Sorption of flavonoids from aqueous acetonitrile solutions containing imidazolium ionic liquids on octadecyl silica gel under RP-HPLC condi-tions
Abstract
Flavonoids are biologically active compounds in medicinal plants, they are widely used in the production of herbal medicines. In this regard, it is important to study the relationship between the structure of sorbates and their retention and to understand the influence of various additives on the chromatographic analysis for their qualitative and quantitative determination by reversed-phase HPLC (RP HPLC). In this paper, we studied the influence of the structure of some flavonoids on their retention in the aqueous acetonitrile solution/imidazolium ionic liquid (IL)/octadecyl silica gel systems. It was shown that the planarity of flavonoids is one of the main factors influencing the retention. It was found that planar molecules of flavonoid aglycones are retained on octadecyl silica gel in aqueous acetonitrile systems containing ILs much better than flavonoid glycosides. It was determined that the nature and structure of the IL used as a specific additive to the aqueous acetonitrile eluent significantly change the retention factors of the studied flavonoids. In particular, it was found that retention factors of flavonoids increase in systems with bromide ILs in the following series: [C4dMIM][Br]<[C4MIM][Br]<[C6MIM][Br]<[C10MIM][Br]. The retention of flavonoids in systems with imidazolium bromide ILs is greater than in systems with similar tetrafluoroborate ILs: ([C6MIM][BF4]<[C6MIM][Br] and [C10MIM][BF4]<[C10MIM][Br]). ILs containing an additional methyl group in the 2-position of the imidazole ring show the opposite trend: the retention factor with bromide ILs is lower than with tetrafluoroborate ILs ([C4dMIM][BF4]>[C4dMIM][Br]). Changes in the retention factors in systems with different ILs are associated with changes in the hydrophobic effect in the system due to the different impacts of the cosmotropic and chaotropic effects of IL cations and anions on the aqueous acetonitrile solution. We analysed the concentration dependences of retention factors using the Snyder-Sochevinsky and Sochevinsky-Wachtmeister models (the range of acetonitrile content was 20÷40 vol.%). We obtained the corresponding linear equations with high values of the determination coefficients, which were used to find the angular coefficients within the considered models.
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