Diffusion and Dispersion in Tubes in Supercritical Fluid Chromatography Using Sub-2 µm Packings

In Situ Rapid Analysis of Squalene, Tocopherols, and Sterols in Walnut Oils Based on Supercritical Fluid Chromatography-Quadrupole Time-of-Flight Mass Spectrometry

Quantification of liposoluble micronutrients in large-scale vegetable oil samples is urgently needed, because their health benefits are increasingly emphasized. However, current analytical methods are limited to either labor-intensive preparation processes or time-consuming chromatography separation. In this work, an online oil matrix separation strategy for direct, rapid, and simultaneous determination of squalene, tocopherols, and phytosterols in walnut oil (WO) was developed on the basis of the lipid class separation mode of supercritical fluid chromatography. A single run was completed in 13 min containing 6 min of column cleaning and balancing. Satisfactory limit of detections (0.05-0.20 ng/mL), limit of quantifications (0.15-0.45 ng/mL), recoveries (70.61-101.44%), and matrix effects (78.43-91.62%) were achieved, indicating the reliability of this method. In addition, eight sterol esters were identified in WO, which have not previously been reported. The proposed method was applied to characterize the liposoluble micronutrient profile of WO samples obtained from different walnut cultivars, geographical origins, and processes.

Diffusion coefficients of an extensive set of pharmaceutical compounds in supercritical fluid chromatography over a wide range of mobile phase compositions

Diffusion data are essential for adequate analysis of the kinetic separation performance of any chromatographic system. Unfortunately, for Supercritical Fluid Chromatography (SFC), very little data is available of the diffusion coefficients in mobile phases typically used in contemporary methods, i.e. with a non-negligible amount of polar modifier such as methanol. In this work, a relative simple method which only requires minor modifications to a standard commercially available SFC instrument is used to determine the diffusion coefficient of an extensive set of pharmaceutical compounds in the range of 10-50 vol% of modifier (methanol) in CO₂. By using a traditional SFC column, the solute is first separated from the sample solvent plug, before entering a long capillary, where the band broadening can be linked to its diffusion coefficient using the Taylor-Aris equation. By using two UV-detectors, before and after the capillary, the effect of the dispersion in the column can be eliminated and the true volumetric flow rate determined. It was found that in the investigated range of conditions, the change in mobile phase viscosity in a first approximation allows to predict the variation in diffusion coefficient. Chemical structure and more particularly functional groups can however have a significant effect on the diffusion coefficient.