Can laboratory-based XAFS compete with XRD and Mössbauer spectroscopy as a tool for quantitative species analysis? Critical evaluation using the example of a natural iron ore

Abstract

While X-ray diffraction (XRD) is a commonly used method for quantification analysis using Rietveld refinement and quantitative Mössbauer spectroscopy is sporadically used primarily for iron speciation, laboratory X-ray Absorption Fine Structure Spectroscopy (lab-XAFS) is rarely applied for the quantitative determination of sample compositions. With the recent developments of laboratory-based XAFS spectrometers, this method becomes more interesting for many applications as well as for quantification. The goal of this study is to compare quantitative lab-XAFS via Linear Combination Fitting (LCF) of reference spectra with XRD and Mössbauer spectroscopy. Iron species analysis with the focus on the determination of the mass ratio alpha-iron(III) oxide (α-Fe2O3)/iron(II, III) oxide (Fe3O4) was used as an example. The examinations were performed on synthetic α-Fe2O3/Fe3O4 model mixtures and, predominantly, on a natural iron ore sample mainly consisting of the minerals hematite and magnetite, thus, these two iron oxides. For the iron K-edge lab-XAFS measurements an X-ray tube-based spectrometer using the von Hamos geometry with Highly Annealed Pyrolytic Graphite (HAPG) mosaic crystal optic was used. The capabilities and challenges of each method are discussed. The quantitative model mixtures examinations by lab-XAFS show results and accuracies similar to those obtained by XRD and Mössbauer spectroscopy. However, while the quantitative results for the iron ore investigations by lab-XAFS are in good agreement (deviation of 2 percent points) with the XRD results, the composition determined by Mössbauer spectroscopy differs clearly from the lab-XAFS and XRD results. Furthermore, the Mössbauer spectroscopic examinations hint the presence of an additional iron oxide species affecting the quantification. Besides the still common challenges in identification, differentiation and quantification of different iron oxides, the results show that quantitative lab-XAFS can particularly compete with quantitative XRD when determining the species composition of one element. This makes lab-XAFS particularly well-suited for routine analytics.

Fig 1. XAFS-spectra of the reference substances

α-Fe₂O₃ and Fe₃O₄, the α-Fe₂O₃/Fe₃O₄ model mixtures and the Mexican iron ore. a unnormalized XAFS-spectra of the sample and the references, with insets of the normalized edge region (a.2) and the pre-peak area (a.1). The complete normalized spectra can be seen in Fig S4a,b (SI). b LCF results performed by the ATHENA software [51] on the normalized spectra with the flatten algorithm of the Mexican iron ore. Fit and data show only minor deviations, indicated by the residual. The LCF results with the Larch software [52] and the in-house algorithm on the unflattened and flattened normalized spectra can be seen in Fig S8 (S1 File ). The LCF results are displayed in Table S7 (S1 File ). c LCF results of the 50/50 α-Fe₂O₃/Fe₃O₄ model mixture (adhesive tape preparation). For each sample/reference the measurements were performed at room temperature with a measurement time of 10 h.

Fig 2. XRD pattern of the Mexican iron ore with quantitative evaluation by Rietveld refinement with Profex 4.2.4/BGMN.

The results of the Rietveld refinement with Topas V6 (Bruker) can be seen in SI, Fig S14a in S1 File. The quantitative results are listed in Table S10 (in S1 File). The measurement time was 8 h. The measurements were performed at room temperature.

Fig 3. Mössbauer spectra of the reference substances, the model mixtures and the Mexican iron ore sample.

a α-Fe₂O₃ micro particles (${\chi }_{\nu }^2=7.748$), b Fe₃O₄ micro particles (sextet site 1: A-site, sextet site 2: B-site) (${\chi }_{\nu }^2=1.110$), c Mexican iron ore (${\chi }_{\nu }^2=4.158$). d 50/50 α-Fe₂O₃/Fe₃O₄ model mixture (${\chi }_{\nu }^2=2.692$) (see SI Fig. S15 in S1 File for 30/70 and 70/30 ratio). The total spectra fits are marked in red and the individual sextets in blue, green and orange. The measurement time was 24 h for α-Fe₂O₃, 72 h for Fe₃O₄, 10 h for the Mexican iron ore and 120 h for the 50/50 model mixture. The measurements were performed at room temperature.