In this work, we applied X-ray Absorption Spectroscopy (XAS) and selective leaching experiments for investigating iron speciation in different dust advections collected on different unwashed quartz fiber filters. XAS analysis evidenced a predominance of Fe(III) in 6-fold coordination for Saharan dust and a trend towards Fe(II) and 4-fold coordination in the order: Saharan dust, mixed Saharan, and non-Saharan aerosol samples. The role of the sampling substrate was evaluated explicitly, including in the analysis a set of blank filters. We were able to pinpoint the possible contribution to the overall XAS spectrum of the residual Fe on quartz as the concentration decrease towards the blank value. In particular, the filter substrate showed a negligible effect on the structural trend mentioned above. Furthermore, selective leaching experiments evidenced a predominance of the residual fraction on Fe speciation and indicated the lowest Fe concentrations for which the blank contribution is <20% are 1 μg for the first three steps of the procedure (releasing the acid-labile, reducible and oxidizable phases, respectively) and 10 μg for the last step (dissolving the insoluble residuals).
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