Evaluation of chemical composition of the core and bast lignins of variety Chinpi-3 kenaf (Hibiscus cannabinus L.) by pyrolysis-gas chromatography/mass spectrometry and cupric oxide oxidation

In order to clarify the chemical composition of in situ lignin of kenaf (Hibiscus cannabinus L.), variety Chinpi-3, the core and bast fibers were fractionated from the 6-month maturated kenaf with small stalks (309.0 cm height × 1.3 cm diameter, lignin content 16.6%) and large stalks (390.0 cm height × 2.3 cm diameter, lignin content 18.8%). The samples were subjected to conventional pyrolysis-gas chromatography/mass spectrometry (500 °C/4 s), and cupric oxide oxidation. The quantitative pyrolysis results showed that (1) the differences in stalk size, when harvested at the same maturity, do not influence the lignin composition, (2) in situ core lignin is a mixed lignin comprising 1.4 parts of syringyl-, 1 part of guaiacyl-, and a small part of p-hydroxyphenyl lignin units, and (3) in in situ bast fiber lignin the syringyl lignin units are present in greater quantities than the guaiacyl lignin units. The cupric oxide oxidation results also supported the pyrolysis results.