Evaluation of iron speciation reference materials by wavelength-dispersive X-ray fluorescence, inductively coupled plasma optical emission spectroscopy, and flame/graphite furnace atomic absorption spectrometry for palaeoredox analysis

Fe speciation is crucial for distinguishing various redox conditions and thus offers valuable insights into the evolution of life and the environment throughout geological history. Monitoring experimental processes using reference materials is essential to ensure the consistency and reliability of Fe speciation analyses across laboratories. In this study, we evaluate the extraction processes and analytical methods used in the analysis of Fe speciation in four marine shale reference materials (WHIT, KL133, KL134, and BHW). A simple and feasible extraction process to extract Fe_carb, Fe_ox, and Fe_mag that enables the simultaneous horizontal shaking of multiple samples by securing centrifuge tubes in foam racks is evaluated. Additionally, we compare the effects of the heating method on the extraction of Fe_R by heating the solution with a boiling water bath and a Bunsen burner. A new droplet method is devised for the instrumental analysis of Fe speciation using samples with high concentrations of total dissolved solids and organic matter matrices using wavelength-dispersive X-ray fluorescence spectrometry. A specialized PTFE support holder is designed to dry the filter paper in situ during sample preparation. We also compare figures of merits of wavelength-dispersive X-ray fluorescence spectrometry (WD-XRF), inductively coupled plasma-optical emission spectroscopy, and flame/graphite furnace atomic absorption spectrometry, including sample preparation, accuracy, precision, method limit of quantification, and linear range. The proposed WD-XRF droplet method will greatly simplify the sample preparation process and improve the efficiency of Fe speciation analysis.