Design and use of a virtual impactor and an electrical classifier for generation of test fiber aerosols with narrow size distributions

Abstract

This report describes the combined use of a virtual impactor and an electrical classifier to produce fibrous aerosols with narrow distributions of fiber diameter and length. These aerosols are useful for calibrating fiber monitors, studying fiber charging mechanisms, and conducting fiber inhalation toxicological studies. Four types of man-made fibers, including carbon fibers, X7484 glass fibers, X7999 glass fibers, and MMVF10 glass fibers, were selected for this study. The first three types of fibers are monodisperse in diameter [count median diameter (CMD) = 1.8-3.8 μm; geometric standard deviation (GSD) = 1.1–1.2] and polydisperse in length [count median length (CML) = 16.5–35.8 μm; GSD = 1.6–2.1], whereas the MMVF10 glass fibers are polydisperse in diameter (CMD = 1.1 μm; GSD = 1.8) and length (CML = 16.1 μm; GSD = 2.4). To classify each of the first three fiber types by size, a one-stage virtual impactor was used to reduce the nonfibrous debris. An electrical classifier containing a unipolar-ion charger and a differential mobility analyzer was used to classify fibers by length. Results showed 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 < 1.30. Classified carbon fibers with CMLs greater than 100 um exhibited bimodal length distributions with the smaller modes containing 5–15% (in number) of fibers shorter than 50 μm. This phenomenon was reduced by using a two-stage, instead of the one-stage, virtual impactor, prior to the electrical classifier. To classify the polydisperse MMVFl0 glass fibers, a two-stage virtual impactor and an electrical classifier were used. The flow rates and nozzle diameters in the two-stage virtual impactor were adjusted to vary the CMDs of the classified fibrous aerosols. Results showed that a two-stage virtual impactor with 50% cutoff diameters of 6.4 um (first stage) and 4.1 um (second stage) classified a monodisperse fraction of MMVF10 fibers based on their diameters (CMD = 1.25 μm; GSD = 1.22). By introducing the classified fibers 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 μm with GSDs from 1.22 to 1.30; the corresponding CMDs and GSDs were 1.22–1.32 μm and 1.21–1.25, respectively. Results from this study demonstrated that the combined use of a virtual impactor and an electrical classifier produced micrometer-sized (in diameter) fibrous aerosols with narrow size distributions in both diameter and length. In addition, combining the use of these two devices has a potential to classify irregularly shaped particles into different monodisperse fractions for aerosol production and size determination.