Effects of lignin structural characteristics on absorptivity in alkaline solutions of the thioglycolic acid lignin

The thioglycolic acid (TGA) method enables lignin quantification even in trace samples by measuring the 280-nm absorbance of TGA lignin (TGAL) in an alkaline solution. Lignin content is calculated using the absorbance coefficient from a calibration curve constructed with samples of known lignin concentrations. It has been reported that the lignin structures vary in plant species, particularly syringyl/guaiacyl (S/G) ratio among angiosperms including hardwood species. If the absorbance coefficient of the TGAL calibration curve changes with the lignin structure, using an appropriate absorbance coefficient in accordance with the lignin structure of each sample may improve the comparability of lignin contents estimated by the TGA method among samples. However, the effect of the lignin structure on the absorbance coefficient of the TGAL calibration curve has not been sufficiently investigated. In this study, we examined the correlation between the absorbance coefficients of the TGAL calibration curve and the lignin structures using two softwood and nine hardwood species with different S/G ratios. The absorbance coefficient of the calibration curve was determined from the relationship between the absorbance of TGAL and the reference lignin concentration determined by the Klason method as the sum of acid-insoluble lignin and acid-soluble lignin. Both measurements were obtained using the same extract-free samples. The absorbance coefficient of the TGAL calibration curve showed a positive correlation with the S/G ratio determined by thioacidolysis method. Therefore, our results indicate that the absorbance coefficient of the TGAL calibration curve varies with lignin structural characteristics represented by the S/G ratio and is closely associated with them, providing insight into the estimation of lignin content in various samples by the TGA method.