From Chitin to Chitosan - Chitosan Based Hydrogels: Functions and Applications - page 11

UV spectrometry is used for analyzing the structure of chitin. Normal polysaccharides
do not have chromophores or conjugated groups. However, chitin is different; it has an
acetyl on C 2 of each residue as chromophore, so that it absorbs UV.
1 H nuclear magnetic resonance is used for identifying the configuration of the glycoside
bond in polysaccharide. Most chemical shifts δ of polysaccharide are within 4.0-5.5 ppm.
Protonic signals of C 2 -C 6 within 4.0-4.8 ppm are hard to analyze; only the protonic signal
of C 1 within 4.8-5.5 ppm is easy to analyze. The chemical shift scope in 13 C nuclear mag-
netic resonance up to 200 ppm is much wider; thus signals can be separated, making the
location determination of carbon atoms and the identification of the configuration and
conformation of a molecule available. The relative height of the peak in resonance is
proportional to the number of carbon atoms; hence, the percentages of different residues
can be calculated according to the relative heights of peaks of different anomeric carbons.
It can be concluded that 13 C nuclear magnetic resonance is more useful than 1 H nuclear
magnetic resonance [50].

1.3.4 Ash Content

Ash content is an important index for the preparation of food-grade and pharmaceutical-
grade chitosans and can be measured by the normal method: carbonizing chitosan in a
crucible by an electric stove and igniting it by a high-temperature electric stove until weight
is constant. The ash index is the ratio of the weight after igniting to the original weight.
The ash content of common chitosan products is as follows: industrial grade ≤1.0, food
grade ≤0.5, and medical grade ≤0.2.

1.3.5 Nitrogen Content

Both chitin and chitosan are polysaccharides containing aminos. The theoretical nitrogen
content of chitin with all N -acetyls, aminos, and acetaminos and without crystal water is
6.9%. The theoretical nitrogen content of chitosan that has 100% DD and is not deaminated
is 8.7%. Nondeamidation is emphasized because wet fresh shrimp and crab shells will
automatically deaminate due to microbes after exposure to air for 1 week, just like protein.
Therefore, DD of the sample not undergoing deamidation can be found in Table 1.1 after
measuring nitrogen content, and the known DD of the sample can be found as well.
Nitrogen content can be determined by the Kjedahl method, like protein. This method
comprises the following steps: damaging organic compounds in the sample by sulfuric
acid and transforming nitrogen-containing compounds into ammonium sulfate, adding
strong alkali to the mixture, distilling to remove ammonia, absorbing ammonia by boric
acid, and titrating the solution by using acid to measure nitrogen content. The sample must
be totally dried until no free water or crystal water exists, and cannot contain nitrogen-
containing compounds such as nitrate or nitrite.

TAble 1.1
Relationship between Nitrogen Content and DD
Nitrogen content

6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8

DD 0 6 11 17 22 28 33 39 44 50

Nitrogen content 7.9 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7

DD 56 61 67 72 78 83 89 94 100