DESIGN AND USE OF A VIRTUAL IMPACTOR AND AN ELECTRICAL CLASSIFIER FORGENERATION OF TEST FIBER AEROSOLS WITH NARROW SIZE DISTRIBUTIONS

This report describes the combined use of a virtual impactor and an el ectrical classifier to produce fibrous aerosols with narrow distributi ons of fiber diameter and length. These aerosols are useful for calibr ating fiber monitors, studying fiber charging mechanisms, and conducti ng fiber inhalation toxicological studies. Four types of man-made fibe rs, including carbon fibers, X7484 glass fibers, X7999 glass fibers, a nd MMVF10 glass fibers, were selected for this study. The first three types of fibers are monodisperse in diameter [count median diameter (C MD) = 1.8-3.8 mu m; geometric standard deviation (GSD) = 1.1-1.2] and polydisperse in length [count median length (CML) = 16.5-35.8 mu m; GS D = 1.6-2.1], whereas the MMVF10 glass fibers are polydisperse in diam eter (CMD = 1.1 mu m; GSD = 1.8) and length (CML = 16.1 mu m; GSD = 2. 4). To classify each of the first three fiber types by size, a one-sta ge virtual impactor was used to reduce the nonfibrous debris. An elect rical classifier containing a unipolar-ion charger and a differential mobility analyzer was used to classify fibers by length. Results showe d that the longer fibers were classified by increasing the total flow rate or decreasing the analyzer voltage in the electrical classifier. Most classified fibers had narrow length distributions with GSDs < 1.3 0. Classified carbon fibers with CMLs greater than 100 mu m exhibited bimodal length distributions with the smaller modes containing 5-15% ( in number) of fibers shorter than 50 mu m. This phenomenon was reduced by using a two-stage, instead of the one-stage, virtual impactor, pri or to the electrical classifier. To classify the polydisperse MMVF10 g lass fibers, a two-stage virtual impactor and an electrical classifier were used. The flow rates and nozzle diameters in the two-stage virtu al impactor were adjusted to vary the CMDs of the classified fibrous a erosols. Results showed that a two-stage virtual impactor with 50% cut off diameters of 6.4 mu m (first stage) and 4.1 mu m (second stage) cl assified a monodisperse fraction of MMVF10 fibers based on their diame ters (CMD = 1.25 mu m; GSD = 1.22). By introducing the classified fibe rs into the electrical classifier, fibrous aerosols with narrow length distributions of different CMLs were produced by varying the analyzer voltage. The CMLs of the classified fibers ranged from 9.0 to 41.5 mu m with GSDs from 1.22 to 1.30; the corresponding CMDs and GSDs were 1 .22 - 1.32 mu m and 1.21 - 1.25, respectively.Results from this study demonstrated that the combined use of a virtual impactor and an electr ical classifier produced micrometer-sized (in diameter) fibrous aeroso ls with narrow size distributions in both diameter and length. In addi tion, combining the use of these two devices has a potential to classi fy irregularly shaped particles into different monodisperse fractions for aerosol production and size determination.