Applying different infrared spectrophotometry methods for determining

Abstract

Chitin, the most abundant biopolymer after cellulose, is a linear, highly crystalline polysaccharide, built from acetylglucosamine monomer units connected by β-1,4 glucosidic bonds. High degree of acetylation (DA) and the crystallinity of chitin molecules make it insoluble in common commercial solvents. Chitosan is the name for chitin substitutes with a low degree of acetylation. The degree of deacetylation in chitin usually ranges from 5% to 15%, and in chitosan from 70% to 95%. This contributes to the solubility of chitosan in aqueous solutions of acids and enables the use of chitosan and its bioactivity. When chitosan biopolymer film properties are considered, DA is an important input, due to the proven influence of DA on chitosan film´s mechanical, physico-chemical and bioactive properties. Developed methods for chitosan DA determination are based on different approaches: infrared spectroscopy, UVspectrophotometry, nuclear magnetic resonance, colloidal titration, and potentiometric titration. Various procedures using infrared spectroscopy technique have been developed to determine DA of chitosan. In this article, these methods were tested for application on chitosan isolated from blue crab shell (Callinectes sapidus). Two different approaches were assessed: 1) determination of the absorption ratio of a probe band and of a reference band and estimation of DA of unknown samples by comparison with samples with known DA and 2) evaluation of several proposed absolute methods that are used in the literature for the DA determination. Results showed that applied absolute methods gave results that varied between the methods applied, but were consistent among different samples within the framework of a particular method. Use of different probes and reference bands for calculation of ratios that would be compared to the reference samples of a known DA was more appropriate method. Selected ratios were A1655/A3450 and A1320/A1420. IR techniques for chitosan DA determination without reference to samples of known DA or calibration curve could be used for qualitative evaluation of DA, while quantitative determination should include mentioned references. Use of absolute methods for determination of DA by IR spectroscopy was shown not to be appropriate in this work