ИСТИНА

Quantitative determination of Rare-Earth Elements (REE) in ore samples is an interesting and challenging task. Laser-Induced Breakdown Spectrometry (LIBS) provides a unique possibility of remote direct analysis (ideally on a conveyor line). However, there are two major problems. The first is line overlapping in the spectra of REE-rich Niobium and Uranium ores. The second problem is a strong correlation between lanthanide contents preventing the use of chemometric approach as a solution for the first problem. Thus, the aim of this work is to solve the abovementioned problems. To solve the problem of lanthanide correlation in standard reference materials we used specialized design of experiment (DoE) based on Latin hypercube sampling (LHS) that already proved its effectiveness in case of XRF analysis [1]. To circumvent spectral interferences in LIBS emission spectra we used Partial Least Squares Regression (PLS-R). Calibration was performed by choosing the lowest RMSECV-LOO and at the same time simplest models in terms of PC number. Naturally the number of principal components (latent structures) strongly depends on the number of elements which lines are interfering in a spectrum. For this reason, we considered narrowing the spectral regions used to build a PLS model according to the positions of interfering lines leaving signals that are only related to the element of interest (and overlapped ones that we cannot separate). Variable selection procedure is an important step in our effort to give the model a reliable prediction power. Thereafter, our multivariate models were validated by direct analysis of lanthanides in 6 niobium (OREAS 460–465) and 4 uranium (OREAS 100a–102a) ore samples. Chemometrics provided quantitative determination (in range of ±10% relative error) of Ce (0.046–3.95 %), La (0.026–2.41 %), Nd (0.015–1.18 %) and semiquantitative results (in range of ±30%) for Pr (47–3800 ppm) and Sm (24–1500 ppm). In comparison a univariate calibration for elements other than Ce provided semiquantitative results at best. This level of accuracy shows that LIBS coupled with Chemometrics is a powerful tool for direct analysis of lanthanides in RЕE-rich ores.