RESUMEN
Aqueous microemulsions (MEs), where an oil coexists with water in the presence of the anionic surfactant sodium dodecyl sulphate (SDS), have been proposed as a solution to decrease the amount of organic solvent in the mobile phase needed in reversed-phase liquid chromatography (RPLC). However, the oil phase of a typical ME is volatile, toxic and flammable, and although it is added in a small amount, it would be desirable to avoid it from an environmental perspective. This is the reason for the proposal of Peng et al. (J. Chromatogr. A 1499 (2017) 132â139) to replace the oil in microemulsion liquid chromatography (MELC) by the apolar ionic liquid (IL) 1-hexyl-3-methylimidazolium hexafluorophosphate ([C6C1IM][PF6]), to analyse neutral phenolic acids at acidic pH. Based on this report, an MELC procedure is here proposed for ß-adrenoceptor antagonists, which are basic compounds where the dominant species is cationic. To verify the formation of MEs containing SDS and IL, and elucidate the interactions between the cationic basic compounds with the SDS anion, and the cation and anion in the IL, an extensive study was carried out with several methylimidazolium ILs containing the cations [C2C1IM]+, [C4C1IM]+, or [C6C1IM]+, combined with the anions Cl-, BF4-, or PF6-, using 1-butanol as co-surfactant. The behaviour was compared with that observed in classical MELC with octane, micellar liquid chromatography with SDS and 1-propanol, and RPLC with mobile phases containing an IL and acetonitrile.
Asunto(s)
Líquidos Iónicos , Aniones , Cromatografía Liquida/métodos , Líquidos Iónicos/química , Aceites , Tensoactivos/química , Agua/químicaRESUMEN
The capability of liquid chromatography with microemulsions (MEs) as mobile phases was studied for the analysis of four parabens (butylparaben, ethylparaben, methylparaben, and propylparaben) and seven ß-adrenoceptor antagonists (acebutolol, atenolol, carteolol, metoprolol, oxprenolol, propranolol, and timolol). MEs were formed by mixing aqueous solutions of the anionic surfactant sodium dodecyl sulphate, the alcohol 1-butanol that played the role of co-surfactant, and octane as oil. In order to guarantee the formation of stable MEs, a preliminary study was carried out to determine the appropriate ranges of concentrations of the three components. For this purpose, mixtures of variable composition were prepared, and the possible separation of two phases (formation of an emulsion) was visually detected. The advantage offered by the addition of octane to micellar mobile phases, inside the concentration range that allows the formation of stable MEs, was evaluated by comparing the retention behaviour, peak profile and resolution of mixtures of the probe compounds, in the presence and absence of octane. The final aim of this work was the proposal of a mathematical equation to model the retention behaviour in microemulsion liquid chromatography. The derived global model that considered the three factors (surfactant, alcohol and oil) allowed the prediction of retention times at diverse mobile phase compositions with satisfactory accuracy (in the 1.1â2.5% range). The behaviour was compared with that found with mobile phases without octane. The model also yielded information about the retention mechanism and revealed that octane, when inserted inside the micelle, modifies the interaction between solutes and micelles.
Asunto(s)
Cromatografía Liquida/normas , Emulsiones/química , Modelos Químicos , Butanoles/química , Micelas , Parabenos/química , Dodecil Sulfato de Sodio/química , Tensoactivos/química , Agua/químicaRESUMEN
In reversed-phase liquid chromatography (RPLC), basic drugs are positively charged at the usual working pH range and interact with free anionic silanols present in conventional silica-based stationary phases. This translates into stronger retention and tailed and broadened peaks. This problem can be resolved by the addition of reagents to the mobile phase that are adsorbed on the stationary phase, avoiding the access of solutes to silanols. Among these additives, surfactants under micellar conditions have provided good silanol suppressing potency through the technique known as micellar liquid chromatography (MLC). The most common example of this is anionic sodium dodecyl sulphate (SDS). When SDS is at moderate concentration in the presence of high organic solvent content, micelles are not formed and the chromatographic mode is known as high submicellar liquid chromatography (HSLC). In contrast, the addition of an oil to an aqueous solution of SDS containing micelles gives rise to microemulsions in a chromatographic mode known as microemulsion liquid chromatography (MELC). A comprehensive comparison of the chromatographic behaviour of a set of basic ß-adrenoceptor antagonists analysed by MLC, HSLC and MELC is carried out in this work, in terms of retention, peak shape and organic solvent consumption. The study shows that high submicellar eluents reduce retention and enhance efficiency with respect to conventional RPLC and MLC. Meanwhile, MELC allows reduced analysis times with less organic solvent with respect to HSLC. The narrower and more symmetrical peaks in MLC, HSLC and MELC, with respect to conventional RPLC, reveal the presence of silanol masking.
