An optimised method to determine the degree of acetylation of chitin and chitosan by FTIR spectroscopy M.L. Duarte a, *, M.C. Ferreira b , M.R. Marva ˜o b , Joa ˜o Rocha c a Department of Chemistry and Biochemistry, CECUL, University of Lisbon, 1749-016 Lisbon, Portugal b Department of Materials, National Institute of Engineering and Industrial Technology, Estrada do Pac ¸o do Lumiar, 1649-038 Lisbon, Portugal c Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal Received 12 November 2001; received in revised form 14 June 2002; accepted 14 June 2002 Abstract Linking up the user-friendly and low-priced FTIR with the more sophisticated and high-priced 13 C CP/MAS NMR spectroscopies, an improved method to determine the degree of acetylation (DA) of chitins and chitosans was outlined. The method was established for the most complex polymorphic form (a-chitin) and for the most problematic range of DA values (most acetylated samples) and can easily be extended to the other polymorphic forms (b- and g-chitins) and to other ranges of DA values. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Chitin; Chitosan; Degree of acetylation 1. Introduction Chitin, poly-N -acetyl-D-glucosamine, is one of the most abundant, easily obtainable and renewable natural polymers, second only to cellulose. Depending on the sources, this polymer has been found in three poly- morphic forms: a-, b- and g-chitin, which differ as regards the arrangement of the chains within the crystalline regions, implying different networks of hydrogen-bonds. The a-chitin is the most abundant and stable polymorphic form, exhibiting the most complex network of hydrogen-bonds. Chitosan, poly-D-glucosamine, is obtained by deace- tylation of purified chitin with concentrated alkali and high temperature treatment, and frequently it is com- mercialised in powder or flake forms. While chitin is extremely insoluble in common solvents, chitosan is soluble in dilute acid solutions, being a much more tractable material with a large number and wide variety of applications, ranging from biomedical to waste water treatment and fibre indus- tries. Despite those specific chemical designations, the names ‘chitin’ and ‘chitosan’ actually correspond to a family of polymers varying in the acetyl content measured by the degree of N -acetylation (DA). The DA dictates the behaviour of these polymers, namely reactivity and solubility, its determination being one of the routine analyses performed for quality control on chitins and chitosans industrially produced. Furthermore, in the case of chitin, the DA determines the suitable conditions of deacetylation, a complex process still requiring investigation in order to assess the feasibility of quicker preparation of chitosan. Thus, the search for quick, user-friendly, low cost and accurate methods to determine the DA has been one of the major concerns over many decades. However, the inherent complexity of this particular system turns this appar- ently simple analytical problem into a very difficult one, which is well illustrated by the extensive number of different techniques, either destructive or non-destruc- tive, yielding direct or non-direct and frequently non reproducible values [1  /5]. It is worth noting that non- destructive methods offer the advantage of avoiding manipulations of the polymers such as hydrolysis, * Corresponding author. Tel.:  /351-217-150-571; fax:  /351-217- 500-088 E-mail address: mlduarte@fc.ul.pt (M.L. Duarte). International Journal of Biological Macromolecules 31 (2002) 1  /8 www.elsevier.com/locate/ijbiomac 0141-8130/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0141-8130(02)00039-9