Combined effects of mobile phase composition and temperature on the retention of phenolic antioxidants on an octylsilica polydentate column

Separation of phenylpropionic acids both by strong anion exchange stationary phase and strong cation exchange stationary phase in supercritical fluid chromatography, using the same additive

This study demonstrates the expanded application of ion-exchange stationary phases (including strong cation exchange, SCX and strong anion exchange, SAX) in supercritical fluid chromatography (SFC), and more importantly, provides a deeper understanding of the retention mechanisms of these two stationary phases when using the same acidic additive. Phenylpropionic acid compounds (belonging to phenolic acids) were selected as probes. On the SCX column, the π-π and polar interactions originating from the bonded benzenesulfonic acid groups were important foundations for prolonging the retention time of solutes, but they were also the main reason for solutes' tailing profiles. It was found that adding 0.1 % phosphoric acid can generate sufficient strength of electrostatic repulsion to obtain satisfactory peak shapes. Here, phosphoric acid can be adsorbed on the surface of the stationary phase to accumulate more negative charges, and at the same time, the phosphate anion and the phenylpropionic acid in mobile phase might combine into the contact ion pair (CIP)- like through the polar interactions to form the apparently negatively charged solute. On the SAX column, phosphate anions generated by ionization of phosphoric acid, were considered as counterions to effectively mask the electrostatic attraction of quaternary ammonium groups, to significantly reduce the retention of phenylpropionic acids, but improve their resolution. Finally, utilizing two developed SFC methods, i.e., SCX with MeOH + 0.1 % phosphoric acid in CO2, or SAX with MeOH + 0.3 % phosphoric acid in CO2, the baseline separation of the extract of Lonicerae Japonicae Flos was achieved within 6 min and 10 min, respectively.

Mechanistic Modeling of Reversed-Phase Chromatography of Insulins within the Temperature Range 10-40 °C

In the many published theories on the retention in reversed-phase chromatography (RPC), the focus is generally on the effect of the concentration of the mobile phase modulator(s), although temperature is known to have a significant influence both on the retention and on the selectivity between the adsorbates. The aim of this study was to investigate and model the combined effects of the temperature and the modulator concentrations on RPC of three insulin variants. KCl and ethanol were used as mobile phase modulators, and the experiments were performed on two different adsorbents, with C₁₈ and C₄ ligands. The temperature dependence was investigated for the interval 10-40 °C and at two different concentrations of each modulator. The model is derived from the expression for the adsorption equilibrium, which assumes that ethanol is adsorbed to the ligands and displaced by the insulin molecules, similar to the displacement of counterions in the steric mass-action model for ion-exchange chromatography. A good model fit to the new linear-range retention data was achieved by only adding and calibrating three parameters for the temperature dependence of the equilibrium. We found that a lower temperature results in a longer retention time for all adsorbates, adsorbents, and modulator concentrations used in this study, indicating that the adsorption process is enthalpy-driven. A comparison of the different contributions to the temperature dependence revealed that the large contribution from the equilibrium constant is dampened by the significant contributions of the opposite sign from the changes in activity coefficients of insulins and ethanol. Neglect of these effects when comparing different adsorbents and modulators might yield incorrect conclusions because the equilibrium constant varies with both, whereas the activity coefficients should be independent of the adsorbent. As expected, the conditions that promote higher retention also give a higher selectivity between the adsorbates. Nonetheless, in relation to its effect on the retention, the influence of the KCl concentration on the selectivity was significantly stronger than that of the temperature or that of the ethanol concentration.

Combined solvent- and non-uniform temperature-programmed gradient liquid chromatography. I - A theoretical investigation

An new class of gradient liquid chromatography (GLC) is proposed and its performance is analyzed from a theoretical viewpoint. During the course of such gradients, both the solvent strength and the column temperature are simultaneously changed in time and space. The solvent and temperature gradients propagate along the chromatographic column at their own and independent linear velocity. This class of gradient is called combined solvent- and temperature-programmed gradient liquid chromatography (CST-GLC). The general expressions of the retention time, retention factor, and of the temporal peak width of the analytes at elution in CST-GLC are derived for linear solvent strength (LSS) retention models, modified van't Hoff retention behavior, linear and non-distorted solvent gradients, and for linear temperature gradients. In these conditions, the theory predicts that CST-GLC is equivalent to a unique and apparent dynamic solvent gradient. The apparent solvent gradient steepness is the sum of the solvent and temperature steepness. The apparent solvent linear velocity is the reciprocal of the steepness-averaged sum of the reciprocal of the actual solvent and temperature linear velocities. The advantage of CST-GLC over conventional GLC is demonstrated for the resolution of protein digests (peptide mapping) when applying smooth, retained, and linear acetonitrile gradients in combination with a linear temperature gradient (from 20°C to 90°C) using 300μm×150mm capillary columns packed with sub-2 μm particles. The benefit of CST-GLC is demonstrated when the temperature gradient propagates at the same velocity as the chromatographic speed. The experimental proof-of-concept for the realization of temperature ramps propagating at a finite and constant linear velocity is also briefly described.