Modifier adsorption in supercritical fluid chromatography onto silica surface

Exploring the application limits of different hold-up time markers in supercritical fluid chromatography

The study focuses on the application range of nitrous oxide as a hold-up time marker in supercritical fluid chromatography (SFC). This compound has been suggested a decade ago to be used as unretained marker, something that the field of SFC was missing for a long time, since its beneficial properties make it an ideal candidate as hold-up time marker. Determination of the hold-up volume and actual volumetric flow rates have always been problematic in SFC due to the compressibility of carbon dioxide and one part of this is the difficulty of hold-up time measurements. Depending on the mobile phase, different methods have been used to measure the hold-up time with varying results. Nitrous oxide and other molecules have been compared in different conditions, mobile phases and stationary phases. In all cases, nitrous oxide gave the lowest elution times. However, detection was difficult in mobile phases containing 10% or more of organic modifier, because most solvents mask the signal of nitrous oxide. Interestingly, the choice of stationary phase also had a slight effect on detection, while different pressure and temperature settings affected each compound in a different manner.

Retention mechanism on phosphodiester stationary phases in hydrophilic interaction liquid chromatography and purely aqueous mobile phase part II: Overloading with limited soluble samples

The adsorption behaviour of caffeine and theophylline under hydrophilic interaction chromatography and purely aqueous conditions was investigated on four phosphodiester stationary phases. Solute adsorption isotherms were determined by frontal analysis or inverse method. The bi-Langmuir model was found to be the best choice to describe the behaviour of caffeine and theophylline adsorption in purely aqueous conditions, whereas the bi-Moreau model describes the adsorption phenomena in HILIC conditions. The results obtained demonstrate that the interaction of caffeine and theophylline with the stationary phase surface varies depending on the mobile phase composition. Both in pure aqueous mobile phase and in HILIC mode, the heterogeneity of the surface of the studied stationary phases is confirmed. In hydrophilic solutions, the sample molecules interact with the stationary phase only. In hydrophobic conditions, a lateral interaction occurs between caffeine or theophylline molecules, which are poorly soluble in acetonitrile-rich solvents. This confirms that the same compound on the same stationary phase can behave rather differently, depending on the mobile phase composition. Thus, the mobile phase may govern and control the retention mechanism.

Covalent immobilization of beta2 adrenergic receptor through trans-methylation reaction by SNAP-tag and its application in anti-asthmatic compound screening from Raphani Semen

Beta2-adrenergic receptor (β₂-AR) is believed as an attractive target for anti-asthmatic drugs. Its crystal structure and pharmacological activity have been clearly investigated. Yet the number of the approved anti-asthmatic drugs has declined in recent years. This work reports on the preparation of an immobilized β₂-AR column through the specific trans-methylation reaction between SNAP tag and the benzyl-guanine derivative and application in anti-asthmatic compound screening from Raphani Semen. The characterization of the immobilized β₂-AR was performed by scanning electron microscopy (SEM) and receptor-ligand interaction analysis by chromatographic methods. SEM analysis showed that the receptor has been successfully coated on the surface of PEGA amino microspheres. Binding constants of salbutamol and terbutaline calculated from frontal analysis within the temperature range of 10-30 ℃ confirmed the feasibility of the method in a thermodynamic viewpoint. Hydrogen bond was verified as the main driving force for drug-receptor interaction analysis. Sinapine was identified as the potential bioactive compound in Raphani Semen that specifically bind with β₂-AR with a specific binding site of Ser 207. Taking together, the immobilized β₂-AR column is promising in exploring drug-protein interaction analysis and anti-asthmatic drug screening.

Advanced Development of Supercritical Fluid Chromatography in Herbal Medicine Analysis

The greatest challenge in the analysis of herbal components lies in their variety and complexity. Therefore, efficient analytical tools for the separation and qualitative and quantitative analysis of multi-components are essential. In recent years, various emerging analytical techniques have offered significant support for complicated component analysis, with breakthroughs in selectivity, sensitivity, and rapid analysis. Among these techniques, supercritical fluid chromatography (SFC) has attracted much attention because of its high column efficiency and environmental protection. SFC can be used to analyze a wide range of compounds, including non-polar and polar compounds, making it a prominent analytical platform. The applicability of SFC for the separation and determination of natural products in herbal medicines is overviewed in this article. The range of applications was expanded through the selection and optimization of stationary phases and mobile phases. We also focus on the two-dimensional SFC analysis. This paper provides new insight into SFC method development for herbal medicine analysis.

Adsorption of N,N,N′,N′-Tetraoctyl Diglycolamide on Hypercrosslinked Polysterene from a Supercritical Carbon Dioxide Medium

The work considers for the first time the preparation of sorbents based on hypercrosslinked polysterene (HCP) and chelating agent N,N,N′,N′-tetraoctyl diglycolamide (TODGA) by impregnation in the supercritical (SC) CO₂ medium. Such sorbents can be applied for further isolation and separation of lanthanides, actinides and other metals. They are usually prepared by impregnation in toxic organic solvents (e.g., methanol, dichloromethane). Our study shows that application of SC CO₂ instead of organic solvents can significantly speed up the impregnation, perfom it in one stage and make the process more eco-friendly. At the same time, the obtained sorbents are close in their parameters to the classical ones. This article presents the results of measuring the TODGA adsorption isotherms on two HCP sorbents (MN202 and MN270) on a wide range of SC fluid parameters. Adsorption measurements were carried out using on-line supercritical fluid chromatography and gravimetry. Based on the sorption capacity parameter, MN202 sorbent was selected as the better carrier for TODGA. An impregnation temperature increase within the range 313–343 K in isochoric conditions (ρ = 0.780 g/mL) reduces the maximum of TODGA adsorption from ~0.68 mmol/g to ~0.49 mmol/g.

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.

Multilayer adsorption in liquid chromatography - The surface heterogeneity below an adsorbed multilayer

A numerical method was introduced for the estimation of the surface heterogeneity below an adsorbed multilayer of the analyte. The calculation procedure is based on the raw adsorption isotherm data points obtained by frontal analysis experiments. To permit the mapping of the nature of the analyte-surface interaction, a numerical procedure was used to pre-estimate the adsorbate-adsorbate interactions occurring during the adsorption process. The surface heterogeneity estimation was carried out using the affinity-energy distribution calculations with assuming local BET isotherm. In the local BET isotherm the pre-estimated adsorbate-adsorbate interaction constant was used, and the surface heterogeneity was described. After the test of the numerical method with benchmark isotherms, the algorithm was tested on several experimental isotherms. The isotherms were measured using phenol as test molecule on reversed phase adsorbents, with different surface coverage of the octadecyl ligands. The surface of the non-end-capped stationary phases showed detectable heterogeneity, while the surface end-capped phases were found to be homogeneous.