Mobility analyzer

Kulju, L.M., K.D. Chu, and P.K. Hopke, The Development of a Mobility Analyzer for Studying the Neutralization and Particle Producing Phenomena Related to Radon Progeny, this volume (1987). 

Attachment coefficient is for neutral daughter products. A cylindrical capacitor used as a mobility analyzer. 

Development of a Mobility Analyzer for Studying the Particle-Producing Phenomena Related to Radon Progeny... 

The determination of the activity size distribution of the ultrafine ions is of particular interest due to their influence on the movement and deposition of Po-218. These ultrafine ions are the result of radiolysis and their rate of formation is a function of radon concentration, the energy associated with the recoil path of Po-218, and the presence of H O vapor and trace gases such as S02 a joint series of experiments utilizing a mobility analyzer, the separate single screen method, and the stacked screen method were conducted to examine the activity size distribution of the ultrafine mode. 

The results obtained from the mobility analyzer are discussed in this paper. 

This diffusion chamber was modified to provide a uniform flow from two channels at the entrance, one for the filtered room air and the other for the gas from the radon chamber. This modified mobility analyzer is schematically shown in Figure 2. The pressure heads are adjusted so that the gas velocities, v, are the same in both channels. An adjustable vertical electric field, E, is provided through the analyzer so that charged particles are drawn toward the detector located at x cm from the entrance. With the known distance, d, between the radon-laden gas channel and the detector implanted plate, the mobility can then be determined from... 

In order to examine the process of ultrafine particle formation, a joint series of experiments were conducted at the Denver Research Center of the U.S. Bureau of Mines. In the Denver radon chamber, the activity size distribution of the ultrafine mode was measured using the mobility analyzer designed by Chu and Hopke (1985), the separate single screen method (Holub and Knutson, 1987), and the stacked single screen method (Holub and Knutson, 1987) for various relative humidities and for various concentrations of SO. The results... 

Figure 2. The mobility analyzer modified from the Erikson spectrometer.

From the data obtained by the utilization of the mobility analyzer it was observed that H O decreased the amount of Po-218 ions and the addition of SC>2 increased particle formation. From Figure 3, it can be seen that with an increase in the relative humidities and absence of SC>2, there is a corresponding decrease in the number of counts. This decrease in the number of counts recorded is a result of an increase in neutralization of the Po-218 ions by water vapor. Since the mobility analyzer is only capable of detecting ions, the mobility spectra obtained in the presence of water are of a different type than those spectra obtained in the absence of water. 

Figure 2 and Figure 3 show the size distribution of these ultrafine particles measured by separate single screens (EML), stacked single screen (USBM), and the mobility analyzer (UI). 

Since the mobility analyzer collects only charged particles, the size distribution derived from the mobility spectrum is only for the charged particles. There are some correlations among these three different methods at both high and low S02 concentrations. 

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. 

The size distribution of the particulate matter in the 0.01-5 ym size range is analyzed on line using an electrical mobility analyzer and an optical particle counter. Samples of particles having aerodynamic diameters between 0.05 and 4 ym are classified according to size using the Caltech low pressure cascade impactor. A number of analytical procedures have been used to determine the composition distribution in these particles. A discrete mode of particles is observed between 0.03 and 0.1 ym. The major components of these particles are volatile elements and soot. The composition of the fine particles varies substantially with combustor operating conditions. 

Particle size distributions of smaller particles have been made using electrical mobility analyzers and diffusion batteries, (9-11) instruments which are not suited to chemical characterization of the aerosol. Nonetheless, these data have made major contributions to our understanding of particle formation mechanisms (1, 1 ). At least two distinct mechanisms make major contributions to the aerosols produced by pulverized coal combustors. The vast majority of the aerosol mass consists of the ash residue which is left after the coal is burned. At the high temperatures in these furnaces, the ash melts and coalesces to form large spherical particles. Their mean diameter is typically in the range 10-20 pm. The smallest particles produced by this process are expected to be the size of the mineral inclusions in the parent coal. Thus, we expect few residual ash particles smaller than a few tenths of a micrometer in diameter (12). 

Electrical mobility analyzers Several types of instruments for measuring particle sizes in the atmosphere depend on the mobility of charged particles in an electric field (e.g., see Yeh (1993) and Flagan (1998) for a review and history of the development of this field). The electrical mobility analyzer developed by Whitby and co-workers at the University of Minnesota, in particular, has been used extensively to measure particles in the range 0.003 to 1 yum (Whitby and Clark, 1966 Eisele and McMuriy, 1997). 

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

There are a number of techniques for generating aerosols, and these are discussed in detail in the LBL report (1979) and in volumes edited by Willeke (1980) and Liu et al. (1984). We briefly review here the major methods currently in use these include atomizers and nebulizers, vibrating orifices, spinning disks, the electrical mobility analyzer discussed earlier, dry powder dispersion, tube furnaces, and condensation of vapors from the gas phase. 

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... 

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