Analysis of lignosulphonates by pyrolysis-gas chromatography/mass spectrometry

Abstract

Lignosulphonates are complex biopolymers which are recovered from the spent sulphite pulping liquids. In the production of wood pulp using a sulphite pulping process, the lignins are sulphonated so they become water soluble. Lignosulphonates are used as binders, dispersants, emulsifiers and sequestrants in a host of products such as gypsum board, animal feed pellets and micronutrient systems. The properties of lignosulphonates are strongly affected by their structure, and therefore whether they are produced from softwoods, hardwoods, or grasses. In this work, we are using analytical pyrolysis coupled to gas chromatography and mass spectrometry (Py-GC/MS) in order to get information on the structure of lignosulphonates. Analytical pyrolysis is a rapid and highly sensitive technique for characterizing the chemical structure of lignin, allowing the analysis of small amounts of sample without prior manipulation and/or isolation. Characteristic features of lignins, including their composition in terms of p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units can be established based upon analytical pyrolysis. Four different lignosulphonate samples (one from hardwood and three from softwoods) were selected for this study: #261 (hardwood calcium lignosulphonate); #264 (softwood lignosulphonic acid, ion-exchanged to H+); #265 (softwood lignosulphonic acid, ion-exchanged to H+); #266 (softwood lignosulphonic acid, ion-exchanged to H+). These samples were analyzed by Py-GC/MS in order to asses the suitability of this technique for their characterization. All the lignosulphonate samples released, upon Py-GC/MS, high amounts of low molecular weight sulphur compounds (sulphur dioxide, dimethyl sulphide, propylene sulphide, dimethyl disulphide, dimethyl trisulphide and methanesulphonic acid methyl ester) arising from the sulphonate group. Lignin-derived compounds with both guaiacyl (G) and syringyl (S) structures, characteristic of a hardwood lignin, as well as some amounts of carbohydrate-derived compounds, were identified in lignosulphonate #261. However, only guaiacyl (G) lignin-derived compounds, characteristic of softwood lignins, were present in the rest of lignosulphonates (samples #264, #265, #266). The presence of only guaiacol and vanillic acid methyl ester in the pyrolysate of the lignosulphonate sample #266 seems to indicate that this sample is more oxidized than the rest.