Determination of the aromatic lignin content in oak wood by quantitative solid state13C-NMR

The problems concerning quantification with cross polarization (CP) and high-field C-13-nmr measurements has meant that, for ligno-cellulosic plant materials, aromatic carbons in lignins are often discriminated against. In this study, the aromatic lignin content of an American red oak sample has been determined at the relatively low field strength of 25 MHz to obviate problems with spinning side bands using both CP and Bloch decay or single pulse excitation (SPE), a more time-consuming acquisition technique but that is, in many cases, considerably more quantitative than CP. The value of 14 mole % carbon from SPE is in close agreement with that of 15 % derived from elemental analysis and the Klason lignin content. Although virtually all of the carbon was observed by both SPE and CP, the latter significantly underestimated the aromatic content at contact times less than 1.5 ms and thus longer times should be used for reliable quantification. The quaternary carbon content was estimated as 11 mole % carbon by combining dipolar dephasing with SPE and CP.