Micro-invasive depth profile analysis by laser-induced breakdown spectroscopy (LIBS): The case of mercury layers on Sasanian coins

Laser-Induced Breakdown Spectroscopy (LIBS) was used for depth-profile analysis of one Sasanian coin of the emperor Khusro II (591-628) from the Coin Collection of the Kunsthistorisches Museum of Vienna. The peculiarity of this coin, also found in analogous samples of the same and other collections, was the presence of an external Hg layer. The thickness and homogeneity of the Hg layer were characterized with the calibration-free inverse method, a variant of the classical calibration-free LIBS approaches in which one sample of known concentration is used to determine the plasma temperature. We demonstrated the feasibility of the inverse method also in cases of unknown samples with matrices different from that of the employed standard. In this work, the standard was a Cu-based alloy and the sample was an Ag-based alloy, also containing Hg and other minor components. The LIBS results were in good agreement with two previous independent micro-XRF measurements performed with a destructive approach, i.e. using a cross section of the same sample. This confirmed the applicability of the inverse method to unconventional matrices, for which the use of matrix-matched standards may be unfeasible. The conclusions of this work strongly support the use of laser ablation techniques for calibration-free and micro-invasive analysis of layered samples, in particular when the integrity of samples has to be preserved.