Iron, the fourth most abundant element in the Earth, commonly occurs in two valence states, Fe²⁺ and Fe³⁺, even within a single mineral. Determination of Fe³⁺/EFe ratios in minerals at sub-micrometre scale has been a long-standing objective in geosciences. One of the most important characteristics of iron is that the charge on the metal is extremely sensitive to its surrounding reduction-oxidation (redox) conditions, which often results in changes in iron valence state reflecting these conditions. The quantification of ferrous/ferric ratios in minerals can therefore provide great insights into physico-chemical conditions of rock formation such as temperature and oxygen fugacity, and allows the determination of redox states for mineral crystallization and the interpretation of geological and geochemical processes. The high spatial resolution available on a (scanning) transmission electron microscope ((S)TEM) combined with the benefits of electron energy-loss spectroscopy (EElS) allows detailed analysis of multivalent element ratios (e.g. Fe²⁺ and Fe³⁺) on the scale of nanometres.

Electron energy-loss spectroscopy is a powerful technique for analyzing the interactions of fast probe electrons with matter, and the energy transferred for a certain excitation process can be measured as an energy loss of the incident electron which reduces its kinetic energy. The probability of inelastic scattering over energy loss is called energy-loss spectrum which results from the excitation of inner-shell, valence or conduction electrons. Excitations are only possible from occupied states below the Fermi level to allowed unoccupied states beyond it. Maxima in the energy loss spectrum correspond to strong electron-specimen interactions. apart from the qualitative and quantitative determination of elements, it is also possible to determine quantitative concentration ratios of