Utilization of 31P PULCON for Quantitative Hydroxyl Group Determination in Lignin by NMR Spectroscopy

Abstract

The amount of hydroxyl groups is a crucial parameter when characterizing lignin as a raw material and when developing lignin-based applications. Currently, the most common method used for quantitative hydroxyl group determination is 31P nuclear magnetic resonance (NMR) spectroscopy after phosphitylation of the hydroxyl groups. The method relies on an internal standard for the quantification. The NMR signals of the internal standard have been shown to either overlap with signals from the lignin or to have poor stability due to degradation of the internal standard. To overcome these drawbacks, we have used PULCON (pulse length-based concentration determination) for the quantification of phosphitylated lignin. The method is based on the fact that the product of the intensity of an NMR signal and the length of the 90° pulse is directly proportional to the concentration of the sample. We conclude that the PULCON method could be used for hydroxyl group determination of lignin, and the results obtained are comparable to those based on utilization of an internal standard. We also show that the simplified method could be automated by using automatic functions in the NMR software and that a stable and accessible compound could be used as external standard.