Effect of the quartz particle size on XRD quantifications and its implications for field collected samples

The aims of the present study were to assess the effect of the quartz particle size on XRD quantifications, and use it to develop models for correcting the measured quartz concentrations of samples collected from the field. Seven nearly mono-dispersed pure quartz dusts, with mass median aerodynamic diameters (MMAD) ranging from 0.70 to 10.84 µm, were prepared by a liquid sedimentation device, and their unit XRD intensities (UI) were measured using the NIOSH Method 7500. The results show that UI increases (from 063 to 1.14) along with the rise in MMAD of the pure quartz dust. To examine the impact of the above results on quantifying field collected samples, both total dust and respirable dust samplings were conducted at seven different workplace environments. The results show that the quartz particles contained in all collected total dust samples (MMAD = 5.18–16.7 µm, GSD = 2.08–2.88) were coarser in their particle sizes than that of the reference quartz standard (NIST-SRM 1878; MMAD = 2.16 µm, GSD = 1.55), and the measured total quartz particle concentrations (Cm) were 16.6–22.5% lower than the corresponding true concentrations (Ct). However, for respirable dust samples (MMAD = 1.37–3.95 µm, GSD = 1.978–2.87), since collected quartz particle sizes could be either finer or coarser than that of the reference standard, both underestimation and overestimation were found in the present study (Cm/Ct = 0.881–1.09). To correct the measured concentrations of field collected samples, correcting models were developed based on the MMADs of the collected quartz particle samples and their corresponding UIs. This study yields correcting factors for the respirable fraction (CRf) as CRf = 1.50 – 0.67 × [1 – exp(–0.69 × MMAD)] (R2 = 0.996, n = 7). However, the obtained CRf should be used with caution if the collected samples were found with quartz particle sizes falling outside the size range of the present study.