Report on the intercomparison run NAT-3 for the determination of trace and minor elements in urban dust artificially loaded on air filters

An intercomparison exercise was organized for two materials, namely air filters artificially loaded with urban dust (dust on filter), and the bulk material (urban dust). 39 laboratories from 31 countries participated in the analysis using the following analytical methods: neutron activation analysis (NAA), particle induced X-ray emission analysis (PIXE), X-ray fluorescence analysis (XRF) and atomic absorption spectrometry (AAS). Results were obtained for 53 elements for dust on filters, and for 57 elements in the urban dust itself. For the dust on filters, more than 4 individual laboratory mean values were reported for 40 measurands. Out of these, there was good agreement for the results obtained by different methods for 12 elements. Results for Au, Cd, Ce, Cs, Eu, Hf, Hg, La, Sb, Sm, Th and U were available only by NAA. Statistical evaluation of the data revealed satisfactory values for As, Ca, Cu, K, Mg, Mo, Rb, and Sc. In other cases, high number of outliers (20%) or the wide range of results did not permit a thorough evaluation. The final results for Br, Cl, Co, Cr, Ni, S, Se and Zr were unsatisfactory due to excessive dispersion observed in the reported ranges. Br in the dust on filter presented a special problem; the values from several laboratories were negative when corrected for blank values for the filter. For the bulk material, 22 elements out of the 39 evaluated showed good agreement between different methods. For Au, Cs, Eu, Hf Sm, Ta, W and Yb only NAA was used. Acceptable values were revealed for Cd, Mg, Mo, S, Sb, and Sc. The results obtained for Fe, Na and Pb were unsatisfactory. The values for Na and Pb showed excessive dispersion. In conclusion, the results from this intercomparison run demonstrated that analysis of loaded air filters is a difficult problem faced by many laboratories. This sort of sample is not a simple matrix and poses several analytical challenges especially for elements of environmental concern. The experience gained by this run indicates that further work is needed to improve the analytical performance in dealing with urban dust on filters as well as urban dust