Optical Emission Spectroscopy is used within the LIBS (Laser-Induced Breakdown Spectroscopy) technique to measure the elemental composition of a sample irradiated by a laser pulse. When the objective is to characterize a known alloy or to analyze the sample semi-quantitatively, standards can be used. This method refers to the ''calibrated LIBS''. When the studied sample is complex (for instance unknown alloy and/or concentration gradients), the modeling of the experimental emission spectra leads to the determination of the laser-induced plasma characteristics including its composition. This second method refers to the ''calibration-free LIBS''.

In this paper, we present the radiative transfer code MERLIN (MultiElemental Radiative equiLibrIum emissioN) aiming at matching experimental spectra of a laser-induced plasma in Local Thermodynamic Equilibrium (LTE) conditions for calibration-free LIBS. MERLIN provides the simulated spectra for any elementary mixture thanks to queries from spectral databases available online. The code is optimized and modular to allow outputs even for a high number of species.

The validation of MERLIN presented in the paper is based on a thorough analysis of the plasma emission performed on the alloy Eurofer97 in experimental conditions providing the LTE. The reconstruction of the observed spectra is performed with MERLIN. For the reconstruction, no adjusted variable is required since all the necessary parameters are derived from experiments.