Modeling the competitive adsorption of sample solvent and solute in supercritical fluid chromatography

Structural elucidation of complex polyesters polyols from bio-lubricant using off-line liquid chromatography x supercritical fluid chromatography coupled with Orbitrap mass spectrometry

Synthetic complex esters and polyol esters are incorporated as partially bio-based and biodegradable alternatives to petroleum base oils in lubricant formulations, to provide specific properties or performance and to help reducing their carbon footprint in certain cases. A sample can contain over 400 molecules of high chemical similarity including numerous isomers. To resolve such complexity, a separation technique with large peak capacity coupled to high-resolution mass spectrometry (HRMS) is essential. In this study, comprehensive off-line LCxSFC hyphenated with an Orbitrap analyzer was used for the structural elucidation of a synthetic bio-lubricant composed of a polyol reacted with fatty acids of varying length or with repetitive units of polyesters of ricinoleic acid. Retention in the LC first dimension was mostly due to the degree of oligomerization of ricinoleic acid within the polyester and to the chain length of the fatty acid. The SFC second dimension highlighted the esterification degree of the polyalcohol and the number and positions of fatty acids double bonds. The combination of both dimensions permitted the separation of isomers. The coupling of SFC with Orbitrap analyzer allowed an accurate assignment of molecular formulas. Finally, the fragmentation in the ionization source confirmed the attributed structures. By introducing a clear distribution of the chemical structures in the retention space, LCxSFC-HRMS provided a powerful analytical method for the comprehensive molecular characterization of the complex polyester polyols sample.

Nonlinear liquid-liquid chromatography: Beyond a constant distribution coefficient

Liquid-liquid chromatography (LLC) is a technique in which the separation of mixture components is achieved due to their different distribution between the two phases of a pre-equilibrated biphasic solvent system. In this work, the LLC operation in the nonlinear range of the distribution isotherm was systematically examined for the first time. The influence of the feed concentration on the elution profiles of a model component (cannabidiol, CBD) was studied in three LLC units of different types and sizes ranging from ∼20 mL to ∼2 L. A series of pulse injections with CBD concentrations varying from 1 to 300 mg/mL was performed with n-hexane/methanol/water 5/4/1 (v/v/v) in descending mode (lower phase as the mobile phase). The elution profiles were simulated using the equilibrium-cell model and an anti-Langmuir-like equation for describing the CBD distribution equilibria. The distribution equilibria equation parameters were fitted to the CBD elution profiles using the peak fitting method. The model was validated and provided good predictions of the CBD elution profiles in the entire concentration range for all three LLC units.

The adsorption of methanol on reversed phase stationary phases in supercritical fluid chromatography

The surface excess isotherms of methanol from carbon dioxide on reversed phase stationary phases under two different operational conditions - which can be considered subcritical and supercritical conditions depending on the molar fraction of CO₂ in methanol - were determined using the minor disturbance peak method. The shapes of the surface excess isotherms were very similar in subcritical and supercritical conditions for the same column. To verify the influence of the sample solvent on the separation efficiency, two solvents methanol and heptane were used as sample solvents for alkylbenzene samples for the separation on the studied columns with pure carbon dioxide mobile phase. The separation efficiency was determined by calculating the number of theoretical plates. On the embedded amide stationary phase with methanol as a sample solvent the efficiency has increased due to the displacement effect of methanol on the solutes which are retained less than methanol. Then the efficiency for the rest of solutes, which coincide with the elution of the methanol peak tail has decreased as a result of the tag-along effect. The surface adsorbent heterogeneity has been discussed; the bonded ligands on the stationary phase surface demonstrated adsorption a big amount of CO₂, while methanol could adsorb with small amount on the residual silanols on the surface of stationary phase and the embedded (amide) polar group in the bonded phase.

Predictions of overloaded concentration profiles in supercritical fluid chromatography

Here, overloaded concentration profiles were predicted in supercritical fluid chromatography using a combined two-dimensional heat and mass transfer model. The heat balance equation provided the temperature and pressure profiles inside the column. From this the density, viscosity, and mobile phase velocity profiles in the column were calculated. The adsorption model is here expressed as a function of the density and temperature of the mobile phase. The model system consisted of a Kromasil Diol column packed with 2.2-µm particles (i.e., a UHPSFC column) and the solute was phenol eluted with neat carbon dioxide at three different outlet pressures and five different mobile phase flow rates. The proposed model successfully predicted the eluted concentration profiles in all experimental runs with good agreement even with high-density drops along the column. It could be concluded that the radial temperature and density gradients did not significantly influence the overloaded concentration elution profiles.