Using Porous Foam as Impaction Substrate for Optimization of Respirable Aerosol Sampler

1. National Taiwan University, College of Public Health, Institute of Occupational Medicine and Industrial Hygiene Using Porous Foam as Impaction Substrate for Optimization of Respirable Aerosol Sampler Chao-Chun Chana; Sheng-Hsiu Huangb; Chih-Chieh Chena aInstitute of Occupational Medicine and Industrial Hygiene, College of Public Health, National Taiwan University, Taipei, Taiwan bInstitute of Environmental Health, College of Public Health, National Taiwan University, Taipei, Taiwan Introduction In order to match the ISO/CEN/ACGIH respirable sampling criteria, applying porous polyurethane foam (PUF) to impactors on particle aerodynamic size fractioning has been studied. However, one of the inherent problems of impactors is particle bounce off the collecting plates, even though foam substrate is applied. Although Grease coating can reduce bounce effect temporarily, it is not feasible if chemical analyses are necessary. The objective of this study is optimizing of respirable aerosol sampler by uncoated foam and identifying foam’s characteristic on sampling efficiency. Methods The Impactor was designed as shown in figure 1. The effect of substrate diameter (3 – 20 mm), thickness (3 – 10 mm) and porosity(45 – 100 ppi , pores per inch) on aerosol penetration curve was demonstrated in this study. Sampling flow rate was set below 5 l/min. Challenge aerosol was potassium sodium tartratetetrahydrate (PST) generated by ultrasonic atomizer (Sono-Tek Inc.). Dried particles was neutralize to Boltzman equilibrium by Americium 241 and spread into testing chamber. Chamber in this study was about 50 l passed by 80 lpm filtered air. Concentration of aerosol was measured by Aerodynamic Particle Sizer (APS, TSI). Upstream concentration was sampled without impactor. Both upstream and downstream sampling time was 60 seconds and repeated over 5 times. Impactor also was tested with silicon oil coated to recognize the bounce effect of the impactor design. Figure 2. The effect of foam’s porosity on aerosol penetration Figure 1. Schematic diagram of designed impactor Result and Discussion Effect of porosity of PUF as collecting substrate on penetration showed higher ppi foam could decrease more particle bounce because of higher collection efficiency showed in figure 2. Data also indicated that PUF of 100 ppi have similar slope of penetration curve to respirable criteria. Figure 3. displayed sampling flow rate not only gave effect on D50 also indicated more significant particle bounce for higher velocity particle with more kinetic energy. Foam of 100 ppi met respirable criteria well at small particle diameter, but had less similarity above 4 μ m since particle rebound form collecting substrate. Study tried to convert thickness of foam to adapt the penetration characteristic, however the slope in figure 4 were still about 0.54 which had little diverse from s=0.6299 of respirable curve (s = (d84/d16)^0.5, where d84 and d16 are particle diameter of which penetration are 84% and 16%, respectively). Diameter of foam larger than 9 mm also indicated less variation on slope, but diameter of 3 mm showed poorer collection efficiency and sharper curve which s=0.696 much closer to target slope as shown in figure 5. Since d50 decreased with increasing flow rate, penetration curve shifted toward smaller diameter and met respirable curve very well by 1.8 lpm in figure 6. Using PUF as collecting substrate reducing particle bounce effect at large particle size extremely in impactor was also proved in this study. Figure 3. The effect of sampling flow rate of uncoated and coated foam on aerosol penetration Figure 4. The effect of foam thickness on aerosol penetration Figure 5. The effect of foam diameter on aerosol penetration Figure 6. The effect of applying D=3mm foam on aerosol penetration Conclusion The designed impactor in this study had an excellent fit with the ISO/CEN/ACGIH respirable sampling criteria when using a PUF of 100 ppi with diameter 3 mm in impactor sampled by 1.8 lpm. Result also showed particle bounce can be minimize by applying PUF as collecting substrate. Correspondent author e-mail: ccchen@ntu.edu.tw (C-C Chen). First author e-mail: r96841015@ntu.edu.tw (C-C Chan).