An International Comparison of the Crystallinity of Calibration Materials for the Analysis of Respirable α-Quartz Using X-Ray Diffraction and a Comparison with Results from the Infrared KBr Disc Method

This paper lists the values recommended by the working group for use with XRD analysis. The values for crystallinity obtained for some of the materials (NIST 1878, Min-U-Sil5 and A9950) were 6-7% lower than the original certification or estimates reported in other comparisons. Crystallinity values obtained by XRD gave a good correlation with BET surface area measurements (r(2) = 0.91) but not with mean aerodynamic particle size (r(2) = 0.31). Subsamples of two of the materials (A9950 Respirable and Quin 1 Respirable) with smaller particle size distribution than their parent material did not show any significant change in their values for crystallinity, suggesting that the area XRD measurement of these materials within the particle size range collected is more dependent on how the quartz is formed geologically or how it is processed for use. A comparison of results from laboratories using the infrared (IR) and KBr disc method showed that this method is more dependent than XRD on differences in the particle size within the respirable size range, whereas the XRD values were more consistent between the different measurement values obtained on each material. It was not possible to assign a value for percentage purity to each material for users of IR analysis. This work suggests that differences are likely to exist between the results from XRD and IR analysis when measuring 'real' workplace samples and highlights the importance of matching the particle size of the calibration material to the particle size of the workplace dust for measurements of crystalline quartz.