Quantitative ¹³C NMR Analysis of Lignins with Internal Standards

Abstract

Novel protocols for acquiring quantitative 13 C NMR spectra of lignins have been developed using the internal reference compounds 1,3,5-trioxane and pentafluorobenzene. Trioxane offers a convenient internal standard for collecting inverse gated proton decoupled 13 C NMR spectra for lignins, whereas pentafluorobenzene can be used to provide information on the amount of methine carbon using the DEPT experiment. In each case, the internal reference compounds provide single, un-overlapped sharp signals in the middle of the spectral region, permitting facile integration. These integrals could be used to determine the amounts of different structural features of lignins, expressed in absolute units of millimoles per gram. The optimum parameters for these experiments were validated for a variety of spectrometer platforms, and standard errors were determined for different spectral areas using lignin model compounds and “standard” lignins. In addition, the data derived for the International Round Robin “standard” lignins showed good agreement with the data from quantitative 31 P NMR spectroscopy and published data, obtained by independent laboratories using independent methods of analysis. Keywords: Lignin; nuclear magnetic resonance spectroscopy (NMR); inverse gated proton decoupled carbon-13 NMR spectroscopy; distortionless enhancement by polarization transfer (DEPT); internal standard; acquisition protocol; International Round Robin lignins; relaxation reagent