Analysis of aerosol samples collected by multi-stage cascade impactors by ED-XRF

Size segregated elemental concentration in atmospheric particulate matter (PM) is commonly measured by cascade impactors. The use of these devices is actually limited by the relevant request of work during sampling and laboratory analyses. As a matter of fact, sampling campaigns using impactors often last a few days with a quite limited number of samples collected. We present here the results of ED-XRF analyses performed on impactor collecting media used by two different devices: the low-pressure 12-stage SDI by Dekati and the 13-stage rotating NanoMOUDI by MSP. ED-XRF has several advantages: it is a multi-elemental, easy-to-use and sensitive technique, which does not require any preparation of the samples. This speeds up the laboratory analyses however the particular geometry of the samples poses some problem. The two devices (SDI and NanoMOUDI) produce samples with different deposit extension and distribution. In both cases, we used a specific geometry to irradiate the whole sample; indeed, ED-XRF systems had to be properly optimised for the analysis of this type of samples, which have non-uniform deposits. For the analysis, an area of uniform sensitivity, big enough to fully enclose the sample deposit, was identified using a small probe material and used for sample irradiation. This ensured the possibility to accurately measure also non-uniform deposits. Each sample was analyzed before and after the sampling to subtract possible contaminations in the coating material. Minimum Detection Limits ranged from 1 ng/sample to 40 ng/sample. The analysis of cascade impactor samples by ED-XRF is not common therefore, as consistency tests, series of samples were also analyzed by PIXE at the external beam facility of the Tandetron accelerator at the I.N.F.N. LABEC laboratory in Florence (Calzolai et al., 2006), where it was possible to scan with a narrow proton beam (1 mm x 2 mm in size) the PM deposit area. A very good agreement has been obtained for all the detected elements in the analysis of both the Dekati-SDI and NanoMOUDI collecting media. The ED-XRF spectrometry has been applied to samples collected during campaigns at sites with different characteristics (in Genoa and Milan) and these results will be also shown. Data inversion was performed using MICRON, to gather information on the size distribution of the different elements.