Differential mobility analyzers

The device resembles a cylindrical differential mobility analyzer (DMA) in that a sample flow is introduced around the periphery of the annulus between two concentric cylinders, and charged particles migrate inward towards the inner cylinder in the presence of a radial electric field. Instead of being transmitted to an outlet flow, the sample is collected onto a Nichrome filament located on the inner cylinder. The primary benefit of this mode of size-resolved sampling, as opposed to aerodynamic separation into a vacuum, is that chemical ionization of the vapor molecules is feasible. Because there is no outlet aerosol flow, the collection efficiency is determined by desorption of the particles from the filament, chemical ionization of the vapor, separation in a mobility drift cell, and continuous measurement of the current produced when the ions impinge on a Faraday plate. 

FIGURE 11.65 (a) Electrical aerosol analyzer (adapted from Whitby and Clark, 1966). (b) Schematic diagram of differential mobility analyzer (adapted from Yeh, f993). 

Rosell-Llompart, J., I. G. Loscertales, D. Bingham, and J. F. de la Mora, Sizing Nanoparticles and Ions with a Short Differential Mobility Analyzer, J. Aerosol Sci, 27, 695-719 (1996). 

Magnesium nitride (Mg3N2) particles on a submicrometer scale were synthesized by evaporating magnesium metal in a pressure of 1 kPa of mixed NH3 + N2 gas flow kept at 800°C by a cylindrical furnace (41). Ge and In UFPs, which were size selected in advance with a differential mobility analyzer, were reacted with NH3 gas in a tube furnace at 1000°C to form size-selected GaN and InN UFPs (42). 

Figure 5. Transients observed in the concentrations of ultrafine particles in smog chamber studies of the photooxidation of dimethyl disulfide. Particles were measured with the electrical mobility spectrometer operating at fixed analyzer column voltages for the 11- and 20-nm sizes and with the differential mobility analyzer similarly operated for the 50-nm particles. (Reproduced from reference 49. Copyright 1991 American Chemical Society.)...

Size-resolved chemical information is much more difficult to obtain. The many applications of the differential mobility analyzer in measuring properties of size-classified particles are important tools for the characterization of aerosol systems, but the approaches demonstrated to date yield limited data. Vapor pressures, surface tension, and optical absorption have been measured on mobility-classified aerosols. Direct measurements of the distribution of chemical composition with particle size are needed. Elemental... 

Mobility diameter Differential mobility analyzer (DMA) Separation by differential mobility of charged particles in electric field +... 

Observations of atmospheric nanoparticle number concentrations have increased greatly during the last five years. This advance has been made possible by new instruments and techniques that can measure particles with diameters as small as 3 nm. The new approaches include pulse height analysis of data from an ultrafme condensation particle counter (UCPC) (Stolzenburg and McMurry 1991 Wiedensohler et al. 1994 Weber et al. 1998b) and the combination of a nano-differential mobility analyzer (nano-DMA) (Chen and Pui 1997 Seto et al. 1997) with a UCPC. 

Electrical mobility analyzers, like the differential mobility analyzer, classify particles according to their electrical mobility Be given by (9.30). The electrical mobility equivalent diameter Dem is defined as the diameter of a panicle of unit density having the same electrical mobility as the given particle. Particles with the same Dem have the same migration velocity in an electric field. Particles with equal Stokes diameters that cany the same electrical charge will have the same electrical mobility. 

Instruments such as the differential mobility analyzer (DMA) (Liu et al. 1979) size particles according to their electrical mobility equivalent diameter. 

Filter collection efficiency was evaluated by comparing filter upstream and downstream particle concentrations measured by an SMPS (Scanning Mobility Particle Sizer) consisting of a long DMA (Differential Mobility Analyzer - Model 3080) and a U-CPC (Ultrafine Condensation Particle Counter - Model 3025). The cordierite wall-flow filters exhibited their well-known soot collection efficiency while the foam sample exhibited a lower collection efficiency. 

Electrical Mobility Equivalent Diameter. The diameter of a particle of unit density having the same electrical mobility as the given particle. It is measured by instruments like the differential mobility analyzer (Liu et al., 1979). 

One potential method for measuring the size of aerosol nanoparticles is a scanning mobility particle sizer (SMPS), consisting of a differential mobility analyzer (DMA) and a condensation particle counter (CPC). Aerosol particles enter the DMA where they are charged using a radioactive source and their size is classified based on the electrical mobility, Z, of the particles in the applied electrical field ... 

SMPS (scanning mobility particle sizer)—combines a DMA (differential mobility analyzer) and a CPC (condensation particle counter) to measure the... 

The hygroscopic growth of pharmaceutical particles is usually less than that of sodium chloride particles. It is, however, not negligible, although it is often neglected in aerosol therapy. This was shown for a number of pharmaceutical aerosols. Particles were produced by nebulization of aqueous solutions of drugs, exposed to dry air and passed through a differential mobility analyzer for selec-... 

Mobility sizers collect, count and determine size distributions for aerosols. These can typically handle particle sizes in the range of about 0.02-1 pm. An example is the differential mobility analyzer, which is described later in this section. 

Aspirator or Flow-Field Filter Designs, Differential Mobility Analyzer... 

---