Asunto(s)
Micelas , Tensoactivos , Cromatografía Liquida , Cromatografía de Fase Inversa , Dodecil Sulfato de SodioRESUMEN
In reversed-phase liquid chromatography, the performance for basic compounds is affected by the interaction of the protonated (cationic) species with the anionic free silanols on the alkyl-bonded stationary phases. Using aqueous-organic mobile phases in the absence of additives, the retention may be too high, and the peaks be broad and asymmetric. The performance is improved by addition to the mobile phase of ionic liquids, from which 1-hexyl-3-methylimidazolium chloride ([C6MIm][Cl]) has especially good characteristics. A recent report has also revealed that the use of the phosphate system as buffer, at varying concentration and pH, may have a significant role in the chromatographic performance of basic compounds, with effects on both retention and peak shape. In this work, this study has been extended to other three buffer systems (acetate, citrate, and formate), at increasing concentrations and pH 3 and 7, in the presence and absence of [C6MIm][Cl]. The results have been compared with those obtained with the phosphate system. The retention increases by addition of larger concentration of all buffers, in both absence and presence of [C6MIm][Cl]. Without additive, peak performance is also enhanced significantly. This effect is minimal in the presence of [C6MIm][Cl], which yields highly symmetrical peaks at all buffer concentrations, due to an effective blocking of the silanol activity.
Asunto(s)
Boratos/química , Cromatografía de Fase Inversa/métodos , Imidazoles/química , Acetonitrilos/química , Antagonistas Adrenérgicos beta/análisis , Tampones (Química) , Cromatografía Líquida de Alta Presión/métodos , Concentración de Iones de Hidrógeno , Líquidos Iónicos/química , Solventes , Agua/químicaRESUMEN
Microemulsions (MEs) are stable, isotropically clear solutions consisting of an oil and water stabilized by a surfactant and a co-surfactant. Oil-in-water microemuslion liquid chromatography (MELC) is a relatively new chromatographic mode, which uses an O/W ME as mobile phase. Retention, selectivity and efficiency can be modified by changing the concentration of the ME components and the ratio between the aqueous and oil phases. This work makes a critical survey on the information found in the literature about the mobile phase compositions that lead to the creation of successful O/W ME mobile phases, as well as the effect of pH for ionizable compounds and temperature. The viability of performing the analyses using isocratic and gradient elution is also considered. The complexity of the composition of a successful ME, and the fact that the different factors interact each other, may require many manipulations during method development to achieve an acceptable separation for complex mixtures. This is the reason of the proposal from several authors of a standard ME as starting point when developing a method for a new separation with no previous reports. Based on these initial conditions, the interest of several authors in applying computer-assisted approaches to optimize the composition of ME mobile phases, and reduce significantly the time and reagent consumption for method development, is described. Some practical tips are given to prepare stable ME mobile phases that yield reproducible results.
Asunto(s)
Cromatografía Liquida , Aceites/química , Agua/química , Cromatografía Liquida/normas , Tensoactivos/químicaRESUMEN
The reversed-phase chromatographic behaviour of six tricyclic antidepressants (amitryptiline, clomipramine, doxepin, imipramine, nortryptiline and maprotiline) was examined in this work with acetonitrile-water mobile phases, in the absence and presence of the ionic liquids 1-hexyl-3-methylimidazolium chloride and 1-hexyl-3-methylimidazolium tetrafluoroborate, which have interesting features for the separation of basic compounds, in terms of peak shape combined with reduced retention. Tricyclic antidepressants are low polarity drugs that strongly associate to the alkyl chains of conventional stationary phases. They are also positively charged in the usual working pH range (2-8) in reversed-phase liquid chromatography, due to their strong basic character. In consequence, they may interact with the residual ionised silanols present in conventional silica-based stationary phases, which is translated in stronger retention, and tailed and broad peaks. A simple chromatographic procedure for the control of tricyclic antidepressants in pharmaceutical formulations was developed using a C8 column and a mobile phase containing 30% acetonitrile/10 mM 1-hexyl-3-methylimidazolium chloride at pH 3, with UV detection. Intra- and inter-day precisions were usually below +1.0%, and intra- and inter-day bias (trueness) ranged between â2.1% and +2.4%, and between â3.0% and +2.3%, respectively. Sample preparation was simple and only required solubilisation and filtration previous to injection.