Negative effect and removal of trace amounts of 1,3-dialkylimidazolium ionic liquids in samples from biorefineries

Ionic liquids (ILs), based on 1,3-dialkylimidazolium cations, are frequently used solvent components or auxiliaries for various types of biomass in biorefinery approaches. Unless washing and sample preparation have been carried out very carefully, analytical samples often contain residual traces of such ionic liquids. These residues can compromise the quality of physicochemical analyses, as was demonstrated for monosaccharide analysis after hydrolysis by gas chromatography, high-performance thin-layer chromatography, or ion chromatography (IC), and even damage analytical equipment, such as gas chromatographic capillaries or IC electrodes. We suggest a simple procedure—short stirring with solid elemental sulfur adsorbed on alumina as the scavenger—as a pretreatment of the analytical samples to remove interfering imidazolium IL traces. The chemistry underlying this pretreatment is the conversion of the 1,3-dialkylimidazolium cation to the corresponding, water-insoluble, neutral, volatile 1,3-dialkylimidazole-2-thiones. Given the negative effect of imidazolium IL impurities, the minor extension of the sample preparation by one short additional step appears to be a small price to pay for an unperturbed and instrument-safe analysis.