Distinction and quantification of inorganic sulfur species including thiosulfate by X-ray fluorescence (WD-XRF)

Sulfur occurs in a variety of inorganic and organic compounds with oxidation states from -II up to +VI. Differentiation of these species in solid geochemical samples can be challenging because of oxidation processes during sample preparation by acidic digestion. Applying pressed powder pellets and an analysis by wavelength-dispersive X-ray fluorescence minimises reactions with oxidants and water. Main subjects of this work were five inorganic sulfur species, sulfide -II, elemental sulfur 0, thiosulfate +II, sulfite +IV, and sulfate +VI, and the determination of their fluorescence energies in the sulfur X-ray spectra. S K-1,K-2 and S K-1 can be observed for all species, S K satellites only for species with coordinated oxygen. The results are in good agreement with previously published data. Yet none of the 38 investigated papers from the past 90years reported S K-1,K-2 of thiosulfate, which was determined as E=2,309.12eV in this work apparently for the first time. Binary mixtures of sulfur species are strongly differing in their ability of being quantitatively differentiated, as a reliable quantification requires a sufficient difference of the respective fluorescence energies. Regression equations for each mixture can be used to calculate the ratio of mass fractions of the investigated species from the evaluated fluorescence energy. If boundary conditions are considered, the presented approaches can be applied for analyses of geochemical samples or quality control of technical products. The main advantage of the described methods is the option of implementation to everyday X-ray fluorescence lab routine without substantial additional effort.