A Rapid and Sensitive Method for the Determination of Chitosan Concentration by NMR Relaxometry

A rapid, reagent-free method for the quantification of chitosan in aqueous solutions was developed using ¹H nuclear magnetic resonance (NMR) relaxometry. Transverse relaxation times (T₂) of chitosan solutions (5-1000 mg/L) were measured with a Carr-Purcell-Meiboom-Gill (CPMG) sequence on a benchtop NMR relaxometer. The data were processed using three approaches: monoexponential fitting (Bruker software), inverse Laplace transformation (CONTIN), and one-component curve fitting (Excel Solver). Calibration curves derived from these models demonstrated high linearity and reproducibility, particularly when concentration intervals were segmented. Among the approaches, the most robust correlation was achieved by plotting the absolute area (AA) of the T₂ distribution obtained from CONTIN analysis versus chitosan concentration, yielding R² values of up to 0.9978 for 5-100 mg/L. Comparative analysis at 40°C and 21°C confirmed the method's temperature stability, with improved sensitivity at elevated temperature. Unlike conventional spectrophotometric or chromatographic methods, the proposed protocol requires no chemical derivatization or complex sample preparation. This technique provides a fast, accurate, and non-destructive alternative for chitosan quantification, especially suitable for material science applications where precise concentration monitoring is critical, such as surface modification of nanomaterials.