Mobile phase effects on single-component and competitive adsorption isotherms in reversed-phase systems

Retention factor and retention index of homologous series compounds in microemulsion electrokinetic chromatography employing suppressed electroosmosis

The retention factor (k) and retention index (I) of homologous series compounds such as alkylbenzenes (BZ), alkylaryl ketones, alkylbenzoates, and alkylparabens in microemulsion electrokinetic chromatography (MEEKC) with suppressed electroosmosis were investigated in a wide range of SDS concentrations ([SDS]), temperatures, and concentrations of organic cosolvents (phi). Using BZ as standards, the retention indices of other homologous series compounds were determined and they were found to be independent of [SDS] and temperature, while are dependent on the types and concentrations of organic cosolvents. The retention factor linearly increases with increasing [SDS], while linearly decreases with increasing temperature. The value of log k linearly decreases with increasing phi for methanol, ethanol, or ACN, while decreases by a second-degree polynomial with increasing phi for 2-propanol. Excellent agreement was found between the observed and predicted values of log k of analytes in MEEKC at given [SDS] and phi, where the predicted values were obtained from modified equations of the linear relationship of log k as functions of [SDS], the number of carbons, and phi. Therefore, both k and I can be used for peak identification of homologous series compounds.

Effect of the mobile phase composition on the isotherm parameters and the high concentration band profiles in reversed-phase liquid chromatography

Single-component adsorption isotherm data were acquired by frontal analysis for phenol on a C18-Kromasil packed column, under reversed-phase liquid chromatography conditions, using various methanol-water solutions (30-60%, v/v, methanol). The isotherm model accounting best for these data was the biLangmuir model. With increasing methanol content, the two saturation capacities decrease, particularly that of the high-energy sites, the adsorption constant of the low-energy sites decreases significantly and that of the high-energy sites decreases strongly. These results allow a quantitative investigation of the properties of the high-energy sites (which are not necessarily the so-called active sites), a feature rarely discussed yet. The band profiles calculated with the numerical values of the isotherm model parameters derived by fitting the frontal analysis data to the model and using the equilibrium-dispersive model agree very well with the experimental band profiles in the whole concentration range.

Numerical estimation of multicomponent adsorption isotherms in preparative chromatography: implications of experimental error

Since experimental methods for measuring multicomponent adsorption isotherms are extremely tedious, numerical approaches are an attractive alternative. Here, the variance in isotherm parameters as a function of experimental error in measured effluent concentrations is quantified. The number of experimental replicates needed to obtain isotherm parameters to a desired level of accuracy is calculated explicitly. After the covariance matrix of the parameters has been determined, Monte Carlo methods are found to be rapid and effective. The use of different kinds of experiments, the effect of resolution and loading, and the impact of the number of measured data points are described.

Fitting adsorption isotherms to the distribution data determined using packed micro-columns for high-performance liquid chromatography

Knowing the adsorption isotherms of the components of a mixture on the chromatographic system used to separate them is necessary for a better understanding of the separation process and for the optimization of the production rate and costs in preparative high-performance liquid chromatography (HPLC). Currently, adsorption isotherms are usually measured by frontal analysis, using conventional analytical columns. Unfortunately, this approach requires relatively large quantities of pure compounds, and hence is expensive, especially in the case of pure enantiomers. In this work, we investigated the possible use of packed micro-bore and capillary HPLC columns for the determination of adsorption isotherms of benzophenone, o-cresol and phenol in reversed-phase systems and of the enantiomers of mandelic acid on a Teicoplanin chiral stationary phase. We found a reasonable agreement between the isotherm coefficients of the model compounds determined on micro-columns and on conventional analytical columns packed with the same material. Both frontal analysis and perturbation techniques could be used for this determination. The consumption of pure compounds needed to determine the isotherms decreases proportionally to the second power of the decrease in the column inner diameter, i.e. 10 times for a micro-bore column (1 mm I.D.) and 100 times for capillary columns (0.32 mm I.D.) with respect to 3.3 mm I.D. conventional columns.

Description of adsorption equilibria in liquid chromatography systems with binary mobile phases

Adsorption of some simple compounds from pure and mixed solvents and of some solvents from mixed binary solvent mixtures on columns used in normal-phase, aqueous-organic and non-aqueous liquid chromatography was investigated. The distribution of the compounds between the liquid and the stationary phase is affected by the composition of the solvent mixture. Although preferential sorption of stronger solvents can often be described by the Langmuir isotherm, significant deviations are observed in some systems. A model was suggested accounting for the deviations from Langmuir isotherm by association on already adsorbed molecules. In most systems studied in this work, a simple competitive Langmuir isotherm did not provide a good fit to the experimental distribution data from mixed solvents when competition between the solute and the strong solvent for the adsorption sites was considered. A competitive isotherm taking into account possible association of solute on its own already adsorbed molecules and on the adsorbed molecules of strong solvent improves the fit to the experimental distribution data and enables description of the distribution data in dependence on the concentration of the strong solvent in mixed solvents in normal-phase, aqueous-organic and non-aqueous reversed-phase systems. Even though we need more data to prove the validity of the present model, we observe a good qualitative agreement of the experimental isotherm coefficients with the relative strength of the interactions expected in various chromatographic systems employing mixed solvents.

Optimization of the recovery yield and of the production rate in overloaded gradient-elution reversed-phase chromatography

To investigate the effects of various parameters on the production rate and on the recovery yield in overloaded reversed-phase gradient-elution chromatography, band profiles of binary mixtures of phenol and o-cresol were calculated using the experimental parameters of the distribution isotherms determined by binary frontal analysis. The effects of the feed volume and of the steepness of a continuous gradient (gradient time) of methanol in aqueous-organic mobile phases on the separation were studied. If the sample feed in a solvent with weak elution strength is used, combined effects of on-column enrichment, frontal chromatography and sharpening of the later eluted bands may enhance the production rate and the recovery yield. Steep continuous gradients offer better results than isocratic elution if the feed is dissolved in water and injected into the mobile phase with a higher elution strength, because larger volumes of dilute feed can be separated with high recovery yields.

Fitting competitive adsorption isotherms to the experimental distribution data in reversed-phase systems

Distribution isotherms of phenol and resorcinol and o-cresol were determined between aqueous solutions with different concentrations of methanol and an octadecyl silica adsorbent. Single-component distribution data can be well described by either the Langmuir or the Jovanovic adsorption isotherms. The competitive Langmuir isotherm with single-component isotherm coefficients can describe moderately successfully the distribution in mixtures of phenol and o-cresol in 20-40% methanol, but it does not fit the experimental distribution data of the mixtures of phenol and resorcinol, which are better described by the competitive Jovanovic and the quadratic isotherms with single-component coefficients. Generally, the quality of the fit of the isotherms improves as the concentration of methanol in the solutions increases. The dependencies of the logarithms of the Langmuir isotherm coefficients and of the coefficients of quadratic and Jovanovic isotherms on the composition of the mobile phase can be described by second-degree polynomial equations over the range 0-40% methanol in water.