Effect of Aerosol Size Distribution on Uncertainty of Analyte Quantification Using X-ray Diffraction
KABIR RISHI, Bon Ki Ku, Alan Dozier, Chen Wang, Orthodoxia Zervaki, Vasileia Vogiazi, Pramod Kulkarni,
Centers for Disease Control and Prevention, NIOSH Abstract Number: 77
Working Group: Instrumentation and Methods
AbstractX-ray diffraction (XRD) measurement is widely used for quantification of mineral aerosol such as crystalline silica (CS) in workplace atmospheres. Standard methods for quantification of particulate analyte must use standard reference materials that have polydisperse size distributions. Measurement uncertainty can be significant if the unknown aerosol has a different size distribution compared to that of the calibration aerosol. Previous studies have found pronounced/strong effect of particle size on crystalline silica quantification in XRD measurements over a limited particle size range.
The objective of this study was to systematically investigate mesurement uncertainty in analyte quantification using XRD measurements. Size classified aerosols in the diameter range 56 nm to 18 µm were of various crystalline materials were used to obtain size-dependent calibration curves relating diffraction intensity with analyte mass. Different materials including α-quartz, α-cristobalite, iron, gold, sodium chloride were used as test aerosols. The diffraction intensity for most materials agreed with Wilchinsky’s theory for predicting variation of diffraction intensity as a function of particle size, except for quartz and cristobalite, where sharp reduction in signal was observed for the sub-micron fractions. Previous studies have attributed such reduction in diffraction intensity to an amorphous layer on silica particles. Examination of surface crystallinity using electron diffraction for smaller particles (<< 1 µm) did not reveal such amorphous layer. We attribute the sharp decrease in diffraction signal to presence of smaller amorphous particles in the standard reference material instead. The overall measurement uncertainty in quantification of CS for varying size distribution of workplace aerosol compared to that of the calibration standard will be discussed.