Discrimination of levoglucosan and its structural isomer in biomass pyrolysis products by iodometry, liquid chromatography, mass spectrometry

Abstract

This study aimed to selectively determine the main products of the pyrolysis of pre-treated lignocellulosic biomass, namely, two structural isomers of 1,6-anhydro-β-D-glucose (AG) – its pyranose (levoglucosan, LG) and furanose (AGF) forms. The influence of AGF on the iodometric titration of LG was evaluated. The presence of AGF had a clear impact on the titration results, but with sample oxidation at pH 9 it was possible to obtain accurate (relative error 1–5%) titration results for total AG, summing up both isomers. More importantly, several liquid chromatography methods were compared for LG and AGF quantification to develop a simultaneous selective determination of the AG isomers. Three columns showed promising results for the selective quantification of LG and AGF – ligand exchange columns with Ag+ or Pb2+ as counterion coupled to HPLC-RID, and a HILIC column coupled to UHPLC-ELSD under gradient conditions. All three methods showed acceptable precision, linearity, and recovery of AGF and LG. UHPLC-ELSD with an amide column could only be used to quantify the summed amount of AG isomers without chromatographic separation. AGF and LG had similar response factors in RID and ELSD methods. Moreover, it was shown that UHPLC-QTOF-MS could be used for qualitative investigations of the sugar derivatives found in biomass pyrolysis products. Samples of fast pyrolysis products were analysed with varying content of AGF (7–55%) and LG (25–64%). The samples were obtained from pre-treated birch wood, and anhydrosugar fractions were separated from the pyrolysis liquids.