Interpreting the temporal emission of LIBS plasmas using advanced chemometric methods
Résumé
LIBS measurements consist of spectrally- and time-resolved analysis of atomic and ionic lines emitted by the plasma. Typically, the gate delay and width are fixed for spectroscopic measurements. Reciprocally, for kinetics studies, some spectral lines are selected which can be a difficult task when dealing with complex spectra. Therefore, either the temporal or spectral dimension of the LIBS signal is usually neglected. In this work, we propose a new way of exploring simultaneously both dimensions of the LIBS signal using innovative multivariate methods. The temporal evolution of the LIBS signal of a pure metal sample was measured between 0.2 and 15 µs after the laser pulse under four different atmospheres (air, nitrogen, neon, and a 50/50 argon/neon mixture). Time-resolved spectra were treated with 3 chemometric methods: Principal Components Analysis (PCA), Mean Field-Independent Components Analysis (MFICA) and Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS). These methods describe the experimental spectra as linear combinations of a small number of components, typically 3 or 4. The observation of scores and components provides a physical interpretation of the temporal emission of the plasma. The interest of this approach was demonstrated with simple spectra of a pure aluminum sample, and then it was applied to much more complex spectra of pure titanium (only in air). Results show that the contributions of ionic, neutral and molecular species are clearly characterized, and their temporal dynamics can be determined. We also see that the evolution of the linewidths is taken into account, and permits to identify, for a given species, groups of lines with different temporal behaviors. In addition, the comparison between the different atmospheres gives an original insight into the effect of this parameter on the temporal evolution of the plasma. Among the three methods tested, we showed that the MCR-ALS technique gives the clearest, most directly interpretable results. This new way of describing the plasma temporal emission using chemometric methods is very powerful, as it takes into account the whole spectrum. It opens the door to accurate optimization of some crucial parameters of LIBS measurements, such as the choice of the analytical line or the gate delay and width. These perspectives will be discussed.
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