Mass differences between samplers for respirable dust and the analysis of quartz: - An international study

Members of an international standards working group for silica measurement (ISO/TC146/SC2/WG7 Silica) have collaborated to assess the differences between analytical approaches for the analysis of respirable crystalline silica (RCS). In addition they have also attempted to assess the relative collection efficiencies of samplers used in this study. Nine participating laboratories from eight countries evaluated thirteen respirable samplers in the study. The samplers were exposed to airborne concentrations of ultra fine and medium Arizona Road Dust (ARD) in a calm air chamber. Subsequently, each participating laboratory analysed samples following their own national method and the Health and Safety Laboratory (HSL) retained a third for verification. On the samples collected, all the methods and analytical approaches applied in this study obtained comparable results (most were within ±12%). The AFNOR method (1995) had a tendency to report lower values when compared to the HSL verification analysis. Correcting for the crystallinity of the calibration quartz dust using a verified value tested against a certified reference material (Stacey et al 2009) has one of the largest positive impacts on the comparability of results. When following good analytical practice, the main factor affecting the comparability of results for RCS analysis are the significant differences in sampler efficiencies. In particular, the conductive sampler from SKC over sampled respirable dust (1.4 - 1.3x) when compared with the average air concentration. The Dorr Oliver, SKC aluminium, CIP10 and IOM head (with polyurethane foam separator) samplers all reported lower respirable dust air concentrations than average with the ultra fine ARD. Their lower collection efficiency compared with other samplers is explainable from sampler information published in literature. The Dorr Oliver sampler also had a tendency to collect a lower proportion of RCS in the respirable dust than others. The working group argue that there is now a case for limiting the number of samplers for analysis of RCS to improve comparability and cross utilisation of exposure data between countries.