Rapid method for monitoring chitosan coagulation using low-field NMR relaxometry

• Time-domain NMR relaxometry was demonstrated to be a simple, rapid method to monitor chitosan coagulation as a function of pH.
• The relaxation times (T1 and T2) were measured in a single experiment using a pulse sequence named CP-CWFPx-x.
• The analyses were carried out in a simple, low-cost benchtop NMR spectrometer.

Time-domain NMR relaxometry was proposed as a simple, rapid method to monitor chitosan (CS) coagulation as a function of pH. The longitudinal (T1) and transverse (T2) relaxation times of three CS concentrations (0.022, 0.22, and 2.2 g L−1) were simultaneously measured by CP-CWFPx-x pulse sequences in a 0.47 T spectrometer. T1 and T2 were shown to be independent of pH as well as to assume values similar to the relaxation time of water (2.7 s) at the lowest tested CS concentration. At the highest concentration, T1 increased whereas T2 decreased as pH varied from 6.0 to 7.0. This indicates a remarkable effect of CS on water relaxation at pH values higher than the pKa of CS amino groups (6.5). Therefore, CS reduced the water mobility at the highest CS concentration and greatest pH values, suggesting a CS supramolecular structure (gel) that entraps the solvent in confined regions. The method proposed here can be further used to study the coagulation of other polysaccharides.