Alginate monomer composition studied by solution- and solid-state NMR – A comparative chemometric study

The potential of using 1H high-resolution (HR) magic angle spinning (MAS) nuclear magnetic resonance (NMR) of alginates suspended in D2O or 13C cross-polarisation (CP) MAS NMR of alginate powders as an alternative method to the traditional 1H solution-state NMR method for the analysis of the alginate monomer composition (mannuronate (M)/guluronate (G) ratio) has been investigated. The MAS NMR experiments can be performed directly on the intact alginate powders and thereby avoiding the relatively time-consuming and labour intensive sample preparation required for the traditional solution-state experiment. A total of 42 different sodium alginate samples were analysed and it was found that the M/G ratio derived from the HR-MAS NMR spectra were comparable to the M/G ratio calculated from the solution-state NMR spectra. However, both these methods overestimated the M/G ratio of samples with high calcium content. In contrast, the 13C CP-MAS NMR spectra were not affected by residual calcium in the samples and thus provided more consistent measurements independent on physico-chemical phenomena such as partial solubility, viscosity and interactions with ions. It is concluded that 13C CP-MAS NMR is a most suitable candidate for accurate analysis of the M/G ratio of intact alginate powders.