Direct Quantification of Lignin in Liquors by High Performance Thin Layer Chromatography-Densitometry and Multivariate Calibration

A method for lignin quantification is presented that is based on densitometry and multivariate calibration by partial least squares (PLS). Densitometry removes interferences otherwise caused by undissolved liquor components and solvents, while multivariate calibration accounts for interferences by other sample components, such as extractives or chromophoric compounds. Method validation predicted a bias very close to 0% and an uncertainty of 10% (kraft) to 20% and less (lignosulfonate). In trials with three industrial kraft liquors, biases from 0 to 5% were observed; for a tested lignosulfonate, a bias of 6% was obtained. By combining this method with thin-layer chromatography, other liquor components can be determined simultaneously with the lignin content, for example monosaccharides in sulfite liquors (total time per sample: 13 min). If only the lignin content is of interest, chromatography is unnecessary, and more than 200 samples can be applied to the plate in a dense pattern (time per sample: 2 min). Compared to other methods, this measurement is fast, gives correct results, is inert toward matrix components, and can be coupled with a chromatographic analysis. Its only prerequisite is the isolation of a suitable reference lignin to be used as an external standard.