A close look at dose: Toward L-edge XAS spectral uniformity, dose quantification and prediction of metal ion photoreduction

This paper represents a call to the broader XAS community to more closely assess dose, as well as the other experimental factors (detection method, temperature, pressure and sample conditions) that affect the rate of X-ray induced chemistry during metal L-edge (as well as other) XAS measurements. While there have been detailed studies on photoreduction within the X-ray community, which have aimed to establish dose-photoreduction relations, it is our view that further standardization of data collection and reporting procedures are required. For molecules as simple as KMnO4 and K-3[Fe(CN)(6)], the literature reported Mn and Fe L-edges, respectively, vary greatly in both overall shapes and spectral distributions. In addition, the reported experimental procedures are often not sufficient to establish the dose-photoreduction relationships. in an effort to more rigorously and quantitatively address this issue, we have measured Mn and Fe L-edge data on KMnO4 and K-3[Fe(CN)(6)] at seven beam lines around the world. We demonstrate that while at a given beam line under identical experimental conditions, there is a dose-photoreduction correlation, this correlation does not hold across different experimental setups or upon variation of the experimental conditions at the same beam line. We introduce a Madab-script XASskindose to calculate the absorbed dose related to a spectrum. By quantifying the rate of reduction (k), half-dose (D-1/2) and the, here introduced, spectroscopically tolerable dose (D-st) values, we find that these values can vary by up to two orders of magnitude for the same compound. Hence, the results presented herein, indicate that a simple dose-photoreduction correlation is insufficient for predicting the rate of X-ray induced changes within a sample. The need to also consider detection method, temperature, pressure and sample conditions are discussed, and correlated to experimental data. A generalized reporting procedure for the broader XAS community is proposed in order to streamline the assessment of reported spectra. In addition, G-values are proposed as a means to predict the rate of metal ion photoreduction prior to an XAS measurement at any beamline by considering the metal ion type, its oxidation state and its stoichiometry in a compound, together with the dose ratio of the compound. (C) 2014 Elsevier B.V. All rights reserved.