Abstract
In reversed-phase liquid chromatography (RPLC), positively charged basic compounds yield broad and asymmetric peaks, as a result of ionic interactions with free silanols that remain on conventional silica-based columns. Diverse solutions have been proposed to mask the silanophilic activity, which is translated to an improved peak shape. In this work, the chromatographic performance of hydrophilic interaction liquid chromatography (HILIC) was evaluated as an alternative to the addition of an ionic liquid (IL) to the aqueous-organic mobile phase used with RPLC columns, for the analysis of eight β-adrenoceptor antagonists. ILs change the behavior of RPLC stationary phases owing to adsorption on their surface. Meanwhile, in HILIC, a layer of adsorbed water is formed on the stationary phase surface. The association of cationic basic compounds with the adsorbed additive ions, hydrophilic partitioning on the HILIC columns, and other interactions, give rise to complex retention mechanisms. The chromatographic behavior was examined in terms of retention, elution strength, selectivity, peak shape and resolution, using acetonitrile-water mobile phases buffered at pH 3. Both chromatographic modes, RPLC with added IL and HILIC, proved to be a viable solution to the problem of poor peak shape for basic compounds.
Highlights
Chromatographic analysis of basic compounds in the reversed-phase mode (RPLC) with conventional alkyl-bonded stationary phases has the handicap of producing broad and asymmetric peaks, due to the direct interaction of the cationic species, formed in the acidic mobile phases, with the residual silanols in the silica supports
This work compared the performance of the chromatographic behavior of β-adrenoceptor antagonists achieved in hydrophilic interaction liquid chromatography (HILIC), with respect to reversed-phase liquid chromatography (RPLC) with and without an ionic liquid (IL)
It is shown that the long retention times obtained in conventional RPLC are significantly shortened, up to obtain practical analysis times, by addition of [C6 C1 im]Cl, or by using a polar column in HILIC conditions
Summary
Chromatographic analysis of basic compounds in the reversed-phase mode (RPLC) with conventional alkyl-bonded stationary phases (usually octyl or octadecylsilane) has the handicap of producing broad and asymmetric peaks, due to the direct interaction of the cationic species, formed in the acidic mobile phases, with the residual silanols in the silica supports. Residual silanols are present in a non-negligible amount on alkyl-bonded stationary phases, due to steric problems in the silica derivatization process [1,2] These groups are weakly acidic, with an average acidity constant (pKa ) approximately in the range 4.5 to 7, which depends on the type of silica used to build the column [1]. As the sorption-desorption kinetics of these interactions is slow, broad and tailed peaks are obtained, with implications in peak resolution [2,3,4] For this reason, the minimization and/or suppression of the “silanol effect” is a challenge in this chromatographic mode
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