Block method: Reference-anchored peak fitting for tracking subtle chemical evolution in sequential XPS spectra

The Block Method is a refinement of spectral subtraction developed for the analysis of complex x-ray photoelectron spectroscopy data. In this method, the fitted spectrum of a reference sample is converted into a structured spectral block and used as an additional component during the fitting of the modified spectra. The differences between the reference and the modified spectra are described by additional peaks with positive or negative areas, corresponding to the appearance or disappearance of chemical species. Unlike ordinary spectral subtraction, the Block Method allows variations in the background while preserving the intrinsic spectral structure of the reference state. While applications of this strategy have been reported in specific systems, a unified formalization of its theoretical framework and practical protocol has been lacking. This work provides a comprehensive formalization and generalization of the Block Method using metallic Sn 3d and its oxidized states as a primary illustrative example. In addition, previously published applications [Sr/Si(001) interface and partially oxidized Si, Ni, Fe, and Cu] are concisely revisited to demonstrate the practical implementation of the method within the AAnalyzer software. By shifting the focus from individual experimental case studies to a general analytical framework, this work demonstrates how the Block Method aids in discriminating emerging species, managing overlapping signals, and providing physically consistent interpretations of complex surface reactions.