FOURIER-TRANSFORM RAMAN ASSIGNMENT OF GUAIACYL AND SYRINGYL MARKER BANDS FOR LIGNIN DETERMINATION

A near infrared fourier transform Raman (NIR-FTR) spectroscopic techni que was utilized to characterize lignin in wood. The Raman bands for C =C stretching derived from 4-hydroxy-3-methoxyphenyl (guaiacyl) nuclei and from 3,5-dimethoxy-4-hydroxyphenyl (syringyl) nuclei exist indepe ndently. The NIR-FTR analysis of a series of lignin model compounds in dicated that a syringyl band was shifted to a lower frequency compared to a guaiacyl band. This shift was also observed in chemically synthe sized lignin (DHP). Syringyl DHP, in which all the aromatic nuclei con sist of syringyl type, exhibited a C=C stretching band at 1594 cm(-1), while guaiacyl DHP exhibited the band at 1599 cm(-1). These bands wer e designated as syringyl and guaiacyl marker bands, respectively. Chem ical and physical treatment of hardwood and softwood exhibited differe nt characteristics. One of the reasons is the chemical structure oi li gnin. Softwood mainly contains only guaiacyl lignin, while hardwood co ntains both guaiacyl and syringyl lignin, and the syringyl/guaiacyl (S /G) ratio varies among species. Under high-resolution conditions (1 cm (-1)), the NIR-FTR spectra of 10 hardwoods (wood meal samples) reveale d that both syringyl and guaiacyl marker bands existed. On the other h and, the spectra of softwoods contained only a guaiacyl marker bands e xisted. On the other hand, the spectra of softwoods contained only a g uaiacyl marker band. The S/G ratio in hardwood calculated from the pea k area intensity ratio of two marker bands shows a linear relationship with the SIG ratio obtained from conventional nitrobenzene oxidation analysis with the correlation factor>0.96. Furthermore, if peak compon ent separation analysis was combined, low-resolution spectral data gav e a similar S/G ratio. Either syringyl of guaiacyl marker bands can be assigned in the NIR-FTR spectra of wood blocks (saw-cut surface). Thi s spectral technique may provide an easy-handling and non-destructive analytical method for lignin determination. (C) 1997 Elsevier Science B.V.