Deep eutectic solvents modified reversed-phase liquid chromatography for the separation of natural products

The composition of natural products is usually very complex. During the separation of natural products， high loading capacity and satisfactory chromatographic resolution are often hardly achieved. To address this limitation， various modifiers were added in the mobile phase to alter the chromatographic behavior. Here， deep eutectic solvents （DESs） were chosen to accomplish this objective on C18 column. Changes in retention factor， overloaded peak shape and adsorption capacity were systematically investigated and interpreted. The application potential of DESs in the separation of natural products was then evaluated. For ionizable compounds， the addition of DESs could compress the peak width and enhance the resolution. This is closely related to the electrostatic interactions provided by hydrogen bond acceptor （HBA） of DESs. For ionizable basic compounds like nortriptyline hydrochloride， they could remain in a cationic state constantly. Consequently， ion-exchange interactions occurred between nortriptyline cation and residual silanol anion， which resulted in severe peak tailing. However， this interaction could be shielded by the addition of HBA of DESs （such as choline chloride （ChCl））. As the pH of the mobile phase increased， the dissociation of silanol groups was enhanced. This rendered the shielding effect more prominent and led to a higher peak compression of nortriptyline hydrochloride. Notably， although the mobile phase was acidic， the dissociation of silanol groups cannot be completely suppressed. It means that this modulating effect of HBA never disappears. Based on these principles， the semi-preparative separation of berberine from Coptis chinensis extracts was successfully improved. The improvement in the separation of acidic compounds by DESs depends on the ion-pair formation. This effect is highly correlated with the mobile phase pH and salt concentration. Here， benzoic acid was chosen as a model to investigate the overload behavior and adsorption. The results indicated that under neutral conditions， ChCl addition converted the sigmoidal adsorption to a linear type. It can be attributed to the ion-pair formation between benzoate anions and HBA cations. Mutual repulsion between benzoate anions was therefore reduced and adsorption capacity correspondingly enhanced. Furthermore， the results of the overload experiment indicated that ChCl can effectively weaken the solute-solute interactions. It is worth noting that the mobile phase pH is barely influenced after the addition of ChCl. Remarkably， these features are of critical importance for the optimization of preparative chromatographic methods. The beneficial effect of HBAs was validated in the semi-preparative separation of chlorogenic acid from Lonicera japonica extracts. For neutral compounds like morroniside， the HBA in DESs cannot affect their chromatographic behavior through electrostatic interactions. However， there are usually many ionizable impurities surrounding the neutral compounds in natural products. Their chromatographic behavior can be modulated by the HBAs in DESs. The separation of neutral compounds in natural products could be indirectly optimized by modulating their chromatographic behavior. This view was also confirmed by the separation of morroniside from Cornus officinalis extracts.