Quantitative determination of chitosans by ninhydrin

A simple method for quantitative analysis of chitosan based on the ninhydri n reaction is described. Only the 2-amino-2-deoxy-beta-D-glucopyranose (Glc N) units of chitosans were found to form coloured products with ninhydrin. This reaction of chitosans with ninhydrin was sensitive and reproducible. L inear calibration curves were obtained in the concentration range of 10-120 mg/l, depending on the chemical composition of the chitosans. The amount o f colour produced per GlcN unit decreased with decreasing fraction of acety lated units (F-A). This imperfect stoichiometry was studied in more detail by comparing a series of beta-(1 --> 4)-linked GlcN oligomers. Dimer, trime r and tetramer produced 82, 67 and 61% of the colour relative to that of th e monomer, respectively. The yield of almost fully deacetylated chitosan (F -A = 0.01) with different molecular weights reached a constant value at 40% when the degree of polymerization increased to more than 10. Differences i n reaction rates were also observed. While monomer, dimer and chitosan with F-A = 0.6 reacted rapidly, other chitosans did not fully complete the reac tion within 30 min. Due to this behaviour, the reaction of chitosans with n inhydrin might be used for quantitative analysis only when a reliable calib ration against a reference of a similar F-A is available. (C) 1999 Elsevier Science Ltd. All rights reserved.