ИСТИНА

Laser-induced breakdown spectroscopy (LIBS) is an atomic emission technique which contactless quantifies elemental composition of solids, liquids or gases using the emission of plasma produced by a high-power Q-switched laser on the surface or in a volume of a sample. Thus LIBS can be used in unique applications such as analysis of Mars surface, ocean floor, or process control of molten metal and dross. In most of those cases it is extremely difficult or even impossible to obtain certified reference materials for calibration. Common approaches to calibration-free (CF) LIBS are based on Boltzmann plots and, therefore, tend to impose constraints on the experiment, namely homogeneity of the source, local thermodynamic equilibrium in plasma and absence of self-absorption of involved atomic lines. Fulfilling these requirements thorough experimental optimisation and preliminary studies may prove impossible in space missions or in deep sea research. We have earlier developed an algorithm based on a stationary model of laser-induced plasma [1] to simulate emission spectra for a given sample composition. In this work, we applied optimisation techniques to fit experimental and model spectra, in which relative elemental concentrations served as variables. For relative simplicity of an implementation, a gradient-free method has been chosen to minimise the sum of squared differences between scaled model and experimental spectra. Various shapes of the loss function are also considered, making it possible to account for analytical lines of very different intensity. We have tested the accuracy by 100 consequent runs of the algorithm with random initialisation (Figure 1). It has been shown that the homogeneous plasma model is suitable for narrow spectral regions, while a multi-zone model with different temperatures and electron densities provides the best results for the full spectral region, although in the latter case the accuracy is lower. The reported study was funded by Grant of the President of the Russian Federation (No. MK-5513.2021.6). [1] Zaytsev S.M.; Popov A.M.; Labutin T.A.; Stationary model of laser-induced plasma: Critical evaluation and applications, Spectrochimica Acta Part B: Atomic Spectroscopy, 2019, 158, 105632.