Chemometric strategies for the study of the complexation of Al(III) ions with model molecule of humic substances from UV-vis data sets

Multivariate curve resolution with alternating least squares (MCR-ALS) is successfully applied to UV-vis spectroscopic data of a model system of metal ion/humic acid interactions, namely Al(III)/caffeic acid. MCR-ALS provides resolution into the contributions of the different equilibrium species. Because of rank-deficiency phenomena, the pure spectra and concentration profiles of all species can only be recovered through matrix augmentation. The data set is thus generated coupling three different experiments, two pH-dependent Al(III)/caffeic acid experiments and one acid-base caffeic acid titration. Under these conditions, the rank-deficiency is suppressed in the matrices of analytical interest and the correct number of chemical species can be detected and modelled. Furthermore, local rank information is derived from the application of evolving factor analysis to the full-rank augmented matrices. This approach provides prior knowledge about the complexation mechanism of Al(III) by caffeic acid and enables the characterisation of the different complexed species from spectrophotometric data.