Discrimination of postal stamps via energy-dispersive inelastic X-ray scattering (EDIXS): Differentiating inks beyond elemental composition

Authenticating postal stamps is crucial due to both their potential financial and historical value. This study explores the efficacy of Energy-Dispersive Inelastic X-Ray Scattering (EDIXS) spectroscopy for discriminating among different stamps, due to its ability to discern atomic environments and oxidation states. While conventional X-ray fluorescence (XRF), coupled with Principal Component Analysis (PCA), can differentiate stamps, its information primarily reflects ink plus paper composition, which varies over time and isn't ideal for a proper classification.

This study analyzes the iron Kα-RIXS peak region in nine Argentine stamps spanning nearly a century. PCA is applied to the EDIXS data, revealing distinct regions in 2D and 3D principal component representations, where each region corresponds to a specific elemental composition and atomic environment of the cancellation mark inks. While some regions overlap, several inks are demonstrably differentiated, highlighting the potential of EDIXS for discriminating inks.

The limitations of the current findings, due to overlapping regions, can be solved with further investigations including increased measurements per point and direct comparisons among stamps. Overall, this study underscores the power of EDIXS in characterizing and distinguishing inks beyond elemental composition, offering a promising potential tool for stamp authentication and combating forgery in philately.