Analysis of atmospheric aerosol collected by multi-stage cascade impactors by Energy Dispersive X-Ray Spectrometry

Size segregated elemental concentration in atmospheric particulate matter (PM) is commonly measured by cascade impactors. The use of these powerful device 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 Energy Dispersive X Ray Fluorescence (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 multielemental, 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 of the SDI supports, an area (diameter about 1cm) 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. PIXE spectra were fitted for 25 elements and the elemental concentrations were obtained via a calibration curve from a set of thin standards of known areal density. In Figure 1 the correlation between concentration data obtained by ED-XRF and by PIXE is shown for some elements in the case of the SDI impactor. A similar agreement has been obtained for the other elements and in the analysis of the 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.