Elemental composition of aerosol particulate matter collected on membrane filters: A comparison of results by PIXE and ICP-AES

Abstract

The elemental composition of aerosol matter (total suspended particles) collected on high-purity membrane filters during 24-h sampling periods with distinctly different pollution levels was analysed by proton-induced X-ray emission spectrometry (PIXE) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The mass density of the material deposited on 34-mm diameter filter areas ranged from 65 to 294 μg/cm 2. Different spots on each sample as well as on blank reference filters were first analysed by PIXE (1.85 MeV protons, beam diameter 1.5 mm). Using a peak-fitting routine developed in-house together with an empirically determined sensitivity function, the background-corrected net spectra were converted to elemental yields. Absolute calibration was achieved by comparison against a thin-film copper standard prepared by evaporation. Following PIXE analysis, the whole filters with the aerosol deposits were digested by high-pressure ashing. A small amount of the dissolved material was analysed by ICP-AES using standard procedures. The mass fractions of elements in the particulate matter ranged from 6×10 −5 for Co to 0.1 for Ca, with significant differences between the sampling periods. For some elements the mass fractions determined by PIXE showed a spot-to-spot variation by up to 30%. In most cases the mean elemental mass per filter determined by the two techniques agreed quite well, with a slight trend towards lower values by PIXE (10–15%). The difference is within the combined uncertainty of about 20%, estimated for elemental mass ratios determined by the two techniques. The Ti content determined by ICP-AES was always about 50% lower than by PIXE, the K content sometimes up to 30% higher. The discrepancy for Ti appears to reflect a digestion problem. The enhanced K content is attributed to a rather high instrumental background in ICP-AES.