Measurement of the Functional Group Contents of Lignins Using FTIR and Partial Least Squares Regression

A method has been developed for the rapid determination of the functional group contents of a number of lignins using a partial least squares (PLS) regression method and Fourier Transform Infrared Spectroscopy (FTIR). The complex structure of lignin requires an IR analysis method capable of interpreting spectra with composite peaks and extensive peak interactions. The PLS multivariate analysismethod uses all of the spectral information that is determined to be statistically significant. For example, a set of well-characterized milled wood lignins (MWL) from hardwoods was used to calibrate the technique to measure aromatic methoxyl content. Data from analyses of organosolv lignins compared very well with methoxyl contents obtained using 13C NMR. The applicability of this technique toother biomass species will be examined by expanding the data set to include isolated lignin from softwoods, grasses, and legumes. The presence of wood extractives (up to 14%) and carbohydrate contaminants was found to have negligible influence on the methoxyl contents measured. The lack of interference by carbohydrate or extractives suggests the potential for a wide application of this method tothe analysis of whole woods. This technique requires minimal sample preparation, requiring purified MWL for calibration only. Other advantages include a reduced sample size and short analysis time.