Influence of the structure of imidazolium ionic liquids on the chromatographic behaviour of some flavonoids under RP HPLC conditions
Abstract
Flavonoids are the most common natural polyphenolic compounds of complex structure, possessing a wide range of biological activities. The determination of the qualitative and quantitative composition of complex mixtures of biologically active compounds (BAC) is an urgent problem of the pharmaceutical industry. The most suitable and used method for this task is reverse phase high performance liquid chromatography (RP HPLC). To increase the efficiency and selectivity of separation of the mixture, additives of various organic substances are introduced into the aqueous-organic eluent. In this study we investigated the effect of adding certain imidazolium ionic liquids (ILs) into the mobile phase on the chromatographic behaviour of a mixture of (+)-catechol hydrate, rutin, hesperidin, and quercetin under RP HPLC conditions. 1-Butyl-3-methylimidazolium bromide [C4MIM][Br], 1-hexyl-3-methylimidazolium bromide [C6MIM][Br], 1-decyl-3-methylimidazolium bromide [C10MIM][Br], 1-methyl-3-octylimidazolium bromide [MC8IM][Br] and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate [C4dMIM][BF4]. It was shown that the order of release of flavonoids does not change depending on the nature of the IL. It has been established that when IL is added to the eluent, the value of the flavonoid retention factor changes. Addition of IL with long alkyl substituents in the imidazole ring ([C10MIM][Br] and [MC8IM][Br]) led to increased retention of flavonoids (with the exception of the highly polar molecule hesperidin). This is explained, firstly, by the possibility of the formation of a hydrophobic ion pair between the analyte molecule and the IL, secondly, by the implementation of the ion exchange mechanism of molecular retention, and thirdly, by a change in the ionic strength of the mobile phase and an increase in the hydrophobic effect. IL with shorter substituents are not prone to such interactions. The ambiguous effect of imidazolium IL on the efficiency of the chromatographic process has been shown: for each sorbate, a different effect of addition of IL to the aqueous-acetonitrile eluent was observed. Chromatographic system with [C4dMIM][BF4] allows to increase efficiency for the three studied flavonoids - catechin, rutin, and quercetin.
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