Electrochemical determination of extractives in wood pulp

Abstract

The objective of this thesis is to establish electrochemical techniques for the qualitative and quantitative determination of pentacyclic triterpenes in wood extractives. These triterpenes, found in wood extractives, possess beneficial biological effects. However, in the context of paper production, their presence can be problematic due to their lipophilic nature, which can reduce paper strength and hinder subsequent processing steps such as the adhesion of coatings and inks. Among the pentacyclic triterpenes, betulin is the most abundant molecule present up to 30% of the dry weight in a Scandinavian species (B. verrucosa) [1, 2]. To address the challenges associated with handling such compounds in paper mills, betulin and stearic acid were chosen as model compounds. This thesis introduces two distinct characterization techniques: potentiometric sensors in combination with principal component analysis, and voltammetric methods including square wave voltammetry and differential pulse voltammetry. Using ion-selective electrodes and principal component analysis the classification between samples containing KCl, KCl and ethanol and KCl, ethanol and betulin was possible. For voltammetric methods, a glassy carbon electrode with an in-situ lead film was used. Both the aqueous and organic solvents were used as media. The solvent used for preparing the standard solution significantly influenced the square wave voltammograms recorded in alkaline media. Therefore, Tris-HCl buffer (pH 7.4) was used as the medium, and dimethyl sulfoxide served as the solvent for the standard solution. The oxidation signal at 1.6 V for betulinic acid, oleanolic acid, and betulin was found to be proportional to the concentration when 5 mM of the stock solution was added. Additionally, differential pulse voltammograms were recorded using 0.1M tetrabutylammonium hexafluorophosphate in acetonitrile considering the benefits such as a wider electrochemical window and compatibility with organic molecules. The preconcentration step enables the detection of analytes with higher signals compared to the DMSO blank. The imperfect proportionality between peak current and analyte concentration, indicated by the similarity in current signals obtained from 0.5 mM working solution and 5 mM stock solution, remained unresolved. Nonetheless, the obtained results demonstrated that higher concentrations of the stock solution yielded higher current signals when compared to the most diluted standard solution and solvent blank. Furthermore, the extracted birch and conifer samples were analysed affirming the method's capability to detect compounds of interest. The outcomes of this study highlight the validity of employing ISEs in combination with chemometric data treatment for the qualitative determination of betulin, and the potential of the voltammetric methods to detect pentacyclic triterpenes in various electrolytes. However, to validate the reliability of the voltammetric methods, it is imperative to undertake further investigations by repeating experiments in non-aqueous electrolytes aiming to discover the underlying causes of the imperfect proportionality. Notably, it is crucial to determine the concentration of analytes by an independent method after chromatographic separation in natural sample analysis.