Laser-induced plasma analysis of copper alloys based on Local Thermodynamic Equilibrium: An alternative approach to plasma temperature determination and archeometric applications

The excitation temperature of Laser Induced Plasmas (LIPs) of copper-based alloys was determined with an alternative approach, which reverses the procedure of calibration-free methods for quantitative analysis through Laser Induced Breakdown Spectroscopy (LIBS). The inverse method here proposed is based on the Local Thermodynamic Equilibrium (LTE) equations, which were applied to simulate the elemental composition of certified samples at different temperatures, assuming that the actual plasma temperature was the one providing the best agreement with certified data. This procedure was validated for a set of bronze and brass standard samples and in different experimental conditions, by changing laser pulse width (7 ns and 250 fs) and laser wavelength (1064 nm, 532 nm and 355 nm). The temperature determined with the inverse method was then employed to determine the elemental composition of archeological findings of different copper-based alloys from Southern Italy sites (from VII century B.C. to VII A.D.), in order to test its validity for the analysis of actual unknown samples. The obtained weight percentages showed a good correlation with those obtained with the calibration line method, which provided a further confirmation of the assumptions made. - Highlights: ► A novel method based on LTE was developed for plasma temperature determination. ► Calibration-free quantitative analyses were optimized with the proposed inverse method. ► The inverse method was tested and validated for standard copper-based alloys. ► Archeological finds were analyzed with the inverse method and with calibration lines. ► The inverse method was proved reliable also for the analysis of unknown samples.