Researchers at LUT University highlight challenges in calculating adsorption values during aqueous chromatography and explore new methods to close this knowledge gap.
Researchers Reko Lehtilä and Tuomo Sainio, based in the Lappeenranta-Lahti University of Technology LUT (Lahti, Finland), recently developed a new approach to monitor adsorption mechanisms surrounding aromatic compounds. Their research was published in the Journal of Chromatography A (1).
Cross-lined dextran separation materials have been used in chromatography since the 1950s (2). A notable property of these materials is that epichlorohydrin cross-linked dextran gels adsorb hydrophobic substances, despite the gel itself being hydrophilic. This proves useful when separating aromatic compounds; further, this property allows water to be used as a solvent, though target chemicals can only be moderately water-soluble.
Previous studies on these substances have not extensively explored differences in adsorption strength or the influence of temperature. Additionally, key thermodynamic parameters related to adsorption equilibrium remain undetermined in similar systems. To address these gaps, some approaches—such as enthalpy-entropy compensation plots—have been employed, enabling comparisons of potential differences in adsorption mechanisms among compounds.
In this study, 12 small aromatic compounds, alongside glucose and cellobiose, were selected to analyze how structural and physicochemical properties influence the adsorption of small aromatic compounds onto cross-linked dextran resin in aqueous chromatography. The group of compounds in question contained structural features common in phytochemicals, such as methoxy, hydroxyl and hydroxymethyl groups and glucosides. The retention factors of these compounds were measured in similar conditions at different temperatures, then compared with their structural and physicochemical information. An enthalpy-entropy compensation plot was used to distinguish groups of compounds that are expected to have differing adsorption mechanisms. At 21 ºC, the retention factors varied for the aromatic compounds, from 1.27, for benzyl ß-D-glucopyranose, to 2.48 for 3-methoxyphenol. When the temperature was increased to 50 ºC, there were mild effects on the strength of π-interaction mediated adsorption. That said, any hydrogen bonding-related adsorption effects in the aromatic compounds were diminished in the process.
The small aromatic compounds that exhibit similar types of adsorption mechanisms aligned along common trendlines, with stronger adsorption being associated with enthalpy (ΔH) and entropy (ΔS) values. Glucosylated aromatic compounds, however, were not found to follow similar trajectories, suggesting that different adsorption mechanisms are used.
When comparing physicochemical properties, the researchers found that lower values for topological polar surface area and sum of hydrogen bond donors were significant indicators of stronger adsorption. Additionally, a higher octanol-water partition coefficient and lower values for surface tension, and the activity coefficient in infinite dilution in water also correlated with stronger adsorption. The scientists deduced that physicochemical parameters can be useful indicators of expected differences in the strength of adsorption for small aromatic compounds. However, this data also is not sufficient to accurately predict retention order or relative adsorption strength. a higher octanol-water partition coefficient and lower values for surface tension, and the activity coefficient in infinite dilution in water also correlated with stronger adsorption.
The structural information of the small aromatic compounds was used to demonstrate that the retention factor (k) can be calculated based on the sum of structure effects, originating from the cumulative effects of their substitution patterns. Resonance, inductive and hydrogen bonding-related effects, along with temperature correlations for the hydrogen bonding strength, are used in an incremental prediction model to obtain similar k values as the observed ones. With this structure-based approach, the model can be a useful tool for estimating relative adsorption strengths and predicting the expected retention order of small aromatic compounds.
(1) Lehtilä, R. L.; Sainio, T. Adsorption Mechanisms of Small Aromatic Compounds in Cross-Linked Dextran Gel. J. Chromatogr. A 2025, 1758, 466162. DOI:
(2) Porath, J.; Flodin, P. Gel Filtration: A Method for Desalting and Group Separation. Nature 1959, 183, 1657–1659. DOI:
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