The simultaneous determination of lanthanide family elements is one of the greatest problems in analytical chemistry, due to the close similarity of their chemical properties. Spectrophotometric methods are generally of limited use, due to the various mutual spectral interferences involved. By using multivariate calibration methods (partial least-squares regression, PLSR), it was possible to obtain a model that adjusts itself perfectly to the values of the mixture concentrations used in the calibration. The model used absorption spectra in the 290-800 nm range for a set of 20 different mixtures of Ce, Pr, Nd and Sm, and made possible the determination of Ce, Pr and Nd concentrations of a commercial rare-earth product, with significantly greater precision than the conventional univariate calibration method. Determination of the Sm concentrations was not possible, since its concentration was below the concentrations used in the model definition.
Peralta-Zamora, P., Cornejo-Ponce, L., Nagata, N., & Poppi, R. J. (1997). Chemometric alternatives for resolution of classical analytical problems: Spectrophotometric determination of lanthanide mixtures. Talanta, 44(10), 1815–1822. https://doi.org/10.1016/S0039-9140(97)00055-6