Effect of initial estimates and constraints selection in multivariate curve resolution—Alternating least squares. Application to low-resolution NMR data

A comprehensive study of the applicability of multivariate curve resolution (MCR) methods to series of T2-relaxation filtered 1H NMR spectra of a cross-linked polymer network is presented. A collection of Hahn-echo NMR spectra is obtained at different echo times, yielding two-way data. In this study the applicability of two different types of orthogonal-projection approach (OPA1 and OPA2) (column-wise and row-wise) were tested. Four different strategies of alternating least squares methods were also examined (ALS1, ALS2, ALS3 and ALS4). These strategies differed on the order of measurement for which the constraints were applied in the final output, and the way in which SSR was calculated to monitor for convergence. In the spectral order of measurement, a non-negativity constraint was imposed, whereas in the time order of measurement, the signal was forced to follow an exponential decay. This yielded up to eight MCR configurations, giving different results. For solid-state NMR, the dissimilarity in NMR profiles is significantly lower than the dissimilarity in signal decays, and therefore OPA2 performed better. A final output with a constrained solution in relaxation time was preferred (instead of a constrained solution in NMR spectra) for practical purposes. Differences between the solutions given from the two ALS configurations can be interpreted as a sign of lack of fit.