Interlab comparison of elemental analysis for low ambient urban PM_2.5 levels

There is growing concern about the accuracy of trace elemental analysis of ambient particulate matter (PM) samples. This has become important because ambient PM concentrations have decreased over the years, and the lower filter loadings result in difficulties in accurate analysis. The performance of energy-dispersive X-ray reflectance spectrometry was evaluated at Harvard School of Public Health using several methodologies, including intercomparison between two other laboratories. In reanalysis of standard films as unknown samples following calibration, the HSPH ED XRF measurements represented good performance: 2% errors in precision and 4% errors in accuracy. Replicate analysis of ambient air filters with low PM_2.5 levels indicated that S, K, Fe, and Ca showed excellent reproducibility, most other quantifiable elements were below 15% error, and the elements with larger percent of flagged measurements had less in precision. Results from the interlaboratory comparison demonstrated that most quantifiable elements, except Na and Al, were quite comparable for the three laboratories. Na performance could be validated from the stoichiometry of Na to Cl of indoor PM_2.5 filter samples.