An infrared investigation in relation with chitin and chitosan characterization
Introduction
Chitin is the most important natural polysaccharide after cellulose found in crustaceous shells or in cell walls of fungi. However, it is not widely used for industrial applications up to now because it is insoluble in many solvents, relatively difficult to isolate from natural sources in pure form and to prepare in a reproducible way under good economic conditions. That is why it is also difficult to characterize this polysaccharide.
Its principal derivative is chitosan, obtained by deacetylation of chitin. It is soluble in aqueous acidic medium due to the presence of amino groups. The name chitosan is reserved to partially or fully deacetylated chitin soluble in acidic aqueous conditions, it usually also means that the average degree of acetylation (DA) is around or lower than 0.5; in addition, the solubility is also controlled by the distribution of the acetyl groups remaining along the chain.
For these different reasons, the characterization of chitin and chitosan is very delicate and has been largely discussed in the literature. Usually, a single technique cannot be adopted to cover the full range of DA, i.e. for chitin as well as for chitosan. For chitin, due to the lack of solubility, solid state NMR can be used [1], [2], [3], as well as infrared spectroscopy on film or powder [3], [4], [5], [6], [7], [8]; for samples in the pure form, elemental analysis can also be used but with lower accuracy [9].
For chitosan, which is soluble in aqueous medium, more methods are available and they have been also often discussed in the literature. The main techniques suggested are potentiometry [10], [11], 1H NMR [12], [13], [14], UV spectroscopy [15], [16], [17] and infrared spectroscopy [3], [4], [5], [6], [7], [8], [18], [19], [20], [21], [22], [23], [24], [25], [26].
The most discussed technique is infrared spectroscopy because of its simplicity, but it needs a calibration versus an absolute technique. Many different calibration relations have been proposed, but they are still under discussion in the literature: some of the typical IR band ratios proposed will be discussed later. In this paper, we intend to improve the use of this technique as a way to characterize the degree of acetylation of chitin and/or chitosan.
The aim of this inter-laboratory study is to discuss the application of infrared spectroscopy for DA determination whatever the degree of acetylation, the source, the salt form, the purity and the solubility of the polymer. It will allow us to propose a more reliable method for using the infrared spectral analysis to determine the composition of these biopolymers.
Section snippets
Experimental
The samples of chitin and chitosan in pure form were prepared in our laboratories as described previously [24]. They were selected from different sources as summarized in Table 1; some of them were commercial samples but purified by us and others were isolated from natural sources in our laboratories. Their DA were determined by NMR for calibration.
d-Glucosamine hydrochloride and N-acetylglucosamine — from Fluka and Janssen Chimica, respectively, — were used as model substances without further
Model analysis with the structural units
Fig. 1, Fig. 2 give the IR spectra of the two molecules representing the repeating units in these polymers; many differences appear, especially when looking for a reference band. Comparing both spectra, it could be appreciated that a specific band appears at 1320 cm−1 for N-acetylglucosamine. The band located at 2900 cm−1, often used in the literature as reference band to analyze chitin and chitosan, must be excluded as, for glucosamine, it may not be distinguished from the background. As
Acknowledgements
The authors thank GESVAL S.A. (Liège, Belgium), the research group on Chitin–Chitosan of the University of Liège headed by Dr M.-F. Versali and CNRS-CONACYT cooperation program for their financial support. WAM wishes to recognize financial support form CONACYT. We are grateful to G. Hernández-Watanabe, K. Garcı́a-Orozco and Dr K.A. Christensen for conducting part of the experimental work of this study.
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