrafine aerosols.

The output of the mobility classifier is monodisperse by mobility classification. The aerosol will then be monodisperse if all the particles are singly charged. A doubly charged particle will have the same mobility as a singly charged particle half its diameter. Occurrence of multiply charged particles is negligible for particle sizes less than 0.02 mm but becomes significant for larger particles. At a mobility size of 0.2 mm, more than 20% of the particles may possess a charge of two or more electrons. This is particularly troublesome because more than two-thirds of the mass of the aerosol particles reside on the particles that are 0.4 mm or larger. This problem in monodispersity can be alleviated by using two or more mobility classifiers in series, but the output will be reduced by losses in the additional classifier. Gupta and McMurry [26,27] have developed a method to minimize this problem of multiply charged particles. A charged condenser was used to remove the charged particles from the aerosol before subjecting the aerosol to a low-activity radioactive source. The particles exiting this arrangement were mostly uncharged or singly charged. At a particle size of 1.0 mm, only 25% of the particles were multiply charged, in contrast to 72% of the particles when the aerosol was at a Boltzmann equilibrium. Although monodispersity is improved, more than one order-of-magnitude reduction in particle concentrations can be expected with this method.

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