Lignin hydroxyl group content by 31P NMR spectroscopy

Determination of hydroxyl group (-OH) content in lignin samples by 31P NMR. The method is suitable for lignin that has or might have free hydroxyl groups.

The lignin is dissolved in a deuterated solvent (typically CDCl₃:Pyridine), and an internal standard is added to the sample. Then, the hydroxyl groups in the sample + internal standard are phosphorylated, and the mixture is analyzed using ³¹P NMR. The number of free hydroxyl groups can be determined from the NMR spectra by comparing the phosphorus signal of the internal standard to the phosphorus signals of the phosphorylated sample material.

The following OH-groups can be quantified (in mmol/g):

• Aliphatic OH

• Phenolic OH

• Carboxylic acid

• Syringyl OH

• Guaiacyl OH

• Catechols

• p-hydroxyphenyl OH

• Total OH

The price can depend on whether your samples require specialized deuterated solvents or preparation conditions (i.e. high temperature or long dissolution time).