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

Abstract

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.

Adding Speed to Lignin Analysis Straight from Black Liquors

“…Switching to higher detection wavelengths (410 and 475 nm) resulted in linear, more robust calibrations, while good signal intensities were still maintained (Figures 1 and S1). Khaliliyan et al 14 reported a similar improvement in lignin quantification in BLs when opting for higher detection wavelengths. Detailed information about the calibration fitting functions can be found in Table S1.…”

“…Fast and robust quantification of the lignin contents in pulping liquors is needed for optimized process control of the evaporation plant, lignin extraction, recovery boiler, or other subsequent biorefinery operations. 13,14 A major bottleneck for the required high analytical throughput is the tedious and timeconsuming isolation and separation of lignin from the pulping liquor prior to analysis. Industrially, lignin isolation is done by acidic precipitation (e.g., Lignoboost, Lignoforce) to separate lignin from the other BL constituents, 15−17 which allows recovery of reasonably pure technical lignin�as the basis of the many future applications.…”

“…Moreover, the solids content of BL depends largely on the sampling location along the processing line of the pulp mill, as significant differences exist between different process stages (e.g., pulp washer vs evaporation plant). Fast and robust quantification of the lignin contents in pulping liquors is needed for optimized process control of the evaporation plant, lignin extraction, recovery boiler, or other subsequent biorefinery operations. , A major bottleneck for the required high analytical throughput is the tedious and time-consuming isolation and separation of lignin from the pulping liquor prior to analysis. Industrially, lignin isolation is done by acidic precipitation (e.g., Lignoboost, Lignoforce) to separate lignin from the other BL constituents, − which allows recovery of reasonably pure technical ligninas the basis of the many future applications. ,− For analytical purposes, acidic precipitation aided by centrifugation is a very popular yet time-consuming method, because the precipitate must be washed and dried prior to analysis. ,,− A high purity of the lignin samples is an absolute prerequisite in most of the methods commonly used in lignin analysis.…”

“…Impurities often cause measurement errors, be it directly by interference with the respective measuring principle or indirectly by reducing the actual sample mass. For these reasons, direct analysis of BL constituents has mainly focused on degradation products, process chemicals, inorganic anions, or organic modifiers so far. , Methods for direct analysis of lignin in pulping liquors are quite scarce, and the few existing ones are usually not universally applicable to different pulping liquors or focus only on one parameter of interest. ,, For instance, methods for lignosulfonates are normally not suitable for KLs and vice versa …”

“…25,26 Methods for direct analysis of lignin in pulping liquors are quite scarce, and the few existing ones are usually not universally applicable to different pulping liquors or focus only on one parameter of interest. 14,19,27 For instance, methods for lignosulfonates are normally not suitable for KLs and vice versa. 27 Along with lignin functionality, molar mass�with its parameters number-average molar mass (M n ) and massaverage molar mass (M w ); molar mass distribution (MMD); and dispersity (Đ M )�is considered a key property for valorization, as it significantly affects the physicochemical properties of KL.…”