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On the Feasibility of a Number Concentration Calibration Using a Wafer Surface Scanner |
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| Authors: | Lin Li Laura Windmuller Miles C Owen Shigeru Kimoto David Y H Pui |
| Affiliation: | 1. MSP Corporation, Shoreview, Minnesota, USA;2. University of Minnesota, Minneapolis, Minnesota, USA;3. U.S. Army Primary Standards Laboratory, Redstone Arsenal, Alabama, USA |
| Abstract: | A new primary standard method for calibrating optical particle counters (OPC) has been developed based on quantitative gravitational deposition on a silicon wafer and accurate counting of the particles by a wafer surface scanner (WSS). The test aerosol consists of 3-μm diameter monodisperse polystyrene latex (PSL) spheres at concentrations in the range of 0.1 cm?3 to 1 cm?3. A key element to the calibration is the ability to generate monodisperse PSL spheres without residue particles by use of a virtual impactor and differential mobility analyzer. The use of these devices reduced the percentage of residue particles from more than 99.98% to about 5%. The expanded relative uncertainty (95% confidence level) in the number concentration determined with a WSS for a deposition of 200 particles is 17.8%. The major uncertainty component arises from the Poisson fluctuations in the aerosol concentration because of the low concentration. This methodology has advantages of a fast scanning time by the WSS of minutes compared to hours or days by microscopy and of counting every particle deposited compared to often only a small fraction via microscopy. The WSS was used in the calibration of an OPC based on 12 depositions with concentrations ranging from 0.1 cm?3 to 1 cm?3 for each deposition. Make-up air was added to the aerosol entering the OPC so that the lowest achievable concentration for the OPC measurement is about 0.01 cm?3 in this study. The detection efficiency of the OPC was measured to be 0.984 with an expanded uncertainty of 13.4%. Copyright 2014 American Association for Aerosol Research |
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