Abstract
The presence of free silanols on alkyl-bonded reversed-phase stationary phases is responsible for broad and asymmetrical peaks when basic drugs are chromatographed with conventional octadecylsilane (C18) columns due to ionic interactions. In the last few years, ionic liquids (ILs) have attracted attention to reduce this undesirable silanol activity. ILs should be considered as dual modifiers (with a cationic and anionic character), which means that both cations and anions are able to adsorb on the stationary phase, creating a positively or negatively charged layer, depending on the relative adsorption. The accessibility of basic compounds to the silanols is prevented by both the IL cation and anion, improving the peak profiles. A comparative study of the performance of six imidazolium-based ILs, differing in their cation/anions, as modifiers of the chromatographic behavior of a group of ten β-adrenoceptor antagonists, is addressed. Mobile phases containing cationic amines (triethylamine and dimethyloctylamine) were used as a reference for the interpretation of the results. Using a mathematical model based on two chemical equilibria, the association constants between the solutes and modified stationary phase as well as those between solutes and the additive in the mobile phase were estimated. These values, together with the changes in retention and peak shape, were used to obtain conclusions about the retention mechanism, changes in the nature of the chromatographic system, and silanol suppression effect.
Highlights
Alkyl-bonded stationary phases with silica as support are extensively used in reversed-phase liquid chromatography (RPLC) due to their capability to separate a wide variety of compounds [1,2]
Surfactant monomers aggregate to form micelles in the mobile phase and are able to adsorb on the surfactant monomers aggregate to form micelles in the mobile phase and are able to adsorb on stationary phase modifying its properties
It should be noted that the peaks are highly symmetrical, especially in the presence of [C6 MIM]+. This suggests that all these additives efficiently hinder the access of the β-adrenoceptor antagonists to the residual free silanols on the stationary phase
Summary
Alkyl-bonded stationary phases (commonly octadecylsilane or C18) with silica as support are extensively used in reversed-phase liquid chromatography (RPLC) due to their capability to separate a wide variety of compounds [1,2]. In the silica derivatization process, free silanol groups remain on the stationary phase in a significant amount due to steric problems which vary with the brand and manufacturer. In typical RPLC columns, the working pH range varies between. This means that silanols are mainly negatively charged and are able to establish additional ion-exchange interactions with cationic compounds, increasing their retention. The kinetics of this process are slow, and, broad and asymmetrical peaks are obtained, which affects peak resolution [4,5,6,7]. The reduction of the “silanol effect” is of special importance in the analysis of this kind of compounds [8,9]
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