Qualitative and Quantitative analysis of monomeric acids in oxidative depolymerized lignosulfonate using gas chromatography

Abstract

Introduction: The modification of lignin to high-value low molecular weight compounds frequently results in complex mixtures of monomeric or oligomeric compounds. In addition, these organic acids can be applied as antioxidants or for the production of bioplastics. 

Background: The composition of lignin is complex. Utilizing technical lignin as a starting point, useful aromatic compounds can be generated by a depolymerization conversion. However, standardized and well-defined methods to quantify non-volatile and thermolabile acids in oxidatively depolymerized lignosulfonate (ODLS) are lacking. As a result, there is an analytical obstacle to the process development. In addition, the mixture containing various of different monomeric acids puts pressure on the efficiency of the separation method. As of today gas chromatography (GC) is commonly applied for quantitative and qualitative analysis of derivatized fatty acids. The acids from depolymerized lignin provide important information about lignin valorization and the chemical process development.

Aim: This study aims to determine if gas chromatography (GC) can offer high detectability and efficiency for the qualitative analysis of acids in (ODLS). In addition, can the SPE-GC-FID method be used reliably for quantification of organic acids, regarding recoveries, detectability, sensitivity, linearity, and repeatability?

Methods: The ODLS samples were prepared by liquid-liquid extraction (LLE) and SPE for preconcentration. The extracted lignin oil was subsequently derivatized using methanolic HCl. Qualitative optimization of the chromatogram was accomplished by applying different temperature gradients to distinguish a gradient that results in the best separation. GC-MS facilitated the identification of different monomeric acids in the lignin mixture. Method validation using GC-FID for quantifying the organic acids was achieved by evaluating recovery, repeatability and sensitivity.

Results: The preliminary GC-MS data indicated a large variety of different monomeric acids. A method for the derivatization of ODLS with methanolic HCl was created after analyzing different methods in the literature. IR ranged between 3402 and 762 cm-1. Method validation was conducted to determine quantification of acids using SPE. Recoveries ranged between 62-123 % for 9 acids of 14 acids and ranges of R2 was between 0.96-0.99 indicating linearity. RSD was under 20 % for 60 ppm showing repeatability, however the concentration of sample was below the limit of quantification.

Conclusion: In conclusion, numerous different monomeric acids were identified by GC-MS for both derivatization methods. Method validation is required for sufficient sensitivity and precision involving optimizing the different methods to minimize the variability in the results.

Popular Abstract

Many studies have been performed to find the use of technical lignin in particular chemicals as regards their economic value. Lignin is a complex polymer found in the cell walls of plants and contains a mixture of aromatic compounds which are bound together by diverse chemical bonds. Lignin is a byproduct of the pulping process in the paper industry which is a method to extract cellulose fibers. Two major processes are sulfite and kraft pulping. In the sulfite pulping process lignosulfonate is a byproduct. Sulfonation of lignin increases its stability and makes it more resistant to degradation. However, it can be difficult to break lignosulfonate into smaller fragments or dissolvement in the majority of solvents. As a consequence of this several depolymerization techniques can be utilized, such as acid-catalyzed, base-catalyzed, oxidative, pyrolysis, microwave-assisted, or sub-supercritical. By the utilization of these methods for instance in oxidative depolymerization the lignosulfonate is broken down into smaller fragments. This modification results in additional high-value low molecular weight compounds such as vanillin. Moreover, these chemical intermediates can be used in the synthesis of a variety of compounds, like bioplastics, resins, and pharmaceuticals.

The analysis of the oxidative depolymerized lignosulfonate is achieved by two different techniques called chromatography and mass spectrometry. To be able to identify and quantify the various acids, chemical analysis methods are required to be developed. Gas chromatography has for an extended time been the leading technique for studying lignin depolymerization. Although, to quantify more non-volatile acids derivatization is required. Derivatization involves chemically modifying the acid to make it more of a volatile derivative such as an ester. However, standardized and well-characterized methods to quantify acids of such nature are lacking. This puts up a barrier to the development of industrial products. Qualitative and quantitative analysis are important tools in chemical analysis. Quantitative analysis is used to determine the amount or concentration of the acids in the oxidative depolymerized lignosulfonate sample. Qualitative is utilized to identify the presence of the different acids in the sample. Together, these two types of analysis provide a complete picture of the chemical composition of the sample. The result of utilizing qualitative analysis led to identifying various acids, such as vanillin, and vanillic acid.

• author: Mahmud, Ujala
• supervisor: Mynta Norberg
• organization: Department of Chemistry
• course: KEMR30 20231
• year: 2023
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: Gas chromatography with flame ionization detection, Gas chromatography with mass spectrometry, Derivatization, Oxidatively depolymerized lignosulfonate, Monomeric acids
• language: English
• id: 9139114
• date added to LUP: 2023-09-29 08:42:40
• date last changed: 2023-09-29 08:42:40