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Effect of Particle Size on Measurement Uncertainty in Quantification of Chemical Components Using Infrared Absorption
KABIR RISHI, Pramod Kulkarni, Bon Ki Ku, Chen Wang, Orthodoxia Zervaki, Elizabeth Ashley, Centers for Disease Control and Prevention, NIOSH
Abstract Number: 517
Working Group: Instrumentation and Methods
Abstract
Infrared (IR) absorption measurement is commonly used for quantification of chemical components of aerosols. Quantification of analyte requires calibration using a standard reference material. Since light scattering, and hence absorption, depends on particle size, the calibration curve generated is specific to the reference material used. Standard methods, such as NIOSH method 7603 for crystalline silica quantification, recommend the use of NIST standard reference material, which have a polydisperse size distribution in the respirable size range. Measurement uncertainty can be significant if the unknown aerosol has a different size distribution compared to that of the calibration aerosol. Previous studies have probed the effect of particle size on analyte quantification in infrared absorption measurements (Bhaskar et al., 1994; Udvardi et al., 2017; Yabuta and Ohta, 2003; Chen et al, 2013); however, they lacked adequate size-resolved measurements over an extended particle size range. The objective of this study was to probe the particle-size induced artifacts in infrared absorption measurements and determine resulting uncertainty in analyte quantification. Aerosol particles of crystalline silica, polymers, and metallic powders were size classified using a cascade impactor in the size range 0.045 µm to 9 µm. Each size selected fraction was extracted into an aqueous suspension and was redeposited on a PVC filter for infrared absorption measurement. For particle sizes ranging from 0.4 µm to 9 µm, an 8-stage Andersen impactor operated at a flow rate of 28.3 L/min was used. To extend the collection to the sub-micron size range, where the particulate mass was << 1 µg, a 6-stage quartz crystal microbalance (QCM) impactor with cut sizes ranging from 0.045 µm to 0.96 µm was used. The measured absorption, at characteristic vibrational frequency, was normalized by the total particulate mass probed by the infrared beam to allow comparison across samples with different particle size distribution. Mass normalized absorption for IR as a function of aerodynamic particle size showed an increasing trend below 1 µm and a decreasing trend above 1 µm, with another broad peak at about 5 µm diameter. Calculations were performed to obtain infrared absorption spectrum using single and multiparticle Lorentz-Mie light scattering theory. Comparison of experimental and theoretical absorption spectra and implications for method calibration and overall measurement uncertainty will be presented and discussed.