Theoretical Foundations of Liquid Adsorption Chromatography with Mixed Eluent

Simple models for the effect of aliphatic alcohol additives on the retention in reversed-phase liquid chromatography

Four retention models for the effect of aliphatic alcohol additives on the retention of analytes in reversed-phase liquid chromatography have been developed following either a semi-thermodynamic treatment or an empirical approach. Their performance was tested using the experimental retention times of six non-polar analytes (alkylbenzenes) and ten o-phthalaldehyde derivatives of amino acids under different isocratic chromatographic runs when a small amount of ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol or 1-heptanol was added to methanol/water mixtures containing a constant amount of methanol. It was shown that for the structurally simple alkylbenzenes all the models can be adopted for retention prediction with good results. In contrast, just one out of four models, that with the fewest approximations, predicts satisfactorily the retention properties of amino acids derivatives. However, the most interesting feature is that this model can predict the effect of an alcohol-additive on the retention properties of solutes, even if this additive was not used in chromatographic runs done for the fitting procedure, provided that it belongs to the same homologous series of alkanols. This feature is also observed in all models described the retention of alkylbenzenes.

A study of the enthalpy and entropy contributions of the stationary phase in reversed-phase liquid chromatography

The goal of this study was to elucidate the roles played by the stationary and mobile phases in retention in reversed-phase liquid chromatography (RPLC) in terms of their individual enthalpic and entropic contribution to the Gibbs free energy of retention. The experimental approach involved measuring standard enthalpies of transfer of alkylbenzenes from typical mobile phases used in RPLC (methanol/water and acetonitrile/water mixtures), as well as from n-hexadecane (a simple analogue of the stationary phase) to the gas phase, using high-precision headspace gas chromatography. By combining the measured enthalpies with independently measured free energies of transfer, the entropies of transfer were obtained. This allowed us to examine more fully the contribution that each phase makes to the overall retention. It was found that the standard enthalpy of retention in RPLC (i.e., solute transfer from the mobile phase to the stationary phase) is favorable, due to the large and favorable stationary-phase contribution, which actually overcomes an unfavorable mobile-phase contribution to the enthalpy of retention. Further, the net free energy of retention is favorable due to the favorable enthalpic contribution to retention, which arises from the net interactions in the stationary phase. Entropic contributions to retention are not controlling. Therefore, to a great extent, retention is due to enthalpically dominated lipophilic interaction of nonpolar solutes with the stationary phase and not from solvophobic processes in the mobile phase. Further, our enthalpy data support a "partition-like" mechanism of retention rather than an "adsorption-like" mechanism. These results indicate that the stationary phase plays a very significant role in the overall retention process. Our conclusions are in direct contrast to the solvophobic model that has been used extensively to interpret retention in RPLC.

Description and prediction of retention in normal-phase high-performance liquid chromatography with binary and ternary mobile phase

The suitabilities of several earlier reported models for description and prediction of retention in normal-phase systems with mobile phases comprised of two organic solvents-a polar and a non-polar one-were tested on the chromatographic behaviour of phenylurea herbicides and alkyl-, aryl- and nitrophenols as sample solutes with a silica-gel column and 2-propanol, n-heptane and dioxane as mobile phase components. With few exceptions, the data obtained from the best-fit three-parameter retention equation differ from the experimental capacity factors at less than 0.1, or 2%, and for most compounds the fit is better than 1%. Of the two two-parameter equations derived from theoretical models, one failed to describe the retention behaviour. The fit of the other two-parameter equation to the retention data of phenols is slightly inferior than with the three-parameter equation. The differences between the fitted data and the experimental capacity factors of substituted phenylureas were < or = 0.2, or 5%. Three-parameter equations were suggested to describe the retention behaviour in three-component mobile phases either at a constant sum or at a constant ratio of the two stronger solvents in ternary mobile phases. To fit the three-component data at any combination of concentrations of the three solvents with the error comparable to that in binary mobile phases, a nine-parameter equation is necessary. Suitability of these equations to describe the experimental behaviour of substituted phenylureas and phenols was verified in ternary mobile phases with various concentrations of 2-propanol, dioxane and n-heptane. Finally, a method was suggested for the prediction of retention in ternary mobile phases with varying concentration ratios of the two polar solvents from the parameters of best-fit equations in binary mobile phases and a single experimental capacity factor in a ternary mobile phase. For most capacity factors, the error of prediction was lower than 0.2, or 5%. Dried solvents were used to improve the reproducibility of the results and the temperature was controlled to +/-0.1 degree C in all experiments. With these precautions, differences between the original retention volumes and the data from repeated experiments measured after ten months of use of the column in the system tested were less than 0.2 ml for 85% of the values compared.