Development of a carbohydrate silylation method in ionic liquids for their gas chromatographic analysis

• A silylation procedure of carbohydrates in ionic liquids, particularly in [EMIM][DCA], is optimized.
• Trimethylsilylimidazole was the most effective silylation reagent.
• Other silylation reagents used under ultrasonic agitation provide variable results depending on the carbohydrate nature.
• Ionic liquids can be used as alternative to non-volatile organic solvents for carbohydrate derivatization.

This paper reports on the feasibility of silylation of low molecular weight carbohydrates dissolved in different ionic liquids (ILs) for their further analysis by gas chromatography (GC). Derivatization reagents (nature and amounts), temperature and time of reaction and stirring conditions were evaluated for different carbohydrates (i.e., glucose, mannose, fructose and lactose) dissolved in 1-ethyl-3-methylimidazolium dicyanamide [EMIM][DCA]. Evaluation of conformational isomerism of glucose dissolved in [EMIM][DCA] revealed the effect of the time of dissolution in the equilibration of α- and β-furanoses (up to 3% and 6%, respectively, after 70 h of incubation) and that 21 h sufficed to obtain results similar to those provided by the reference method involving pyridine. Once optimized, the proposed derivatization procedure provided satisfactory yields (i.e., close to 100%) using 100 μL of trimethylsilylimidazole (TMSI) at mild conditions (25 °C) for a relatively short time (1 h) for most of the investigated carbohydrates. Under these experimental conditions, linear responses (i.e., R2 better than 0.974) were obtained in the tested range of 0.25–1 mg of the derivatized target compounds. Other reagents, such as N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) + 1% trimethylchlorosilane (TMCS), were successfully used under ultrasonic conditions for aldose monosaccharides and disaccharides derivatization, while BSTFA was useful for ketose monosaccharides. The possibility of using the proposed method for the derivatization of selected carbohydrates dissolved in different ILs and the efficiency of the method applied to the analysis of carbohydrates present in real samples (fruit juices) have also been investigated.

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