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.

Quantitative 2D HSQC NMR determination of polymer structures by selecting suitable internal standard references

“…These observations again demonstrate that the methods of applying an internal standard reference compound of low molecular weight in a polymer sample for quantitative DEPT and 2D HSQC NMR analyses is of little value. 4,13 If one of the clusters of signals from the lignin polymer, such as the signal G (Fig. 3), is used as the secondary internal standard reference, the DEPT90 NMR analysis gives the same integration result as that of the quantitative 13 C NMR analysis (Table 1).…”

“…a low molecular weight compound was added to the polymeric sample as internal standard. 4,13 The resonances from an internal standard reference compound of low molecular weight can, however, have rather different 1 J X -H values, different X-nuclear chemical shift values and, most importantly, different proton and X-nuclear T 2 profiles from those of the polymeric substrate. To achieve accurate signal quantification in such a system, errors of all the types mentioned above need to be corrected.…”

Recent innovations in analytical methods for the qualitative and quantitative assessment of lignin

“…The unparalleled structural detail obtained using NMR has prompted researchers to routinely utilize this method in lignin monomer analysis, and S/G ratio determination [61,64,78,81,128,134,137,[141][142][143]146,154,156,158,161,166,167,196,318]. HSQC 2D-NMR was used to probe lignin monomer composition in young and adult eucalyptus trees [156].…”

“…Xia et al used 13 C and 31 P-NMR to quantitate hydroxyl and methoxyl functionalities in model compounds as well as RoundRobin standard lignins including Indulin kraft pulping, steam explosion yellow poplar, and steam explosion aspen lignins [318]. 1,3,5-trioxane and pentafluorobenzene were employed as internal standards to facilitate comparison with the results of other laboratories.…”

A fast track for the accurate determination of methoxyl and ethoxyl groups in lignin

“…36 Over the last few decades, the development of 13 C-NMR techniques, providing well-resolved spectra in which methoxyl group signals are not overlapped, and the application of appropriate nonoverlapping internal standards have permitted facile integration and methoxyl group quantication. 37 Further developments have led to signicant decreases of experimental time 38 and demonstrated a strong correlation with values determined by the conventional wet chemistry GC-FID approach. 32 Nevertheless, the availability of NMR hardware, the necessity of high purity, and sufficient solubility of lignin samples in an appropriate solvent, as well as still rather long experimental time, do not allow considering these approaches as routine methods for simple methoxyl group content determination.…”